Friday, February 17, 2012

Presidential Pow?

President's weekend is upon us.  Aspects of the forecast are easy.  There will be tourists, traffic, and crowds in the Cottonwoods.  How about powder?

The latest GFS forecast brings a cold front into northern Utah Saturday night.  We should get a period of widespread snowfall in the mountains and the valleys as the front moves through.

If the timing of the GFS forecasts is accurate, snowfall rates in the Cottonwood Canyons will be highest late Saturday and Sunday morning.

This looks to be a right-side-up storm, with decreasing snow water contents over time, as often occurs with frontal passages.  Our snow water content algorithms, when directly applied to the GFS output, suggest an initial snow water content of ~8% at Alta, falling to ~4% later in the storm.

When applied to the liquid precipitation forecast from the GFS, this yields an accumulation of about 8" of snow at Alta.  However, the GFS does not fully account for local orographic effects.  Thus, going for something ~10–14" at Alta by Sunday afternoon is probably more appropriate.

Update 2:50 PM: To clarify, late Saturday should be late Saturday night - JS


  1. Hi Jim - I'm doing the USW forecast and comparing forecasts and model outputs from a few different sites. Most notable, the NWS seems to be a huge outlier - . This forecast calls for 19" of snow in a 12 hour period and then 8" through sunday night. What could cause the NWS forecast to be so far off from Trevor's NAM and WRF outputs? Even as a zone forecast, isn't Alta the site that is forecasted for and the nws program interpolates that data around the zone? Thanks!

  2. Expecting more than produced by the GFS and NAM makes some sense given their poor terrain representation, but 19+8 probably lies on the outer edge of the envelope of possibilities. I won't complain if it verifies (actually, I probably will as that's probably too much of a good thing), but it is not likely. I just pulled up their digital forecast, which was updated at 2:55 PM and it is calling for 5-9" on Saturday night and 5-9" on Sunday at Alta.

  3. I love the title. Bring on the Presidential Pow!!!

  4. I don't know if this is just a typo in the script, but it looks like the initialization for the WRF on Trevor's page seems to be for a time back in January. Is he working on some updates? The other stuff looks good though!

  5. Alex - the WRF simulations are a low priority for us. We were using them primarily for our lake-effect field activities, which have ended. Trevor is busting hump trying to get his thesis done and using CPU time on that system for his thesis related work. Thus, fall over to the 4 km NWS WRF runs or those from the University of Washington.