Warm, Wet, and Juicy

It felt a good deal like spring yesterday with the sun now higher in the sky compared to its winter solstice minimum and mild temperatures from the valleys to the mountains.  Yesterday's high of 55 at the Salt Lake City airport was not exceptional, but combined with the brilliant sunshine, it was a beautiful day to be outside.  

We have one storm on tap for the coming week and it's looking like a warm, wet, and juicy one.  It is associated with an inland penetrating atmospheric river.  The ECMWF HRES forecast valid 0000 UTC 25 February (5 PM MST Tuesday) shows the atmospheric river (bottom right panel) over the Bay Area curving clockwise through northern Nevada and Utah.  Integrated vapor transport (IVT) values in our area are > 250 kg/m/s, which puts is in the low end of atmospheric river categories, although that's a fairly high value for the western interior.  

It's a relatively slowly evolving pattern. As can be seen above, the ECMWF puts our mountains in precip Tuesday afternoon and that continues overnight.  The forecast valid 1200 UTC 25 February (5 AM MST Wednesday) shows atmospheric river conditions persisting in mild westerly flow. 

Atmospheric rivers in northern Utah can be fickle.  Some are heavy precipitation producers.  Others do little.  This one does have something going for it and that's large-scale confluent flow, meaning that there are two air streams merging together over northern Utah, one with a more northern origin and the other with a more southern origin.  This often creates and environment of large-scale lift that helps to generate a larger-scale cloud and precipitation shield to get precipitation going.  

Unfortunately, this storms has something that we don't want and that's warmth.  HRES forecast 700-mb temperatures over the central Wasatch are around -1°C Tuesday afternoon through about 5 AM Wednesday, afterwhich they decrease slowly.  That's a recipe for high snow levels and low-to-mid elevation rain.  Some models are warmer than that.  The 12Z HRRR for example has a temperature of about +1°C at 700 mb at 0500 UTC 25 Feb (10 PM MST Tuesday).  That would mean rain above the base of Alta, possibly to mid mountain. 

Indeed the HRRR-derived Little Cottonwood forecast is really pumping up the warmth.  It has the wet-bulb zero above 9600 feet, roughly mid-mountain at Alta, from 2 PM MST Tuesday to 2 AM Wednesday.  The snow level is usually several hundred feet below that, but I would not be surprised if it briefly got to 9600 feet or higher Tuesday night if this forecast verifies.  

The 12Z HRRR is one of the warmer models, but even the 06Z RRFS ensemble has some members pushing the wet-bulb zero to 10000 feet or higher Tuesday night (bottom left panel).  Water equivalents in that ensemble vary from 1 to 2.25" for Alta-Collins, but one member has only 3" of snow, which tells you that at least one member producing rain or mixed rain-and snow at Alta-Collins.

So expect a wet, warm, and juicy storm to develope Tuesday afternoon and persist into Wednesday, although temperatures and snow levels will slowly decrease on Wednesday.  Snow levels could be quite high Tuesday night, probably getting above the base of Alta and maybe getting to 9600 feet or even high if the warmer model forecasts verify.  Hopefully the cooler forecasts will verify, but even those are fairly mild.