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|
|Fig 2 -- 1500 Z Nov. 22, 2010 RUC 500 mb geopotential height and wind analysis|
|Fig 3 -- 3-hour pressure changes and wind vectors from 18Z, Nov 22, 2010|
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|
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.|
|Fig 6 -- 18Z sounding from KDVN, from the SPC website.|
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.|
Exciting afternoon in the world of weather!