Critical fire weather concerns today and Sunday due to breezy and dry conditions

October 5, 2019 – Near normal high temperatures today, then warming Sunday with above normal temperatures through Tuesday. Areas of breezy north to east wind this weekend leading to critical fire weather conditions. Near normal high temperatures return midweek with breezy conditions again and elevated fire weather concerns.

Discussion

Northwesterly flow aloft as EPAC surface high extends inland across Oregon into the Great Basin. KSAC-KRDD surface pressure gradient around 6.5mb attm and trending up. KDAX/KBBX VAD profilers showing 30 to 40 kts of northerly wind at 2k ft agl. As stronger winds aloft mix down this morning, valley surface winds will increase.

Areas of breezy northerly wind can be expected along the west side of the Sacramento Valley, extending into the Coastal Range. Near 4 mb surface pressure gradient between KSAC and KRNO is also producing locally breezy conditions through favorably oriented canyons in eastern foothills and over ridges of the Sierra Nevada and Western Plumas mountains. Increased subsidence and low level downslope flow will combine to warm the AMS today.

Gusty wind with low humidity and drier fuels along the west half of the Sacramento Valley into the Delta and in the Coastal range will increase fire weather concerns in these areas today and Sunday.

Wind decreases in the Sacramento Valley this evening as boundary layer decouples, but remains locally breezy overnight in the adjacent foothills and mountains. Wind in the Central Valley picks up again Sunday morning with locally breezy conditions again tomorrow, mainly on the west side. Afternoon temperatures warm an additional 3 to 8 degrees Sunday resulting in highs in the mid to upper 80s in the Central Valley. Lower latitude closed upper low drifts from the SW towards Central CA tomorrow but will have little impact as associated moisture is scarce and feature weakens as it moves inland Monday.

Heights/thicknesses continue to trend up over NorCal Monday pushing max temperatures in the Sacramento Valley into the upper 80s to around 90. Lighter wind expected Monday with weaker pressure gradients. Slight cooling forecast Tuesday, but continued above normal temperatures, as upper ridge weakens and another short wave trough digs from VRISL into the PacNW.

Extended Discussion (Wednesday through Saturday)

An upper level high pressure ridge will slide across the west coast Wednesday through Saturday keeping most of the extended period under fair skies and near normal temperatures. An upper trough is forecast to drop into the northern Great Basin Wednesday. Surface high pressure nosing in over the Pacific Northwest combined with northwest flow aloft will bring a period of breezy north winds Thursday and Friday. Moderately low RH values during this time will bring elevated fire weather concerns.

Winds drop off on Friday as the upper ridge axis centers over the west coast. A weak upper trough is forecast to move into the Pacific Northwest next Saturday bringing a slight cool off and some cloud cover the the northern CWA but at this time, any precipitation is expected to remain north of the forecast area.