Mid May Powder Prospects

OK, the cat is out of the bag.  An unusually cold airmass is on its way to Utah.  The question is, will we get some decent mid-May deep powder skiing?

Let's start with the temperatures.  The loop below shows GFS 700-mb (about 10,000 ft above sea leavel) temperature analyses and forecasts from 1200 UTC 14 May (0600 MDT Sunday) through 1200 UTC 18 May (0600 MDT Thursday).  Note how the tightly wrapped core of cold air over the Gulf of Alaska slides down along the Pacific coast and then moves inland to directly over northern Utah.

Air that cold is pretty unusual in our part of the world in mid to late May.  In fact, 700-mb temperatures below -10ºC have never been observed at the Salt Lake City airport after May 12th, although we've been close.

Source: SPC

We'd have a shot at getting a -10ºC in the record books except soundings are only taken twice a day, at 1200 UTC (0600 MDT) and 0000 UTC (1800 MDT) and it appears right now that the coldest air at 700-mb will be here between the sounding times.  Right now, the forecast for 1200 UTC are -9.6ºC for the NAM and -9.0ºC in the GFS, so we'll have to hope things come in a bit quicker or colder to set a new mark.

Next, let's talk about precipitation.  Today we'll see some scattered showers and thunderstorms, which may produce snow in the highest elevations (emphasis on highest).  Things pick up overnight with the approach and passage of the cold front, which is expected to be pushing into the Salt Lake Valley around 0900 UTC (0300 MDT) tonight.  

Precipitation will be in the form of rain initially in the valleys, but if current forecasts hold, snow levels will fall to bench level and probably even the valley floor prior to sunrise.  That will be at the tail end of the frontal band, so most of the precipitation will fall as rain, but don't be surprised if you see snow on colder surfaces tomorrow morning.  The weather story graphic below from the National Weather Service summarizes the situation quite well.  

Source: NWS

After that, we may see some additional periods of rain or snow showers through early Thursday.  Right now, none of that seems very organized, so I'll call it "scattered", but it's worth paying attention to forecasts, especially if perhaps the lake can get involved Wednesday night.   

For the mountains above 8000 feet, the models are putting out numbers that are tantalizingly close for good powder skiing conditions.  At Alta, the NAM pumps out about 2 inches in scattered storms through midnight, and then another 5 inches overnight, with about 7 inches through 9 AM tomorrow.  The typically more bullish GFS is laying down quite a bit more with the frontal passage, with a total of 13" through 9 AM tomorrow.  NCAR ensemble plumes total about 1 to 2.25", although that includes dribs and drabs from last night that in some members that were probably overly done.  

I'll go for perhaps an inch or so in scattered showers through early tonight and then 5-10 inches through 9 AM tomorrow at Alta-Collins.  Good deep-powder conditions will require something at or above the top end of that distribution and getting turns in the morning tomorrow as  I think we'll see mostly scattered snow showers tomorrow during the day and some potential for things to get manky. There's a possibility of additional snow late Thursday and Thursday night, but it's really a crap shoot at that long lead time and the odds of a major dump are low (but non zero).  

Unfortunately, we're not receiving data from Alta-Collins at the moment.  If that continues, some guesswork might be needed in the morning concerning totals at upper elevations.