June 21, 2018 – Fire Weather Watch from Saturday morning through Sunday morning for Carquinez Strait and Delta-Central Sacramento Valley in Glenn, Colusa, Yuba, Northern Sutter, and Butte County Below 1000 Ft-Eastern Mendocino NF-Eastern Portion of Shasta/Trinity NF-Lake County Portion of Lake-Napa-Sonoma Unit-Northern Motherlode From 1000 to 3000 Ft. Includes portions of Nevada-Yuba-Placer-Amador and ElDorado Units-Northern Sacramento Valley to Southern Tehama County Line Below 1000 Ft-Northern San Joaquin Valley in San Joaquin and Stanislaus Counties Below 1000 ft-Northern Sierra Foothills from 1000 to 3000 Ft. Includes portions of Shasta-Trinity and Butte Units-Southeast Edge Shasta-Trinity NF and Western Portions of Tehama-Glenn Unit-Southern Motherlode From 1000 to 3000 Ft. Includes portions of Calaveras-Tuolumne Unit-Southern Sacramento Valley in Yolo-Sacramento Far Western Placer, southern Sutter and Solano County Below 1000 Ft.

Excessive Heat Watch from Friday afternoon through Sunday evening for Carquinez Strait and Delta-Central Sacramento Valley-Clear Lake/Southern Lake County-Motherlode-Mountains Southwestern Shasta County to Northern Lake County-Northeast Foothills/Sacramento Valley-Northern Sacramento Valley-Northern San Joaquin Valley-Southern Sacramento Valley.

Clear skies across interior NorCal early this morning. Onshore flow continues into the Central Valley and the marine layer remain around 1K feet deep with satellite imagery showing stratus along the coast extending locally inland in the Bay Area. Current temperatures are running several degrees cooler compared to 24hours ago thru much of the valley while readings elsewhere are slightly milder.

Effects from yesterday’s short-wave will linger this morning, but anti-cyclonic flow aloft will return this afternoon leading to slightly warmer temperatures compared to Wednesday as onshore flow is expected to slacken a bit.

Widespread triple digit heat is expected on Friday as surface flow becomes northerly and the Delta Breeze diminishes. Temperatures will be as hot or perhaps slightly hotter on Saturday. Given the potential for heat-related illnesses with this sharp but relatively brief heat stretch, an Excessive Heat Watch has been issued across the Sacramento and northern SJ Valleys, along with adjacent foothills.

Extended discussion (Monday through Thursday)

Upper trof moving through the Pac NW on Mon will bring some cooling to NorCal as Sun’s ridge axis is pushed inland by the trof. Lowering 5H heights and synoptic cooling should lower max temps some 5-10 degrees most locations. Slight differences in the modeling beyond Mon into the middle of the week results in some uncertainty as to the amount of cooling (or lack of cooling) over NorCal as the Mod Trend Tool shows a rather stable forecast 5H pattern over NorCal until Wed when there is just a slight variation.

Although there should be an increase in onshore gradients by midweek which should provide some cooling to the Delta influenced areas, the warmth should continue elsewhere. Have adjusted most of the temps upward during the EFP wrt to the MEX guidance. However, more significant height falls on Thu should offer a little more cooling to our CWA. The closed low descending into NorCal next Sat (30th) per the ECMWF looks overdone, but can certainly buy off on a more progressive trof per the GFS given the time of year.