textproduct: San Diego

SYNOPSIS

High pressure remains locked in place in the near term as hot days and near-record warm nights continue through Friday night. Slight chance of showers and thunderstorms for the mountains this afternoon and Friday afternoon. High pressure weakens and slides east this weekend with temperatures cooling to near average by Sunday and below average by early next week. For Tuesday and Wednesday of next week, increased high cloud cover is likely along with a 20 to 30 percent chance of precipitation from a tropical system that has yet to develop.

..UPDATED AVIATION DISCUSSION

The strong upper ridge centered over Arizona will remain in place through Friday with 500 mb heights hovering around 594 dm over southern California. High temperatures generally overperformed yesterday with daily record high maximums set in a few inland valley and desert locations. This overperforming trend will have to compete with increased cloud cover today, courtesy of the remnants of Tropical Storm Iselle, so forecast highs are few degrees lower than yesterday's temps. Regardless of exact temperatures, hot days in combination with near-record warm nights will produce major to locally extreme HeatRisk through Friday for inland valleys and lower deserts. Moderate to locally major HeatRisk is expected for the mountains, High Desert, and areas near the coast.

There is a slight chance (10-20%) for thunderstorms over the mountains this afternoon and Friday afternoon. There is ample moisture present (PWATs around 1.4-1.5"), but the 12z KNKX sounding indicates the presence of substantial mid-level capping, which will keep convection quite limited.

By Saturday, the ridge begins to weaken and slide eastward as a robust upper trough pushes into the PNW. This will usher in a welcome cooling trend across the region. Temperatures are expected to return to near average by Sunday with highs further cooling to around 3 to 7 degrees below average early next week. This pattern change will also produce breezy westerly winds for the mountains and deserts and deepen the marine layer for Sunday and Monday.

From next Tuesday onward, the forecast becomes more uncertain. The synoptic set up will remain generally unchanged on the synoptic scale with troughing over the PNW and ridging over the south-central CONUS resulting in southwesterly flow the region. Simultaneously, global models indicate that a tropical system will pass well to the southwest early next week. The exact evolution of this large scale set up and the track of the tropical system will determine how much moisture will be drawn into this region for the middle of next week. Most ensembles indicate a notable increase in PWATs over southern California next week, but only about 20-30% have any precipitation at this time.

AVIATION

280545Z...Coast...Low clouds 600-800 ft MSL have formed along parts of the Southern California coastline, moving intermittently into coastal areas. Clouds will become more widespread as mid to upper level clouds move northwards out of the area, though likely will remain patchy for most of the night. Best chances for impacts for coastal sites is 10-15Z, with cigs likely lowering slightly to 400- 700 ft MSL and vis restrictions 2-5 SM over higher coastal terrain. Low clouds expected to clear by 16Z, with low clouds of similar bases/patchiness forming after 09Z Sat.

Mountains/Deserts...Cumulus based around 9,000ft MSL over the mountains Friday 20Z to Sat 01Z with isolated CB cloud tops rising to 40,000ft MSL. There is a 10-15% chance of iso TSRA/SHRA over the mountains and a 5-10% chance in the deserts until 01Z Sat. Any TS that develops could produce gusty, erratic winds.

Regionwide...High clouds AOA 15,000ft MSL will continue tonight and Friday, decreasing through Friday evening.

MARINE

No hazardous marine conditions are expected through Monday.

BEACHES

A fresh 3 foot swell from the south-southwest (200 degrees) at 15-17 seconds moves into the region Saturday-Sunday. This will bring localized elevated surf of 4-6 feet to southwest-facing beaches, mostly in north San Diego County and Orange County.

SGX WATCHES/WARNINGS/ADVISORIES

CA...Heat Advisory until 8 PM PDT Friday for Orange County Coastal Areas-Riverside County Mountains-San Bernardino County Mountains-San Diego County Coastal Areas.

Extreme Heat Warning until 8 PM PDT Friday for Apple and Lucerne Valleys-Coachella Valley-Orange County Inland Areas-San Bernardino and Riverside County Valleys-The Inland Empire- San Diego County Deserts-San Diego County Mountains-San Diego County Valleys-San Gorgonio Pass near Banning-Santa Ana Mountains and Foothills.

PZ...None.