Friday, March 16, 2012

Get Me Some Tums!

The past several posts I've mentioned that this is a difficult forecast.  There are numerous reasons for this, but for educational purposes, we'll talk first about the forecast for tonight when a band of precipitation is expected to develop somewhere in southern Idaho and northern Utah.

The loop below moves sequentially from long-range (78 hour) to short range (18 hour) forecasts produced by the NAM over the past few days, all valid at 0600 UTC (0000 MST) Saturday 17 March (i.e., midnight tonight).  Note how the band was initially forecast to be over the Salt Lake Valley and central Wasatch Mountains, then over extreme northwest Utah, and, in the most recent forecast, just to the northwest of Salt Lake City.

This helps illustrate why a precipitation forecast can sometimes be so difficult.  Nailing down position, intensity, and timing of these small scale precipitation features is very difficult.  In the case above, imagine if you were forecasting for the Bear River Range east of Logan in northern Utah.  You have some model forecasts calling for heavy snow, and others calling for nothing.  Further, there is no consistent trend as the forecast lead time decreases.  In other words, the NAM is not converging on a consistent solution, further eroding forecast confidence.

For Saturday, I still expect most of the action to remain to our north and for us to be windy and warm, as discussed yesterday.  We could see some mountain snowshowers (although snow levels will be high) and maybe even some thunder, but I don't expect it to add up to much.

Saturday night is when things get interesting.  Both the GFS and the NAM are now producing a band of heavy precipitation over the Wasatch and Uinta Mountains, as illustrated by the NAM forecasts below.

That begin said, this is a really bizarre pattern.  Initially, the band is in the southerly flow ahead of the surface trough (top panel), although the two features eventually nearly merge (bottom panel).  Note the southerly and southeasterly flow over eastern Utah, which, should it verify, might mean this is one of those events where the east side does well, such as Deer Valley.  On the other hand, not shown here is the flow at crest level, which is south-southwesterly.  This is going to be a very interesting period.   We don't really have a good conceptual model for a pattern like this and much will depend on how the larger scale flow and precipitation bands set up.  I'll go for 4-8 inches at Alta by first chair on Sunday, but look forward to seeing how it all pans out.  

1 comment:

  1. You think the winds are going to be an issue Sunday into Monday?