Critical fire weather conditions continue through early Tuesday across the Delta and Valley and adjacent foothills locations from gusty northerly winds and low minimum relative humidity. Dry and generally seasonable weather then continues through midweek, with a warming trend expected late week into next weekend.
Key Points
- A Red Flag Warning is in effect until 8 AM Tuesday 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 Thursday)
Mostly clear skies are evident over interior NorCal via latest GOES-West imagery as of early this morning. Despite breezy north to west winds persisting overnight, low temperatures still look to reach the mid 50s to low 60s throughout the Valley and foothills, with upper 30s to low 50s at higher elevations. Moving into today, an upper level trough currently tracking through the Pacific Northwest will continue to push eastward, while digging southeastward slightly as well. This upper level pattern will be conducive for reinforcing dry northerly winds today. Strongest winds are anticipated from mid morning into the early evening, where gusts of 30 to 35 mph are expected at Valley and Delta locations, with a few gusts to 40+ mph not out of the question (particularly across the western Sacramento Valley).
Given the general northerly component to the winds, any overnight humidity recovery will likely be overcome, with afternoon and early evening minimum relative humidity values expected to fall into the upper single digits to mid teens for most of interior NorCal. These dry conditions in tandem with the gusty northerly winds will result in critical fire weather conditions continuing today for the Delta and Valley and adjacent foothills locations below 2000 feet. Despite the dry and typically warming northerly wind that is expected, broad troughing and anomalously low heights aloft will keep temperatures generally seasonable, with 80s in the Valley and adjacent foothills and 60s to upper 70s at higher elevations for afternoon high temperatures today.
As the trough exits toward the Intermountain West moving into Tuesday, gusty northerly winds should become breezy (gusts 20 to 25 mph) late this evening into Tuesday morning, before further subsiding throughout the day on Tuesday. Additionally, broad troughing aloft looks to remain in place from Tuesday through Thursday across interior NorCal. This flow pattern aloft should usher in the return of onshore flow to the region across the mid to late week period, with periodically breezy, late day south to west winds. Despite this, temperatures will be on a gradual warming trend as well as heights aloft slowly begin to rise throughout the week.
Still, high temperatures are expected to remain generally seasonable, with 80s to 90s in the Valley and adjacent foothills, and upper 60s to low 80s at higher elevations.
Extended Discussion (Friday through Monday)
Despite cluster analysis continuing to favor broad troughing aloft over interior NorCal moving toward the weekend, heights aloft look to continue rising as a transient ridge attempts to build in over the Great Basin and Intermountain West. This will result in a continuation of the midweek warming trend from Friday into the early weekend.
High temperatures in the upper 90s to low triple digits are anticipated on Friday across the Valley and foothills, with 70s to 80s at higher elevations, culminating in generally widespread moderate HeatRisk. Saturday then looks to be the peak of heating at this time, with widespread high temperatures in the triple digits across the Valley and adjacent foothills, alongside 80s to 90s at higher elevations. While primarily widespread moderate HeatRisk is expected, some pockets of major HeatRisk are being indicated throughout the foothills on Saturday as well. A pronounced cool down is not anticipated by Sunday, but the highest heights aloft do look to shift eastward, cooling afternoon high temperatures by 2 to 4 degrees and bringing most areas back down to moderate HeatRisk to round out the weekend.