Saturday, September 28, 2013

Special Long Range Lookout: September 28, 2013

This special Long Range Lookout will address the expectation of a colder trend in the weather in the 20+ day forecast range, and the effects of the current weather pattern on the winter ahead.

The current weather pattern involves a highly meridional (wavy) jet stream across North America, with a deep trough stretched across the West and strong ridging spread over Canada and the Great Lakes. The deep troughing is likely due to an increased stream of storms that have been shifting across the northern Pacific. The 500 millibar anomaly chart on the left displays a strong system currently stationed over the Bering Sea and into the Gulf of Alaska, and I suspect that system will move south towards the West Coast, possibly also into the West US. This train of storm systems has changed the sea surface temperature pattern across the northern Pacific as well, as the image below shows.

There is a large body of below normal sea surface temperatures stretched from the Aleutian Islands to waters well offshore of the Pacific Northwest. This development has occurred only very recently, and it was only a few weeks ago that the entire northern Pacific was well above normal in SST anomalies. The continuing storm train over the northern Pacific, aided by a favorable Madden-Julian Oscillation (which we will discuss more in-depth later), should assist in further lowering sea surface temperatures across the waters in and southeast of the Bering Sea. A potential long term implication of this new SST anomaly could mean an Aleutian Low, which would be favorable for harsh winter prospects in the East US. Despite that potential, a decisively negative PDO pattern has been present recently, and this could discourage such cold weather in the East and Central US.

We're going to dive right into some hardcore weather with an analysis of the Relative AAM tendency. The relative AAM tendency shows what its name says; the chart helps predict the tendency of the AAM index. Right now, the AAM tendency is positive, and this fits well with the current observation of a slight positive AAM. In the medium range, this should lead to a tendency for the positive phase of the East Pacific Oscillation, which encourages the formation of above normal temperatures in much of the United States, as illustrated by blizzardof96 on the AccuWeather forums.

Long term AAM projection from Nicholas Schiraldi shows a gradually lowering AAM over the next 360 hours, to the point that we see a neutral or slightly negative AAM at the end of the forecast period. This fits in well with ensemble projections of a lowering EPO in the long range, which could lead to chillier temperatures for the United States. 2-3 week lag time between the AAM suggests not only an incoming warm trend at some point in October, but also a cooler trend towards the end of next month. Exact timing is to be determined and will depend on the strength of the AAM at its low point.

On to the more interesting concept of this special Long Range Lookout, the Madden Julian Oscillation, or MJO. The MJO involves the anomaly of tropical convection along certain regions of the Pacific and Atlantic. A certain placement of this tropical convection indicates a different phase, with the MJO having 8 phases in all. Each phase has different implications on the weather here in the United States. Model guidance projects the MJO meandering around in a weak-moderate Phase 6 for the next several days. The Canadian model guidance set prefers to retrograde the MJO back into Phase 5, but agreement between the American, European and United Kingdom guidance systems says we will most likely stay in Phase 6.

Now, the MJO at Phase 6 helps to enhance tropical cyclone activity over the West Pacific, which we certainly have seen in the last couple of weeks. This above normal tropical activity in the West Pacific has greatly helped in the negative SST anomaly trend in the north central Pacific, which we discussed earlier. Should the MJO continue to work its magic with the West Pacific, there is little reason to think that this cooling trend in the waters of the North Pacific will not continue.

A key point of interest with Phase 6 of the MJO includes the persistent blocking pattern that sets up in the 10-30 day timeframe following the MJO entering Phase 6. The circled area shows these positive height anomalies as the blocking regime that would present itself between the 90W and 30W longitude areas of the 70N and 55N latitude lines. In other words, the blocking would be present across eastern Canada and much of Greenland. The positive height anomalies present in the same region in the 15-0 days prior to the MJO entering Phase 1 have verified in the last couple of weeks, with the current weather pattern image at the top of this post confirming that suspicion and strengthening the argument for this long range idea of a blocking pattern that may resemble a negative North Atlantic Oscillation.

On another note, if you look closely, you can see a swath of positive height anomalies between the 150W and 120W markers between the numbers 10 and 20 on the left side of the image. This represents a trend of ridging across the Gulf of Alaska, which appears to eventually weaken and shift west. Depending on if this Gulf of Alaska ridging does set up, the East US may have some trouble with maintaining more seasonal temperatures.

Another composite image for the weather pattern 20 days prior to a significant upper latitude blocking event nearly guarantees this idea that we will see some long term blocking in the northeast region of North America. This graphic shows 500 millibar anomaly contour lines as well as OLR anomalies in color (OLR= tropical convection, where blue indicates enhanced tropical convection and yellow portrays below normal tropical convection). The blue anomalies stationed due south of the Bering Sea suggests the MJO is supposed to be in Phase 6 twenty days prior to the significant blocking event. Model guidance confirms that we are already seeing this play out. 500mb anomalies show steadfast high pressure across the far upper latitudes, with negative height anomalies in the northern Pacific. The September 24th 500mb height chart below shows how good of a match the two images are, further helping the argument for blocking in the next 10-30 days. The 'Slow MJO' phrase on the top of that graphic above just tells that the MJO should be moving slowly through Phase 6 in order for this situation to work out, and model guidance assures us that the MJO is in no hurry to get through Phase 6.

September 24, 2013 500mb pattern
So, what can we draw from all of this?

Well, in the short term, expect this very wavy jet stream to continue, due mainly to the continuing storm train across the northern Pacific. Inconsistent but strong bouts of ridging will occur in the Central and Eastern US in response to any storm systems that bottom out in the Southwest and enhance the meridional flow of the jet stream. In the short range, warmth looks to be the buzz word. But we are just a week or two away from what could be a sizable shift in the temperature trend. If the MJO composite works out as I explained above, we should see a tendency for a decisive cool weather trend in the Plains and portions of the western Midwest, with some iffy temperature patterns in the eastern Midwest, Ohio Valley and much of the East US. Depending on how influential the ridging in Canada and Greenland is on the United States, we could see above normal temperatures in portions of the eastern half of the US. A general stormy pattern may evolve over the Great Lakes in the next 7-14 days, but I'm a little skeptical due to low confidence from some other composites I've looked at. Probably won't see any long-term wintry weather potential in much of the Central and Eastern US outside of expanding frost zones for portions of that area. The West should keep an eye out for additional wintry weather possibilities in coming days.