Tuesday, November 11, 2014

Will Wacky Weather Pattern Bring Mountain Snow?

As illustrated by the loop below, which shows the observed infrared satellite imagery and 500-mb pattern for the past two days followed by that in the GFS forecast through the middle of next week, shows the high-amplitude long-wave pattern that will dominate the weather over North America over the next several days.  The pattern is characterized by a persistent ridge over the west and trough over the east.  

The ridge is far enough north to let some westerly flow and moisture sneak into the western United States.  It's a bit of an odd ball pattern as storms weaken in the strong flow split over the eastern Pacific, but are able to maintain some intensity in the confluent upper-level flow over the central U.S. Rockies, as illustrated in the forecast for 0000 UTC 15 November below.

The latest GFS time-height section for Salt Lake City shows warm frontal passage late Thursday followed by a prolonged period of moist westerly–northwesterly flow through Monday.

There's not much in the way of large-scale forcing in the forecast, but the mountains have an influence.  In the low-res GFS (below) you can see the importance of terrain forcing, with interior accumulations through Sunday afternoon greatest over the high mountain region that includes the Wasatch of Utah and Tetons of Wyoming and also further east over the western Colorado Rockies.  

Although this doesn't currently look like a huge or intense event, it has duration on its side and periods of snow may add up over time.  We'll take a closer look as the event approaches and we gain greater confidence in the gory details, especially the position of the moisture plume.

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