Saturday, July 21, 2012

The Tropical Connection

Thursday and Friday brought extraordinary thunderstorm activity to the Northwest, with torrential rain hitting both sides of the Cascades.  I can not remember such a period of such sustained summer convective activity in our area.

It turns out that there is a tropical connection to this outbreak, a fact noted to me by Dr. Jim Steenburgh, an ex-Husky who is a professor of atmospheric sciences the University of Utah and probably the greatest booster of Utah snow in existence.  And he has a wonderful weather blog (Wasatch WeatherWeenies)

Let me warm you up (or wet you down) by showing a plot of 24-h rainfall ending 9 PM on Friday (see image, click to enlarge).   Loads of places over the Cascades and Okanogan highlands got over 1 inch...some close to 2.  Here in Seattle 3/4 inch totals were plentiful...that is more than our normal MONTHLY rainfall.


As mentioned above,  yesterday's plentiful precipitation was juiced by cool moist, unstable air moving northward out of now defunct Hurricane Fabio (see an image below from a few days ago)


 Let me show you a sequence of satellite images from the water vapor sensor on the GOES geostationary weather satellite run by NOAA.  Water vapor imagery shows you the amount of moisture in the mid to upper troposphere--the lower layer of the atmosphere.  You will see how the moisture streams northward (white and greens)






There was also a modest feed of moisture at lower levels from off the Pacific...a weak "atmospheric river" type structure.  You can see this from the simulated vertically integrated water vapor from the WRF model (red, white and blue are the most H2O):

 Pulling this all together was a strong upper level trough, which provided upward motion and southeasterly flow (helping to pull the topical moisture up to us).  Here is the 500 hPa chart for mid-day Friday:

Mamma Mia...that's a nice summer trough!

  Everything came together to give us moisture, instability, and lift....the ingredients for robust summer convection.  And we probably won't see any more of that for a long time.

Tomorrow a strong closed low will reach us and clouds/cool temperatures will return for a few days.  Showers coming in later tomorrow and into Monday...sorry.

1 comment:

Mannes said...

Hi Cliff

A couple of questions:

(1) Why has our summer broken down? Between July 5-13 we had warm and sunny weather which has since disintegrated into a series of soggy and stormy events.

(2) Is it possible that a warmer than usual Pacific ocean water and atmosphere could carry extra moisture and send much of it our way - ironically causing our weather to be wetter and cooler than usual?