Tuesday, November 14, 2023

A Tricky One

For the past several days, some of our computer models have been producing a somewhat unusual banded precipitation event for northern Utah on Thursday.  I'm not sure what to make of it, but let's take a look.  

We'll begin with the setup.  Below is the GFS forecast for 1200 UTC 15 November (5 AM MDT Wednesday).   The upper-level flow off the Pacific coast is best described as "split" with a branch of the jet south of a deep low off the coast of California and a second branch over the Pacific Northwest.  The latter has a short-wave trough embedded in it (sashed line).  Over the Pacific Northwest, those two branches come together.  This is referred to as confluence by meteorologists.  

By 0600 UTC 16 November (11 PM MST Wednesday), the upper-level trough in the northern branch of the jet has moved over Idaho and a second trough that is pinwheeling around the low has is over the Sierra Nevada.  The confluent flow has shifted downstream to over northern Utah, Southern Idaho, and Wyoming.  

This looks like a fairly innocuous pattern, and up until this forecast time it is.  However, if you look at the precipitation pattern in the upper-right panel above, you'll see a weak precipitation band extending across Northern Nevada (see red circle).

That band then strengthens and by 1200 UTC 16 November (5 AM Thursday) it extends right across northern Utah (see red circle below) as the upper-level trough that was previously over the Sierra Nevada phases with the upper-level trough in the northern branch of the jet.  This band develops right in the confluent flow region, which is probably not a coincidence.  Such flow patterns, given the right environment, can generate circulation patterns that can lead to banded features of this type.  

The GFS is not overly excited about this storm.  The 6Z run spits out 0.25" of water and 2.2" of snow for Alta-Collins.  Most of the SREF members are in the 3-6 inch range, but about a quarter of the members are a bit more excited and generating 8-10 inches of snow.  

This is a tough forecast.  In the days before numerical weather prediction models, we would probably have no idea that such a band could develop and the forecast would probably be for fair weather.  The models are suggesting, however, that one will develop.  It is, however, fairly narrow, so what happens at a given point, say Alta, is dependent on both band intensity and location.  In the upper-end SREF forecasts, both of those things come together.  In the low end (and if you look carefully there are 3 members that do practically nothing), none of those things come together.  The GFS and SREF resolution is such that they probably overestimate band width.  In reality, things may be even more localized than they suggest.  

Keep expectations low (dust-on crust is most likely) but hopes high (maybe we can get lucky and get several inches).  Note that this will be a warm storm, with snow levels near 8000 feet to start, possibly falling to 7000 feet by late Thursday morning.  


  1. For what its worth, the European model has been consistently beefier with the QPF, in the "chaotic" multiple wave pattern, which produces low confidence in the forecast beyond 72 hours. For hat its worth, the 18z run of the GFS has started to come around to the ECMWF with 0.50-0.75" QPF amounts. Keeping fingers crossed. :)

  2. I saw a YUGE grey, fat joint shaped lenticular over wilson peak this am and say this one goes upper end projections.