A look ahead toward Thanksgiving and beyond
You may have noticed chatter about potential for wintry weather around or just after Thanksgiving at AccuWeather and on the Baltimore Sun's weather blog. It's too early to be speculating about forecast details more than a week away, but we can talk about how the large scale weather patterns are evolving and what kind of weather they might support.
Over the past couple of weeks, we've discussed La Nina and the North Atlantic Oscillation (NAO) and noted that one of the big uncertainties in this winter's forecast was how the two might interact.
The latest forecasts from the European Center model and NOAA's global model (the GFS from National Center for Environmental Prediction, NCEP) both now showing a high latitude blocking pattern developing over the North Atlantic - which signals the potential for cold air to settle into the region. Above, I've posted the NOAA's ensemble mean forecast for 216 hours from Tuesday evening which is valid Thanksgiving evening.
On the left panel, note all the red colors at the higher latitudes and the blue area to the southeast and east of Nova Scotia. That's a classic negative NAO look, a classic pattern which usually increases the chances of snow and cold in our area. However, it's only November and snow this early is relatively rare but not unheard of. You may recall snow around Thanksgiving in Washington in 1989.
The map posted above is from the same time range when the operational NCEP high resolution model from yesterday morning was predicting a possible snowstorm or wintry mix like pattern for Washington around or a little after Thanksgiving.
The low that was supposed to give us the storm has shifted westward and northward on each of the last two runs. So can anything be said about a potential snow storm?
The panel on the right in the image above, called a spaghetti plot, gives an estimate of how predictable the pattern is. The same model is run multiple times with slightly different initial conditions to see how much variation occurs. The more variation in the forecasts, the lower the confidence in being able to intelligently choose which solution might be the correct one . The closer the lines are clustered together, usually the more confident you can be of a model forecast. When all the lines look like a small child was scribbling on a piece of paper (like in this case), the forecast is highly questionable; not necessarily wrong, but it is almost impossible to draw any real inference from it.
For those interested in trying to assess the predictability of the pattern themselves you can view such products on Penn State's weather wall. As suggested by the spaghetti plot, at such long time ranges, most potential snowstorms are illusions
Then what can we say about the pattern?
The pattern looks more predictable through around next Tuesday. By that time we already have the negative NAO but also have a strong reverse (or negative) Pacific North American pattern (rPNA), characterized by a ridge over the eastern Pacific ocean and trough over the western U.S. (cold air) - which tends to build a ridge to the East Coast (warmer air).
While there still is quite a bit of spread in the lines on the right panel in the above image, the magnitude of the mean ridge (red area) over the Pacific and of the trough over the West indicates there is considerable agreement about these two critical components of the rPNA pattern. The same can be said about the negative NAO.
As we've discussed before, La Nina years like this one favor an rPNA pattern like the one shown above. That same article discussed the considerable impact of a negative NAO on the temperatures during a La Nina.
The rPNA and negative NAO will be battling for supremacy over our area for the next couple of weeks making longer range forecasts really tricky. How these two features wax or wane will have a significant influence on our weather.
Complicating the situation is that during fall, the PNA tends to have more of an impact on the weather over the U.S. than the NAO but as we head into December, the NAO's influence typically waxes. The southerly flow across our area and orange colors on the map above suggest that next Monday and Tuesday might give us well above normal temperatures unless there is cloud cover.
The forecast shift of the cold air east and the area of below normal heights (the blue area) eastward away from the west after Thanksgiving is predicated on the changes forecast to occur in the Pacific by both the GFS and ECMWF ensemble mean forecasts. Compare the position of the ridge/hill in the lines near Alaska (red area with the solid line) on the earlier 156 hr prediction to the two maps below showing the patterns on the evenings of Thanksgiving and Black Friday.
Note that the ridge near and just south of Alaska is replaced by a trough (the dashed line). The strongest ridging (red area) develops farther west in the Pacific which forces changes to the pattern further east. The new trough (dashed line) just south of Alaska helps promote the development of another ridge or bump near the West Coast which in the models forces the blue area eastward as shown on the 216 and 240 hour forecast. This weak ridge near the West Coast helps force the cold air east.
If the strongest ridging in the Pacific does not shift westward as much as currently forecast by the various models, then the pattern over the U.S. may be slower to change than shown on any of the models.
Last night's runs of both the ECMWF and GFS were a little slower with the transition than on the previous day but both eventually bring the pattern change.
My guess is (and it's certainly not much more than a guess) is that the changes in the Pacific will occur enough to force changes over the U.S. leaving the mid-Atlantic states in a battle ground for how far south the colder than normal temperatures reach after Thanksgiving. Right now both model ensemble mean products are calling for below normal temperatures across our area. The individual ensemble members continue to show considerable spread suggesting there is little that can be said about any potential storms.
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