Wednesday, November 20, 2013

From calm to cold with cut-off lows

Looks like we're in for a major synoptic pattern change over the next week or so.  This morning's 12Z analysis from the ECMWF model at 500mb finds us with a nice zonal (east-west) pattern.  There are a few minor shortwave troughs but nothing very large in scale.
By tomorrow morning (Thursday), that is forecast to change.  The weak troughing over the eastern Pacific and western US is forecast to deepen rapidly into a strong trough.
And by Friday morning, the trough has deepened to become cutoff low--completely separated from the main jet stream winds (shown by the enhanced colors on the map) that are blowing to the north.
It's pretty amazing how over the course of 48 hours the upper-level pattern can change so quickly.  As I noted in previous blog posts, cut-off lows like this tend to stick around for a while.  Because they are separated from the main jet stream winds, there's nothing to really force them to move along from west to east.  So they usually just sit there.  This low will hover around for just a few days though, before transitioning back to an open-wave trough by next Monday as seen here:
But then, yet another cut-off low is forecast to re-develop off the west coast by next Wednesday!
I should also note that this particular upper-air pattern forecast for next Wednesday is bad news for the east coast--you have a deep trough covering the northeastern quarter of the country and then another shortwave trough racing through the southeast US and about to emerge over the Gulf Stream.  Nor'easter type storms are often born with this sort of setup as cyclogenesis occurs off the Carolinas.  This is still a long way out in the forecasts (168 hours), but it's something to watch...

Another aspect of this cut-off low moving across the US this week into next is that it will open the door for colder air currently over Canada to sneak down into the US.  Compare this morning's 12Z surface temperature analysis from the GFS:
With the forecast for Friday morning's low temperatures:
Much colder air moving in.  Pretty strong front at the leading edge of it too.  There are chances for some severe weather tomorrow as the front crashes through the plains, followed by a round of wintry precipitation. We're going from tornadoes this past Sunday to winter weather by the end of the week.  Such is autumn...

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