Monday, December 20, 2021

An Active Holiday Week Ahead

The upper-level pattern is setting up to make the holiday week a stormy one in northern Utah.  

Below are the average 500-mb height anomaly forecasts from the GEFS ensemble valid at 0000 UTC 25 December (5 PM MST 24 December) and 0000 UTC 29 December (5 PM MST 28 December).  The 500-mb level is a good proxy for the upper-level flow.  Anomalies are departures from the long-time average 500-mb heights.  Positive anomalies indicate anomalous ridging, negative anomalies anomalous troughing.  The images below show persistent ridging over the central Pacific and troughing along the Pacific Coast, a pattern that can be very good for northern Utah snowfall provided it doesn't become too amplified (i.e., the trough along the west coast becomes too deep).  


Source: NOAA/ESRL/PSD

Consistent with the upper-level pattern, our downscaled NAEFS ensemble product for Alta-Collins shows a major shift in the pattern beginning around Thursday December 23rd.  Until then, high pressure will predominate, but after that, the pattern becomes quite active.  


The NAEFS mean water-equivalent and snowfall through 0000 UTC 27 December (5 PM MST 26 December are about 3.65" and 60" respectively.  There is a wide range, but over 80% of the members are producing over 2" and 30".  The downscaled NAEFS seems to have a high bias at Alta this year, but even adjusting for that, this looks like the start of a pretty healthy storm cycle that may continue deep into the holiday week.

Ho ho ho.

3 comments:

  1. I don't envy those who have to make a precip-type forecast for the valleys and benches Wednesday night/Thursday. Looks like all sorts of moving parts with the strength of inversions/flow aloft/when precip develops. Sort of surprised NWS hasn't really mentioned a chance of freezing rain outside their discussions as it seems like a definite possibility especially in some of the colder valleys.

    Also, any thoughts on why the inversion has been so strong in Delta this cycle? I feel like those west-central valleys are usually better at mixing a bit and warming up in the afternoons. Until today even Logan was warmer than it was there.

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    Replies
    1. The area around Delta is completely snow covered. Probably a big help.

      https://worldview.earthdata.nasa.gov/?v=-118.59078026663357,36.11092897035177,-104.9877407176838,44.73434344377447&t=2021-12-20-T20%3A18%3A13Z

      Jim

      Delete
  2. Hopefully this one PUMMELS pc Valley!

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