Thursday, November 26, 2015

Long Range Forecast (Posted 11/26/15)

This is the long range forecast, published on 11/26/15.

 The 500mb pattern over the last week showed a dominating ridge in the Gulf of Alaska, leading to negative geopotential height anomalies in central North America. This was exhibited by the strong storm system(s) that passed through the region since our last outlook.

 500-millibar chart shows a strong upper level low stationed over the Southwest, as the closed blue contours indicate. Southerly lower-level winds are advecting moisture northward into the Plains and Midwest, which will serve to kick off wintry weather on Thanksgiving.

 Polar water vapor imagery shows an active pattern in the North Hemisphere. We see two upper level lows with a Rossby Wave signature in the Gulf of Alaska, as well as another upper level low impacting the Aleutian Islands. Hurricane Sandra is shown near Mexico, and this will deliver moisture to the US later on.

 Analysis of the jet stream shows how the upper level low to the southwest of the Gulf of Alaska will likely be cut off from the jet stream in the near future, while a strong jet streak plows into the Pacific Northwest. Another jet streak pressing northwest of Arizona to the southeast of the trough tells us the upper level low in the Southwest is preparing to eject eastward. We also see another jet streak ribbon exiting Japan, which should provide us with a storm opportunity about 7-10 days down the road.

 Current tropical forcing across the globe shows upper level divergence from the area east of Madagascar, as well as northeast of Australia. This forcing pattern in the central Pacific is typically favorable for warmer weather, although additional convection near Africa helps dilute its influence so forcing is not too useful in its current position.

 Expecting a re-amplification of the pattern over the next 7-10 days as we see increased storm chances. Watch for a moderation in temperatures over the next several days, as tropical forcing indicates the atmosphere will be favorable for generally warmer than normal weather.

Storm concerns … potential storm monitored for 12/2 – 12/6. Increased risk for stormier than normal pattern about 12/5 – 12/15, though confidence is low.

 Continuing with previous outlook, conditions do still look somewhat cooler and stormier around the Christmas timeframe. New Years Day and the first week of 2016 should see a departure from this cooler/wetter pattern back into more moderated temperatures.

The Next Long Range Outlook will be published on December 2nd, 2015 at 5:00 PM CDT.


Thursday, November 19, 2015

Long Range Forecast (Posted 11/19/15)

This is the long range forecast published on November 19, 2015. 

Over the last seven days, we’ve seen strong negative geopotential height anomalies over the Gulf of Alaska, leading to ridging in Canada and a warmer pattern here in the U.S. A strong storm system is currently moving out of the United States, showing the reasoning behind that small swath of negative anomalies in the Plains.

The SLP map (brown lines) and 500mb contours (blue lines) show our upper level low finally moving off into Canada, with the cold front continuing east into the East US, as infrared satellite presentation shows.

Water vapor imagery over the North Hemisphere shows a strong Pacific jet stream blasting into the Pacific Northwest, delivering the next chance at active weather. We also see an upper level low in the Bering Sea, which is expected to contribute some energy to our pattern over the next week or two.

Analysis of the jet stream over the Northern Hemisphere shows a rather inactive pattern in the North Central Pacific, but a strong jet streak or two over East Asia indicate the potential for some stronger storm systems over the next 10-20 days in the United States.

Current tropical forcing imagery over the globe shows strong divergence across Central America into the United States. Additional divergence is seen in southern Africa and near India. This pattern is conducive to cooler conditions in the United States (Phase 8-1 MJO).

Pattern should quiet down over Thanksgiving, with tropical forcing moving into areas more favorable for warmer weather for much of the country into the opening 5-10 days of December. Heading into mid-December, tropical forcing should re-organize in a pattern more favorable for cooler weather in the United States.
Storm concerns… Anticipating storm system around 11/20 – 11/25. Watch for a potential storm system around 11/30 – 12/5, as well as a second system around 12/4 – 12/9.

Early analysis of pattern around Christmas does favor another round of stormy and cooler weather in time for the holidays. Tropical forcing indicates we may need to monitor for a significant storm system not unlike the one currently moving into Canada, which brought tornadoes and heavy snow to the Central US this past week.

Next Long Range Outlook
November 26th, 2015 at 5:00 PM CDT


November 20-22 Potentially Significant Winter Storm

Model guidance continues to increase snowfall amounts in the winter storm expected to impact Friday through the weekend.

Instant Weather Maps
The latest GFS model has shifted snowfall amounts further to the north since we last analyzed the system yesterday. We now see forecasted totals in the 3-6" range across northern Nebraska into extreme southern South Dakota. Amounts in the 6-10" range are then forecasted in most of Iowa, with lesser amounts in the southern portion of the state. Amounts exceeding 12" are projected to hit southern Wisconsin, including Madison and Milwaukee. Northern Illinois experiences an intense gradient, with the city of Chicago receiving under 3" and areas along the Illinois-Wisconsin border receiving 12".

Instant Weather Maps
The latest NAM model takes a different approach to the storm. It drops 3-10" across southern South Dakota, with the lower end of the gradient in the western portion of the state and heavier amounts in the eastern part of the state. Northeast Nebraska is in line for 6-10" of snow according to this model, with a sharp gradient to the south of that heaviest snow axis. Iowa is projected to be slammed with snow across the board, with heaviest amounts in the western and northern portions of the state. Amounts then drop off as the storm moves into the Midwest. 6-8" amounts are forecasted to hit in a thin line along the Illinois-Wisconsin border, heading right into downtown Chicago, Illinois. Lesser amounts in the 3-6" range are then found in southern and central Michigan.

To summarize:

- A potentially significant winter storm is expected to impact the Plains, Midwest and Great Lakes Friday into Sunday.
- Considerable uncertainty still exists with respect to the track of this system.
- Amounts in excess of 6" are possible, especially in Iowa, southern Wisconsin, and northern Illinois.


Thanksgiving Potential Wintry Storm System

There does appear to be at least a chance of a wintry storm system in the Thanksgiving timeframe.

Tropical Tidbits
The GFS model has a messy solution evolving for Thanksgiving Day, as the above image shows. We see two centers of low pressure, one on the Oklahoma/Colorado/Kansas border, and another on the South Dakota/North Dakota/Minnesota border. They are both about the same strength, with the southern one at 1004 millibars and the northern one at 1005 millibars. The graphic here shows widespread (albeit light) snowfall in Montana and North Dakota, with heavier snowfall in eastern North Dakota near the low pressure system.

In the warm sector, we see a line of rain and thunderstorms extending from southern Texas through Oklahoma, Missouri, and into Wisconsin and Minnesota. Going by this image, it's possible we see some stronger storms in the Texas and Oklahoma areas, into Arkansas as the day continues. Heavy rain would still be possible in the Midwest and Great Lakes, making for a soggy Turkey Day for locations such as Chicago, Milwaukee, St. Louis, and Little Rock.

Tropical Tidbits
The GEM model shows a singular storm system placed in southeast Oklahoma in the early morning hours of Friday, immediately after Thanksgiving. This would include heavy rain, and possibly thunderstorms, in Missouri, Arkansas, and along the Gulf Coast. Lighter rain would be anticipated in Iowa and Illinois, while a rain-to-mix solution could evolve for Kansas, Nebraska, and Iowa. A snow-mix situation would unfold across the Upper Midwest and North Plains. It's too early to talk accumulations, especially since there is no model agreement on strength, track, and precipitation.

To summarize:

- Model guidance is indicating the chance for a wintry storm system on Thanksgiving Day.
- Low confidence still exists in this situation.


Wednesday, November 18, 2015

November 20-22 Winter Storm

A winter storm is expected to impact portions of the Plains, Midwest and Great Lakes over the November 20-22 timeframe.

Tropical Tidbits
The GFS model is insistent on a strong, yet compact low pressure system bringing accumulating snowfall to the North US. Using a 10:1 snow-to-liquid ratio, the 12z run of this model drops 4-8" of snow in northeast Nebraska and central Iowa, with 2-5" spread across southern South Dakota, central Nebraska, and the northern and southern sides of Iowa. Amounts in the 6-10" range are then forecasted in northern Illinois, northern Indiana, and southwest Michigan, with lesser amounts into southern Wisconsin, central Illinois, central Indiana, and central Michigan. Some lake effect snow after the storm could enhance totals on the eastern side of Lake Michigan.

Tropical Tidbits
The GEM model has a slightly different scenario. It paints a swath of 5-10" across northeast Nebraska into much of Iowa. Southern Iowa could see amounts near 12" according to this model, before amounts back in the 5-10" range are forecasted in north Illinois, north and central Indiana, and southern Michigan. This model is notorious for over-doing snowfall amounts, and has historically not performed well compared to other model guidance with respect to snowfall amounts, storm strength, and storm track.

It does appear the GFS and GEM models are in a snowier 'camp' together, with both of them laying down a swath of significant snowfall. The NAM model is against this snowier forecast. Since the system responsible for this threat will not be onshore until Thursday evening, and these early-season storms can have low ratio snows, it would be wise to stick with lower-end guidance for the time being. These concerns are also reflected in the Area Forecast Discussion from the NWS office in Chicago:


The Chicagoland office expects 4 to 5 inches of snow for that area, using 7:1-8:1 to 9:1-12:1 ratios across their forecast office. I do believe this is a reasonable forecast, given the low-ratio nature of the system.

To summarize:

- A winter storm is expected to impact the area over the November 20-22 timeframe.
- Snowfall amounts generally in the 2-6" range can be expected in portions of the Plains, Midwest, and Great Lakes.