KEY MESSAGES

1. A cooling trend will occur across Central California as an upper level high pressure system begins to weaken over the next few days. Afternoon temperatures will remain below normal today and Thursday.

2. Low daytime humidity and poor overnight recovery will provide for an elevated risk of grass fires in eastern Kern County and over the West Side Hills through early this week.

3. An upper level trough is projected to drop southward into the western United States midweek, bringing near normal temperatures to Central California. There is a 80-90% chance for afternoon highs to remain below 90 degrees and 40-60% chance to be below 85 degrees in the San Joaquin Valley on Thursday.

4. Monday and Tuesday brings a chance of rainfall to the San Joaquin Valley, Sierra Nevada thunderstorms, and very high elevation snow.

DISCUSSION

A deep trough over the Pacific Northwest and Northern California early this morning will continue to slide into the Northern Rockies by early Thursday morning. This will continue the trend of below normal temperatures for the San Joaquin Valley for today and Thursday. Average high temperatures are in the low to mid 90s and will use 93 degrees as the benchmark. The probability of exceedance (PoE) of 93 degrees is less than 25 percent area wide today. When viewing the probability of staying below 90 degrees there is 30 to 80 percent chance today.
Tuesday it goes up to 90 to 100 percent chance. Looking down to 85 degrees or below mark for Thursday it is 25 to 75 percent for the San Joaquin Valley. A significant departure from the record highs this weekend.

Friday through Sunday the pattern remains as a general trough over the western states. Upper heights and thickness value do increase. Temperatures will rebound to near normal on Saturday with the PoE 93 degrees being the highest on Saturday with 20 to 65 percent probability in the San Joaquin Valley.

Monday and Tuesday the upper trough over the region gets a refresh from a diving shortwave from the Northern Pacific. This will lower heights even more for the area. Cluster analysis for the period has fairly high confidence in the pattern. Only differences is on the spread of the anomalies of the low heights. Looking at anomaly tables is not unheard of to get systems like this in September. Return intervals are less than five years. Still is a fairly significant system with the first widespread valley rainfall in awhile, albeit very light, this meteorological fall. Mid-level lapse rates and low top thunder is probable in the Sierra. The Sierra has a 10 to 15 percent probability of thunder, and even the Valley north of Bakersfield has a 5 to 10 percent probability of thunder. Not going to introduce that yet and will wait for a clear trend before turning the blinking thunder light on for the San Joaquin Valley. Even the probability of snow comes into play for the highest elevations including the Tioga Pass region with a 40 to 50 percent probability of measurable snow. Otherwise the story will be cool for the region with 10 to 15 degrees below normal.
The probability of staying below 80 on Monday and Tuesday ranges from 50 to 85 percent valley wide.

AVIATION
Areas of MVFR and local IFR visibilities near wildfires.
Otherwise, VFR conditions will prevail across the central CA interior over the next 24 hours.

AIR QUALITY ISSUES
ISSUED: 09/10/2024 14:31 EXPIRES: 09/11/2024 23:59 None.

CERTAINTY

The level of certainty for days 1 and 2 is high.
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 and/or to provide feedback.

HNX WATCHES/WARNINGS/ADVISORIES
Wind Advisory from 11 AM this morning to 11 AM PDT Thursday for CAZ338-339.