Sunday, March 18, 2018

Warm Atmospheric River Event Possible Later This Week

Atmospheric rivers are narrow corridors of strong atmospheric moisture transport, often connected in some way to moisture exported into the midlatitudes from the tropics or the subtropics.  Although Utah sometimes sees the remnants of weakened atmospheric rivers, bonafide atmospheric river conditions in Utah happen only a few times each cool season.

However, we have a shot at one making it to the state Wednesday night and Thursday if model forecasts hold.  The average integrated water vapor transport in forecasts produced by the Global Ensemble Forecast System for 0600 UTC 22 March (0000 MDT Thursday) shows strong values extending from the Pacific Ocean northeastward into Utah.  Values exceeding 250 kg/m/s (outlined by red line) indicate atmospheric river conditions, and most of Utah is covered.

Source: NWS
Two ingredients favor atmospheric river penetration into Utah in these forecasts.  The first is that the atmospheric river is strong on the coast, with integrated water vapor transport values over 750 kg/m/s.  The second is that the flow moves up the lower Colorado River Basin, missing the southern Sierra Nevada and other high terrain features that typically generate precipitation and deplete the water vapor content of airmasses moving into the western interior.

It is still a bit early to discuss details of where, when, and how much precipitation will fall.  Not all atmospheric river events are big precipitation producers in Utah, but some are.  Much depends on the orientation of the atmospheric river, the orientation of the flow relative to major terrain features, and the efficiency of the precipitation processes embedded in the flow.  Also important is the duration of the event.  Right now, it looks like we'll see a warm, humid (by Utah standards) precipitation event developing first in southwest Utah late Wednesday and then in other parts of the state Wednesday night and Thursday.  Until then, enjoy the spring break and monitor forecasts.

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