After a winter of disappointing model forecasts it is hard to get too excited about the potent looking storm cycle on the horizon. However, the models have been fairly consistent over the last few days in positioning a longwave trough over Utah for the end of the week and weekend. Here is the 500mb GFS ensemble mean and spread for friday afternoon
The blue colors on the left image mean upper level heights will be below average, which usually means cooler and stormy weather, and the agreement of the various ensemble members on the right means there is confidence in this forecast. For the past few weeks (and most of the winter) Utah has been under a upper level ridge so this will be a welcome pattern change. However, large scale upper level support is only one ingredient for stormy weather. The operational forecast shows another necessary ingredient: smaller scale energetic shortwaves that will dumbbell through the trough axis to help enhance precipitation. Here is the GFS analysis for tomorrow afternoon. Notice the shortwave in the top 2 panals over Utah
Then another shortwave 18 hours later:
And another 24 hours after that:
Until finally the trough cuts-off and a closed low forms somewhere in the vicinity of Utah by the end of the weekend
Unfortunately, it is difficult to predict the timing and placement of these smaller scale features making forecasting when and where the heaviest precipitation will fall very challenging.
In terms of moisture, there does appear to be a atmospheric river associated with this trough, but most of the moisture appears to go south of us. Here is the precipitable water (amount of water in the atmosphere) for Friday:
So a prolonged period of unsettled weather is ahead beginning tomorrow afternoon and lasting through the weekend. Somewhere in Utah will likely receive a lot of precip, but exactly where and how much is yet to be determined. After the weekend, another ridge looks to build in so hopefully this storm produces.
-Jeff (Jim is out of town for the week)