A Pattern Change to Start October

The models are advertising a significant pattern change to start October, one that will curl the smiles of snow-safety teams in the Wasatch Range.  

Over the past several weeks, the large-scale pattern has been characterized by persistent troughing along the Pacific coast and ridging over the south central United States, as shown by the mean 30-day 500-mb heights for the period from 31 August to September 29. 

Source: PSD Maproom

This general pattern is consistent with the North American Monsoon, but these two major circulation features were stronger than on average.  In other words, the Pacific trough was deeper and the ridge over the south central United States were stronger than observed.  This "opened the door" for frequent monsoon surges into northern Utah and occasional severe convective storms due to the stronger flow and wind shear between the two circulation features.  

The pattern is changing, however, for October.  The ECMWF AIFS analysis for 0000 UTC 30 September (1800 MDT last night) showed a closed upper-level low over southwest Arizona and a broad, low-amplitude, upper-level ridge over the upstream eastern Pacific.

Once that trough exits the area, the broad ridge will move in and we will have a much drier and very fall-like pattern settle in with clear skies, sunny days, and cool nights.  The forecast valid 1200 UTC 3 October (0600 MDT Saturday) shows the ridge over Utah and monsoon moisture well to the south and confined to southern Baja California and central Mexico.  

Extended forecasts like the one below valid 0000 UTC 10 October are also ridgy and generally dry.

Thus the start of October looks to have seasonal or above average temperatures and below average precipitation.  It is not impossible for a weak trough to slip through the ridge and give us a few showers, but by and large the next 10 days look dry.  

This is just what the snow-safety teams want as early snow often turns into weak snow on north aspects if we get a significant storm in early October followed by clear weather.  That weak snow can then act as a persistent weak layer when it finally starts snowing again.  A best-case scenario would be for this pattern to predominate through the month (I have no idea if it will) and then have an active storm track kick in to quickly build a deep snowpack in November.

Updates to weather.utah.edu

We are in the process of upgrading weather.utah.edu.  Most of the changes will be implemented tomorrow (Thursday October 1) afternoon.  Some down time and delayed products are possible and it may take us a couple of days to get the bugs worked out.  After the changes, weather.utah.edu will have a new look and hopefully improved reliability.  Below is a peak at the look of the new site.