Area Forecast Discussion
Issued by NWS San Diego, CA
Issued by NWS San Diego, CA
429 FXUS66 KSGX 300357 AFDSGX Area Forecast Discussion National Weather Service San Diego CA 857 PM PDT Sun Sep 29 2024 .SYNOPSIS... Hot temperatures weather will occur through the week continue through the coming week for inland areas with a shallow marine layer keeping temperatures at the coast moderated, with low clouds and patchy dense fog each morning. High pressure will slowly ease its grip by the end of the week, with a subtle decrease in temperatures. && .DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES... Evening update... Low clouds stuck to the San Diego County coast again today, but have started to retreat this evening. The 00Z KNKX sounding shows the marine inversion based near 900 ft MSL, down slightly from this morning. While some stratus development is expected overnight, less inland extent will occur as surface pressure gradients trend offshore (mainly to the north) and the marine layer remains more shallow. Patchy dense fog is possible where clouds develop over higher coastal terrain and on the coastal mesas. Otherwise skies will be clear tonight, except for areas of smoke in the vicinity of the Line Fire. Previous discussion... .SHORT TERM (Through Wednesday)... Our inversion has held strong today, keeping low clouds near the beaches early this afternoon. Slow lower level mixing will allow for clearer skies to occur over the next couple of hours. A weak low is crossing our region overhead, leading to the coolest day for most areas away from the coast as head into the work week. High pressure off the coast today will move eastward across the desert southwest, bringing increased heat impacts to our region this week. Temperatures will increase near 5 to 10 degrees from what we`ve seen today for inland valleys west of the mountains into the high desert with a moderate to high chance in seeing triple digit readings in the Inland Empire. The marine layer will become slightly more shallow, keeping low clouds and areas of dense fog closer to the coastal areas by Monday morning. This coastal fog pattern will continue into at least Wednesday. The ridge of high pressure will strengthen further into Tuesday and Wednesday when the hottest weather is expected overall as 850 mb temps rise near 28-30C. Latest forecast guidance continues to trend hotter, so forecast highs have been increased to account for this as our confidence grows in this forecast. We have also issued another Excessive Heat Warning across the Lower Deserts and Heat Advisories for the Inland Empire and San Diego Co valleys as major heat risk is expected from Tuesday through at least Thursday. As of now, Wednesday looks to be the hottest day across the region. NBM probabilities are moderate to high in seeing temperatures of at least 105 degrees across the Inland Empire. Probabilities for triple digit heat across San Diego county foothills like Ramona have similar chances as well. 80s and 90s will be common away from the immediate coast in Orange and SD Counties. Please be safe in the heat by avoiding outdoor recreation in the afternoon hours and drink plenty of fluids if you must be outside. && .LONG TERM...(Thursday into Next Weekend)... By Thursday, models are in good agreement of the high weakening some as a weak trough lingers farther off the coast. This should help increase onshore flow, leading to subtle cooling for areas west of the mountains. Confidence is still low on how much we are able to cool, but one thing is certain that temperatures will continue to remain much above normal through the end of the week. The Pacific trough that will near that PacNW that we have been mentioning the past few days that may help us cool off...well, that is looking less likely. Long range models are now coming into fairly good agreement of the ridge anchoring over the desert southwest into the end of the week. We may be able to be far enough away from center of the high pressure and closer to the weak trough off the coast, which would help us cool off a bit more by late week, but confidence is low on where the ridge and the trough set up. && .AVIATION... 300330Z...Coasts/Valleys...Low stratus clouds have developed along the coast of San Diego County this evening, bases around 800-1000 ft MSL, making their way inland up to 10 mi. Cigs will lower overnight to 500-700 ft MSL with tops to around 1200 ft MSL. Clouds will likely be patchy with intermittent cigs and periods of scattering through the night due to a weak low pressure system moving into the region. Similarly intermittent FG across the western valleys and coastal terrain with VIS down to 2-5SM, locally dense areas down to 1SM or less for the higher coastal terrain. Low clouds clearing to beaches by 15-17Z. Mountains/Deserts...Mostly clear with generally unrestricted VIS through Monday. Localized areas of VIS reductions possible near and to the north of the Line Fire in the San Bernardino Mountains due to FU. && .MARINE... No hazardous marine conditions are expected through Friday. && .SKYWARN... Skywarn activation is not requested. However weather spotters are encouraged to report significant weather conditions. && .SGX WATCHES/WARNINGS/ADVISORIES... CA...Heat Advisory from 10 AM Tuesday to 8 PM PDT Wednesday for San Bernardino and Riverside County Valleys-The Inland Empire- San Diego County Valleys. Excessive Heat Warning from 10 AM Tuesday to 9 PM PDT Thursday for Coachella Valley-San Diego County Deserts. PZ...None. && $$ PUBLIC...SS/APR AVIATION/MARINE...CSP