National Weather Service - Discussion

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Area Forecast Discussion

National Weather Service - San Joaquin Valley / Hanford, CA
Issued: Friday, September 24 2021 - 4:05 PM PDT

UPDATE...Updated AIR QUALITY ISSUES section.

SYNOPSIS...Continued dry conditions, above average temperatures, and periods of smoke will persist over the area through the weekend. A cooling trend will commence on Monday and continue through at least the middle of next week as temperatures fall to below average by Tuesday. Increased winds are possible on Tuesday as the cooler air pushes into the area.

DISCUSSION...Interaction between an upper ridge nudging into Norcal and an upper low center over northern Baja are producing an northeast flow over central CA which has been pulling smoke from the Windy Fire and the KNP Fire southwest over the south end of the San Joaquin Valley. Other than the smoke, skies are mainly clear across our area with generally light winds and temperatures running close to yday/s values.

The low is progged to continue moving west tonight and be situated off the northern Baja coast on Saturday with the upper ridge being firmly established over Norcal. This will keep on offshore flow over our area on Satruday which will result in a continuation of smoky conditions across area downwind of the Windy and KNP Fires as well as above normal daytime temperatures which are expected to change little from today. SREF and NBM guidance are indicating that the moisture and dynamics associated with the upper low to our south will remain to the south of our area. Latest probabilistic guidance is indicating that chances for measurable rainfall on Saturday are 5 percent for Springville and 3 percent for Three Rivers so the potential for precipitation on the large fires does not look very good. In fact, no portion of our CWFA has a greater than a 10 percent chance of measureable precipitation on Saturday so have decided to keep the forecast dry based off of NBM probabilistic guidance.

The low will then eject eastward from off the Baja coast on Saturday night and Sunday while the upper ridge strengthens over our area on Sunday. However, a light offshore flow will likely provide for a continuation of the smoky conditions across the San Joaquin Valley downwind of the large fires in the Sierra Nevada.

The ridge will weaken on Monday as a large trough deepens off the PAC NW and Norcal coasts. This will result in slightly cooler temperatures for our area on Monday. The medium range models and their ensemble means are indicating that the trough will push inland through Norcal on Tuesday which will result in much cooler temperatures for our area with temperatures expected to plunge to below normal values. Latest NBM probabilistic guidance is indicating that Fresno has a 44 percent chance of having a maximum temperature below 80 Deg F on Tuesday while Bakersfield has a 53 percent chance of having a maximum temperature below 80 Deg F on Tuesday. The other concern for Tuesday is that with an increase in p-grads on Tuesday and Wednesday, a period of gusty wind is possible across the West Side Hills as well as across the mountains and Deserts of Kern County. The increased onshore winds will provide for the benefit of blowing the smoke out of the San Joaquin Valley on Tuesday and the presence of the upper trough swinging through will allow for much of the remaining smoke to mix out. The moisture and dynamics with sufficient for precipitation with this trough are expected to remain to the north of our CWFA. At this time only a slight chance of precipitation is mentioned for the Yosemite National Park area on Tuesday afternoon.

The trough is progged to moves east of our area on Wednesday with another dry upper ridge building inland into CA on Thursday. The 12Z models are now picking up on another trough approaching the Norcal coast on Friday so any recovery in temperatures occuring late next week might not last that long as another shot of cooler air might follow by the first weekend in October.

AVIATION...In the San Joaquin Valley, the foothills and higher elevations of the Sierra Nevada, areas of MVFR visibilities can be expected due to smoke and haze with local IFR visibilities persisting in the vicinity of wildfires in the Tulare county mountains during the next 24 hours. Elsewhere, VFR conditions will prevail for the next 24 hours.

AIR QUALITY ISSUES... Please see SFOAQAHNX for an Air Quality Alert

On Saturday September 25 2021...Hazardous in Sequoia National Park and Forest. Unhealthy in Tulare County. Unhealthy for sensitive groups in Fresno and Kern Counties. Further information is available at Valleyair.org

CERTAINTY...

The level of certainty for days 1 and 2 is medium. The level of certainty for days 3 through 7 is medium.

Certainty levels include low...medium...and high. Please visit www.weather.gov/hnx/certainty.html for additional information an/or to provide feedback.

HNX WATCHES/WARNINGS/ADVISORIES... None.