Saturday, November 20, 2010

Why this morning's low-pressure center is a dud for Washington

All the buzz up here in Seattle right now is on the possibility of lowland snow (that is, snow in the city itself and not just up in the mountains) for Sunday and Monday.  But, as I was asked recently, this morning there appeared to be a storm sitting off the coast of Washington today--so, why aren't we concerned about it?

Currently, there is indeed an upper-level trough whose axis lies just off the coast of western Washington.  Note the jet streaks on either side of the trough in the image below.
Fig 1 -- 300mb geopotential height and winds from 12Z, Nov 20, 2010.  From the HOOT website.
Now if we examine a very low-level chart, we notice that there is a low-level cyclone also situated just off the Washington coast:
Fig 2 -- 925mb geopotential height and dewpoint depressions from 12Z, Nov 20, 2010.  From the HOOT website.
Notice how the 925mb low is directly beneath the upper-level trough.  If you remember one of my blog posts from last week, you might recognize this situation as being a stacked low.  Stacked lows are weakening systems (since they tend to lack strong temperature gradients near their centers).  To confirm this, take a look at this morning's sounding out of Quileute (on the Washington coast):
Fig 3 -- Sounding from Quileute, WA (KUIL) 12Z, Nov 20, 2010.  From the HOOT website.
We see a low tropopause height of around 450 mb, but this makes sense as Quileute is near the deepest part of the upper-level trough.  Within the troposphere (under the tropopause), we see little in the way of organized directional wind shear--winds are generally out of the southeast at all levels.  This is another indication of a stacked low--if the trough aloft and the cyclone at the surface were displaced at all, we'd expect winds to be changing direction with height.

So what do we expect to happen?  Well, in a stacked low situation, we would not expect the low-level cyclone to be strengthening in its current location.  So we have to re-examine our analyses and try and figure out where would be a better spot for low-level cyclogenesis.

My very first blog post talked about diagnosing upper-level divergence (and consequently rising motion and usually cyclogenesis at the surface) using  jet streaks.  Back then I talked about how in cyclonically-curved jet streaks, we typically saw upper-level divergence in the exit region of the jet--where air is leaving the jet streak.  Let's try applying this to our 300 mb map above.

Fig 4 -- 300 mb geopotential height and winds from 12Z, Nov 20, 2010.  Annotated with a basic jet-streak analysis.
Above I've annotated the 300 mb map, drawing a long red arrow along the axis of the cyclonically-curved jet streak and using the default cloud-thought-bubble shape to outline the exit region of the jet in brown.  This is the area where we'd expect to find the most divergence aloft.  Remember--divergence aloft leads to rising motion which leads to lower pressure below as air lifts out of the levels below.  We'd expect better chances of cyclogenesis (i.e. falling surface pressure) down in that region, off the coast of southern Oregon and northern California.  Also notice that if we had a surface low develop there, the lower-and upper-level cyclone/trough centers would no longer be stacked.  So, if we were to make a prediction, based solely on analysis--without using any model output at all so far, we would predict that our weakening surface low off the Washington coast would be replaced by a strengthening surface low off the coast of northern California.

So what do models say?  Here is the GFS surface pressure field at its initialization at 12Z this morning (the same time as the maps above).  Note our nice surface low centered off the coast of Washington.
Fig 5 -- GFS sea-level pressure, surface temperature and winds, analysis from 12Z, Nov 20, 2010.
Now here is the GFS's 6-hour forecast for that same model run:
Fig 6 -- GFS sea-level pressure, surface temperature and winds, 6 hour forecast valid 18Z, Nov 20, 2010.
The GFS has forecast the center of the surface low to have moved to just offshore of the Oregon-California border.  But we just predicted this above!  So you can see now why Washington shouldn't have been concerned about the low looming off of their coast.  A simple look at the upper-air analyses shows that the low was going to weaken off the Washington coast and strengthen much further south.  And this was all without looking at model data to come to that conclusion.  I find that to be pretty cool.

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