Sunday, May 1, 2016

La Nina Expected for Winter 2016-2017

A La Nina is expected to develop this summer, and should reach maximum intensity during the winter of 2016-2017.

NCEP/NOAA
A look at sea surface temperature anomalies across the central Pacific reveals a change in progress. Over the last several months, we have been in a notable El Nino event, which is characterized by above to well-above average temperature anomalies in the waters along the Equator, west of Ecuador. However, in the last few weeks, we've started seeing those warmer water anomalies wane, and even be replaced by cooler than normal anomalies. This switch is occurring mainly in the waters immediately offshore Ecuador, as well as a little farther out to sea, between the 120W and 110W longitude lines.

CPC
We can trace this cooling to an upwelling event ongoing across the Equatorial Pacific. Shown is a cross-section diagram of the ocean at the Equator, from a depth of the surface to 450 meters. Ever since February, note how we've been seeing well-below normal water anomalies pushing towards the surface, and recently breaching the surface near Ecuador. This is the root cause behind those cooling anomalies on the surface image discussed earlier in this post, and so long as this upwelling of cooler waters continue, the rest of the Equatorial Pacific should continue to cool down from this El Nino.

But what about the future... specifically the winter ahead?

IRI/CPC
A chart showing the forecasted sea surface temperature anomalies in a portion of the Equatorial Pacific known as Nino region 3,4 shows our decline from an El Nino into 'ENSO-Neutral' conditions around May and June. You may recall that Neutral conditions are when the waters are too warm for a La Nina, but too cool for an El Nino, and this is defined when water temperatures in Nino region 3.4 are between +0.5 degrees anomaly and -0.5 degrees anomaly. We begin to enter a La Nina around August or September, when the average of model guidance dips below -0.5 degrees anomaly, the threshold for a La Nina. You'll also notice it reaches maximum intensity as we head into the winter of 2016-2017.

Now the big question: what does this mean for winter 2016-2017?
The big answer: we don't exactly know. But we do have an estimate of what things will look like if this forecast holds.

NCSU
The above graphic shows typical weather conditions in a La Nina pattern. In the U.S., La Nina's tend to bring cooler than normal conditions to the Pacific Northwest, northern Plains, and Midwest. Wetter/snowier than normal conditions also tend to evolve in the Midwest, Ohio Valley, and portions of the Great Lakes. The Southern U.S., however, generally can expect warm and drier than normal winters out of La Nina's. This isn't a set-in-stone interpretation of what the coming winter will be like, but we could see something similar to this graphic set up.

To Summarize:

- A La Nina is expected to develop this summer, and intensity into the coming winter.
- As is typical in La Nina winters, the Ohio Valley may be wetter/snowier than normal, while cooler weather overspreads the North U.S. The South may remain warm and dry.

For more weather updates, follow me on Twitter: @akracki and @TheWxCentre.

Andrew

Thursday, April 21, 2016

April 26 Potentially Significant Severe Weather Event

A potentially significant severe weather event continues to show up in model guidance for Tuesday, April 26th over the Central and Southern Plains.

Storm Prediction Center
The Storm Prediction Center's long range outlook for Tuesday places a 30% chance of severe weather within 25 miles of a given point across northern Texas, much of central and eastern Oklahoma, and southern Kansas. A 15% chance of severe weather within 25 miles of a given point is situated from central Texas north into Oklahoma (outside of the panhandle), much of central and eastern Kansas, and extreme western portions of Missouri and Arkansas.

CoD
This event will begin taking shape on Monday, April 25th, when the storm system that will act to ignite this potential outbreak makes landfall on the West Coast. 500-millibar speed max near 100 knots barreling into southern California and western Arizona will be associated with a vorticity max rounding the base of the main trough, whose energy will be extended from northern California offshore Oregon and Washington by Monday morning. This speed max will shunt the aforementioned vorticity max east, where it will then round the lee side of the trough and act to pull the main trough northward by Tuesday morning.

The main energy associated with the trough will continue moving southeast into the Southwest U.S. and strengthen as another speed max rounds the base of the trough, this time making it negatively-tilted. Recall that a negatively-tilted trough means the highest wind speeds are 'pointing' the trough in a southeast direction. This indicates the trough is mature, and is optimal for severe weather.

CoD
As mentioned above, by Tuesday night at 7 PM Central Time, we see a speed max just breaching 70 knots traversing the Central Plains, indicating the trough is now negatively-tilted and at max strength. The trough here is in prime positioning for a severe weather event to unfold across the Central U.S. in the evening and overnight hours on Tuesday, and will set the stage for continued opportunities for severe weather as the week continues.

CoD
Strong moisture flow prior to the trough's entrance to the region will allow surface dewpoints to surge into the 70s by Tuesday evening, as the above graphic shows. As such, instability on the order of 4000 j/kg to 5000 j/kg is anticipated across Oklahoma into northern Texas, approximately in line with the 30% delineation in the Storm Prediction Center graphic at the beginning of this post. This ample moisture will allow for cloud bases to drop below 1000 meters- soundings from points across central and northern Oklahoma suggest cloud bases (also known as LCLs) will be around 600-800 meters off the ground, which can be conducive for tornado development. 800 meters off the ground is a little high for prime tornado formation conditions, but ample instability may act to offset a portion of this.

CoD
Lastly, supercell composite forecasts off the 18z GFS suggest the environment most conducive for supercell formation will be from southern Kansas across central Oklahoma, just into north Texas, again in line mainly with that 30% severe threat corridor. Although supercell composite values are high in central Kansas, the best axis of instability will be displaced in extreme southern Kansas and central Oklahoma, and it is this latter region where the best potential for severe weather lies. In this region, taking current model guidance at face value, all hazards of severe weather would be possible. This would include a threat for strong, possibly long-track tornadoes, very large hail, and damaging winds.

To Summarize:

- A severe weather event is being forecasted for April 26th.
- Substantial uncertainty exists regarding storm initiation, threat level, and placement of best environment, among other things.
- As such, confidence is low with this event.
- Going by current model guidance, a potentially dangerous severe weather situation could evolve across northern Texas, Oklahoma, and southern Kansas on April 26th.

Andrew

Wednesday, March 30, 2016

Severe Weather Discussion for March 30, Posted March 30

A severe weather event is forecasted to unfold across the Plains and Mississippi Valley today, March 30th.

Prognosis…
           Large upper level low currently stationed across the Western US will provoke a potential severe weather event in the Plains and lower Mississippi Valley today. Situation is complicated as convection later in the day will be heavily influenced by morning convection, residual cloud cover, as well as dryline placement later in the day.

Overnight…
               Stout upper level low centered across Nevada and Utah will begin to flatten as energy currently located in the Dakotas is ushered northeastward in pace with the jet stream, which will be arching north of mid-level ridging currently stationed in the lower Mississippi Valley. This energy in the Dakotas will be drawn towards another system in south-central Canada, resulting in a band of elevated vorticity extending from Canada back to Nebraska at 12z today. Further, the vorticity max currently in southern California, extending in a semicircle offshore the West Coast into Oregon, will keep the upper level low positively tilted and aid in this flattening-out.
    Moisture flow will continue to amplify across the Southern Plains, with water vapor imagery showing the subtropical jet feeding a moisture plume into the region. As such, cloud development occurred across the area earlier Tuesday and continues to persist at the time of posting. To this extent, continued moistening of the planetary boundary layer through isentropic ascent, notable 850-millibar moisture plume advection, and substantial low-level warm air advection will result in areas of drizzle to showers overnight, particularly across Texas and Oklahoma. 3/30 00z KOUN sounding showed strong temperature inversion between 800-millibar and 700-millibar levels, and I suspect this inversion will be strong enough to suppress anything more than broad, generally-weak precipitation. Surface plots at 0700z show areas of mist and light rain across central and eastern Texas, in line with favored model guidance. Model guidance troubles will be discussed more in depth in the following sections. Uncertainty with this particular subject of morning precipitation will feed into the morning and afternoon discussions below.

Early Morning to Noontime…
         Convection-allowing model guidance has been promoting the formation of showers and thunderstorms along the Red River, most notably in southeast Oklahoma in the mid-morning hours today, following expected overnight precipitation and continued moistening of the lower levels. Not completely convinced we see hail threats as advertised by some, including the threat of up to tennis ball-sized hail as per NWS OUN in their hazardous weather outlook posted yesterday. However, as the atmosphere continues to become more conducive to severe weather through continued warm air and moisture advection overnight, will not rule out severe weather tomorrow morning, especially given expected values of elevated instability that may favor stronger storms. I do believe hail is a primary threat with these storms, especially in their elevated nature, but again, I am not completely sold on the idea of severe and/or significant hail.
         Primary point of this timeframe discussion is to what extent precipitation is ongoing by and after 12z. Model guidance has been hinting at potential convection across portions of Oklahoma, especially south and east in the morning hours, but has also left the door open for weaker, more broad precipitation in central and northern Oklahoma. Regardless, expect overcast cloud cover given continued lower-level warm air and moisture advection, as well as taking into account 3/30 00z soundings across Texas and the western Gulf Coast. I am not taking the HRRR model into account for this discussion, as although it is currently handling surface precipitation (or lack thereof) rather well, it brings surface temperatures across Oklahoma and Texas into the 80s and 90s, well above current and more-realistic temperature forecasts introduced by the NWS. Additionally, am not using the NCAR ensemble, as all members have weak, broad, and scattered precipitation at initialization (3/30 00z) across Texas, which was proven to not be the case when compared with 3/30 00z radar. In terms of morning precipitation, am currently favoring 00z 4km WRF-NMM model, as well as SPC SREF system, as both models handle initialization conditions at the surface with regards to precipitation coverage well, and perform well with expected high temperatures later in the day. Given high model uncertainty with this event, however, making these two forecasts ‘favored’ is not exactly as affirmative a word as it is made out to be.
          In all, expectation is for precipitation, possibly convective in nature, to be ongoing in portions of Texas and Oklahoma by daybreak today and in the morning hours in general. Most favored region for this activity, as promoted by model guidance and SPC probabilistic thunderstorm outlooks, is southeast Oklahoma into northeast Texas, east into Arkansas. Weaker and more broad precipitation also possible for central Oklahoma, where uncertainty is greatest.

Afternoon and evening…
        Remarkably complex situation will unfold for the afternoon and evening hours. It has been noted that the upper level low in question has been progressing east more slowly than anticipated yesterday, and this has been feeding into a further-west trend with the dry line positioning later on in the afternoon today. Case in point, 4.0km WRF-NMM guidance shows convective initiation in the afternoon hours along the dry line, which is projected to be planted almost immediately east of the Oklahoma panhandle in due north-south orientation. This is the most westward solution I’ve seen, and while it has handled overnight precipitation well so far as of this posting, and the upper level low has been progressing slower than thought, the far westward positioning of this dry line is something to monitor. To speak more frankly, this solution is suspect, although this model still remains among favored guidance as of posting time. 3/29 21z SPC SREF takes a nice middle ground between this westward solution and further-east guidance, placing the dryline approximately along a north-south line of Lawton, Oklahoma to Burlington, Oklahoma by the afternoon hours. As such, thunderstorm initiation along the dryline would threaten the Oklahoma City metro area, and eventually may threaten the Tulsa region.
        Concerns have been brought up frequently in discussions around the lack of convective initiation on the dry line in some model guidance, despite the presence of an uncapped, supercell-favorable storm environment. This would be the result of a lack of trigger mechanism, it appears, as some guidance shows weak convergence along the dry line. I am personally opposed to this solution, and while it may very well pan out, I am more pressed to believe favored guidance, which does initiate convection along the dry line in the afternoon hours.
In the event convection does initiate along the dry line, it is expected that thunderstorms would quickly become supercellular, with all modes of severe weather possible. The new SPC Day 1 outlook has placed central and southern Oklahoma in the Enhanced Risk of severe weather, primarily for potentially significantly large hail, in addition to damaging winds and a non-zero, yet relatively low tornado threat. Although it still retains unrealistically-high surface temperatures tomorrow afternoon, new runs of the HRRR model continue to gradually increase the coverage and intensity of convective initiation along the dry line in central Oklahoma. Initiation here is along the Lawton-Burlington line previously mentioned, but I am not willing to put stock into the HRRR currently given its unrealistic surface temperatures.
       Outside of Oklahoma, best tornado dynamics outlined by SPC Day 1 projection and SPC SREF guidance will be placed in Louisiana/Arkansas/Texas area, where a 10% hatched delineation has been posted in the Day 1 outlook. Despite this, as mentioned previously, non-zero tornado threats will exist in Texas and Oklahoma in addition to these more favorable dynamics.

        To summarize, potential severe weather event is on the table today for the Southern Plains into the lower Mississippi Valley, with high uncertainty still included in the forecast. Broad, weak precipitation expected to develop and move into Texas and Oklahoma overnight into the morning hours will likely temper a full-on severe weather outbreak for the Plains, but afternoon clearing and favorable dry line positioning may allow the formation for strong to severe storms in the afternoon across central and eastern Oklahoma. Widespread convection is expected for the eastern Texas / northern Louisiana / Arkansas region, where tornado dynamics will be best. Large hail will be a day-long threat for the Oklahoma area, particularly with a round of thunderstorms in the morning that may form in the midst of the weak, broad precipitation to produce hail, and then again in the afternoon when potentially-supercellular development occurs along the dry line.

Friday, March 4, 2016

Severe Weather Discussion for March 7th, Posted March 4th

This is a discussion for the potential severe weather event on March 7th, 2016 in Oklahoma.

Models Used: 18z GFS, 12z ECMWF, 15z SREF

    Synopsis
    Energy currently elongated near the Gulf of Alaska, beginning to round the base of a strong upper level low in the Gulf, will continue to be pushed east as the strongest portion of the upper level low will retreat north and west over the Aleutian Islands by Sunday. System of interest will make landfall in California around 15z Sunday, digging as it does so to form a ridge across the Northern Plains. Small piece of energy will be sheared off into the Aleutian Islands upper level low as this landfalling occurs, and this will weaken the primary energy enough until it begins to re-organize while progressing into the Four Corners region. By 00z Tuesday, this strong shortwave will be located over western Kansas, with a much stronger upper level trough digging well south into Baja California, and eventually into Mexico. This stronger upper level feature will force a slight ridge over the Four Corners region, and also act to pump a large ridge over the Midwest and Eastern U.S., in response to the general long wave trough pattern in the Western U.S. into the Plains. As the shortwave rides the western fringe of this ridge, weakening will occur before the energy is ingested by another vorticity maxima riding the Canada / United States border.


    Oklahoma
    Potential severe weather event will become set up with the strong shortwave moving into western Kansas, attaining what will pass as a negative tilt in the process. 500-millibar jet streak AOA 60 knots will form by 18z 3/07 over the Texas and Oklahoma Panhandles into western Kansas as the shortwave moves into the Plains. Jet streak will move east by 00z, with speeds weakening slightly to around 55 knots out of the southwest over much of Kansas and Oklahoma. Best wind features on the 700-millibar level at 00z 3/08 will be in the terms of a jet streak positioned in central / eastern Kansas, Oklahoma (outside of the panhandle and southwest portion of the state), northwestern Arkansas and western Missouri, with speeds up to 55 knots out of the west-southwest. 850-millibar wind field finds southerly winds across the Plains, with speeds of 40 to 50 knots in the Oklahoma area. Also will make note of sustained lower-level warm air advection, most intense from about 00z 3/07 until the time of the severe weather event. This advection will see 850-millibar dewpoint temperatures rise to the 10-12 degree Celsius mark, enabling moistening of the PBL. Most intriguing feature is 925-millibar wind direction at 00z 3/08 out of the south-southeast at speeds AOA 40 knots for much of Oklahoma, and slightly higher into Kansas. This veering wind profile is confirmed when observing projected surface winds at the same time to be almost out of the southeast, if not south-southeasterly.
    Surface low with strength of approximately 992 millibars in south-central Nebraska by 00z 3/08 will enhance moist airmass fetch from the Gulf of Mexico, tracing surface and 850-millibar wind fields back to the Galveston, Texas to Mobile, Louisiana region. Surface dewpoint projection from the 18z GFS show values nearing 60 degrees Fahrenheit by 12z Tuesday, although 00z Tuesday values see a lower, but still impressive swath of dewpoint numbers AOA 55 degrees.

    Storm Prediction Center currently outlooks western Oklahoma into central Kansas and eastern Nebraska, as well as much of central Texas for a 15% chance of severe weather on the long range Day 4-8 outlook for this event. Latest SPC SREF run shows a maxima of over 50% probability of supercell composite values AOA 1.0 occurring simultaneously with over 0.01” of precipitation falling - basically the probability of strong thunderstorms occurring - along the Red River south into the region immediately northeast of Abilene, Texas at 03z, with values gradually increasing and pushing almost due north towards Woodward, Oklahoma by 06z 3/08. The presence of this 50% to 70% maxima advecting northward during the night conveys uncertainty exhibited by the SPC SREF members, but also the confirmation that strong thunderstorms are possible in this environment. Projected 0-6 kilometer shear will increase past 00z Tuesday, as the nocturnal lower-level jet kicks in and the aforementioned veering wind profile continues to take shape across Oklahoma.
    GFS suite has taken a more aggressive tone in this event since a particularly bullish 00z run on 3/04, which triggered a potentially tornadic environment signal for a good portion of Oklahoma for this Monday night event. Analysis of forecasted soundings using the 18z GFS continue to indicate a potentially tornadic environment near Lawton, Oklahoma with SBCAPE exceeding 1800 j/kg amidst close to 60 j/kg CIN. Surface to 6km shear forecasted at 50 knots, combined with 0-3km storm-relative helicity values near 300 m2/s2 only acts to confirm this strong thunderstorm environment for the southwest Oklahoma area, the same region highlighted by the latest run of the SPC SREF suite for potential strong thunderstorms. Examination of forecasted soundings over Norman shows lower instability on the order of 800 to 900 j/kg SBCAPE, although values slightly higher for MLCAPE on the order of AOB 1000 j/kg. Storm-relative helicity and shear values generally consistent with Lawton numbers, although weaker instability and still-present PBL temperature inversion may pose problems for convection in central Oklahoma. Despite this, veering wind profile affirms potential for strong thunderstorms in the general state of Oklahoma, particularly if the inversion can be broken. Environment in Norman becomes far more favorable for convection by 06z 3/08, with SBCAPE and MLCAPE both taking a significant jump to AOA 1200 j/kg, although the presence of a now-weakened surface temperature inversion on the magnitude of 10-20 j/kg CIN still presents some concern. Additionally, substantial lowering of bulk shear and storm-relative helicity values, despite maintaining a veering wind profile on this forecasted sounding, highlights how factors may line up for strong to severe thunderstorms, but not necessarily an explosive severe weather event according to current projections.
    In terms of tornado potential- lowering LCLs as convection approaches, combined with sufficient instability and SRH / shear lead me to believe this event will pose the first notable threat for rotating thunderstorms this spring season. Not incredibly impressed with the potential for a large-scale - nor a large-number - tornado event, but the environment should be supportive of potentially tornadic thunderstorms. This will be further addressed as the timeframe of this event approaches.

    Overall, this event is showing signs of supporting strong to severe thunderstorms over Oklahoma. Highest concern from most recent model runs rests with western Oklahoma, particularly along a corridor of Eldorado to Comanche, to Buffalo to Medford. It is this corridor that should see the best forcing for thunderstorms, and will likely have the best threat for tornadic thunderstorms. Further east, in the Oklahoma City / Norman region, severe thunderstorms will still be possible, albeit higher uncertainty is present with potential capping and forcing concerns. Will re-evaluate this portion of the state in the next discussion to try and clear up this uncertainty, should successive model runs follow suit.

Andrew

Sunday, February 28, 2016

March 1, 2016 Severe Weather Discussion for Oklahoma

This is a severe weather discussion valid for the morning of March 1, 2016 for the state of Oklahoma.

Models used: 00z NAM, 00z NAM-4km, 00z GFS, 21z SREF

Synopsis
Energy that will trigger this event is currently pushing into the Pacific Northwest / British Columbia region, moving directly into a small ridge positioned immediately due east of the system. Expectation is for the main body of energy to ride down on the lee side of a ridge that will build in across the Pacific Northwest as the energy continues moving eastward. Immediately south of this main piece of energy, at approximately 18z 2/29, emergence of higher mid-level vorticity values will commence across the Southwest, immediately ejecting southeastward into the Southern Plains. It will be this new piece of energy that will help to trigger the potential severe weather event.

Oklahoma…
Concentrated lower-level warm air advection will commence in the morning hours on Monday, with 850-millibar winds out of the west by 12z. Winds shift to southwesterly in the late morning, marking the warm air advection as high pressure shifts to the east, away from the Plains. Warm air advection aloft intensifies by 21z, and continues to build in magnitude through the late evening on Monday, up until the early morning hours on Tuesday when the potential severe weather event begins to unfold. Overall wind field for this event is messy. 500-millibar jet streak will round the base of the main trough as the event unfolds, reaching speeds AOA 80 knots in Colorado, as per latest model guidance.

Despite muddying of synoptic environment from previous discussion to this one, overall severe weather set-up has not deteriorated, and has actually strengthened in a handful of regards. Thunderstorms in this event will need to be elevated, due to strong temperature inversion consistent on forecasted soundings around the 900-millibar level. Surface CINH depicted at approximately -265 j/kg on latest NAM guidance, certainly conducive to a stable environment. However, 850-millibar to 500-millibar lapse rates of 7.8 degrees Celsius/km show a much more favorable environment above the PBL. As such, MLCAPE of just below 1400 j/kg with 87 j/kg MLCINH displayed at 06z Tuesday over KOUN. Wind profile shows substantial veering, perhaps with slight backing near 500-millibars, but overall much improved from previous model runs a few days ago. Surface to 3km storm-relative helicity values over KOUN progged at 435 m2/s2 from latest guidance, in addition to 0-6km shear around 50kts exemplify this improved wind field from previous model runs. LCLs outlooked at around 700 meters, certainly supportive of tornado formation, though perhaps slightly high in my opinion for any real tornado potential. SARS product over KOUN at 06z from 00z NAM depicts 65% chance of tornado, utilizing favorable MLCAPE and improved wind fields. Still skeptical of PBL temperature inversion, as well as depicted isolated storm coverage on 00z NAM-4km.

Most concerning point about this event, in addition to PBL temperature inversion, is projected coverage and evolution of convection. 00z NAM-4km projects evolution of thundershowers in extreme northern Texas, intensifying into either a group of strong cells or a single severe thunderstorm by 04z Tuesday, crossing into Oklahoma by 05z. Complex appears to strengthen and begin to bow out by 07z Tuesday, and begins to impact OKC/OUN at 08z, albeit in a slightly degraded severity. Complex continues to bow out and degrade through 12z, turning to the right and bowing out in the process as it moves into southeast Oklahoma. Scattered strong thunderstorm cells begin to fire at approximately 10z in eastern Oklahoma out ahead of the weakened segment of thunderstorms, and these could also pose a risk for marginal severe weather, in line with the latest Storm Prediction Center outlook (Slight Risk across most of central Oklahoma). In terms of probabilities, latest SREF suite places a 50% to 70% likelihood of thunderstorms occurring in an area with a supercell composite at or above 1. SREF members notably less enthusiastic with MLCAPE values over Oklahoma for this potential event, on the order of AOA 500 j/kg from about KOKC south. This uncertainty calls into question the bullish appearance of the 00z NAM, but will wait for the 03z SREF suite to see if uncertainty holds or is reduced using new 00z data. Reduction in uncertainty is certainly a possibility, as SREF suite will likely be influenced by more aggressive 00z NAM solution.

Overall, situation is a difficult one to assess. Not entirely convinced mixed layer will be conducive to severe weather event, especially given unfavorable temperature inversion around 900-millibars. In addition, very scant coverage by 00z NAM-4km only adds to concerns about any convection actually forming to begin with. Do believe that if convection is able to form, strong thunderstorms will become a good probability given favorable wind fields and potentially-favorable mixed layer environment. Severe thunderstorm formation also possible in the event convection can form and be sustained. Tornado risk somewhat low but not entirely zero in my opinion, given slightly high LCLs despite favorable wind profile, including veering winds, rather high 0-3km SRH values, and good surface to 6km shear values. Best threats will include hail and strong winds, with perhaps a low threat for a tornado if mixed layer environment can be fully realized.

Andrew

Friday, February 26, 2016

March 1, 2016 Oklahoma Severe Weather Discussion

This is a severe weather discussion for March 1, 2016 for Oklahoma.

Models Used: 2/26 00z GFS

Synopsis
Energy currently located out over the North Pacific, west of Alaska, will ride in on strong Pacific jet stream (over 190 knots as of 06z 2/26) into the Gulf of Alaska by 12z 2/28. Storm system will then drop to the southeast on lee side of a ridge just offshore of the West Coast of North America by 2/29. Slight interaction with energy immediately north of Montana/Canada border will aid to wrap system to a negative tilt by the day of the event. Vorticity max will be located over western Kansas by morning of 3/01 as system pushes east and severe weather event commences.

Oklahoma…
Similar to severe weather event on 2/23 and 2/24, a negatively-tilted trough will move into Central/Southern Plains on the morning of 3/01. Mid-level jet streak in excess of 110 knots will wrap around base and lee side of trough in the morning of March 1, providing anomalously supportive dynamics for a severe weather event. Unimpressive jet streak at 250-millibar level (90 knots) should be negated by the aforementioned mid-level dynamics. Lower-level wind fields also impressive, with nocturnal lower-level jet streak at 700-millibars nosing into southern Kansas by 09z 3/01 AOA 80 knots. 925-millibar wind field generally broad-brush 40 to 50 knots across Oklahoma out of the south, with temperatures up to +20ยบ C in this warm sector. Surface temperatures in excess of 60 degrees F being advected northward by sub-990 millibar surface low centered in northwest Kansas combines for southerly surface winds AOA 25 knots.

Severe weather set-up not enticing from current model guidance, despite anomalous mid and lower level features. Primary factor looks to be the mere factor that it will be the morning when this event may occur (06z - 15z 3/01), and as such an inversion will be in place. Current guidance suggests favorable mid-upper level lapse rates but anemic lower-level lapse rates with CINH progged to exceed 200 j/kg in the morning hours, particularly in western and central Oklahoma. Forecasted soundings from KOUN indicate excessive CINH, as well as somewhat dry lower levels. Narrow corridor of uncapped instability forms in central Oklahoma by 12z 3/01, with just below 1000 j/kg MUCAPE available in the midst of CIN AOB 20 j/kg. LCLs around 700 meters, combined with 0-3km SRH exceeding 200 m2/s2 and surface-6km shear nearing 50 knots, do foretell a possible supercell threat with any storms that do manage to form amidst this rather stable environment. Wind profile generally veering on forecasted soundings, save for a slightly backed look around 700-millibars.

All in all, expect that the environment will not be conducive to anything more than scattered severe thunderstorms on the morning of March 1st. Any storms that do manage to form will have the potential to be severe, thanks to impressive lower and mid-level wind fields and sufficient helicity and shear. Current thinking is while there is a marginal tornado threat with this environment (SARS-indicated, in addition to favorable LCL and sufficient wind profile), isolated nature of storms in a more or less unfavorable instability-related environment should suppress any threat of a larger-magnitude severe weather event.

Tuesday, January 19, 2016

January 22-25 Potentially Historic Winter Storm

A potentially historic winter storm looks to be unfolding for portions of the Eastern U.S.

Tropical Tidbits
A low pressure system is forecasted to develop in the South US and push northeastward to the area just offshore the Mid-Atlantic, with this coastal storm fully unfolding around January 23rd-24th. The above image shows the GFS 500mb geopotential height contours and mean sea level pressure values for the morning of January 24th. We see the snowstorm located fairly out to sea, at approximately 985 millibars. This track would produce the snow map below, taking this solution verbatim.

Tropical Tidbits
This solution would bring about 6-12" of snow to the Kentucky and Tennessee region, extending into extreme southern Indiana and a good chunk of southern Ohio. We also see similar amounts into northern Arkansas and southern Missouri, as well as North Carolina. Amounts on the order of 12-24" are outlooked for West Virginia, particularly the southern part of the state, into the majority of central Virginia, New Jersey, and eastern Pennsylvania into southern New York, with some slightly lower amounts further east into Massachusetts. Long Island is also forecasted to receive upwards of 12" if this solution were to verify.

Tropical Tidbits
To emphasize how critical the placement of this system will be, take a look at the ECMWF model's 500mb geopotential heights and mean sea level pressure values for the morning of January 24th, the same timeframe as the GFS graphic. We see the system at 984 millibars, pretty similar to the GFS, but the system is clearly displaced further west. Given how anomalously warm the Atlantic currently is, this shift could easily push those two to three feet amounts inland, and leave the coastline with rainy conditions. This run of the ECMWF has snowfall amounts of over 30" in central Virginia, but lesser accumulations the further north and east you go.

Tropical Tidbits
The GEM model shows amounts in the 6-12 inch range for most of Missouri into the southern parts of Illinois, Indiana and Ohio, as well as much of Kentucky and northern Tennessee into western North Carolina. Amounts in excess of 30" are forecasted from Maryland into Pennsylvania, certainly an incredible amount of snow, but not entirely off the table given how strong and persistent this system may end up being. Again, the placement of this storm will be critical to who gets the heaviest accumulations, and we likely won't know who will receive this jackpot until the storm is actually happening.

Tropical Tidbits
One last bit of advice, do not bite on any particular model run for this storm. The strength of this storm, as well as how long-lasting it will be, among other factors, makes me way too uneasy to trust any single solution, hence why I presented a plethora of models here. The above graphic shows GFS ensemble spread in MSLP values for this storm. The warmer colors to the left and right of the system indicate increased uncertainty with where the storm will track. Again, this emphasizes how no single solution is set in stone, and thus should not be taken at face value. A lot of change will happen in the next several model cycles, and the heaviest amounts could plausibly shift a substantial bit as these changes unfold.

To summarize:

- A potentially historic snowstorm is forecasted to occur along the East Coast over the January 22-25 period.
- Currently, the Maryland/Virginia/Pennsylvania region is on track to receive the heaviest amounts.
- The heaviest accumulations could reach or exceed 30".
- Enough uncertainty still exists to change the storm track substantially, as well as shift the heavy snow axis and amounts.

Andrew

Friday, January 8, 2016

January 9-11 Potentially Significant Winter Storm

A potentially-significant winter storm appears to be unfolding in the January 9-11 timeframe.

Before we even dive into this storm, it's crucial to acknowledge that there are two model guidance 'camps' for this storm. As a result, there is unusually high uncertainty, so keep that in mind while reading this post.

Tropical Tidbits
The first solution we'll look at is the east-and-weaker idea. This is the latest GFS model's projected snowfall accumulation through Sunday morning. In this forecast, we see a swath of about 4-8" from southwest Missouri into extreme southwest Illinois, with a stripe of 2-4" on a northeast path until about Gary, Indiana. From that point, amounts on the order of 4-8" are outlooked for much of Michigan, particularly from the extreme southwest part of the state into the northeast part of the state.

This solution has been gaining ground in the last few model cycles, especially against the second model solution, which we'll analyze next.

Tropical Tidbits
The other solution being analyzed is a further northwest and stronger storm track. This is the 12z RGEM model, and it shows a solid 4-8" across much of the southern half of Missouri, with a small jackpot zone of 12-16" from extreme southwest Illinois into central Illinois. A zone of about 5-10" is outlooked for the Chicagoland area into the western half of Michigan.

This solution would appease many in the Midwest and Great Lakes regions looking for snowfall, but this particular solution is been losing traction to the weaker/southeast track in the last few model cycles.

Despite high model uncertainty, and the fact that I've seen stranger things happen, I'm expecting that weaker & southeast track to win out.

To summarize:

- A potentially significant winter storm is expected in the January 9-11 timeframe.
- High model uncertainty exists.
- A 2-6" snowfall swath could impact a southwest-to-northeast line from southwest Missouri to northeast Michigan.

Andrew

Tuesday, January 5, 2016

Stratospheric Vortex Under Attack, But Holding Strong

The stratospheric polar vortex looks to come under attack in the next several days, and possibly beyond, but all indications are that this attack will be weak, and the polar vortex aloft will remain strong.

We'll begin with a current look at the stratosphere.

CPC
(Refresh page if animation stops looping)
This animation above shows temperature anomalies at the 30-hPa level of the stratosphere, just above the middle part of the stratosphere. Note how over the past month we've seen a few instances of minor warming occurring at 'lower' latitudes, but never reaching a strength adequate enough to be an actual stratospheric warming event. Consequentially, the stratospheric polar vortex has remained pretty much undisturbed. There is some warming commencing over the Himalayan mountain region, and this could incite another minor warming event to spread over the north Pacific, as has happened before in the last 30 days on this animation. Whether it builds into anything substantial remains to be seen, but as the analysis below shows, I'm not optimistic.

FU-Berlin
The first image we'll analyze is a longitude-by-height chart, forecasted by the ECMWF model for five days' time. This chart is forecasting the presence of a geopotential Wave-1 event in the stratosphere. As the phrase 'Wave-1' indicates, the forecast here is for the risk of a single body of high pressure (hence Wave-1) forming aloft to try and displace the polar vortex. The legend on the left indicates this risk is highest from the 1-millibar to 10-millibar region, where geopotential values are highest and the warm colors are most prominent. This is a pretty strong Wave-1 event, but because it is centered above the 10-millibar level, and it will weaken shortly after this time period five days out, I'm not expecting any long-term damage to the stratospheric polar vortex.

FU-Berlin
Another tool we can use are time-by-height charts for different levels of the atmosphere to see how much, if any pressure is being applied to the polar vortex. It looks a little intimidating at first, but we'll break it down below.

The top panel shows us the forecast for Wave-1 (explained above) and Wave-2 temperature attacks at the 10-millibar level. The Wave-2 event is where one body or more of high pressure / warm temperatures form aloft (in this case, at the 10-millibar level) and try to split the polar vortex into two vortexes, hence the '2' in Wave-2. Notice the forecast for a Wave-1 event peaking at that 5-day forecast period, which we analyzed above, but then that Wave-2 line starting to rise up towards the end of the forecast period, ten days away. That could be something to monitor, but when we consider Wave-2 events are weaker than Wave-1 events, I'm not particularly encouraged by it.
This description of that top panel also applies to the third panel from the top, except now valid at the 30-millibar level. In this case, however, notice a sustained elevated Wave-1 attack throughout the forecast period, but also a strengthening Wave-2 parameter towards the end of the forecast period. Again, perhaps something to watch, but I'm not exactly keen on it impacting the vortex significantly right now.

Panels 2 and 4 from the top are nearly identical to the first and third panels described above, but these new panels show us Wave-1 and Wave-2 attacks from a geopotential (ridges instead of warm temperatures, although in essence they tend to occur together) standpoint. Note in Panel 2, showing us the forecast at the 10-millibar level, a sustained Wave-1 attack, and a pretty strong one at that, throughout the forecast period. We also see a strengthening Wave-2 event at the end of the forecast period.
The situation in the fourth panel, showing geopotential values at the 30-millibar level, is different than the 10-millibar level. We see a pretty strong Wave-1 event, but a weakening Wave-2 episode. This piques my interest, just because I want to see how that evolves more than anything, and may be something to watch down the road.

To summarize:

- Minor warming has occurred in the stratosphere over the last month, but has had very little impact on the polar vortex.
- Additional minor warming is forecast to occur over the next several days, again with little impact in the lower stratosphere.
- Long-range guidance indicates some items of interest beyond Day 10 forecasts, but I'm personally pessimistic as to the chance of a stratospheric warming event.
- This may keep the chances for a large-scale, significant cold weather event relatively low, speaking strictly from a stratospheric viewpoint.

Andrew

Brief Pattern Change Expected Next Week

A change in the current weather pattern, albeit a brief one, is expected next week.

PSU
Image Valid: January 12
Ensemble forecasts see strong ridging building over Alaska and extreme northwest Canada, extending from northern Siberia all the way across the Arctic Circle to the waters immediately south of Greenland. Consequentially, this will force the tropospheric polar vortex to lower latitudes, and it just so happens that ensemble guidance sees a piece of this vortex will dip south into Canada, and will exert its influence on the United States.
It's important to note that the tropospheric polar vortex will not enter the United States, it will stay well north of the Canada/US border. Cooler air will still result, but extreme cold should not be expected right now.

PSU
Image valid: January 15
By January 15th, the pattern is progressive enough so that the piece of the tropospheric polar vortex shifts east into eastern Canada. We also see strong ridging dominating the Arctic Circle, which will drop the Arctic Oscillation (AO) index well into negative territory, as we'll see a little later on in this post. Note the strong trough modeled over the Gulf of Alaska- this will maintain a positive phase of the East Pacific Oscillation (EPO). While I won't discuss the EPO in this post, that trough in the Gulf of Alaska is what will return our pattern to a more seasonal, and possibly warmer than normal pattern after this brief change to chillier temperatures.



Above 3 images from CPC
Just a brief look at the teleconnection forecasts, we see the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) both expected to drop into negative territory. This is a sign for cooler than normal weather, and as we saw earlier in this post, ensemble guidance believes a brief shot of colder weather will hit the Central and East US. The Pacific-North American (PNA) index is expected to stay positive, possibly heading more towards  neutral territory in the medium-long range, but generally should remain positive. This is likely a result of that trough in the Gulf of Alaska. That trough will keep the pattern progressive, which means the ridge that forms over the West US as a result (the positive PNA indicator) will likely drift east and spread warmer weather east.

To Summarize:

- A brief flip to colder weather is expected next week, mainly between a January 10-15 period.
- Warmer weather is expected to return after January 15th.

Andrew

Monday, January 4, 2016

January 9-12 Potential Winter Storm

There appears to be the risk for a storm system to impact the East US, particularly the Northeast, in the January 9-12 timeframe.

Tropical Tidbits
We'll first go over the forecast of the storm, and then get into it at a more analytical level. This forecast panel shows the 18z GFS forecast of precipitation type and intensity, as well as mean sea level pressure contours and 1000-500mb thickness values for the early morning of January 11th. We see a strong storm system just offshore the Northeast, producing widespread moderate to heavy rain right along the coast from New Jersey on northeastward, with light snow located in inland New York into east Pennsylvania. This solution could give copious amounts of rain and/or snow to the East Coast.

As expected, the internet is biting hard on this storm, given it could be the first real wintry system for the East this winter. Not to burst anyone's bubble, but below explains why I'm not expecting a real snow threat with this storm, and why not to bite on any forecasts yet.

Tropical Tidbits
This graphic shows us the 500-millibar vorticity values, forecasted for 12z (6 AM CST) today, Monday. That red box, all the way out near the Aleutian Islands, encompasses our storm system. That's right, the system that people are looking at is currently hundreds of miles away, closer to Russia than mainland United States. That's the first sign that we shouldn't take any model guidance at face value right now. Heck, the latest 00z GFS run came in and showed the storm pushing well out to sea, not even affecting most of the East US. I just chose to show that 18z GFS graphic so I could show what I'm talking about, and to make a few points, like this one.

Additionally, check out all the pieces of energy out ahead of our main system. If you count closely, just looking at this graphic, I can identify at least three separate storm systems downstream of our system of interest near the Aleutian Islands. Three systems is a lot for model guidance to sort out, and it doesn't help that one of those systems is also well out at sea, not even close to the radiosonde network which would help model accuracy. In sum, models are going to suffer a lot with this storm, both with the distance the storm is from the U.S., and how many systems are downstream of our storm itself.

Tropical Tidbits
This next point is specifically for the snow enthusiasts in the Northeast already getting excited for this storm. This is the 850-millibar temperature forecast for the morning of January 9th. At this point in time, our system is located somewhere in the South Plains, though it is not well-shown on this map. That's alright though, because we're looking at this image for the temperature spread across the Central and East US. All colors green and warmer are temperatures above freezing at this layer of the atmosphere (about 5,000 feet off the ground). That means that most areas east of the Mississippi River, save for Maine, Vermont, and New Hampshire, should be comfortably above freezing in the couple of days, even several hours prior to the storm actually beginning to impact the East (if it does at all).

For a good snow set-up, no matter where you may live, you want an established cold air mass before the storm begins. We saw that guideline in play with our last storm system, which dumped copious amounts of snow in the Plains into Iowa and Wisconsin, even laying down a few inches of sleet near Chicagoland. It was all that sleet and freezing rain that fell because there was no antecedent cold air mass. This go-around, we also will be lacking a pre-established cold air mass, and that really concerns me with snow prospects.

CPC
While I'm not set on the idea of a snowstorm, I am open to the concept of this actually being a storm for the Northeast. The North Atlantic Oscillation (NAO) will be negative when this storm occurs, albeit only slightly negative. This should encourage the storm to curve up the coast, though just how much 'encouragement' there is remains in question. It's completely plausible the storm goes straight out to sea, as the 00z GFS portrayed, and it's also possible it curves up the coast. I'm not confident in it going one way or the other right now, but both solutions are plausible.

To Summarize:

- There is the chance for a wintry storm system in the Northeast between the January 9-12 timeframe.
- Personally, I see a very realistic chance that this storm produces more rain than snow as a result of no antecedent cold air mass.
- Model guidance should not be trusted this far out.
- This storm could plausibly go out to sea rather than up the coast.

Andrew