Thursday, September 25, 2014

Outlier Mode Begins

Although there remain some differences in timing and details between the models, we remain on track for what could be an extremely wet period in northern Utah and other portions of the state.  Given the heat today, I'm officially declaring the start of "Outlier Mode."

The record high for today is 92ºF, which is also the latest 92ºF day in the record books at the Salt Lake City International Airport.  As of 1:35 PM, it was 89ºF at the airport.  I'm guessing a 91ºF max is probably most likely, but maybe we can squeeze out a record.

Tomorrow there's a chance of showers and thunderstorms as the upper-level trough approaches, afterwhich your ark-construction needs must be complete as the system taps into juicy air over the eastern Pacific and Gulf of California.  The forecasts below show the forecast integrated water vapor transport produced by the GFS (bottom) and the average of all members of the Global Ensemble Forecast System (GEFS) on Saturday morning.  There is a narrow plume of strong water vapor transport extending from Baja into Utah.

Source: NWS
Moisture transport of the intensity forecast is fairly unusual in the Intermountain West this time of year, with return intervals of about once every 10 years in late September and early October.  

Source: NWS
Not surprisingly, given the moisture and strong forcing associated with the approaching upper-level trough and frontal system, the models are going berserk for precipitation.The graph below shows the accumulated precipitation produced by the 21 members of the Short Range Ensemble Forecast (SREF) system.  The mean (black line) calls for nearly an inch of precipitation at the Salt Lake city airport from about 0000 UTC (6 PM) Friday afternoon to 0000 UTC (6 PM) Saturday afternoon.

There is, however, quite a bit of spread and much is going to depend on timing and location.  By and large, Saturday looks like a fairly wet day.  Those of you heading to the Utah–Wazzu football game should keep an eye on the forecast and hope for a break.  Coach Wittingham might want to consider having his players practice with wet footballs today and tomorrow.  There's even a possibility of thunderstorms, so hopefully the University has an appropriate lightning safety plan in place as we have discussed previously.  Kudos to the University of Michigan for taking proper precautions last week.

In case you are wondering, it appears that there have only been five calendar days during which more than 2 inches of rain was recorded at the Salt Lake City Airport (or probably downtown for really old observations), but two of those days are in September.

May 3, 1901
June 5, 1885
July 13, 1962
September 5, 1970
September 26, 1982

September is a good month for a deluge because of the combination of monsoon moisture with cool-season storm systems.  I won't be surprised if we see some locations with 2+ inches on Saturday.  


  1. One interesting aspect of this moisture surge, based on satellite and model forecast data, is that it appears to be primarily a low-level surge from the Gulf of California rather than the type of larger-scale mid and upper-level moisture influx like we typically see during the monsoon. Satellite images currently show very little cloudiness in the area around the Gulf of California, even though 70+ dew points near the surface are already moving up into the lower Colorado River Basin today. Model forecasts also don't show too much reflection of the moisture at 700 mb initially, until it gets ingested into the frontal boundary further north over the eastern Great Basin.

  2. I'm confused as to why the NWS has yet to issue a flash flood watch when we are now ~12 hours away (in southern Utah) from the start of an outlier event. The GFS and NAM have been remarkably consistent with their forecasts of widespread 1-2 inches of rain with this event.