textproduct: San Joaquin Valley

KEY MESSAGES

1. 10-20% probability of light showers or sprinkles today from southern Kern county north through the Coastal Range to San Luis Reservoir.

2. Moderate to Major Heat Risk on Tuesday and Wednesday.

3. 30% probability of thunderstorms along the high Sierra and eastern Kern deserts through the week.

4. Slow moving mountain thunderstorms can produce excessive rainfall.

5. Dry lightning strikes in the high Sierra this week, 10% proabability of occurrence, away from the excessive rainfall.

DISCUSSION

Textbook pattern for infusion of moisture from the Gulf of California and eastern Pacific, with push and pull between Pacific Northwest trough and closed upper ridge centered over the Dakotas. California lies in the transition zone of these two systems, allowing deep southwesterly and southerly flow at all levels to push precipitable water totals to 200% of normal, per the entire model suite, starting late today and lasting through much of the week, especially along the spine of the Sierra Nevada.

This anomaly will lend enough moisture in the mid levels of the atmosphere to create enough instability for 20-30% probability of thunderstorms in the high Sierra much of the week. There is a 5% Probability of excessive rainfall leading to flash flooding within 25 miles of a point along a north northwesterly line from Barstow to Risgecrest to Sequoia National Park to Yosemite National Park, on Tuesday. Slow moving thunderstorms in these areas can lead to the excessive rainfall. However, the probability of greater than 0.10" of rainfall during the week is 30% or less, meaning that excessive rainfall from thunderstorms will be localized or over a small area. This, sets up a 10% probability for dry thunderstorms, outside of the excessive rainfall where lightning can still strike, potentially leading to fire weather impacts.

Elsewhere, the anomalous moisture, not unusual for a southwest summer monsoon, will contribute sprinkles or light rain, at most wetting the pavement, through the coastal range and southern Kern county, now through the next few days. For the Central Valley, a bulk of dry surface air will translate to abundant mid and high clouds with sunny breaks.

Uncertainty resides with how these clouds will affect heat Tuesday and Wednesday for the Central Valley. Worst case scenario, these clouds thin by mid morning and temperatures trigger a High Heat Risk, with a major risk of heat-related illnesss for much of the population, but especially those without air conditioning, as elevated moisture will keep overnight temperatures in the 70s. Best case, clouds persist through the day keeping the heat down, but still hot enough for a moderate heat risk. For the hottest urban locations, there is a 10% probability of Max Temperatures 105F or hotter. At this point, leaning more towards a potential Heat Advisory. But, keep in mind, these will be the hottest temperatures of the summer.

As the whole upper air pattern shifts east, so does part of the monsoonal moisture plume late work week, but still a 10-20% probability of high Sierra thunderstorms.

Looking at the end of the forecast, a majority of ensemble solutions erode the west coast troughiness and retrograde the center of the ridge from the Dakotas to the Four Corners region. Likely this is why the Climate Prediction Center builds back the heat over California during next Sunday and the following days, with a slight (20-40%) probability of extreme heat.

AVIATION

High Sierra thunderstorms could lead to mountain obscuration, although VFR is expected through 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 high.

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.