textproduct: San Diego

SYNOPSIS

Another very hot and humid day is on tap for Southern California today. Major to Extreme HeatRisk expected in the coastal basin and low deserts with moderate HeatRisk elsewhere. Isolated to scattered showers and thunderstorms expected over the mountains again today and a slight chance of showers on Thursday. A slow cooling trend will begin Thursday, though hot and humid conditions will be slow to subside. Moderate to Major HeatRisk will continue through Thursday for the coastal basin and through Friday in the low deserts. The cooling trend will continue through the weekend with temperatures dropping below normal from Sunday into early next week. Accompanying this cooldown will be marine layer clouds for the coastal basin and afternoon breezes for the mountains and deserts.

DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE

SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES...

The stage has been set for another dangerously hot and humid afternoon and evening in Southern California. Several locations in the coastal basin astonishingly did not drop below 80 degrees this morning. Strong upper ridging is peaking today and tonight with 500 mb heights around 595-596 mb. Also, surface analysis reveals a thermal trough sitting near or just offshore as of this morning, which will hinder the attemps of the seabreeze to cool the coastal basin. Temperatures this afternoon are currently ranging from 100-110 for inland areas and generally in the 90s near the coast. The combination of heat and humidity have pushed heat indices above 100 even near the coastline this afternoon, producing dangerous conditions for those without air-conditioning. In fact, heat indices are locally exceeding 115 in the western valleys of San Diego and Orange Counties. Overall, major to extreme HeatRisk is occurring or imminent in the coastal basin today and will last well into the evening hours. Major HeatRisk is also expected in the Coachella Valley this afternoon where temperatures will peak near 105-110. Moderate HeatRisk elsewhere, except for mountain locations above 6000 ft, where it should be a rather pleasant next few days.

Speaking of the mountains, isolated to scattered thunderstorms are expected again this afternoon. Storm activity exceeded expectations yesterday, so we are hedging on the side of more activity again today. There is less instability and greater capping today as indicated by the 12z KNKX sounding, but PWATs remain very high near 1.8 inches. Isolated rainfall rates near 0.5 in/hr will likely occur today along with potential for gusty winds with any storm that develops. Southeasterly flow will permit a slight chance (10-20%) for a storm or two to drift into the eastern valleys. For Thursday, instability will decrease yet again. Therefore, expecting only isolated showers over ridgetops at this time.

A slow cooling trend begins on Thursday as the upper ridge begins to weaken. However, this humid, tropical airmass will remain in place through Friday. Therefore, areas of major HeatRisk will continue through Thursday for the coastal basin and through Friday for the low deserts. By Saturday, an upper trough to the northwest will begin to push the weakening ridge out of the region bringing more relief from the heat and humidity. High temperatures will fall 5 to 10 degrees below normal across the region from Sunday into early next week.

Morning marine layer clouds and fog will likely begin to reform over coastal waters as moderate onshore flow returns from Friday onward. There is a chance for fog Saturday morning for coastal areas and western valleys. Thereafter, the marine layer should deepen further into early next week with low clouds filling most of the coastal basin each morning. Additionally, breezy conditions are expected for outer coastal waters, mountains, and deserts over the weekend. By late next week model solutions diverge, but the mean of the guidance tilts toward high pressure and monsoonal moisture building back into the region from the east.

AVIATION... 091800Z

Mostly VFR conditions expected across the region through Thursday morning with SCT-BKN clouds between 10-20 kft. CBs based at 9 kft MSL with tops reaching to 35 kft will lead to some SHRA/TSRA mainly over mountains, but locally into deserts or inland valleys 19-01Z. Localized gusty winds and heavy rain in and near any TSRA.

MARINE

No hazardous marine conditions are expected through Sunday.

BEACHES

High tides peaking near 7 feet this evening could produce tidal overflow, coastal flooding, and localized beach erosion. Check the Coastal Hazard Message for details.

SKYWARN

Skywarn activation is not requested. However weather spotters are encouraged to report significant weather conditions.

SGX WATCHES/WARNINGS/ADVISORIES

CA...Coastal Flood Advisory until 11 PM PDT this evening for Orange County Coastal Areas-San Diego County Coastal Areas.

Extreme Heat Warning until 8 PM PDT this evening for Orange County Coastal Areas-Orange County Inland Areas-San Bernardino and Riverside County Valleys-The Inland Empire- San Diego County Coastal Areas-San Diego County Valleys- Santa Ana Mountains and Foothills.

Heat Advisory from 8 PM this evening to 8 PM PDT Thursday for Orange County Coastal Areas-Orange County Inland Areas-San Bernardino and Riverside County Valleys-The Inland Empire- San Diego County Coastal Areas-San Diego County Valleys- Santa Ana Mountains and Foothills.

Heat Advisory until 8 PM PDT Thursday for Riverside County Mountains-San Diego County Mountains.

Extreme Heat Warning until 8 PM PDT Friday for Coachella Valley- San Diego County Deserts-San Gorgonio Pass near Banning.

PZ...None.