Thursday, December 1, 2011

When high pressure gets crazy...

I haven't posted in a while, mostly because I've been busy traveling and teaching.  But I'd like to get back to the current weather today briefly, as we have an unusual set of circumstances that's causing some unique weather.

First, anyone who has followed my Facebook status may have noted that Seattle (or, rather, SeaTac airport) reached a record high pressure of 1043.4 mb last night.  This extraordinary high pressure supported light winds and clear skies last night, allowing us to cool down into the upper 20s.  Pretty cold for Seattle.

But our locally high pressure here is just one facet of the far-reaching consequences of this dome of high pressure.  If we check the anomalies in 850mb geopotential height for this morning (which is close enough to the surface to reflect much of the surface pattern), we get a pattern that looks like this:
What is this showing us?  Anomalously high heights (which tends to translate down to high pressure at the surface) over much of the northern west coast and into the northern plains and Great Lakes.  This is contrasted with anomalously low heights over the southwestern US.  We can see that this low anomaly is associated with a low height center in the desert southwest in the actual 850mb map for this morning:

The problem is that when you have anomalously high pressure right next to anomalously low pressure, you get strong pressure gradients.  On the anomaly map above, the entire corridor from central California through northern Utah and into northern Colorado is marked by a sharp gradient between these two anomalous centers.  This is reflected in a strong pressure gradient at the surface.  Here's this morning's surface analysis from the GFS model:

The black contours are the isobars--lines of constant pressure.  You can see the sprawling high centered over Seattle contrasted with the low pressure center over western Arizona.  In between there are lots of isobars, indicating that the pressure is changing rapidly over that area.  Strong pressure gradients mean strong winds, and looking at the wind barbs on that map there really are some high winds in that corridor.

Usually our wind tend to follow what is known as "geostrophic balance".  This simply means that winds in the northern hemisphere tend to blow with high pressure (or height) to their right.  This also explains the counterclockwise flow around low pressure centers and the clockwise flow around high pressure centers.  However, this sort of balance tends to break down near the surface, particularly over rough terrain.  The more friction between the air and the surface, the more this disrupts the geostrophic balance.  Considering how mountainous the west is, you can see based on the wind barbs in the map above that the winds are blowing more directly from the high pressure in the north to the low pressure in the south.  This is just an example of surface friction at work.

The winds still tend to have a slightly easterly component, however, and that has huge ramifications for what that means for the weather.  It's causing a lot of trouble in two places.

1) Central and southern California.  Easterly or northeasterly winds here are descending down the western slopes of the Sierra Nevada and the coastal mountains of California, accelerating as they do so.  Here's a map of the amazing winds currently being reported throughout southern California:

There are several areas with 30+ knot sustained winds being reported, particularly in the central valley and in the mountains just northeast of Los Angeles.  Notice how dry the air is too--dewpoints (the blue numbers) are down in the teens and low 20s while the actual air temperatures (the red numbers) are in the upper 50s or 60s.  This is another symptom of downslope winds--they tend to be very dry.  Some damage has been produced by these strong winds in southern California, as the news outlets are reporting.

2) Upslope flow in the Colorado Rockies.  With an easterly component to the winds, that means that air is running into the eastern side of the Rockies and is being forced upward.  As that air rises, it cools, condenses, and (because it's cold enough) we get snow.  Lots of snow.  With strong winds on top of it.  Here's the latest surface map from the central Rockies:
Strong 20-40 knot winds are being observed in many locations throughout the mountains.  And notice all those little pink asterisks next to a lot of observations in central Colorado?  That means snow is falling at that location.  The more asterisks, the heavier the snow.  So far, between 6-12 inches of snow have been reported in the Boulder area, and it's still coming down.  Winter storm warnings and advisories are up for most of the Front Range.  It's turning out to be an impressive event.

And all of this is happening--without any real fronts!  It just goes to show that we don't need to have our deep lows with powerful cold fronts to still get some impressive weather

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