textproduct: San Diego

SYNOPSIS

Areas of dense fog on the coastal mesas and western valleys this morning and again tonight into Monday morning. Well above normal high temperatures with periods of weak offshore flow through Thursday. Gradual cooling with more widespread night and morning low clouds and fog Friday into next weekend.

DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE

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

Marine layer low clouds and fog have spread into the western valleys this morning. Several sites on the coastal mesas and valleys in San Diego County are reporting 1/4 mile visibility in fog, and a Dense Fog Advisory is in effect for the San Diego County coastal areas and western valleys through 9 AM. Clouds and fog will clear back to the coast by late morning, though some beaches could remain overcast through the afternoon once again. Otherwise scattered high clouds today and high temperatures around 10-15 degrees above normal inland, and near normal along the coast.

A weak short wave trough will pass over the region today into tonight. This will help deepen the marine layer slightly, allowing low clouds to spread a little further inland and resulting in less widespread dense fog. Once this wave goes through, surface pressure gradients turn offshore Monday morning with north to northeast winds gusting to around 30 mph developing below the passes and canyons. Weak offshore flow persists into Tuesday. Upper level ridging and the offshore flow will combine to bring warmer and drier conditions Monday and Tuesday, especially west of the mountains. If enough dry air makes it to the coast, the marine layer will struggle to develop each night, limiting the risk of fog development. By Tuesday, highs will be close to 15-20 degrees above normal in the inland areas again with highs in the low to mid 80s across the valleys and low deserts.

A weak upper level wave passes by to the north on Wednesday and onshore flow briefly returns for minor cooling and perhaps a return of low clouds and fog along the coast. This is quickly replaced by another round of weak offshore flow on Thursday as surface high pressure builds over the Great Basin behind the departing wave. Winds are currently forecast to be even weaker than Mon/Tue, so no impacts expected with that other than another little bump up in high temperatures west of the mountains.

We finally begin to enter more of a pattern change Friday into next weekend as the upper high breaks down and broad zonal flow develops across the entire CONUS. A weak Atmospheric River is forecast to move into Northern and Central California next weekend, but for now the moisture looks to be too far north for any precipitation here, so PoPs remain below 10%.

AVIATION

140900Z...Coast/Valleys...Low clouds based 300-800 ft MSL have pushed into the inland valleys, including the western Inland Empire. Widespread VIS reductions down to 2-5SM for coastal areas. Widespread VIS reductions 0-2SM and FG over elevated coastal terrain and inland areas along the I-15 corridor. VIS to continue to reduce over coastal areas with periods of 1/4SM possible after 10z. VIS will begin to gradually improve between 15-17z with VIS eventually reaching above 5SM for most of the coastal basin after 19z. Clearing of clouds will occur for much of San Diego County 18- 20z, 20-21z for Orange County, with stratus lingering at certain beaches beyond this point. Low clouds with slightly higher bases push inland again as early as 00-02z Mon for SD County, and after 04z for Orange County. VIS to drop rapidly to 0-3SM for inland areas when low clouds approach, lowering gradually to 2-5SM for coastal sites.

Mountains/Deserts...Clear skies and VFR conditions expected through the TAF period.

MARINE

Patchy low clouds with visibility generally above 1 nautical mile will continue into early Sunday afternoon. Otherwise, no hazardous marine conditions are expected through Thursday.

SKYWARN

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

SGX WATCHES/WARNINGS/ADVISORIES

CA...Dense Fog Advisory until 9 AM PST this morning for San Diego County Coastal Areas-San Diego County Valleys.

PZ...None.