Thursday, October 27, 2011

Phillyweather.net 2011-12 Winter Forecast

It's our sixth stab at forecasting winter in Philadelphia -- some of the years have been pretty successful in terms of forecasts...others, well, not so much.   The theme this year, as our earlier series on "Laying The Grounds" discussed, is that this is a winter where La Nina rules the roost again.  The combination of the La Nina, cooler ocean waters in the North Pacific, relatively strong Arctic and North Atlantic blocking patterns in recent months, and other considerations leads to a forecast that will shape up as another cooler-than-average winter across the Delaware Valley.   The devil in the details is timing when we see a persistent colder than average pattern set up -- however, the thinking here is that mid December through mid January will shape up as a four week period of our coldest temperatures of the season compared to average.

Sandwiched around this cold period will be a mild, but variable beginning and end to meteorological winter.   If you remember the winter of 2008-2009, there may be a lot of similarities to that winter in terms of brief, significant warmups in advance of storm systems that bring rain, followed by a cool down, with a yo-yo pattern of sorts around a cold January.   This winter may shape is in similar regard  -- early and mid December featuring mild but variable weather (there will be some cold shots mixed in with a few warm days) before the pattern loads up with colder air for the latter half of December and the beginning of 2012.  The holidays will likely be colder than average, with the coldest weather of winter occurring just after the New Year.






As we switch to January, the Midwest and Great Lakes will be quite cold compared to average as a trough lines up over the eastern part of North America.  The first half of January could be the coldest part of the month before we moderate somewhat as the storm track becomes a bit more active.      Climatology places the best snow chances in mid January through mid February...we'll get at least some snow out of this period but unlike last winter, the "big snows" do not seem as likely as Pacific energy will be a bit more muted compared to last winter.

The latter half of February, in general, will be mild.  Again, variability and variety will be part of the weather landscape.  When we warm up, we'll be quite mild...in mid February we could see some 60 degree weather at times as early teases of Spring hit the region.  However, cold shots will lurk and we, again, could end up in some snowy scenarios as we transition into March.  February will be a month where winter chill takes a bit more of a break and we thaw out.

The monthly calls are included in addition to the overall "theme" of winter as an idea of what we think will happen from month-to-month.   The timing of patterns is a bit harder to nail although general winter trends are easier to spot outside of exact snowfall.   Locally, I project this winter to be a cooler-than-average winter on the whole, with January the coldest compared to average on the whole, with December coming in near normal and February coming in above normal.   Temperatures for the winter will average to around a degree below average, with relatively average to just below average precipitation across the Eastern US.

December and February shape up as around average in precipitation.   I'm nudging a bit out on a limb but I think January ends up a touch below average on precipitation (the thinking is the first half of January will be cold and dry) but because of the colder weather, we can probably coax a decent amount of snowfall out of the month.   Timing is always the devil in this -- a storm track that doesn't properly time with cold air in place will not yield much in the way of snow locally.  




In terms of snowfall, we're projecting slightly above average snow for the region this winter -- between 24 and 29 inches overall.  One can't deny the trend towards snowier winters of late but Philadelphia has never had three consecutive winters of more than forty inches of snow.  I'm sure the snowstarved would love to see it -- unfortunately, I don't think it happens.  Most everyone ends up a bit above average on the whole -- nothing dramatic for the winter but the suburbs should get to around 30" this winter, with the Lehigh Valley getting a shot at 40" of snow this winter if everything breaks right and the Poconos having a shot at a 55-60" snowfall winter.   Not bad...for some, not as great as last winter, but the odds lean towards slightly above average snow.   Snow is always the big wild card though!

In short, expect a "typical" winter in the Delaware Valley for most of the winter season after this unusual weekend that may be ahead of us, with a cold holiday and early January that averages the winter out to a bit cooler than average once we're all said and done.