Satellite imagery shows mid and high clouds continuing to stream northward into NorCal from the subtropics along and west of 120W. Current temperatures are milder compared to 24 hours ago across most of the area and generally range from the mid 50s in the mountain valleys, to the mid 60s to mid 70s across the Central Valley.
Anti-cyclonic flow persists across the region again today as the closed low remains off the PacNW coast with a weak trough extending southward off the California coast. Looks like the better chance for showers and thunderstorms will remain to the north and east of the forecast area today as ridging suppresses deeper convection.
Highs today are expected to be similar to Monday’s readings as there will be sunshine making it through the higher clouds, and the marine layer depth remains around 1K ft in depth with onshore gradients yet to strengthen.
Gradual cooling is forecast to commence on Wednesday and continue through the remainder of the week as the low edges closer to the West Coast resulting in stronger onshore flow and a deeper marine layer initially. Highs are expected to cool to below average Thursday and Friday.
Non-zero potential for some elevated convection early Wednesday for the southeastern half of the forecast area with late-day deep convection limited to the northern Sierra south of Lake Tahoe. Deeper southwesterly flow will likely push the thunderstorm threat to the east of the Sierra crest on Thursday and Friday, though shower and thunderstorm chances will increase across the northwest corner of the state on Friday as the closed low approaches.
Extended Discussion (Saturday through Tuesday)
A cool upper low moves inland early Saturday and then becomes an open trough. This system will bring below normal temperatures due to synoptic cooling and increased onshore flow. This will also bring some breezy winds.
Some rain showers are possible over northern Shasta County on Saturday, with the rest of the area remaining dry. The best potential is in the morning hours, when ensembles and deterministic models are in good agreement with the core of the low passing through Oregon. Instability looks marginal over our area and timing is not ideal for convection, so don`t currently anticipate thunderstorms.
High temperatures will be seasonably cool Saturday, 7 to 14 degrees below normal over the area. Valley highs will remain in the 80s, even over the northern Sacramento Valley, where there will be some cloud cover through the day associated with the low.
Upper ridging then rebuilds into the area Sunday into Monday. This should bring near normal temperatures for Sunday and then temperatures warming to several degrees above normal on Monday.
By Tuesday, highs will be 3 to 6 degrees above average, with Valley highs around the triple digit level. Morning lows should be generally in the 60s, though, so there should be some overnight relief from the heat.