Quiet weather before an active weather system bringing the potential for showers/thunderstorms, gusty winds, lightning, and small hail, primarily north of I-80. Below normal temperatures this week with hot weather returning early next week.
Discussion
Key Points
- Robust weather system impacting the area Thursday and Friday
- Gusty winds 20-35 MPH on Thursday; Strongest in the foothills and Delta
- Chances for isolated to scattered showers/thunderstorms with 15-30% chance of thunderstorm development mainly north of I-80 Thursday – Friday. Highest chances on Friday.
Today – Wednesday
Fairly benign weather continues with below normal temperatures continuing through this week. Highs will be in the upper 80s to low 90s in the Valley with 80s and 70s over the foothills and mountains respectively. Winds continue to be breezy each afternoon with gusts up to 15-25 MPH.
Thursday – Friday:
Moving toward the late week period, a brief, but notable pattern shift is anticipated. A robust weather system moving in from the Pacific NW moves down the coastline and then over Northern CA by Friday. With this, cooler temperatures, gusty winds, and increasing precipitation and thunderstorm potential are all expected as this system impacts the region.
Initially on Thursday, gusty winds look to be the primary concern with a secondary concern of isolated showers and thunderstorms across Northern Shasta County (25-50% chance of showers;5-15% chance of thunderstorms) Gusts peak in the afternoon and evening with gusts of 25-35 MPH throughout the Valley and up to 40-45 MPH over the Sierra. Probabilities show the highest chance for gusts greater than 30 MPH are across the Sierra and Delta areas (55-90% chance)
As the low continues southward over Northern CA, shower and thunderstorm chances intensify by late morning Friday into the afternoon and evening hours. Chances for precipitation are 50-75% over Shasta and Tehama counties and 25-50% elsewhere. For thunderstorm potential, NBM probabilities are showing 15-30% chance from southern Plumas County to the Coastal Range/Northern Shasta County and southward across portions of the Sierra, including the area of the Park Fire. Probability of seeing greater than 0.10 is 45-75% from Hwy. 32 north and west and broadly 10-30% elsewhere. There is a slight uptick over the Delta with 25-40% chance but is very localized.
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 also 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 is just 10-25%, up to 20-40% at the highest peaks.
Temperature highs on Thursday and Friday cool to mid 70s to mid 80s, making out for a pleasant afternoon for those unaffected by precipitation or winds.
Extended Discussion (Saturday through Tuesday)
Ensemble guidance indicates some uncertainty in the exact timing of the trough ejection over the weekend, with better agreement in the upper level pattern moving into early next week. As a result, depending on how the trough ejection evolves, some lingering precipitation potential will be possible on Saturday. Highest precipitation probabilities (20% to 40%) remain primarily confined to the Sierra, with similar areal extent of thunder as well on Saturday afternoon but marginally less chances (10-20% of thunderstorms).
As the trough does eject over the weekend, it looks to lift northeastward rather than further deepening into interior NorCal. This will still likely induce breezy northerly surface winds late weekend into the early week ahead, but current probabilities of wind gusts greater than 20 mph are around 15% to 30% at this time.
The more notable product of this upper level transition will be a pattern shift back toward hot and dry conditions, with 25% to 50% probabilities of high temperatures exceeding 100F on Monday jumping up to 45% to 75% by Tuesday as broad upper level ridging moves westward and heights aloft increase across interior NorCal.