Unsettled and uncertain the next few days as monsoon moisture builds over the region. This will bring scattered thunderstorm chances over the mountains with Valley shower and thunderstorm chances at times. Mountain thunderstorm chances continue mid-week onward but should not be as widespread. Cooler the next few days with a gradual warming trend into the middle part of the week.
Discussion
We will be entering an unsettled weather pattern the next few days as high pressure over the four corners region and low pressure over the Pacific push deep monsoon moisture into northern California. Short waves will spin around the high and that will be the main forcing to kick off activity along with orographic lift in the mountains. The main uncertainty will be the timing of these short waves and how widespread activity will be with them.
Today the main forcing will be orographic lift keeping the thunderstorm chances to the mountains. We should see a bit more coverage today with the activity compared to yesterday but are not expecting widespread thunderstorms. Timing of thunderstorm chances will be noon to 8 pm with the best chances in the afternoon.
The first short wave then spins up around the ridge late tonight into Monday. This will bring better chances for activity including some chances in the Valley. A lack of mid level instability should keep activity more isolated in the Valley but activity is expected to be scattered over the mountains where we will see higher instability. There is some uncertainty on the amount of instability especially over the Valley, the NAM shows quite a bit while the GFS shows very little, this will have effect on how widespread activity becomes.
Things should quiet down into Monday night with a few showers lingering over the mountains. We will then see another short wave for Tuesday. Models are not as excited with this short wave with there being less instability over the region but it will once again bring renewed chances for showers and thunderstorms.
The majority of the activity will be over the mountains and we once again should see scattered thunderstorms during the afternoon and early evening. Meso Analysis currently has PWATs 0.60 to 1.2″ over the region with them 2.0″+ over the Mojave Desert. As the short wave spins up it will push some of these higher PWATs north and we should see PWATs across the area between an impressive 1.0 and 2.0″ Monday and Tuesday.
Heavy rain will be likely with any thunderstorms the next few days given this amount of moisture in the atmosphere. If a thunderstorm would happen to track over a recent burn scar a Flash Flood Warning may be needed.
Dewpoint depressions aren’t overly impressive in the Valley either and given this amount of moisture we could see brief heavy downpours even in the lower elevations. It will also feel somewhat humid the next few days as dewpoints will range between 58 and 64 in the Valley and foothills.
The ridge does look to push back west on Wednesday. This will diminish some of the shower and thunderstorm chances and will keep any short waves to our west. Higher moisture levels will remain in place though and we could see a few afternoon and evening thunderstorms over the mountains. Cooler temperatures are expected the next few days with gradual warming into mid-week.
Extended Discussion (Thursday through Sunday)
Not expecting a major pattern change as we head into the extended period. The four corners ridge will remain in place with an upper level low over the Pacific. We don’t see any major forcing over the region but moisture will remain in place. This will bring chances for afternoon and evening thunderstorms for the mountains. Temperatures will be seasonal throughout the period.