June 20, 2019 – Cooling today with daytime temperatures near normal then a little below normal on Friday as a low pressure system drops through the Pacific Northwest and into the northern Great Basin. Breezy north winds behind the departing low pressure system will bring elevated fire weather concerns tonight through Saturday. Warmer than normal over the weekend with cooling again next week.
A strong delta breeze continues this morning with coastal status making it into the delta and even into parts of the southern Sacramento valley. The Sacramento executive airport is showing overcast cloudiness at this time. Elsewhere, skies are clear this morning under upper level high pressure ridging. Temperatures are running several degrees cooler this morning than 24 hours ago as a low continues to drop into the Pacific Northwest. The increased onshore flow will bring a significant cool off today with daytime highs expected to come in around normal for this time of year.
By late this afternoon the upper low center is predicted to be over the northern Great Basin with northerly flow developing over the north state. Breezy winds will begin to develop today but mainly over the northern Sacramento valley. Coverage of these increased winds will be limited so the timing of the fire weather watch has been trimmed out of the today forecast period.
Northerly surface pressure gradients and wind pick up tonight especially along the west side of the Sacramento valley. This will keep humidity recovery limited in this area where a fire weather watch remains in place. Higher RH outside of the windiest areas should preclude the need for a fire weather watch until RH values drop on Friday.
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Daytime temperatures cool a little more on Friday and should see highs a little below normal. Combination of breezy north winds and low humidity will still elevate fire concerns, however. Combination of northerly surface gradient and good upper level support from upper ridge pushing back in over the west coast will keep breezy north winds going through most of the day Saturday.
Airmass warms a bit bringing daytime highs back up above normal and keeping humidity relatively low for late June. Upper level high pressure ridge pushes in over the State on Sunday bringing a little more warming but lighter winds. This should allow humidity to climb a bit and combined with the lighter winds should reduce fire weather concerns slightly.
Extended Discussion (Monday through Thursday)
Unseasonably deep 552 DM upper low from GOA digs slowly southeast towards West Coast Monday into Tuesday, then shifts into the PacNW Wednesday/Thursday. This will result in increasing synoptic cooling and Delta breeze as heights/thicknesses lower over NorCal. Not expecting precip with increased troughing, but should see a deepening marine layer and enhanced onshore flow. High temperatures cool from about 2 to 5 degrees above normal Monday to 4 to 8 degrees below normal Wednesday into Thursday. Low to mid 80s expected by the middle of next week in the Central Valley, with 50s to 70s for the mountains and foothills.