textproduct: San Joaquin Valley

KEY MESSAGES

1. Lake Wind Advisory for San Luis Reservoir and Lake Isabella through 11 PM PDT Thursday, July 9. Beware of unsettled waters.

2. Slow but steady warmup this week, with widespread Moderate Heat Risk by the weekend and isolated areas of Major Heat Risk.

3. Hot, dry, and windy conditions will create elevated fire risk, particularly in eastern Kern and Tulare Counties.

4. Increased chance for Sierra Nevada thunderstorms late this weekend into Monday with monsoonal moisture pushing into our region.

5. Practice safety around lakes, rivers, and streams, as water is still running cold.

DISCUSSION

Zonal synoptic flow has set up over much of California, with the region caught between a low pressure system to the north off the coast of British Columbia and an area of high pressure centered over the Four Corners region of the southwestern US. The current flow is resulting in seasonal conditions over the area, being clear and dry with temperatures around average. These conditions will continue for Wednesday before the high pressure gradually slides westward over southern California. The rising pressure will also cascade over central California, raising temperatures into the triple digits for the San Joaquin Valley as early as Thursday, though probabilities are more likely (70 to 80 percent) on Friday and Saturday. The warming temperatures will be linked to a drying trend across the area, with minimum relative humidity expected between 15 and 20 percent across the Central Valley Thursday through Sunday. Lower minimums are expected for the Sierra Nevada and Kern Desert at 5 to 10 percent. A tighter pressure gradient on Thursday will bring occasional breezy winds for the valley areas, but will keep stronger gusts going at San Luis Reservoir, Lake Isabella, and in the Mojave Desert Slopes. These gusts will gradually wane into the weekend as the high pressure begins to erode.

In the latter part of the weekend and into early next week, a low pressure trough will push into the Pacific Northwest, nudging the aforementioned high pressure eastward over the Great Basin. This will turn the prevailing flow over central California to the southeast, maintaining and potentially rising temperatures into the low 100s for the lower elevations. An influx of monsoon moisture will also accompany the warm-up, and will result in chances for thunderstorms, mainly across the Sierra Nevada starting Monday. This is due to the moisture rising and condensing as it interacts with the elevated terrain in the region. Current chances for thunderstorms in the Sierra sit at 20 to 30 percent for both Monday and Tuesday of next week.

As we get further into next week, the ridge of high pressure over the Great Basin appears likely to retrograde westwards, strengthening again over California and raising temperatures back to around five degrees above season averages.

AVIATION

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

FIRE WEATHER

Warmer and drier conditions are expected for the region through the weekend due to high pressure building in from the southwest. As stated above, minimum relative humidity values for the Central Valley will be around 15 to 20 percent Thursday through Saturday, with values between 5 and 15 percent for the Sierra Nevada and the Mojave Desert. Values in the foothill areas are expected around 20 percent. These minRH values will improve slightly in the lower elevations early next week, and dramatically around 40 to 50 percent in the Sierra Nevada due to monsoonal moisture moving into the region. However, the moisture adds risk for thunderstorms across the higher elevations which could result in new fire starts on cured fuels.

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

Lake Wind Advisory until 11 PM PDT Thursday for CAZ300-332.