Tuesday, November 19, 2019

Wait and See Forecast

Tomorrow is a very difficult forecast day due to the fact that we have multiple systems potentially affecting the weather of northern Utah. 

The NAM forecast valid 2100 UTC (2 PM MST) Tuesday shows a complex upper-level pattern with a 500-mb closed low over southern California with an inverted trough extending through Nevada and connecting with a trough in the westerlies over southern California (500-mb contours and wind barbs in black below).  A short-wave trough is moving through northern utah with southwesterly flow south of the Great Salt Lake and southerly flow over southern Idaho.  Meanwhile, at low levels, high pressure is pushing through Wyoming (sea level pressure color contours below). 

Because of that high pressure,  low level winds forecast by the NAM over southwest Wyoming are easterly, and northeasterly, with flow splitting apparent where the cold air impinges on the Uinta Mountains.  The resulting easterly branch pushes across the Wasatch Range and into the Great Salt Lake Basin. 

All of this means we'll see mountain snow showers and valley rain showers pushing northward across the region.  At the same time, we could see downslope flow developing from the east.  The where, when, how strong, or how much is difficult to answer.

For the winds, much will depend on the gory details.  The NAM forecast above suggests we'll see moderate but not strong easterlies.  Other models give different solutions.  It's the sort of pattern where I'd bring in the patio furniture and keep an eye on the forecast in case the low probability outcome of stronger winds were to verify.  Certainly anytime there's a closed low drifting around the Las Vegas area one needs to be alert.  

For precipitation, let's look at Alta-Collins.  Through 5 PM MST tomorrow (0000 UTC 21 Nov), the SREF has a range from nearly 0 to about 7 inches of snow with a mean of 4 inches.  For the event, the range is about 5 to 20 inches.  Note the staircase like structure of the plumes in the lower left, indicative of the storm coming through in bits and pieces, with the timing of those bits and pieces varying.  

My take is precipitation in fits and starts form tomorrow through Thursday.  Total at Alta Collins of 3-6 inches for a best guess.  More if we get a really productive precipitation period going.  These numbers are a bit lower than indicated by the SREF above.  I think that reflects the fact that this is not a pattern that typically produces huge amounts of snow unless something well developed moves slowly over the Wasatch Range.  

That being said, I'm not sure exactly how this will play out, so I'm calling it a wait and see forecast.  

1 comment:

  1. All sorts of precip records are falling in southern Utah this morning, but the biggest one is that St George broke the record for its wettest November 24-hour period ever, with 1.29 inches: https://twitter.com/NWSSaltLakeCity/status/1197191949763002368

    As of the latest NWS update, the winner appears to be Hurricane with 1.52 inches, but more is coming and I wouldn't be surprised to see some locations break 2".