Thursday, January 26, 2012

A slow-moving storm and looking toward the weekend

It's been a while since I last posted on the blog as we've scrambled to recover from last week's winter weather here in Seattle.  I had a grand total of four inches of snow at my place, topped with a quarter inch of freezing rain, topped by another two inches of snow.  All in all, a pretty good storm--enough to close down the university for a few days.  However now we're returning to more seasonable weather--periods of rain with highs in the 40s.

A different kind of storm has been slowly making its way across the southern US these past few days.  Here's this morning's radar mosaic over the southeast:
A line of strong storms with some very heavy rain is moving across the area.  This is really the same line that moved across east Texas and western Louisiana yesterday, spawning a few tornadoes in the process.  Here is a map of the Storm Prediction Center storm reports from yesterday.  The red dots indicate tornado reports and the blue dots indicate severe wind reports.
I've been surprised at how slow-moving this storm has been, particularly since organized lines of storms like that tend to move eastward pretty quickly.  However, the upper-level structure shows that this surface low is being supported by a cut-off low aloft.  Here's yesterday morning's 300mb analysis:
And here's this morning's 300mb analysis:
Not a whole lot of change.  The center of the cut-off low at 300mb has moved east from west Texas to more over central Texas, but that's about it.  We call this a "cut-off" low because it's basically cut off from the main jet stream.  Note the strong jet streaks across the northwest and the northeast, but the 300mb low is well south of that stream.  Without those strong winds moving through, cut-off lows tend to move more slowly and stay around for a while.

Interestingly enough, the upper-level forcing from this trough really isn't communicating all that well with the surface.  Here's this morning's GFS surface analysis:
There is no strong surface low analyzed over the southeast--just a diffuse area of lower pressure along a baroclinic zone (a zone where there is a strong temperature gradient).  Cool (but not particularly cold) continental air is somewhat organizing behind this baroclinic zone as a cold front, and as this air encounters warm, moist air brought up from the Gulf of Mexico, we're seeing storms.

This storm looks to slowly lift northeast over the next 24 hours.  Here's the GFS forecast surface map for tomorrow morning:
The cut-off low aloft is actually forecast to begin folding back into the main flow today and, as that brings the surface low north, the surface low will begin interacting with that strong jet streak over the northeast that we saw in the 300mb map above.  This extra support aloft looks to deepen the low and really sharpen that cold front.

There are some other interesting mountain-driven features on this forecast surface map as well.  Notice the big, sprawling high-pressure area centered over Washington state. With clockwise (anti-cyclonic) flow around that high, you can see that it's bringing winds from the northwest to southeast over the Rockies in Montana, Wyoming and into Colorado, and it's also bringing winds that are trying to come from the east over the Cascades and Coastal Ranges in California.  In both cases the air is descending from high mountains down to either the lower Great Plains or out to the Pacific Ocean. When air descends down mountain ranges, it tends to warm up (sometimes rather significantly) and that lowers its pressure.  You can see that, as a result of this, there's a trough of lower pressure extending down the east side of the Rockies (or, rather, over the northern and central plains) and also an inverted trough of lower pressure extending up the coast of California.  See how there is that series of upward bulges in the pressure contours along the west coast?  That's areas of lower pressure within the larger sprawling high pressure.  This is still considered a trough--an area of lower pressure--even though it's not pointed in the direction we normally expect a trough to be.  You'll sometimes see this phenomenon (particularly on the west coast) referred to as a thermal trough.

Anyhow, moving ahead in the forecast, on Friday evening a quick Alberta-clipper type low is forecast to move through the upper midwest, bringing a shot of snow.  Here's Saturday morning's forecast surface map:
Behind this low is colder air, but not frigid--here you can see temperatures only forecast to be in the teens and low 20s for much of the upper midwest.  Certainly cold, but not extremely so.  Much colder air is pooling to the north in central Canada, but longer-range forecasts keep that cold air to the north throughout the weekend.

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