textproduct: San Diego

SYNOPSIS

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

DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE

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

Satellite and ground observations at 9 PM were showing areas of low clouds and fog for the San Diego coast and valleys. Visibility had fallen to a quarter mile in a few locations. Patchy dense fog is expected to continue into Monday morning most likely in the valleys. Low clouds and fog become unlikely for Monday night into Tuesday with a brief uptick in offshore winds. Highs on Monday are expected to be a few degrees warmer than today for most locations.

From Previous Discussion Issued at 2 PM December 14...

High pressure aloft begins to build in from the west and offshore flow develops on Monday, with winds gusting to around 30 mph below the passes and canyons. Weak offshore flow under high pressure aloft will persist into Tuesday and mitigate or nullify the influence of the marine layer so that low clouds and fog are unlikely to return for Mon night/Tue morning or Tue night/Wed morning. It will also bring drier and warmer conditions into the coastal areas, resulting in daytime high temperatures on Tuesday as much as 10 degrees above seasonal averages in the coastal areas, and up to 20 degrees above average in the far inland valleys. Tuesday is likely to be the warmest day overall, with high temperatures reaching the 80s in the inland valleys and lower deserts.

Onshore flow returns on Wednesday as a shortwave trough moves inland to the north and the upper level ridge weakens and flattens out. This could bring some cooling to the coastal areas and far western portions of the inland valleys, and potentially a return of low clouds and fog to the coastal areas Wed night. However, this is a fast-moving trough and offshore flow quickly returns on Thursday as a sfc high follows the departing trough into the Great Basin. The offshore winds will bring drier and warmer conditions but are unlikely to be strong enough to produce any other impacts.

As we approach next weekend, a more vigorous trough will displace the upper high to the south, establishing a high-zonal pattern over CONUS. This pattern is expected to bring a weak atmospheric river to central and northern CA next weekend but SoCal will likely remain dry but cooler with more of a marine layer presence as onshore flow returns. Temperatures for next Sat and Sun will range from near normal at the coast to about 10 degrees above normal in parts of the inland valleys.

AVIATION

150330Z...Coast/Valleys...Low clouds have sufficiently pushed around 10 miles inland for San Diego County, and continue to spread north into Orange County. Bases currently 600-800ft MSL, may fall closer to 400-700ft MSL over the next few hours. VIS generally 3-6SM, but will be down to 1-2SM for coastal terrain and valleys. Periods of VIS down to 1/4SM in FG possible for more elevated sites, especially after 08z. Clouds and FG may clear a bit quicker Monday morning with offshore flow returning, possibly as early as 14z. Clouds are expected to linger at sites closer to the coastline, including KSAN, through 18-19z.

Mountains/Deserts...Clear skies and VFR conditions expected through the Monday evening.

MARINE

Low clouds will bring areas of low visibility around 2-5 nautical miles through early Monday 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...None. PZ...None.