Incoming?

Presidents weekend and week are on our doorstep.  Brace yourselves!  Fox13 is reporting that the Salt Lake City International Airport is expecting Monday, the day after the All Star Game, to be the busiest day of the year.  It will be a busy weekend downtown.  I wonder if that will increase or decrease the usual holiday pulse to the mountains?  Time will tell. 

On the weather side, storms are incoming, but with some forecasting issues that give me indigestion early next week.  Saturday looks dry, but a passing cold front and upper-level trough will bring a quick hitter storm Saturday night and early Sunday.  

Then we transition into a moist northwesterly flow for Monday and Tuesday.  The GFS integrated vapor transport valid 0000 UTC 21 February (5 PM MST Monday) shows large vapor transport indicative of an atmospheric river wrapping clockwise from Hawaii around a high pressure system centered near 40˚N and then into the Pacific Northwest.  Although the flow is from the northwest in our region, in a strange sort of way, this is actually a pineapple express with moisture originating from near Hawaii.

Source: CW3E

Mountain snowfall on Monday will depend strongly on the where we are relative to the inland penetrating atmospheric river and associated upper-level jet stream.  The GFS provides one solution in which Monday we get some periods of mountain snow showers (top image below, valid 5 PM MST Monday), but things really pick up on Tuesday (bottom image below, valid 5 PM MST Tuesday) as a deep, developing trough in the northwesterly flow moves onshore, a cold front pushes across the Intermountain West, and the jet sags southward.

The storm would continue with the cold frontal passage Tuesday night and then periods of post-frontal snow showers on Wednesday.  

The ECMWF high-res is drier, which is not unusual, with less precipitation on Monday, but it also brings the goods Tuesday afternoon (forecast below valid 5 PM MST Tuesday) and Tuesday night.  

It's always good to consult ensembles, but if you want spread, last night's 52-member downscaled NAEFS plume for Alta is ridiculous and caused me to get a good laugh.  There's the weak system on Sunday and then some models give us something on Monday, others not, and then for the big storm on Tuesday and Wednesday (or into Thursday) the spread is enormous.  In general, the GEFS is wetter than the Canadian (CMCE) members, but the driest member of the NAEFS produces less than a half inch of water and 10 inches of snow for the entire 168 hour forecast and the wettest over 6 inches of water and 110 inches of snow!  

If we were to throw in the ECMWF ensemble, which I can't show here, undownscaled it comes in with a mean for Alta at just over 1.5 inches of water through 6Z 23 Feb (11 PM MST Wednesday), which covers most of the storm period.  This is near the downscaled Canadian mean.  

The cause of all the ensemble spread is the deep, developing trough in the northwesterly flow.  It essentially develops out of nothing, in response to two intense cyclones that form over the northwest Pacific Ocean, through a process known as downstream development.  The loop below shows the evolution of sea-level pressure (color contours) and 500-mb heights from 1200 UTC 19 February (5 AM MST Sunday) through 1200 UTC 22 February (5 AM MST Wednesday) and illustrates the development of the intense cyclones at left over the northwest Pacific and the subsequent development of the trough over western North America.  

There tends to be a great deal of sensitivity of the forecast in the downstream development region, which is why there is such a huge range in the NAEFS ensemble.  The amplitude and track of the upper-level trough will ultimately affect precipitation in our region.  Hence my indigestion.  

I suspect we will see more clustering of the model solutions emerging in the next couple of days.  Right now, I think things look good for a significant storm early next week, possibly a big one, but best to say that details remain uncertain until we see more agreement.