Wednesday, July 25, 2012

A Monsoon Break for Northwest Utah?

Overnight, somewhat drier air has moved into northwest Utah, as shown by the loop of 700-mb wind and integrated precipitable water below.

Integrated precipitable water is the depth of water that you would have if you condensed all the water vapor in the atmosphere.  Higher values (warmer colors above) indicate more integrated water vapor, lower values (cooler colors above) indicate less.

Integrated precipitable water can be calculated using data from upper-air soundings (i.e., weather balloons), but those are only launched twice a day and frequently contain biases that can be quite large.

Alternatively, we can measure and track changes in integrated precipitable water using ground-based GPS receivers.  This is because the time it takes for a GPS signal to travel from the satellite to the receiver is partly a function of the atmospheric water vapor content.  Clever wizards (a.k.a. scientists) figured out how to use the signal delay to infer the integrated water vapor.

Based on these GPS measurements, we can examine trends in integrated precipitable water at the Salt Lake City airport.  Note how the integrated precipitable water peaked yesterday at almost 3.5 cm, but now sits at just under 2.5 cm, consistent with the analysis above.

Partly because of the decline in integrated precipitable water, I suspect we'll see less widespread convective activity in northwest Utah this afternoon.  The odds of a thunderstorm aren't zero, however, as we may see a few isolated thunderstorms pop up.  Nevertheless, we'll call this a monsoon "break" given the expected reduction in thunderstorm coverage.

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