The flow was fairly weak yesterday, but inspection of the 700-mb winds in the Rapid Refresh analysis showed a weak trough draped across northern Utah and central Nevada with a tongue of higher precipitable water air extending from southern Nevada into western Utah. This moisture tongue became the locus for the development of convective rain showers yesterday afternoon, which proceeded to move across central Utah and produce a few showers as far north as Little Cottonwood Canyon.
The moisture tongue was entirely unresolved by the conventional upper-air observing network, which provides observations only at Elko, Salt Lake, and Las Vegas. The fact that the Rapid Refresh picked up on such a feature is a testament to the tremendous advance in numerical weather prediction and data assimilation that has occurred in recent years. The Rapid Refresh assimilates everything from radar reflectivities to aircraft moisture observations, making it a powerful tool for weather forecasting.
If you really want to blow your mind, check out the experimental High-Resolution Rapid Refresh (a.k.a., the "HRRR"), which is presently under development at NOAA's Earth System Research Laboratory. Running at a grid spacing of 3-km (compared to 13 km for the Rapid Refresh) and incorporating state-of-the-art assimilation of radar data, the HRRR will likely revolutionize short-range weather analysis and forecasting over the western United States.
This moist plume had been sitting off the coast for a few days, looks like it is part of the remnants of Tropical Storm John.
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