Area Forecast Discussion
Issued by NWS San Diego, CA
Issued by NWS San Diego, CA
995 FXUS66 KSGX 291604 AFDSGX Area Forecast Discussion National Weather Service San Diego CA 904 AM PDT Sat Jun 29 2024 .SYNOPSIS... Gradual warming through this weekend followed by minor cooling on Monday. A prolonged heat wave is expected for inland areas Tuesday into at least next weekend, including for the Fourth of July holiday. Night and morning low clouds and fog will continue along the coast and far western valleys through much of next week. && .DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES... Update... The 12z Miramar sounding showed a 9C degree inversion based near 1500 feet and a saturated profile as low as 1060 feet. Low clouds extended through the coast and western valleys but are quickly eroding as of 9 AM, with most areas expected to clear by late morning. Some patches of low clouds could persist at the coast due to the strength of the inversion, but most areas should be mostly sunny today. Today will be warmer than on Friday, with highs generally topping out 2-5 degrees above normal. An interesting synoptic-scale pattern sets up this weekend as longwave troughing envelopes central California through the Pacific Northwest and a ridge builds across the Southern Plains, with increasing heights nudging westward into SoCal. Mid-level monsoonal moisture could make it into Imperial County and eastern Riverside County tomorrow, increasing mid-level cloud cover over the mountains and lower deserts. Some high-resolution guidance shows small precipitation chances south of the border in the mountains tomorrow afternoon, so we will continue to monitor trends to see if cells could pop in our mountains. Further warming is likely on Sunday across all areas. Previous Discussion (Issued 250 AM PDT Sat Jun 29 2024)... A warming trend will take place this weekend as an upper level high centered over TX expands westward towards So Cal. By Sunday high temperatures will be around 5 to 8, locally up to 10, degrees above normal. Highs in the lower deserts will be as much as 114 degrees, with highs in the low 100s across much of the Inland Empire. The persistent marine layer will keep the coastal areas and to a lesser extent the inland valleys somewhat moderated, with highs in the 70s near the coast and mid 80s to low 90s for inland Orange County and much of the San Diego County Valleys. An Excessive Heat Watch remains in place for the lower deserts beginning on Sunday morning. Brief cooling occurs on Monday as a weak upper level trough moves inland across the West Coast and causes the upper ridge to retreat eastward. Most areas will see highs around 2 to 5 degrees lower than Sunday. An upper level high over the East Pac slowly shifts east over CA and eventually into the Great Basin as it strengthens through the week. However, there are still a lot of spread amongst the global ensembles regarding the placement and strength of the upper level high, especially beyond Wednesday. For the Fourth of July, The warmest solutions which accounts for 20 percent of the ensemble space has the high centered over CA and NV, while 69 percent of the ensemble members have the high still centered off the coast/not as hot in So Cal. By next weekend it is more decidedly warmer with about 80 percent of the ensembles placing the high somewhere across the southwest or Great Basin. The GFS remains the most aggressive with the strength of the high, peaking at 600 dm next weekend, though only 10-20 percent of the GEFS members show heights of 600 dm or more. Meanwhile, no EPS members show heights that high. Needless to say, there is still a bit of uncertainty with regards to the exact high forecast temperatures - add in potential monsoon moisture in the lower deserts and the uncertainty increases even more. Given the position and strength of the high, thunderstorm probabilities are fairly low - generally 10 percent or less. Regardless of the forecast uncertainties, we are in for a long duration heat wave whether temperatures end up falling in the low end or high end of the current forecast range, especially in the deserts. The Excessive Heat Watch for the lower deserts currently extends into Friday, while the timing for the high deserts has been adjusted to start on Tuesday and lasting into Friday. Heat Advisories will likely be needed for additional areas as the event draws closer. && .AVIATION... 291550Z...Coast/Valleys...Low clouds currently extend about 15 miles inland by with local VIS 2-4 miles on higher coastal terrain, bases 600-1500 FT MSL and tops to 2000 FT MSL. Expect clearing to the coast 16Z-18Z but patches of low clouds could persist at the beaches through this afternoon. Low clouds returning to coastal TAF sites around 05-08Z Sunday, bases at or slightly below 1000 FT MSL expected. Mountains/Deserts...Mostly clear skies with unrestricted VIS will continue through this evening. Westerly winds gusting locally 25-35 kt will return to the desert slopes and adjacent deserts and through San Gorgonio Pass this afternoon/evening. && .MARINE... No hazardous marine conditions are expected through Wednesday. && .SKYWARN... Skywarn activation is not requested. However weather spotters are encouraged to report significant weather conditions. && .SGX WATCHES/WARNINGS/ADVISORIES... CA...Excessive Heat Watch from Tuesday morning through Friday evening for Apple and Lucerne Valleys. Excessive Heat Watch from Sunday morning through Friday evening for Coachella Valley-San Diego County Deserts-San Gorgonio Pass Near Banning. PZ...None. && $$ PUBLIC...EA AVIATION/MARINE...Adams