Area Forecast Discussion
Issued by NWS San Diego, CA

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000 FXUS66 KSGX 182024 AFDSGX Area Forecast Discussion National Weather Service San Diego CA 123 PM PDT Sun Apr 18 2021 .SYNOPSIS...
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Santa Ana weather will persist into Monday, but winds will weaken along the coastal foothills, and cooling will begin along the immediate coast. It will remain quite warm inland on Monday, with cooling making better inland progress through midweek. Onshore flow will strengthen late Monday ahead of another trough dropping south through Nevada, deepening the marine layer. Expect more clouds, fog, patchy drizzle, and perhaps some light rain showers Wednesday into Thursday morning. Temperatures will be below average through midweek, then rise back to average and above into next weekend.
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&& .DISCUSSION...
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FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE... SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO COUNTIES... At 1 PM PDT...Skies were clear over SoCal. The offshore sfc pressure gradient was weakening and wind reports, while gusty in many areas, including the deserts, were falling below Advisory strength. So the Wind Advisory has been cancelled. Still, it was very dry, and where it was breezy, the combination of wind and very low RH was creating elevated to near critical fire weather conditions in a few spots(See Fire Weather discussion below). Winds of 20-30 MPH with higher gusts to around 40 MPH will be confined to the wind-prone pass and foothill areas for the remainder of the day. Today will be the warmest day of the week along the coast, even with a weak sea breeze. On Monday, winds will begin the day weakly offshore, but turn back onshore and become gusty through the passes and along the desert slopes later in the day. It will be warmer tomorrow inland, but coastal areas should begin to cool as marine air gradually replaces dry, continental air. For Tuesday through Thursday, a broad trough will become reestablished over the SW, supporting stronger onshore flow and a deepening marine layer. Most ensemble members of the CMC/EC/GFS models have some light precip falling over the coastal basin either Wednesday or Thursday. Dynamics are weak, so any accumulations will be light. Once again, some of the lower coastal slopes could see some of the larger accumulations, due to better convergence in the shallow moist layer. As we head into the weekend, a weak ridge develops over the SW and the ensembles favor at least slight warming over SoCal ahead of another developing EastPac trough next week as the marine layer shrinks back toward the Coast. Uncertainty increases considerably for next week as a number of ensemble solutions of both the CMC/ECMWF turn wet. The ECMWF deterministic is the most bullish, indicating a sizable late season storm here. The entire GFS ensemble spread however remains dry for that period.
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&& .AVIATION...
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182030Z...Clear skies will prevail through Monday with unrestricted vis. Areas of N-NE winds 15-25 knots with local gusts 30-40 knots will continue from the mountain crests through the foothills and into the valleys through 22Z with gradually diminishing winds through 02Z Mon. This will include VCNTY KONT, KCNO, KRAL, KRIV, KAJO. Areas of MOD up/downdrafts and LLWS will also occur.
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&& .MARINE... Northwesterly winds will locally gust around or slightly over 20 knots in the outer coastal waters this evening and then again Monday evening. Otherwise, no hazardous marine weather is expected through Thursday. && .FIRE WEATHER...
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Several sites across the Inland Empire and into the San Gorgonio Pass were hovering near the edge of critical fire weather criteria at Noon today with RH at about 10% and wind gusts to 35 to locally 45 MPH. These winds will weaken this afternoon, but RH will remain low. Some modest RH recovery will occur overnight, and coastal areas will continue to recover on Monday, while inland areas fall back to or below 10%. Winds will turn back onshore by midday, and become gusty from the west through and below the passes, along the desert slopes and into the deserts. These areas will see elevated fire weather conditions by late afternoon. The onshore flow and increasing marine influence will minimize fire weather conditions along and west of the mountains for the remainder of the week.
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&& .SKYWARN... Skywarn activation is not requested. However weather spotters are encouraged to report significant weather conditions. && .SGX WATCHES/WARNINGS/ADVISORIES...
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CA...NONE. PZ...NONE.
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&& $$ PUBLIC/FIRE WEATHER...10 AVIATION/MARINE...Maxwell

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