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:
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:
This storm looks to slowly lift northeast over the next 24 hours. Here's the GFS forecast surface map for tomorrow morning:
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: