While Reno is looking to break 100 year old records for the warmest September on record along with similar conditions over Eastern Ca…mother nature has been quietly planning a change to cooler…possibly wetter weather with even the chance of some snowfall in the highest elevations of the back country the following weekend.

Retrogression of the long wave ridge currently extends from West Central Canada then south over the inter-mountain west is forecasted to back up to about 140West; (1400 miles west of Mammoth) by the following weekend.

In the meantime;  as the upper ridge in the mean backs up….it will grow warmer over the high country through Tuesday.

1. There is a weak mid latitude upper low west of South Central Ca. that is pulling up some moisture from west of Baja. A 80 knot upper jet at 250mb is forecasted to spin up the next 24 hours as it moves into Northern Baja.  The front left exit region favors the Tehachapi’s and Southern Sierra with spokes of vorticity remaining west of the crest. The model’s QPF however is light and focused upon the Southern and Central Sierra this afternoon and evening.  Most instability is South and west of Mammoth.  The forecast for this afternoon and evening is for a 20% chance of showers & thunderstorms.

2. By Saturday….the upper high strengthens and caps most convection. Warmer temps will be the rule through Tuesday with highs in the mid seventies the first few days of October.

RECAP:    With the upper ridge migrating west, ridging will strengthen over the next few days with record high temps a possibility over Eastern Ca this weekend into early next week.  There has been a weak mid latitude upper low off the Ca coast, far enough west to channel a small amount of the remains of Miriam just off the Southern Ca Coast.  This mornings 12z guidance has several spokes of weak vorticity coming into the Southwest coast.  Some of that moisture as it combines with the anomalously warm temps and weak divergence to the south will help to destabilize the atmosphere and allow isolates TSRW’s to form today. As the upper ridge strengthens further, the atmosphere will “stabilize” to cap off convection for the weekend.  At the same time the upper low backs up as high pressure aloft over California puts the squeeze on the upper low westward into early next week. Temps in the meantime will range pretty much in the low 70s for the Mammoth and June area this weekend and rise to the mid 70s Monday and Tuesday. Low 90s in Bishop by Sunday.

Longer range next week:

The upper ridge should hold through October 3rd….Wednesday.  As the long wave ridge continues its retrogression…Short wave energy will begin to drop SE toward the Pacific Northwest…. “Possibly”coupling with the mid latitude upper low and drawing it northward, as cooling begins and moisture streams into California later Thursday. Thereafter, if coupling is truly in the cards…..subsequent short wave energy will deepen the upper low and move it east into California,  providing further cooling and precipitation along with a chance of the first real dusting over that weekend.   Beyond that…..indications are that warming will return for the high country with Indian Summer weather in the offering….

“Another Scenario” does not couple the upper low to the west of us, with short wave energy coming in as an inside slider, bringing just cooling and gusty winds.  Either way it’s looking significantly cooler by the following weekend.


The Dweeber………………………….:-)

Reference Glossary of Weather Terms
Disclaimer: I have been a hobbyist meteorologist for over 30 years here in the Mammoth area and I do this for my personal enjoyment. The National Weather Service saves lives every day . . . I do not. When making important planning decisions please use information provided by the National Weather Service as they are the most knowledgeable and accurate information source available today.