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Northern CA Forecast Discussion
FXUS66 KSTO 181928 AFDSTO

Area Forecast Discussion
National Weather Service Sacramento CA
1228 PM PDT Sat May 18 2024

SYNOPSIS...
Seasonable temperatures will continue through the weekend and into early next week. Mostly sunny and dry conditions with periods of breezy winds expected through the next 7 days. Isolated showers and thunderstorms over high Sierra south of Highway 50 today.


.DISCUSSION

... Morning marine stratus deck still lingering around portions of the Valley at the time of this writing. Analysis of GOES-18 Satellite reveals the stratus deck is beginning to erode, albeit slowly, as we approach the afternoon hours, where it should finally lift out of the area. Sunny and mostly dry conditions should prevail for the remainder of the day across interior NorCal, except for a few isolated showers and possible t-storms over the high Sierra south of Highway 50. Breezy onshore winds are expected to continue throughout the day as well, which will help usher in more seasonably high temperatures for the area. The best chances (15-30%) of an isolated shower or thunderstorm to develop this afternoon and into the evening hours exists in southeastern Tuolumne County. Recent runs of CAMs (Convective Allowing Models) reveals decent levels of instability, and limited inhibition over the aforementioned area. Only forcing mechanism will be the mountains themselves, so main impacts with any thunderstorm development will be brief heavy rain, lightning, locally gusty winds, and small hail. Any storms that do develop should move eastward after forming. Recent National Blend of Models (NBM) runs have introduced a slight (10-20%) chance of a stray shower forming over northern Butte County and southeastern Tehama County later in the late evening hours as well, so we cannot rule out a stray shower or two in those areas this evening. Getting into the overall upper level pattern for the area, our current westerly flow will begin to transition into a northwesterly component tonight and into tomorrow as a trough to our north in British Columbia begins to slide southward and phase with a closed low in SoCal on Monday. This will setup a northerly component Monday and stronger pressure gradients over the area, which will result in a breezy northerly wind day for the region. The NBM is still advertising around a 50-75% probability of wind gusts exceeding 40 mph Monday in the usual wind prone area in the western Sacramento Valley, along the I-5 corridor. Minimum relative humidities (RH) values will fall to below 15% Monday, so locally elevated fire weather conditions may exist throughout the morning and evening hours. Tuesday, the pressure gradient will weaken and allow for northerly winds to weaken slightly, and allow high temperatures to warm into the mid 80s to low 90s for the Valley/Delta and upper 60s to low 80s in the foothills/mountains.


.EXTENDED DISCUSSION

(Wednesday THROUGH Saturday)... Wednesday the area will be under northwesterly flow aloft, which will help keep temperatures warm in the 80s for the Valley/Delta and 60s to 80s for the foothills/mountains. On Thursday, an upper level low is progged to move into the PacNW which will extend troughing into NorCal. Global ensembles differ with the strength, location, and timing of troughing moving into our area, so precipitation chances remain relatively uncertain at this point. Cluster analysis is in relatively good agreement of the upper level pattern change, however there are discrepancies in location and strength as well. The NBM begins to introduce precipitation chances Friday afternoon/evening for the northern and eastern foothills through Saturday evening. Right now, current NBM probabilities of exceeding 0.50" inches are less than 15%, highlighting the uncertainty with regards to the location/strength of the troughing. High temperatures should cool through the extended period, with near normal temperatures returning Friday then below normal temperatures on Saturday. Saturday temps may not break the 80 degree mark in the Valley, with cooler 50s to low 70s in the foothills and mountains. Stay tuned for forecast updates as we head into the Memorial Day weekend.






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