May 28, 2014

Why Thunderstorms in the Desert?

Today was a very unstable day around the area, with heating causing the air to percolate and convect. Convection, which is associated with cumulus and cumulonimbus clouds, is generally associated with a large change in temperature with height and forced  lift (upward vertical motion).  And we had plenty of both today in the area.

Take eastern Washington.  Here are three visible satellite images over Washington State and northern Oregon today at 9 AM, 11 AM, and 4 PM.  At 9 AM it was virtually clear east of the Cascades crest and down into the Columbia basin   This is not unusual because that area is the zone of downslope flow during normal westerly flow aloft.  By 11 AM, some shallow cumulus was forming on the hills of the eastern slopes and a few big cumulonimbus clouds were firing off over the Okanogan.  But by 4 PM, major convection was firing off over the normal dry eastern slopes and the Columbia basin.

Here are the radar images for roughly 9 AM and 4 PM.  Big difference, with heavy showers over SW Washington and over the Columbia Basin during the afternoon.  Radar coverage is poor over the eastern slopes, so the radar imagery is underplaying it.

So what was going on?   Aloft, we had an upper trough over us that was associated with cool temperatures. To show this, here is the 500 hPa (about 18,000 ft) forecast from the UW WRF model for 2 PM..  Colors are temperature and solid lines are heights (similar to pressure).  The blue colors are cold temperatures.

The sun is very strong now (we are less than a month from the summer solstice) and thus has great potential to heat the surface.  Cold aloft and warm at the surface produces a big temperature change with height (or temperature gradient), which in turn leads to atmospheric instability and overturning.  A frequently used measure by meteorologists for the potential instability of the atmosphere is CAPE (Convective Available Potential Energy!) and here is the plot of it for 2 PM.  Because we don't get very warm at the surface (and because our humidities are low--which also helps convection), we generally have wimpy CAPE compared to the central and eastern U.S.   Today's CAPE was pretty high for us: values getting to around 500.  But look how much higher it was in eastern Montana (2000-3000!).

So the sun had a good shot over the clear zone of eastern Washington this morning, leading to warming and substantial instability.    But there was something else.

Convection is substantially aided by a little lift to push parcels of air upward.  Sort of priming the pump of instability.   As the upper trough moved eastward today, it created lift that encouraged the release of convection.  Lightning detection networks indicated dozens of strikes over eastern Washington;  perhaps this is the origin of a substantial fire that begin near Wenatchee around 4 PM (see image courtesy of the Wenatchee World newspaper)

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