Intricacies of Medium-Range Monsoon Surge Forecasts

I don't know about you, but I look forward to another rainy day or night and have started to surf through the forecasts looking for hope.  

There is some potential for later this week and weekend, but subtle differences in the large-scale pattern could make a big difference.

To illustrate this, I'll compare the GFS and the ECMWF (a.k.a. Euro) forecasts valid 0000 UTC 19 August (6 PM MDT Friday) and 1200 UTC 21 August (6 AM MDT Monday). 

The GFS forecast for 0000 UTC 19 August (6 PM MDT Friday) shows two key large-scale circulation features: (1) ridge centered over Oklahoma and (2) a trough off the SoCal coast at 500 mb (top left panel below).  Southerly flow between these two features transports moisture from the Gulf of California region up the lower Colorado River basin and into Utah.  

The tricky thing for the Salt Lake City area is that there is a sharp contrast cross northern Utah between air that originates from northern Mexico and the Gulf of California, which is relatively moist, and drier that originates of the California and Baja California coast, which is drier.  This leads to a sharp contrast in 700-mb humidity (lower left panel) and model forecast precipitation (upper right) across northern Utah.  A small shift in position could make a big difference for Salt Lake City.  Note that this situation is not all that unusual during the monsoon.  

The ECWMF forecast is remarkably similar on the large scale, but the moisture contrast is just a bit farther west.  If this were to verify, the odds of showers and thunderstorms in the Salt Lake area would be a bit higher.  

Pushing things out to 1200 UTC 21 August (6 AM MDT Monday), the GFS is positively giddy about a strong monsoon surge.  As a short-wave trough that was located over the Queen Charlottes digs into the western US, it strengthens the moisture transport up the lower Colorado River valley and into western Utah.  There may be some contributions from a tropical cyclone that is expected to develop over the eastern Pacific the next couple of days in the form of a moisture surge up the Gulf of California to supplement things.  

We might even get some severe convective storms from a forecast like that.

The Euro, however, is not as excited for Salt Lake City.  Really, the forecast isn't tremendously different.  It has a trough over the Pacific Northwest swinging in and it has a tropical cyclone in the eastern Pacific.  However, these are both more progressive, meaning that they are moving downstream faster.  This leads to the eastward moving northwest trough being a bit farther inland and the westward moving tropical cyclone just a bit farther offshore.  As a result, northwest Utah is in a drier airstream and the Gulf of Califonia surge is essentially non-existent.  There is still a monsoon surge, but it's not as potent and it may result in the action being a bit farther east. 

These two forecasts illustrate some of the sensitivities of monsoon surges to small differences in the large-scale forecast. In terms of the overall pattern, these forecasts are not that different, but the magnitude of the moisture transport and the location of moisture plumes and precipitation differ, and that matters in this case for the Salt Lake Valley.  

We will see how this all shakes out.