Friday, February 10, 2012

A really big high brings winter back

A lot of snow and rain is currently impacting the eastern third of the country.  This morning's radar composite sums it up nicely.
Heavy rain for southern Louisiana with scattered heavy showers throughout the southeast.  Further north where it's colder, snow (enhanced by the lake effect) is being reported in the Chicago area and northern Michigan.

All this active weather...but where is the surface low?  Here was this morning's 12Z GFS surface analysis:
The only really organized center of low pressure is analyzed well to the northeast in eastern Quebec.  There is, however, a general trough of low pressure extending back through the Great Lakes and down into the southern plains.  This trough looks to be simply a consequence of being caught between two high pressure centers--one weaker high off the Carolina coast, and another, stronger, sprawling high pressure center in the Canadian Prairies.  Notice the very cold temperatures associated with this high pressure center--well below zero Fahrenheit a the surface this morning in parts of central Canada and the northern plains.  There's also a pretty sharp boundary between this colder air and slightly warmer air being pulled north in that low pressure trough. So, even in the absence of a strong surface low, we still have such a strong high pressure center that good enough thermal gradients are set up to help produce some significant weather.

This high pressure center looks to be here to stay--by tomorrow morning it is forecast to have set up shop in the central plains, bringing down much colder air than we've been seeing in the central and eastern US as of late.  Here's tomorrow morning's GFS forecast:

Notice the very strong temperature contrasts still on the leading edge of this high pressure center.  Furthermore, as this cold air moves down over the land surface, it encounters much warmer air over the open waters of the Gulf of Mexico and off the east coast.  This is setting up some very strong-looking temperature gradients that follow the coastline.  It also looks to be driving some cyclogenesis off the east coast.  Furthermore, with such a strong high pressure center, the pressure gradients are also very strong, and this means strong winds--you can see some 15-20 knot winds forecast over the upper Mississippi valley and into the southeast.  This means when the cold air arrives, it's going to arrive with quite the blast.

This high continues to linger into Sunday, according to the 48-hour GFS forecast:
Nos the strongest pressure gradients look to be across the middle Atlantic states.  Should be quite the blowdown.

With this huge high pressure center crashing the party, it looks like the weather pattern is finally turning to something more winter-like.  Seattle is once again back to having its rounds of rain with relatively cool weather, frigid temperatures are building back across the northern Plains, and with the presence of this high pressure center things look to dry out a bit from the rainier-than-normal conditions some places in the south and east have seen lately.  Perhaps these La-Nina-based seasonal predictions will come true after all...

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