September 20, 2018 – Breezy north winds and low humidity will bring increased fire danger Sacramento valley and coast range today. Remaining warm on Friday then cooling to near normal by the end of the weekend. Warming slightly again early to middle of next week. No precipitation expected interior NorCal through extended period.
Red Flag Warning until 7 PM PDT this evening for Central Sacramento Valley in Glenn, Colusa, Yuba, Northern Sutter, and Butte County below 1000 Ft
Discussion
A northerly surface gradient has developed in the wake of an upper trough that moved through NorCal on Wednesday. Currently the surface gradient from MFR to SAC is over 9 mb. Breezy north winds have developed over the northern Sacramento valley and along the coast range this morning. Upper level flow a bit more northwest so upper level support is limited. Therefore breezy north winds will be spotty. RH values in these windier areas have shown fairly poor recovery this morning and minimum RH expected to drop down to as low as single digits some areas this afternoon. Therefore a Red Flag warning for the western portions of the CWA will remain in place.
Upper level ridge amplifies over the west coast by this afternoon bringing a few degrees of warming today. Daytime highs are expected to come in a few degrees above normal. Upper level ridge slides quickly across the west coast on Friday bringing decreasing winds and similar airmass on Friday. Therefore, expecting daytime highs Friday remaining a few degrees above normal.
An upper trough is forecast to move into the Pacific Northwest on Saturday bringing more onshore flow and cooler temperatures. RH values should see some increase as well. Overall airmass cools a bit more Sunday as the upper trough swings across the north state and into the Great Basin. Daytime highs Sunday should come in right around normal for this time of year. Precipitation and even significant cloud cover from this system should remain well north of the forecast area.
Extended Discussion (Monday through Thursday)
Warm and dry weather is expected through the extended period as a Rex Block pattern develops over the eastern Pacific, and high pressure builds over California. The latest ECMWF model would suggest a period of gusty north winds Sunday into Monday as an upper trough drops into the Great Basin, but the GFS is much weaker and farther east with this system. Regardless, look for Valley high temperatures generally in the 90s through the extended period, with 70s and 80s over the mountains.