Bad pattern for snow through end of February
But need to watch possible system early next week
Yesterday Ian wrote an article summarizing snow chances moving forward based on the climate record . Today, I'll look at the forecast pattern and how it bodes for our snow chances during the next couple of weeks.
In summary, the pattern is not one that is particularly favorable for snow. But while the pattern is not that good overall, it still could produce surprises - with a system to monitor early next week.
We are currently stuck in the reverse Pacific North American Pattern (RPNA, more information on the PNA and RPNA) as illustrated to the right. Such a pattern is composed of a ridge in the jet stream off the west coast (high pressure at the surface), a trough (a southward dip in the jet stream) and cold weather over the West and a ridge and warmth over the Southeast.
The combination of a western trough and southeastern ridge helps direct storms that form just east of the Rocky Mountains northeastward towards the Great Lakes or Ohio Valley regions. Such a track generally keeps the D.C. area on the warm side of storms as the counterclockwise flow around the storms cause winds to have a southerly component across the region.
Also, as storms approach from the west, the ridge gets suppressed and nudged back to the south as the low passes which can contribute to strong southwesterly or westerly winds developing as a squeeze play happens between the high pressure system to the south and the low to our north. That's what happened on Monday.
As shown above, the GFS model ensembles (in red) are uniform in predicting a reverse (negative) PNA pattern through the end of February. Recently these forecasts have been pretty accurate through day 10 with the skill of the forecasts decreasing rapidly by day 14. The strongly negative values being forecast suggest that the pattern in the Pacific will probably hold through the month. If so, look for more storms to track towards the Ohio Valley and for storms to reform off the New Jersey coast rather than off the Virginia Capes or Hatteras. The storm track is not a good one for producing snow storms across our area without help from a favorable phase of the Arctic or North Atlantic Oscillations (AO and NAO)
Earlier this year I discussed how the phase of the AO could modulate temperatures during a La Nina. Essentially, when the AO and NAO are positive during a La Nina, the temperatures across our region end up being above normal. However, when index drops into the negative range, our temperatures generally swing back and forth between being above and below normal and end up averaging around normal. Only when the NAO and AO are strongly negative like they were earlier this winter, do our temperatures average below normal.
This week's warmth is therefore not surprising. The AO and NAO have been positive with the rPNA pattern holding firm. Such a pattern can lead to temperatures torching like they did on Monday and might again on Friday.
So what happens moving forward?
Unfortunately, the models are much less certain in their prognostications of the AO and NAO in the longer time range. Some keep the NAO positive while some edge it back into the negative range. However, none of the negative members drop it very far from neutral. The uncertainly in the forecasts of the NAO makes 8-10 day temperature forecasts iffy and probably is the reason why CPC temperature forecasts are for us to average around normal in the 6-10 and 8-14 day ranges.
My own guess is that there will be more warmer than normal days than colder than normal ones through the remainder of the month but that's a low confidence statement. I also wouldn't be surprised by additional days that flirt with 70 degrees not including Friday's warm day. The pattern also suggests a below normal probability of snow through the remainder of the month.
While the pattern is not that good for snow overall, it still could produce surprises.
Last night's European model forecast offers hints at how. The forecast valid at 7PM on Feb. 22, shown above, is a case in point.
The model shows more blocking in the north Atlantic than the latest GFS forecast or its ensembles. It initially takes a low towards the Great Lakes but because of the blocking the low ends up getting squeezed to south and ends up as a weak wave that tracks to our south as shown above.
The freezing line is right over the DC as the dry slot moves into the region suggesting that some light precipitation during the previous 12 hours might have fallen as snow. This morning's Canadian model has the same look as the European model and would support the possibility of light snow especially north of the city.
I'm not ready yet to jump on the European and Canadian solutions as there still is uncertainty about the blocking. The bulk of last night's GFS ensemble members would argue for rain and have a warmer look than the European model for that storm. However, having two operational models hinting at a period of light snow sometime early next week makes the system worth tracking. They give snow lovers a glimmer of hope.
In summary, the pattern is not one that is particularly favorable for snow. However, there is a weather system that will be approaching next Monday or Tuesday that is worth monitoring. The pattern across the Pacific and much of the U.S. is not one that is very favorable for snow without help from a negative NAO and its associated blocking. Early next week the European and Canadian models show enough blocking to maybe supply that help.
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