An anomalously strong weather system arrives later this week bringing the potential for showers/thunderstorms, gusty winds, lightning, and small hail, primarily north of I-80. Below normal temperatures persist before hot weather and moderate HeatRisk returns early next week.
Discussion
Key Points
- Robust weather system impacting the area Thursday and Friday
- Gusty winds 20-35 MPH on Thursday into Friday; Strongest winds will be over the higher elevation Sierra, gaps in high terrain, and the Delta.
- Isolated to scattered showers/t-storms with 5-20% chance of thunderstorm development mainly north of I-80 Thursday-Saturday. Highest chances will be on Friday, becoming more localized to the Sierra crest on Saturday.
Today:
Clear skies are currently being observed across interior Northern California with only a few straggling stratus clouds hanging around the northern Sacramento Valley and northeastern foothills of the Sierra. Cooler, early morning temperatures are kicking off the start to the last day of quiet weather before a strong weather system begins to impact the region tomorrow. Valley highs today will be in the mid 80s to low 90s alongside foothill/mountain highs ranging in the low 60s to low 80s. But across the board, temperatures will be 3 to 10 degrees below-normal. Periodically breezy afternoon and evening onshore winds today, gusting 10-15 mph in the Valley and 15-25 mph in the Vicinity Delta.
Thursday – Saturday:
As we approach the late week period, a pronounced weather system moving down from the Pacific Northwest will bring anomalously cooler temperatures, gusty winds, and the potential for scattered showers and thunderstorms.
Thursday (Gusty Winds & Isolated Showers/T-Storms) The main impact will be periods of gusty winds 20-35 mph, stronger than what we’ve experienced so far this Summer. Winds funneling through gaps in mountainous terrain and over higher elevations in the Sierra have a 40-60% chance of exhibiting gusts greater than 45 mph, but overall expect higher terrain gusts to be in 30-45 mph range. Winds will be breezy throughout the day, before peaking in the afternoon/evening hours. A secondary concern is the growing potential of scattered showers and isolated evening thunderstorms across Northern Shasta County (25-50% chance of showers;5-10% chance of thunderstorms). This becomes the main impact heading into the Friday-Saturday period as the area of coverage increases.
Friday-Saturday:
Gusty Winds, Scattered Showers & Isolated T-Storms. As the low pushes further south over Northern CA, gusty winds (especially over higher terrain) continue into Friday along with shower and thunderstorm chances increasing by late Friday morning lasting through the evening. Gusts will similarly be 20-30 mph with locally higher gusts 35-45 mph over higher terrain.
Chances for precipitation are 50-80% over Shasta and Tehama counties and along the Sierra crest; 25-50% elsewhere. For thunderstorm potential, NBM probabilities are showing 10-20% chance from southern Plumas County to the Coastal Range/Northern Shasta County and southward across portions of the Sierra, including the Park Fire burn area. Probabilities linger into Saturday, but remain localized to the northern Sierra foothills and along the crest north of I-80. Probability of seeing greater than 0.10″ is 55-85% from Hwy 32 north and west over a 48 hour period. Probabilities elsewhere for 0.10″ of rain or more are 40-60%.
The main threats with any thunderstorm are: lightning, brief heavy rain showers, gusty winds, and small hail.
Lastly as the low moves through the area, it does bring a nonzero chance of snowfall accumulations of around an inch along the Sierra above 8000 feet but CONFIDENCE is LOW as probabilities of occurrence are only 10-25%, up to 20-40% at the highest peaks.
Extended Discussion (Sunday through Wednesday)
Ensemble guidance is in generally good agreement that by Sunday, the trough will have progressed far enough eastward for any lingering precipitation impacts to subside across interior NorCal. As the trough lifts northeastward on Sunday, surface winds are expected to shift to a northerly direction. Given the orientation of the ejecting trough though, only breezy winds are anticipated, with probabilities of gusts greater than 20 mph only around 15% to 30% for areas outside of the Delta vicinity.
Otherwise, broad ridging aloft is expected to build in from Sunday into early next week. This will result in a rapid return to above normal high temperatures across interior NorCal through midweek. Current probabilities of high temperatures exceeding 100F for the Valley and foothills sit around 10% to 20% on Monday, before jumping up to 40% to 60% for Tuesday and Wednesday.
Corresponding areas of resultant moderate HeatRisk return to Valley and foothills locations on Tuesday and Wednesday at this time as well.