Critical fire weather conditions from gusty northerly winds and low relative humidity continue through midday today across the Delta, Valley, and adjacent foothills locations below 2000 feet. Dry and generally seasonable weather then persists through midweek, with a warming trend late week into the weekend.
Key Points
- A Red Flag Warning is in effect until 12 PM PDT Today across the majority of the Delta region, Valley, and adjacent foothills below 2000 ft, due to low daytime relative humidity, gusty northerly winds, and poor overnight recoveries.
- Generally seasonable temperatures continue through most of the week, with a gradual warming trend late week into the weekend.
- Widespread Moderate HeatRisk is expected from Friday through Sunday, with some Major HeatRisk in the foothills on Saturday.
Short-Term Discussion (Today through Friday)
Another morning of mostly clear skies is evident across interior NorCal via latest GOES-West satellite imagery. Winds have trended down from yesterday, although still breezy northerly winds persist this morning, primarily across the northern and central Sacramento Valley. Otherwise, early morning low temperatures should continue to fall a few more degrees into the upper 50s to low 60s throughout the Valley and foothills, with upper 30s to low 50s at higher elevations.
The primary troughing feature that produced the gusty northerly winds yesterday has since shifted eastward toward the Intermountain West. A subtle mid level shortwave moving through a similar flow pattern aloft today though will result in a brief period of gusty northerly winds increasing early this morning and continuing through midday today. Strongest winds, with gusts of 15 to 30 mph, are anticipated across the northern and central Sacramento Valley. Some modest overnight RH recovery is anticipated, but this additional period of heightened northerly winds will work against any overnight recovery. As a result, critical fire weather conditions are expected to persist through midday today and the attendant Red Flag Warning has been extended through 12 PM PDT Today.
This mid level shortwave is expected to transition eastward by the afternoon, with winds expected to rapidly drop off as this occurs. Broad troughing then looks to remain in place over interior NorCal through the remainder of the week, although heights aloft will gradually be lowering as well. This will result in the return of onshore flow and breezy to gusty, late day, south to west winds from Wednesday onward, with strongest winds across the Delta and its vicinity. Otherwise, high temperatures are expected to remain more or less seasonable, with 80s to 90s across the Valley and foothills and 70s to 80s at higher elevations through Thursday.
Moving into Friday, ensemble guidance is in good agreement that the broad troughing aloft will become more nebulous as heights aloft rise and a ridge begins to build in across the Great Basin. This combination of factors will result in a warming trend from Friday into the weekend, with high temperatures in the upper 90s to triple digits throughout the Valley and foothills, and 80s to 90s at higher elevations by Friday afternoon. This will result in fairly widespread moderate HeatRisk for much of interior NorCal.
Extended Discussion (Saturday through Tuesday)
Cluster analysis continues to favor the somewhat muddled upper level pattern over interior NorCal across the early weekend period as heights aloft continue rising and ridging continues to become more pronounced across the Great Basin. While the ridge axis looks to remain well to the east, temperatures on Saturday are still expected to be the warmest of this upcoming event. Fairly widespread triple digit readings are anticipated throughout the Valley and adjacent foothills locations by Saturday afternoon, with current probabilities of exceeding 100F in the 70% to 95% range in these areas. This will still muster primarily widespread moderate HeatRisk for the region, although a few pockets of major HeatRisk will be possible throughout the foothills on Saturday.
As quickly as this heat arrives however, current ensemble trends indicate broad troughing being reinforced across interior NorCal from late weekend into early next week. Resultant forecast high temperatures are then expected to fall back toward the 90s to low triple digits for most Valley and foothills locations, with 80s to 90s at higher elevations into early next week. Widespread moderate HeatRisk looks to hang on through Sunday, before reverting to predominantly minor HeatRisk by early next week.