Tuesday, December 14, 2010

MD for the NW

It's not often at all that you see something like this out here in the Pacific Northwest:
Fig 1 -- Mesoscale Discussion 2128 from the Storm Prediction Center on Tuesday, Dec. 14, 2010.
A mesoscale discussion for this region?  Not only that, but a discussion about an isolated tornado threat for the Puget Sound lowlands down through the Willamette Valley? Very intriguing.  To see the full text of the mesoscale discussion, you can visit the SPC's page for it here.  They note that there probably won't be a watch because the threat is so limited.  However, there already has been a tornado report near Salem, OR.

Right now, there's just some scattered showers moving through the region.
Fig 2 -- 0.5 degree base reflectivity from KRTX radar at 2207Z, Dec. 14, 2010.
Some stronger convective elements are present, particularly in the storm east of Portland.  Some of the showers coming over the Coast Range to the west are also showing stronger cores.  Added orographic lift as the southwesterly winds aloft are forced to rise over the low mountains there seems to be helping to get some of the showers convecting.  A view looking south from the top of the Atmospheric Science building at the University of Washington in Seattle shows the rain to the south (but clear skies over Seattle!).
Fig 3 -- Southward view toward Mount Rainier (though you can't see it) from the roof of the UW Atmospheric Sciences building at 2:14 PM PST.  From the UW Northwest Observations website.
So why this isolated tornado threat for the area?  The big story is the wind shear.  Here's this morning's 12Z sounding out of Salem, OR.
Fig 4 -- 12Z sounding from Salem, OR, on Dec. 14, 2010.  From the SPC website.
Focusing on the winds, it's clear there's a lot of wind shear, both directional and speed-wise, going on in the lowest levels.  Winds go from southerly at 10 knots at the surface to westerly at 40 knots at 850mb.  That's a fairly large amount of wind shear, particularly for this region of the country.  In terms of instability, the lapse rate is conditionally unstable--it's steeper than a moist adiabat but not quite as steep as a dry adiabat.  The absence of any strong temperature inversion also helps make this a tempting sounding for instability.  There's clearly a lot of moisture and with just a little warming at the surface, a fair amount of CAPE could be generated.  There are many ways to get warming at the surface.  One way is through warm air advection--wouldn't you know that the surface to 850mb directional wind shear represents a veering of winds with height.  That's a sign of warm air advection going on.  You can also see in the sounding above that the SPC analysis tools have  picked up on this as well--the vertical bar chart labeled "Inferred Temperature Advection" shows red bars (indicating warm air advection) at low levels with blue bars (indicating cold air advection) in the mid levels.  Warming below and cooling above will tend to destabilize the lapse rate.

Of course another thing that will help warm the surface is clear areas where sunlight can get through.  Here's a look at the latest visible satellite image over Washington.
Fig 5 -- Visible satellite image over Washington state at 2215Z, Dec. 14, 2010.  From the College of DuPage website.
There's still a lot of stratus cloud cover over southwestern Washington and northern Oregon (you could even see that on the webcam image above).  But, Puget Sound is definitely clear.  There's also some isolated patches of clearing on the northern Oregon coast between areas of very convective-looking clouds.  This makes sense as strong convective updrafts usually have compensating downdrafts (or subsidence) that tend to clear the air around the storm.  This clearing lets sunlight get through and can further help to warm things up.

So, perhaps we'll see some more destabilization this afternoon--there's still a few hours of sunlight left.  And with that wind shear, we'll have to watch extra carefully to see if we spot any rotation.

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