Friday, February 21, 2014

Long Range Outlook: Late February, March 2014 Forecast

This post will detail the expected weather conditions for the end of February into most of the month of March, 2014.

We'll begin with the weather over the North Pacific and East Asia. Shown above is a previous ECMWF model forecast, showing 500mb height anomalies over the aforementioned regions. If we take a look over at Japan, we see some negative height anomalies present, valid on February 20th. If we use the idea that weather anomalies over East Asia can "re-appear" in the United States 6-10 days later, I would expect a stormy and cool weather pattern for the February 26-March 2nd period. But if we look just to the west, we see strong ridging behind that storminess in Japan.

We eventually see that ridging fill into Japan a couple of days later, as this 500mb height anomaly forecast from the most recent ECMWF forecast shows. With ridging expected to hit Japan on February 24th, a warm-up is probable for the March 2nd-6th period. It's worth watching for another storm threat, as we see a depression in the contour lines right over Japan that indicates the presence of a weak storm system. It is possible we see a storm drag warm air north from the Gulf of Mexico to set-up a severe weather threat, but that's all speculation at this point.

Following that initial warm-up in the first week of January, model guidance (the ECMWF Ensemble set, shown above here) wants to bring in a strong ridge of high pressure right over Japan. It looks like this ridge has the potential to bring some seriously warm air into the United States when we see the correlation kick in. With the graphic here valid for February 26th, look out for a warm, quiet weather pattern around the March 4th-8th time period. It's quite possible we see a sustained warm period from March 2nd to the 8th, if these ridges hold together.

Looking out to the last day of February we see the ECMWF bringing in a suppressed ridge into Japan. This tells us to look for that warm pattern to continue in the United States through the March 10th period. Remember to keep track of that Rossby Wave in Alaska, denoted by the powerful ridge.

The weather pattern will then do a flip in the second week of March, as model guidance sees the entrance of a strong trough into Japan on the 3rd day of next month, as the image here shows. This would likely see a return of cold and stormy weather somewhere in the March 9th-13th time period, as we use that 6-10 day gap between East Asian weather and United States weather.

Moving out into the long-long range, we start to key in on the synoptic pattern, and not so much individual features like we were previously analyzing. We've got a good handle on the anticipated weather pattern for the first two weeks of March, so now we can begin forecasting the remainder of the month. This graphic shows you the predicted 500mb height anomalies over the Northern Hemisphere on March 8th. Note the ridging over the Arctic Circle indicating the presence of a negative Arctic Oscillation (-AO) phase, which heightens threats for cold weather in the lower latitudes (including the US). Also note the ridging starting to come back over the west coast of North America. This is part of the Rossby wave train, which is created when several rounds of strong high pressure are pumped poleward. In this case, the train has maintained itself over the northeast Pacific, at times shifting west into the waters just south of the Bering Sea. The way the pattern is aligned here, you'd be hard-pressed to not find a cold forecast for the Central and Eastern US. The flow across the Northern Hemisphere strongly favors a cold middle of March, with the Rossby Waves continuing to form and push northward along the West Coast, leading to cold weather for the populated areas out East. Since this is a 360 hour forecast, things will obviously change, but you get the idea: after the early March warm-up, additional cold weather is possible.

Why is all of this happening? The primary reason is the swath of above-normal sea surface temperature anomalies over the northeast Pacific, as this graphic shows. One of the best indicators of long-term weather patterns is sea surface temperatures. Apart from some sea surface temperature anomalies (SSTAs) defining various oscillations that also affect our weather, these swaths of above or below normal SSTAs correlate to positive or negative 500mb height anomalies, respectively. Thus, it's of little surprise that we are seeing strong ridging over the Northeast Pacific, where this big warm water mass resides. With the warm water still firmly entrenched in the waters south of Alaska, look for a cool spring to also be in store.

What we could end up seeing, if this warm pool becomes a long-term feature, is a northwest flow pattern for the spring season. The screenshot above, from a Storm Prediction Center publication, shows 500mb height contours in two different northwest flow situations, with the severe weather outbreak area circled. As you can see, severe weather events would be more prone to occurring further north during northwest flow severe weather outbreaks, and it's possible we see something like one of these situations evolve if that northeast Pacific warm pool continues to work its magic. I'll detail this more in my upcoming 2014 severe weather season outlook.