October 21, 2020 – Dry weather and breezy northerly flow will continue this week with periods of stronger winds resulting in elevated fire weather conditions. Cooler temperatures by the weekend with potentially another strong north to east wind event Sunday into Monday.
Discussion
Nearly cloud-free skies cover the region early this morning. Moderately strong northerly surface pressure gradient remains across NorCal early this morning while the easterly gradient has decreased a bit compared to 24 hours ago. Local northerly gusts in the teens to mid 20s mph are present across the northern Sacramento Valley and surrounding terrain, and the increase in wind is keeping temperatures in the mid 70s across the Redding and Red Bluff areas.
The persistent ridge over NorCal is forecast to flatten somewhat today as the upper trough drops down the BC coast into the PacNW. A dry cold front will push into far northern California this afternoon, and then sweep southward tonight and Thursday. The northerly surface pressure gradient is forecast to increase again this afternoon and tonight with more widespread breezy conditions expected to return to the Sacramento Valley.
Easterly gradient will increase tonight as the front moves south, so we`ll see an increase in winds again across the northern Sierra foothills. The tight surface gradient will linger Thursday before subsiding on Friday. Gusts of 40-50 mph will be possible across the windier locales, and combined with low RH and very dry fuels, critical fire weather conditions are nearly certain.
A brief lull is expected on Saturday, then the next batch of short-wave energy is forecast to drop down from the north on Saturday night into early Sunday as the upper pattern amplifies. Another dry cold front is forecast to move south Saturday night bringing a renewed increase in surface pressure gradients.
Extended discussion (Sunday through Wednesday)
A cold upper level trough will be digging into the Great Basin for the start of the extended period. This will bring active weather to NorCal mainly in the form of strong winds across the Sacramento Valley Sunday and then over the Sierra for Sunday night into Monday. Currently looking like a 10+ mb MFR to SAC gradient by mid Sunday afternoon and a SAC to RNO gradient of 11+ mb Sunday night. This should result in strong winds. We will also see a push of cold air with this trough and overnight lows Sunday night will be quite chilly falling into the teens over the Sierra and into the 40s in the Valley. The cold air in place over the Sierra and over NV will likely enhance some of the gusty downslope winds Sunday night. Wind will be the main impact of the trough but a few light rain/snow showers will be possible in the high Sierra I-80 south Sunday. Forcing looks weak and there will be a lack of moisture and therefore little to no rain or snow accumulation is expected.
As we head into the Monday the trough will be digging into the Desert SW. This will leave an increased gradient over NorCal with breezy winds continuing. They do not look to be as strong as Sunday though. Very dry daytime humidity and poor to moderate overnight recoveries are expected along with the winds and this will lead to elevated fire weather concerns during this period.
Quieter weather settles in for the rest of the period as upper level ridging begins to build in. High temperatures will remain near to just below normal along with chilly overnight lows. Daytime humidity will remain very dry with moderate overnight recoveries.