A Forecast Gone Bad

A few days ago I announced the arrival of a monsoon break and suggested that it would likely remain dry for several days (see Arrival of the Monsoon Break).  Well, we got a few days of dry weather, but a monsoon surge snuck into Utah last night and rain is falling this morning across northern Utah.

Source: NCAR/RAL

This forecast bust provides a nice example of how subtle changes in the large scale forecast can bite you in the rear during the monsoon.  The GFS forecast from 6Z last Friday called for an elongated upper level trough to extend from the eastern Pacific into northern California, placing Utah in a relatively dry southwest to westerly flow aloft.

Instead, the trough verified along the California coast, placing Utah in southerly to southwesterly flow that tapped into moisture from the monsoon region.  Really, this was a full blown forecast bust for all of the southwest U.S. it is also far wetter in Arizona than forecast a few days ago.

The surge of moisture into northern Utah last night shows up well in integrated precipitable water measurements from the Salt Lake City International Airport.  Note how values spike from 1.5 cm yesterday afternoon to nearly 3 cm this morning.

Source: NOAA/ESRL

So, moisture has returned to northern Utah.  Looks like we can go a few more days before turning the sprinklers on.