textproduct: San Joaquin Valley

KEY MESSAGES

1. Heat Advisory issued for the San Joaquin Valley, Sierra Foothills and West Side Hills along the Coastal Range.

2. Medium to high probability (70 to 80 percent) for triple- digit highs in the San Joaquin Valley Thursday.

3. Decreasing confidence in precipitation Wednesday. Medium probability (40 to 50 percent) for measurable rainfall in the Sierra Nevada.

DISCUSSION

The warming trend that began today will continue through this week as a ridge slowing builds in from the east as a low pressure cut-off is forming off the coast of California. This trend is expected to peak on Thursday with the Valley reaching highs of 101 to 103 degrees and the Desert seeing highs of 103 to 106 degrees. The nighttime lows from Wednesday into Thursday will also be elevated as Valley temperatures will only cool into the mid to upper 60's in the rural areas and in the low to mid 70's in the urban areas, leading to inadequate cooling for those without air conditioning. This Moderate to Major Heat Risk affects much of the population. The Desert is also likely to see lows in the mid to upper 70's. A Heat Advisory has been issued for the Valley, West Side Hills and Sierra Nevada foothills due to these conditions from 10 AM Wednesday through 10 PM Thursday.

At the same time, moisture from the current tropical system Marie (though by this time in the forecast, has become a cut-off area of low pressure) may enter our area on Wednesday into Thursday, causing a small amount of rain to fall over the highest elevations of the Sierra Nevada. Current chances for measureable precipitation is 40-50%, though these chances have been decreasing in the last few model runs, suggesting rainfall will be minimal, if it occurs at all.

Looking to the end of this week into early next week, temperatures are expected to cool down starting on Friday and continue through Sunday as a trough moving down from British Colombia pushing the ridge back east as it moves through Northern California. After this trough leaves the west coast, more zonal flow will take over, causing a small rise in temperatures before becoming stable around normal values seen for this time for year (in the low 90's for the Valley). The long term forecast currently shows the region leaning above normal for temperatures in the 6 to 10 day and 8 to 14 day periods as a potential weak ridge moves back in from the east, though triple digits become less likely as we move further through September.

AVIATION

VFR conditions expected for the next 24 hours.

AIR QUALITY ISSUES

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

Heat Advisory from 10 AM Wednesday to 10 PM PDT Thursday for CAZ300>317-319-321.