Monday, November 22, 2010

Tornado and Snow

Well--quite the day.  And at least from the Seattle end of things, it's just beginning...

But first--unfortunately, this morning's tornado possibility in northern Illinois verified.  A strong tornado formed just east of Rockford, Illinois, and caused significant damage in the town of Caledonia.  This same supercell produced additional tornadoes along its path as it moved across northern Illinois and southern Wisconsin.  I grew up riding my bike through Caledonia all the time--I can only hope that there are no fatalities and that the damage isn't too bad. Tornado watches continue along this frontal boundary as it pushes east, so we're not out of it yet...

I'm still trying to get some radar images from the Milwaukee radar as the tornado strengthened.  I was watching as this went on but was unable to grab any screen captures.  There was a loosely-defined hook structure in the reflectivity field.  However, the true strength really came through in the velocity field.  It should be noted that these storms were moving northeast at almost 50 knots--fast movement can often mask internal rotation signatures.  This is why it's always important to use storm-relative velocity fields when trying to assess rotation..

I also wanted to update the snow situation.  We're looking at winter storm warnings throughout the Puget Sound region, with strong northerly winds and snow expected to continue this evening.  Here are the surface observations for western Washington from
Fig 1 -- Surface Observations from 100Z, Nov 22 2010.
The lowest pressure currently observed is just under 1000 mb, which is stronger than we were expecting from our models.  The low pressure center is actually on the western side of the Olympic Mountains and slowly tracking southeast.  However, note this isn't very well reflected in the wind field--we can see that the winds along the coast are indeed curving counter-clockwise around the low like we'd expect.  But--the winds throughout Puget Sound are very strong and out of the north.  This is opposite of what we'd expect for being on that side of the low.  So what's going on?

Take a look at the wind profiler images for the lowest 3.5 km for the last day or so.  Particularly focus on the most recent four profiles (the four left-most profiles).
Fig 2 -- Wind profiler history for the lowest 3.5 km at Sand Point.  Current as of 200Z, Nov 23, 2010
The labels at the bottom are of the form date/Zulu-time.  Notice how we had relatively light winds until 2300Z.  Then the winds became northerly near the surface while the winds aloft stayed relatively out of the west-southwest.  As the hours have gone on since then, we see that the layer of northerly winds has gotten deeper and deeper with southwesterly winds aloft.  This represents a strong wedge of cold air that's moving south across the Puget Sound region near the surface.  Aloft, we still see westerly to southwesterly flow--which is more the direction we'd expect from having a low-pressure to the west-southwest of the area!

Why is this important?  Because a setup like this is going to enhance snowfall amounts across the Puget Sound region.  We have a wedge of cold air moving in at the surface, but above the low-pressure center is still advecting in moist air that's coming in off the Pacific Ocean.  As this moister air is lifted over the cold wedge, we'll see the moisture condense out and more precipitation.  Furthermore, since the air new the surface is getting so much colder (cold air advection associated with the northerly winds), that precipitation is going to fall as snow--and any liquid near the surface will soon be frozen.  I believe that we're seeing an area of enhanced isentropic lift in association with this cold wedge coming south--a topic I'll get into in a later blog.

In the meantime, time to stay indoors and stay warm in Seattle tonight!

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