textproduct: San Joaquin Valley

KEY MESSAGES

1. There is a high confidence in a significant change in the overall weather pattern with temperatures continuing to cooldown and precipitation mid-week.

2. The first wetting rainfall throughout the entire area is likely with a 50-65% chance of at least a half an inch of rainfall by Thursday evening.

3. Snowfall is likely above 8,000 feet in elevation with a 55-70% chance of at least 4 inches of snowfall (probabilities increase further at higher elevations).

4. The Storm Prediction Center (SPC) has western Kern County in a level one out of five risk for severe weather with a 5-14% chance for severe winds tomorrow.

DISCUSSION

A strong trough with a deep area of low pressure has begun to move into our area off the Pacific Ocean. This is leading to much cooler temperatures across our region and our first chance of widespread precipitation this fall. Temperatures will be cooler today than yesterday and will likely be near to slightly below average. The probability of temperatures exceeding 80 degrees today will be around 25-40% in the northern Valley and 65-80% in the southern Valley. Temperatures will be much cooler tomorrow as a low pressure system moves off the Pacific Ocean and begins to have a widespread influence in our region. The probability of temperatures exceeding 70 degrees tomorrow is only about 35-50% across most of the Valley, with cool and well below average temperatures remaining very likely for the middle to end of the week. There will be a warm-up in temperatures however by the coming weekend with the chance of temperatures exceeding 70 degrees at around 85-95% across the Valley on Saturday and Sunday.

In terms of precipitation, there is a 10-20% chance of general thunderstorms today along the foothills, Sierra Nevada mountains, and Mojave Desert in eastern Kern County this afternoon through evening. Tomorrow, moisture will move off the Pacific Ocean and into our area bringing the first widespread rainfall in the Valley since this past spring. Precipitation will likely begin in our area sometime between the mid- morning and mid- afternoon. In terms of precipitation amounts, the probability of a half of inch of rainfall in the Valley is at around 45-65% with slightly higher probabilities along the Sierras, and similar to slightly lower probabilities in the Mojave Desert. The probability of three quarters of an inch of rainfall is reduced to around 30-50% across much of the Valley. The worst case- scenario, or the most precipitation possible in the Valley would likely be around an inch and a quarter. An area particularly vulnerable to rainfall is in the Mojave Desert in Kern County, and this rainfall forecast is therefore of particular importance in that area. There is a marginal chance, at least 5% chance, of excessive rainfall in through much of Kern County and along the southern Sierra Nevada Mountains. In addition, to the excessive rainfall, there is a chance for severe thunderstorms with this influx of moisture tomorrow. Thunderstorms with severe damaging winds are possible (5-14% chance) with the Storm Prediction Center (SPC) having western Kern County in a level one out of five risk for severe weather tomorrow. There is a general thunderstorm risk however throughout the entire area with ample moisture off the Pacific Ocean and a strong jet streak coming into our area. Precipitation could linger and remain in our area through much of the day Thursday, but is then expected to taper off sometime late in the day.

In terms of snowfall, there is a chance for at least 4 inches of snowfall above 8,000 feet in the high Sierras (55-70% chance). Being still October, snowfall will remain at high elevations. There is the potential for even more snowfall at higher elevations with a 50-65% chance of at least 6 inches of snowfall above 10,000 feet. In terms of wind gusts, isolated wind gusts greater than 30 mph remain possible from late Thursday night into late Friday night within the Sierra Nevada mountain range and the Mojave Desert (60-75% chance), as the low pressure system begins to move out of our area.

The Climate Prediction Center (CPC) has a 33-40% chance for above normal temperatures in the next 6-10 days and a 40-50% chance for above normal precipitation. However, in the 8-14 Day Outlook, the CPC has us with near normal temperatures and a 50-60% chance for above average precipitation. Previously mentioned chances for precipitation next weekend and early next week have dwindled, and there is only about a 5% chance for any rainfall at all within the entire area.

AVIATION

VFR conditions will prevail 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

None.