Unusual weather continues...

So far, April has brought record heat to the western U.S. with much cooler, wetter weather in the East, especially when compared to the record heat there back in March.  A strong upper-level ridge of high pressure brought sunny, warm weather to the immediate West Coast this past weekend.  San Francisco didn't break any records with highs in the 70s.  Many locations in the Central Valley, the Southwest, and the Pacific Northwest did break daily record highs over the past few days though, and some locations in the Great Basin even set new record highs for all of April.  This ridge is slowly moving east, and brought more record heat today for the Rockies and Plains.

Meanwhile, a strong trough of low pressure has been parked over the Northeast, resulting in cool temperatures and over the weekend produced heavy rain along the coast.  Over parts of western New York and Pennsylvania, heavy, wet snow fell, with over a foot in some locations.  For some parts of the Northeast, this was the biggest snowfall of this otherwise mild winter season.

Also incredible is that after the record warmth across the Midwest back in March, April is now on track to be cooler than March was.  Such an occurrence is extraordinarily rare, but will likely happen this year in Chicago, Milwaukee, and other nearby locations.  The last time this happened in Chicago was 1907.  And, this April will likely be 2º to 3º above normal for Chicago but will still be cooler than March, demonstrating how unusually warm March 2012 was.

In San Francisco, we've obviously cooled down since this weekend as a low-pressure system approaches from the Pacific.  Today was still fairly nice though, with mild temperatures and not much wind because onshore pressure gradients have weakened compared to Sunday and Monday.  As a quick measure of onshore gradients, we look at the pressure difference between San Francisco and Sacramento.  Outside of winter, SF normally has higher pressure and Sacramento has lower pressure (due to stronger heating in the Valley).  The approaching low pressure system offshore though has disrupted the usual onshore gradient, keeping the gradient almost neutral today.  So, we've had a relatively mild evening (image on right) without the usually-strong westerly winds.  Anyway, the bulk of the rain with the approaching system will be focused on southern California, but we will see some showers and possibly a thunderstorm in the Bay Area Wednesday afternoon and night.  Then, this weekend looks sunny and seasonable, with highs in the low 70s for downtown SF but cooler near the beaches with onshore flow.