Area Forecast Discussion

KEY MESSAGES

1. Warming and drying trend continuing tomorrow with temperatures rising to above average.

2. An elevated risk for fire starts in Kern County due to low humidity and increased winds, particularly across the Mojave Desert.

3. An approaching cut-off low brings periods of heavy rainfall and mountain snow by early Tuesday, with favorable conditions for convective weather afterwards.

DISCUSSION

As the shortwave ridge continues east over the CWA, highs are expected to rise about 10 degrees above normal this afternoon. This ridge is expected to weaken slightly by Monday afternoon, leading to slightly cooler (though still above average) temperatures. By Monday evening, ridging will have been pushed east by the next system to impact our region this week.

By early Tuesday morning, a cut-off low pressure system is expected to move through Central California. There is increasing ensemble agreement for fairly significant precipitation to much of the CWA, especially the Sierra Nevada, its Foothills, and the northern San Joaquin Valley by Tuesday afternoon into the evening. Current probabilities during that time period for an inch of rainfall in the mountains below 7000 feet north of Kings Canyon is 60-80%, and 40-60% for the rest of the Sierra Nevada.

Looking to the northern Valley, chances for half an inch of rain are 50-60% for Fresno northwards, while the southern Valley is expected to be rain shadowed by the Coastal Range as the chances for the same amount of rainfall drop to 20-30%. This places the entire CWA sans Kern County and the southern halves of Kings and Tulare County under a marginal risk for excessive rainfall through 5AM Wednesday.

Snow levels with this system are expected to be around 6000-7000 feet for the majority of the precipitation Tuesday evening, and are expected to come down to around 5000-6000 feet early Wednesday morning. Snow totals with this system are expected to be around 6 to 10 inches for the Sierra Nevada south of Yosemite NP, with locations northwards expecting 12 to 18 inches with the largest amounts above 9000 feet. As a result, a Winter Storm Watch is in effect from 2AM PDT Tuesday to 5PM PDT Wednesday.

Along with the stratiform precipitation band expected Tuesday morning, the clearing skies behind that band will create an environment favorable for thunderstorm development as the extra dynamics interact with the newly dropped rainfall and the heating in the afternoon. As of the newest model runs, thunderstorm probabilities on Tuesday are 20-30%, beginning on the east side of the Valley before running into the Foothills later in the day.

As this system moves east, temperatures are expected to rise above normal once again by Friday, though only by a few degrees as weak, broad troughing remains aloft. By next weekend, another cut-off trough may make its way through California, though current model runs are seeing a weaker system compared to the upcoming system this Tuesday. The Climate Prediction Center continues to predict likely above normal precipitation for both the 6 to 10 and 8 to 14 day outlooks across the CWA.

AVIATION

VFR conditions expected across the area for the next 24 hours.

FIRE WEATHER

A strong warm-up is expected for this weekend with afternoon temperatures approaching ten degrees above averages on Sunday. Humidity will continue gradually drying across the region as a result. Elevated fire risk is expected particularly for the Mojave Desert with a decrease in minRH values between five and ten percent. Wind gusts will increase for the Mojave Slopes during the afternoons. An approaching system brings a pattern change late Monday, with precipitation across much of the region anticipated from then until Wednesday afternoon.

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

Winter Storm Watch from late Monday night through Wednesday afternoon for CAZ323-326>328-330.