July 25, 2019 – Hot temperatures this week with widespread triple digits at the lower elevations. Excessive Heat Watch in effect for this weekend. Temperatures cooling to average next week. A few thunderstorms possible through Friday.
Satellite imagery shows some mid and high level moisture beginning to creep northward into the southern portion of the forecast area, otherwise skies remain clear across interior NorCal. The coast near the Bay Area is stratus free and profiler data indicate the marine layer has shrunk to around 500 ft in depth. Current temperatures are mild ranging from the upper 40s and 50s in the mountain valleys to the mid 60s to mid 70s elsewhere (except around 80 milder portions of the Central Valley and foothill thermal belts).
High-based moisture and instability will continue to work its way northward around the periphery of the Southwest high into NorCal later today and tonight. IR imagery shows some enhancement off the central and southern California coast and lightning data even indicated a few strikes in that area Wednesday evening.
Models continue to point toward tonight and Friday morning as seeing the best potential for elevated convection across NorCal as another short-wave approaches from the west. The best potential for diurnal deep convection will remain over the high-Sierra south of Tahoe today and again Friday.
Temperatures today and Friday are expected to be pretty similar to Wednesday`s, or possibly down a bit in some areas as the approaching short-wave induces a stronger Delta Breeze.
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For the weekend, ensemble guidance remains in good agreement that an area of high pressure will be centered over southern California with ridging extending into the interior of NorCal. This will allow temperatures to increase significantly Saturday and Sunday. Highs will be around 8 to 12 degrees above average Saturday and Sunday with Valley and foothill temperatures around 100 to 110 degrees and the mountains around 75 to 95 degrees.
Onshore flow will be cut off with this pattern, so overnight lows on Saturday will only cool to the upper 60s to lower 80s in the Valley and foothills.
An Excessive Heat Watch has been issued for Saturday and Sunday as these temperatures can be hazardous, especially for those groups most sensitive to heat such as children, the elderly, people spending extended periods of time outdoors, and those with chronic ailments.
Extended Discussion (Monday through Thursday)
Reprieve from the Valley triple digit temperatures comes Monday as weak troughing approaches the west coast pushing the upper high pressure system eastward. Highs Monday will be roughly 5-10 degrees cooler than those of Sunday, and will continue to decrease back to near normal values by mid-week.
The Delta Breeze returns next week as well helping to cool overnight temperatures back into the low 60s and upper 50s for areas around the Delta. Dry weather continues through the period.