Monday, July 18, 2016

Wrong Side of the Monsoon "Dryline"?

Often during the North American Monsoon, Utah is located in a region of large-scale confluence where airstreams of different origins are drawn together between the large-scale trough along the Pacific Coast and upper-level ridging over the south-central US.  This often leads to a strong contrast in moisture and thunderstorm activity from east to west across the state.  It's a bit of a stretch, but you could call it the Monsoon "Dryline."

We have such a situation today with confluent flow over Utah leading to a strong contrast in precipitable water [a measure of the total water content of the atmosphere (color contours below with warmer colors indicating higher values)] across the state.  The dry air over western Utah and neighboring Nevada originates over the subtropical eastern Pacific and streams into the region from the southwest, whereas the moist air over eastern Utah and neighboring Colorado originates over the tropical Pacific and Gulf of California as well as the Gulf of Mexico.

Forecast precipitation by the GFS is confined to eastern and central Utah, but does nose it's way into portions of the Wasatch Front this afternoon.

This pattern will predominate over the next couple of days.  The exact location of the transition will probably vary some from day to day.  Right now, I think Salt Lake City may be just a bit too far north and west to get in on the thunderstorm action, but it's close enough that it's worth keeping an eye to the sky.  The likelihood of thunderstorms will increase as you move eastward, and that's worth keeping in mind if you are recreating in eastern Utah or the Uinta Mountains.

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