Monday, November 22, 2010

Seattle Snow and Illinois Tornado Watch

Quite the day of contrasts here--I wasn't sure what exactly I should focus on.  On one hand, I live in Seattle now, and Seattle is currently having their first significant snowfall in two years.  On the other hand, I'm from northern Illinois which is now under an unusually late-season tornado watch.  So, we'll briefly look at a little bit of both.
After reading several different articles from various people over the last few days about how it was definitely not going to snow in the Seattle lowlands today, I was pleasantly surprised to find a half-inch of snow on the ground outside my house this morning--with the snow still coming down.  The weather service, for their part, did a pretty good job at calling this, actually.  In spite of some model evidence to the contrary, they decided to err on the side of caution and it's looking like this has paid off. Continuing that trend we now have a winter storm warning for tonight.

Let's take a look at the general weather setup across the US right now.  Here we have the HPC's objective analysis for the surface from this morning.
Fig 1 -- 1500Z, Nov. 22, 2010 Surface Objective Analysis from the Hydrometeorological Prediction Center
Though it's difficult to glean relative strengths from this map, we can see our two lows of interest--one just off the Washington coast and one over northern Missouri.  Based on what I talked about in my last blog post, are these lows in positions favorable for strengthening?
Fig 2 -- 1500 Z Nov. 22, 2010 RUC 500 mb geopotential height and wind analysis
Both lows are located under the exit regions of jets aloft (there might be some debate over whether these are actually "cyclonically curved" or straight jets, but in general the lows are not that far out of good position). So, we can conclude that in general these lows should be deepening.  Another product we can look at with respect to the strength (and also movement) of surface cyclones is by looking at observations of 3-hour pressure falls.
Fig 3 -- 3-hour pressure changes and wind vectors from 18Z, Nov 22, 2010
Solid contours surround areas of increasingly negative pressure changes, and therefore these are areas where the surface pressure has fallen over the past three hours.  Dashed contours surround areas where the pressure has risen over the past three hours.  This graphic really makes our areas of interest stand out.  We're limited off the coast of Washington because there are very, very few observations over the ocean.  However, even on the Washington coast there have been significant pressure falls over the last three hours.  Oddly enough, though, there area areas of pressure rises to the east of the Cascades on this graphic.  With a strengthening pressure gradient as the low moves onshore, we can expect some pretty strong winds tonight in the Puget Sound area, which is exactly what's being forecast.

Over the midwest, though, we see an interesting feature in the pressure changes--a sort of couplet, with an area of strong pressure rises over the Kansas City area and an area of strong pressure falls over central Wisconsin.  If you go back to figure 1 above (the HPC analysis) you'll see that the surface low-pressure center is right between the two (keep in mind that the HPC analysis above is from three hours before the pressure change map).  This couplet of pressure rises--pressure falls shows us how the low pressure center is moving.  As the low center approaches, we'd naturally expect pressures to be falling.  As the low moves away, you'd expect pressures to rise again.  Therefore, a good way of finding the direction of movement of a low pressure center is to draw a line from the center of the pressure rises to the center of the pressure falls.  The low should roughly follow that line.  We see above that this means our low pressure center is going to be moving through eastern Iowa and into central Wisconsin.

So--more snow in Seattle?  It's difficult to tell because we lack a really good measure of moisture aloft.  There are no soundings over the Puget Sound region (the nearest are at Quilute out on the other side of the Olympics on the coast and way over in Spokane) so trying to figure out where our moisture is aloft is difficult.  We could try a water vapor image...
Fig 4 -- GOES-W water vapor channel from 1830Z, Nov. 22, 2010
We do see moisture "aloft" but it's difficult to see much in the way of structure.  We can see the subsidence (and consequent drying) associated with that jet streak coming in from the northwest, so at least that lends some validity to our upper-air map above.  However, there is no organized source of moisture (like a so-called "atmospheric river") fueling the cyclone from the south.  There's definitely still moisture there, though, and so if models are showing more snowfall this afternoon and tonight (which they are) reason to doubt that...

However, in the midwest, we DO have a sounding in the middle of our area of interest.  One thing I want to point out is how quickly the lower troposphere can change in advance of this cyclone.  This is this morning's 12Z sounding out of Davenport:
Fig 5 -- 12Z sounding from KDVN, from the SPC website.
This sounding is a tour of different layers all stacked in a fun way.  From the surface up to around 900 mb there was a strong inversion--the atmosphere was almost isothermal (at the same temperature) through that entire layer.  From 850 mb to 700 mb, the atmosphere becomes dry adiabatic--a highly unstable layer.  Above 850 mb, the profile becomes moist adiabatic (good for heavy rain if there's moisture--which there is).  The entire boundary layer up to 850 mb is saturated, so naturally once we get to the unstable dry adiabatic layer there's a healthy amount of CAPE.  However, to get more severe convection, we'd have to be able to draw on more of that boundary layer moisture which means breaking that strong surface inversion. But six hours later---
Fig 6 -- 18Z sounding from KDVN, from the SPC website.
  Wow!  Talk about some warming.  Two things helped here--there's a fair degree of warm-air advection in the area (winds are strong out of the south throughout the state of Illinois) and that helped to warm things up considerably.  More importantly, though,not only did warm air get advected north, but moisture as well.  This has allowed us to not only erode the inversion, but also to stay saturated as well--a rare combination!  Note we are now nearly dry adiabatic from the surface up to 800, with a (much smaller) inversion, above which we have another dry adiabatic layer and then a conditionally unstable layer all the way up to the tropopause.  Wich such steep lapse rates and saturation near the surface, CAPE values are naturally high--2411J/kg of surface based CAPE in this sounding.  Very unstable, very primed for severe weather.

However, to get tornadic weather, you need healthy amounts of wind shear as well.  Clearly this is not lacking in the sounding above.  South-southwesterly winds slightly veering with height (another sign of warm-air advection!) and increasing in speed from 10 knots at the surface to 50 knots around 850 mb.  That's both directional and speed shear.  Excellent environment for maintaining discrete storms with rotation capabilities.  No wonder the SPC is putting out tornado watched.

As of 1930Z, the radar looked like this:
Fig 7 -- Radar image from KDVN, VCP 212, 1923Z, Nov 22, 2010.
Based on where the wind shift is in the surface observations, the cold front still seems to be the primary lifting mechanism.  But, if the 18Z Davenport sounding is indicative of the airmass over Illinois and southern Wisconsin, I wouldn't be surprised if more discrete storms popped up ahead of the front this afternoon.  Those storms could be somewhat intense.  Also note that even with the front as the forcing, the storms are remaining independent so far and not conglomerating into a big line.  This is due to the extraordinary wind shear which is helping to keep the updrafts separate.

Exciting afternoon in the world of weather!

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