October 22, 2020 – Gusty winds with low humidity and record dry fuels will continue to bring critical fire weather conditions through Friday afternoon. The strongest winds are expected through mid afternoon for the Valley, tonight for the mountains and foothills. Temperatures will cool to near or below normal by late this week. There is also the potential for a few light showers in the Sierra south of I-80 on Sunday as a cold system moves into the Great Basin. This system will bring a stronger, north/downslope drying wind event and critical to potentially extreme fire weather conditions across Northern and Central California Sunday into early Tuesday. This looks to be the strongest fire weather event of the year so far, and will be closely monitored. We will update the forecast as conditions change.
Discussion
Trough continues to carve out to our northeast over the northern Rockies while strong ridging remains in place across the eastern Pacific. NorCal remains in-between these two features with intermittent bouts of moderate to strong north to east wind events this week. Another north wind event is ramping up early this morning as the north surface pressure gradient tightens (MFR- SAC is now over 14 mbs) behind the dry cold front. Widespread gusts of 25-35 mph are already occurring across the northern Sacramento Valley and will spread southward this morning.
The strongest winds today will be in the Sacramento Valley with gusts of around 40-45 mph possible over the northern half of the valley, but winds will increase tonight across the foothills of the southern Cascade Range and far northern Sierra where gusts of 25-35 mph will be possible (locally higher in wind prone gaps, canyons and ridges) as the east surface pressure gradient tightens. The north and east breezes are expected to gradually decrease Friday as the pressure gradient relaxes. Wind advisory is in effect for the northern half of the Sacramento Valley into this evening, and the red flag warning remains through Friday afternoon as critical fire weather conditions persist due to the winds, very dry airmass and record/near record dry fuels.
Some RH recovery, along with cooler temperatures, will be possible Saturday as flow briefly reverses to onshore ahead of the next upstream short-wave. There is still some uncertainly on its track, but the northern Sierra may see a few showers on Sunday as it approaches. The more significant impact from this system will be the potential for an extreme northeast wind event beginning Sunday night as upper flow comes into better alignment with a tightening easterly surface pressure gradient.
Extended discussion (Monday through Thursday)
Active weather is expected for the start of the extended period as we may be in the strongest off shore wind event of the season so far. A cold upper level trough will be digging out of the Great Basin into the Desert SW. The pressure gradient from SAC to RNO Sunday night is looking to be greater than 12 mb.
This will result in strong gusty downslope winds and we will also see a strong north to south gradient across the Sac Valley keeping strong winds in the Valley Sunday night into Monday morning. This trough will also have a strong push of cold air with it 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.
The gradient will be weakening on Monday as the trough closes off over the Desert SW and begins to weaken. Extremely dry humidity will be accompany these winds as overnight recoveries Sunday night may not make it higher than the lower teens in some areas and daytime humidity Monday will be widespread single digits. If this forecast holds this will be an extreme fire weather event, be sure to keep up with the latest forecast updates and any possible changes.
Much quieter weather settles in Tuesday as upper level ridging begins to build in. This will bring a warming trend with highs pushing back above normal. Overnight lows will remain chilly though. Less wind is expected after Tuesday but humidity levels will remain low during the day with moderate to locally poor overnight recoveries.