textproduct: San Diego

SYNOPSIS

Cloudy and warm today today with periods of very light rain or sowers, not enough to wet the ground. Gusty Santa Ana winds early this afternoon will weaken by this evening across the mountains and western valleys. A low pressure system will bring periods of showers for New Year's Eve and New Year's Day with the potential for more cloudy and wet weather by the end of the week into early next week.

DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE

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

This afternoon...Sfc pressure gradients are weakening. Currently, northeast to east winds are gusting 25-40 mph in the wind-prone areas. The offshore winds at the surface will continue to weaken this afternoon, becoming light tonight. A very dry layer of the atmosphere extends upward from the sfc to about 12,000 ft. Above that, widespread clouds in a deep, moist layer are moving from south to north in a warm advection process on the eastern side of an upper level low about 650 miles southwest of San Diego. These mid to high level clouds are producing light rain/showers but a percentage of the precip is evaporating in the dry sub-cloud layer. As a result, the precip reaching the surface is likely not enough to make the roads wet except possibly at higher elevations where the dry layer is not as deep. This pattern has also produced generally warmer conditions at the sfc, and high temperatures will be in the low to high 70s - possibly reaching 80 - west of the mtns.

This warm overrunning pattern will continue through Wednesday as the low to the southwest moves northeast. The deep, moist layer aloft will gradually extend lower in the atmosphere and over time, more rain is likely to reach the ground but still not enough to produce significant impacts. Temperatures on Wed will be lower but will remain near or slightly above seasonal averages.

For Wed evening through Thu...This is the period of time when most of the rainfall will occur as the closed low to the southwest accelerates to the northeast and becomes an open wave. It will produce enough dynamic forcing for deep convection, resulting in periods of heavier showers which will be more efficient at extracting moisture from the atmosphere. Rainfall rates during heavier showers will likely range from 0.3"-0.5" per hour, with about a 50% chance of exceeding 0.5" per hour. The greatest accumulations will be in the San Bernardino Mountains and on the coastal slopes where totals will range from about 1.9" east of the Cajon Pass to about 5" near the LA County line. Elsewhere, rainfall amounts will range from 0.9"-1.8" in the coastal areas, 0.9"-1.8" in the inland valleys, 1.1"-2.2" in the San Diego and Riverside County mtns, 0.25"-0.75" in the low deserts, and 0.25"-0.9" in the high desert.

There will be a break from the precip on Fri as a weak dynamic ridge builds between the trough moving east and a low pressure system moving southeast out of the Gulf of AK. Temps will rise slightly, especially in the low deserts where temps could climb back into the 70s.

The low from the Gulf of AK will resume the precip for Sat-Sun. This system will bring colder air but will have less available moisture. Temps on Sat and Sun will be in the 60s west of the mtns and precip amounts will range from a trace to 0.25" in the San Diego and Riverside County mtns as well as areas west of the mtns. The deserts will be lucky to get any measurable precip and the San Bernardino Mountains will likely get about 0.2"-0.8"...greatest west of the Cajon Pass.

A reinforcing major shortwave trough moving into the base of the existing trough over the west coast will continue the unsettled weather and bring chances for additional precip for Monday and Tuesday. Most precip will fall in the San Bernardino Mountains where Mon-Tue totals will range from about 0.2" east of the Cajon Pass to about 0.7" west of the Cajon Pass. Elsewhere, amounts will range from a trace to 0.25", except the low deserts which may get no measurable precip.

Snow levels will likely remain above 8,000 ft through Friday before dropping down to about 6,500-7,500 ft for Sat through next Tue. Snowfall estimates near resort levels are for a trace to about 4 inches between Sat and next Tue.

AVIATION

302100Z...Generally VFR conditions today and tonight TAF period with BKN-OVC high clouds 8000-12000 ft. Intermittent very light -RA/-SHRA moving northwards today and tonight, but is unlikely to cause any cig/vis impacts. Localized offshore gusts near 20-30 kts in the mountain passes and coastal slopes through early evening before offshore winds diminish late evening. -SHRA become more widespread and slightly heavier Wed morning after 14Z, with intermittent reduced vis 5-6 SM and widespread clouds 5000-7000 ft MSL. RA chances increase and spread substantially Wednesday evening, bringing lowered vis 1-4 SM in heavier showers and local cigs 1000- 3000 ft MSL.

MARINE

Southeasterly winds start to strengthen Wednesday afternoon, with conditions hazardous to small craft Wednesday evening through Thursday afternoon, bringing south to southeast gusts to 25 knots. Winds calm late Thursday afternoon, then no hazardous marine weather conditions expected through Saturday.

SKYWARN

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

SGX WATCHES/WARNINGS/ADVISORIES

CA...Flood Watch from Wednesday evening through Thursday evening for Orange County Coastal Areas-Orange County Inland Areas-San Bernardino County Mountains-San Bernardino and Riverside County Valleys-The Inland Empire-Santa Ana Mountains and Foothills.

PZ...Small Craft Advisory from 7 PM Wednesday to 3 PM PST Thursday for Coastal Waters from San Mateo Point to the Mexican Border and out to 10 nm-Waters from San Mateo Point to the Mexican Border Extending 10 to 60 nm out including San Clemente Island.