Tuesday, September 20, 2016

Summary of the Action over the Next Few Days

Source: e-trailer.com
Some weather excitement is on the schedule for later this week as the first significant mid-latitude trough of the fall taps into monsoon moisture to give us a wild ride.  

Phase 1 covers today and tonight, although northern Utah isn't really in on the action.  An upper-level trough dropping into the western U.S. helps to coax a monsoon surge and the remnants of Hurricane Paine (doesn't that just roll of the tongue) into the southeast US, with showers and thunderstorms affecting a broad area from SoCal and BaCal through southeast Utah and California.  

Although for us its "No Paine No Gain" today and tonight, the monsoon surge establishes a reservoir of moisture over the southwest that is pulled into northern Utah as the upper-level trough subsequently develops along the west coast and the surface cold front impinges on northern Utah.

Thus, tomorrow (Wednesday) will be at transition day as moisture streams northward.  After that, I'll describe the weather as "highly unsettled" as showers, thunderstorms, and a slow moving front impact northern Utah Wednesday night, and Thursday.  I hesitate to attempt to forecast gory details as there is a complex stew of ingredients coming together.  I do know that the weather often gets interesting around here (heavy rain, strong thunderstorms, etc.) when midlatitude troughs toy with monsoon moisture in the fall, so I look forward to seeing how things play out.

Amongst the new tools for us to examine on weather.utah.edu this winter are downscaled forecasts from the Short-Range Ensemble Forecast System (SREF).  Forecasts from the 26-member SREF for Salt Lake City show the activity picking up Wednesday evening and night and continuing into Friday.  Variations in the slopes of the accumulated precipitation lines show uncertainties in the timing of precipitation and the wide range through the period (from about 0.3 inches to 1.7 inches) shows uncertainty in the total accumulated precipitation for this portion of the storm.
Nevertheless, if my quick math is correct, we've had only 0.43" of rain in the past 3 months and it looks like we have a good shot of exceeding that, perhaps by a lot, by Friday afternoon.  For what it is worth, I moved my automated sprinkler system to the "off" position this morning.

The most recent model forecasts call for snow levels to drop Thursday night.  Right now it's looking like snow at times in the upper Cottonwoods through Friday and possibly Saturday.

Yesterday's GFS was calling for something like 28" of snow for Alta.  Although it's easy to be seduced by a forecast like that, remember it's a long-range forecast and the GFS has been prone to overforecast since it was upgraded to an effective grid spacing of 13 km.  In fact, I find the precipitation produced by the GFS to be of limited value (in the summer, it is nearly unusable!).  The NAEFS ensemble from last night shows most members in the 5-10" range for Alta, with a few larger-accumulation outliers.

Further fueling anticipation is that this is the first possible accumulating storm of the year.  Let's keep those emotions in check and see how things look in a couple of days.  

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