July 27, 2019 – Well above normal temperatures expected this weekend with a heat advisory in effect for the Central Valley and surrounding foothills. Cooling to near or slightly below normal early next week. Slight chance of an isolated thunderstorm near the Sierra Nevada crest this afternoon.
Discussion
Weak upper trough drifting across CWA this afternoon combined with monsoonal flow around 4 corners high will result in a slight chance of a thunderstorm near the Sierra Nevada crest. HREF probability of greater than 40 dBZ suggests main activity will be east of the crest in NV. Satellite and radar are confirming this. Afternoon temperatures are running near to or lower than 24 hours ago with greatest cooling in the Delta, due to increased onshore flow with upper trough.
Upper wave pushes into the Great Basin Saturday causing ridge axis to discontinuously retrograde over Interior NorCal. Increased subsidence will shunt mountain convection and warm AMS, pushing temperatures up 5 to 15 degrees, higher in the Delta, above normal. Triple digit heat is expected in the Central Valley and portions of the surrounding foothills. Forecast temperatures however look to remain anywhere from about 2 to 8 degrees below record values. A heat advisory is in effect to address this. Limited overnight cooling expected Saturday night into Sunday with lows in upper 60s to lower 80s for advisory area.
Ridge axis shifts into the Great Basin Sunday as next weak upper trough in EPAC pushes inland. Little change in AMS Sunday over Interior NorCal with widespread afternoon temperatures again exceeding the century mark in the Central Valley. Delta breeze will be weak Sunday allowing for continued above normal temperatures in those areas influenced by it.
Significant cooling begins Monday as upper troughing over NorCal deepens, pushing ridge axis further east. Depth of marine layer should increase in response to troughing with strong Delta breeze expected Monday night into Tuesday. Near normal high temperatures Monday, cool further to slightly below normal Tuesday. Increasing southwesterly flow aloft will push any afternoon mountain deep moist convection east of the forecast area.
Extended Discussion (Tuesday through Friday)
Model ensembles are consistent in showing upper level high pressure ridging shifting back eastward early next week to be centered over the 4 Corners area. This will bring high temperatures to slightly below normal levels, generally in the 90s in the Valley and the 70s in the mountains. Dry weather will persist next week with no evidence of the Monsoon returning at this point. Broad troughing will persist along the coast, bringing onshore flow and Delta breezes.