Wednesday, July 31, 2019

Will This Monsoon Surge Produce?

An impressive surge of monsoon moisture is currently occurring over Utah.  Below is a loop of High Resolution Rapid Refresh (HRRR) model analyses and forecasts of precipitable water for the period from 0400 UTC 31 July (2200 MDT yesterday) through 0200 UTC 1 August (2000 MDT this evening).   Precipitable water is the depth of water you would have on the ground if you were to condense out all the water vapor in an atmospheric column.  Thus, it is a measure of the total integrated water vapor in the atmosphere.  Note how high values move northward from Arizona and the lower Colorado River Basin into Utah.  Values in Salt Lake City increase from about 12 mm (0.5 inches) to about 30 mm (1.2 inches). 

Surface dewpoints overnight increased steadily with the surge, as illustrated by meteograms for St. George and Salt Lake City. 

The latest (1505 UTC/0900 MDT) satellite and radar analysis shows extensive upper-level cloud cover over Utah.  Relatively weak and spotty radar returns are evident over northern Utah, although much of that is probably not reaching the ground yet. 

Concerns for the forecast today include where will the moisture go and how will the forcing for convection and thunderstorms evolve.  The HRRR simulated reflectivity forecast does bring some showers into northern Utah, some of which may be strong enough to generate lightning and thunder.

No model today can reliably predict the timing, location, and intensity of monsoon precipitation and thunderstorms.  Be aware of the possibility of rain, which could be locally heavy, as well as the possibility of lightning and thunder.  Below is the latest summary from the National Weather Service.

Also, remember that individual storms can sometimes produce more precipitation than indicated by precipitable water values.  This is because storms process water vapor not only from the column overhead, but also from the surrounding area.  Thus, precipitable water should not be interpreted as the maximum amount of precipitation that could fall, but instead simply as a measure of how much water vapor is in the atmosphere. 

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