textproduct: San Diego

SYNOPSIS

Light isolated showers in the San Bernardino Mountains this afternoon become more widespread and heavier overnight into Friday morning. Highest precipitation favored in the northern mountains, with lighter totals closer to San Diego. Shower activity weakens late Friday night, fully tapering off Saturday morning. Drier and warmer weather early next week with a possible Santa Ana event peaking Monday. Showers could return from the south some time the latter half of next week centered around New Year's Day.

DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE

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

Aviation discussion updated.

Today through Saturday...

Today features a lull between the moderate to heavy rainfall observed across the region yesterday and a secondary surge of moisture set to move through late tonight into Friday afternoon. Very isolated showers remain possible through this afternoon for the San Bernardino Mountains with relatively light accumulations through midnight. The more defined band of precipitation will move through the region beginning after midnight, spreading moderate rainfall eastward, which may be heavy at times in the mountains. While rainfall rates within this band of only 0.4-0.6"/hr expected, of particular concern is the overly saturated portions of the San Bernardino Mountains and adjacent drainage basins spreading into the Inland Empire and High Desert regions. 4-8" of rainfall observed yesterday (and locally up to 10") will likely lead to lower intensities of rainfall required to lead to further instances of flash flooding, landslides, and debris flow. With this, a Flood Watch has been issued for the San Bernardino Mountains and adjacent areas into the northern Inland Empire and High Deserts from midnight Friday morning until 4 PM Friday afternoon. Behind the main band of rain, light showers remain possible through Friday evening and overnight, eventually tapering off by Saturday morning.

Expected additional rainfall totals from this band of rain is about 1-3 inches (and locally up to 4") for the San Bernardino Mountains, 1-1.5 inches for Orange County and portions of the Inland Empire adjacent to the San Bernardino Mountains, with about 0.25-0.75" for San Diego County coasts/valleys and the remainder of the Inland Empire. Snow levels will generally remain between 7500-8000ft for a bulk of the precipitation tomorrow morning/afternoon, through snow levels will rapidly fall to around 5000ft by Saturday morning as the upper low moves overhead. 2-3 inches of snow is possible near 7000ft with 8-12 inches possible near mountain peaks.

Sunday through Late Week...

Much drier conditions expected come the beginning of next week after this weekend's system departs. However, a potent inside slider upper trough helps to strengthen the offshore gradient, with a Santa Ana Wind event looking more likely for Sunday into Tuesday. CW3E WRF ensemble means suggest this could be a moderate strength event with a ~60% chance of a moderate to strong event. Given the recent rainfall, there is not as high of a fire concern, but the potential for strong wind gusts through and below mountain passes is worth noting. Weak upper level ridging coinciding with the downsloping offshore winds will trend temperatures up to 5 to 10 degrees above normal.

Attention quickly turns to cut-off low out in the south-central Pacific that could bring the next shot of rain to southern California. Global ensembles diverge significantly on the resolution of this storm system, so uncertainty for this event is quite high. Keep an eye on the forecast to see how this pans out, but early indications hint that this will be a much less intense event than yesterday's atmospheric river.

Region-wide

A band of moderate to locally heavy rain moves into the region from the west starting around 12Z Fri, though exact timing is still uncertain, and moves across the area through the daytime. A few hours of MVFR to locally IFR cigs/vis are expected as the rain moves through. SCT -SHRA in the wake of the main band will create intermittent cig/vis reductions into Saturday night.

MARINE

South to southwest winds strengthen slightly again late Thursday into Friday, with gusts intermittently exceeding 20 kts. Winds then turn northwest and gradually decrease late Friday into Saturday.

Beaches

Westerly swell will arrive tonight leading to surf building 5 to 8 feet Friday-Saturday. Highest surf for west-facing beaches. A Surf Advisory is in effect now through Saturday afternoon.

HYDROLOGY

Isolated light showers remain possible through this afternoon across the San Bernardino County mountains, with expected rainfall generally insignificant, but up to 0.25" possible through midnight. More widespread showers and a more defined band of rainfall is expected to push through the region tonight into Friday. Maximum rainfall rates with this band of 0.4-0.6"/hr expected. Total additional rainfall of 1-3 inches (and locally up to 4") for the San Bernardino Mountains, 1-1.5 inches for Orange County and portions of the Inland Empire adjacent to the San Bernardino Mountains, with about 0.25-0.75" for San Diego County coasts/valleys and the remainder of the Inland Empire. Given the 4-8" (locally up to 10") that fell in the San Bernardino Mountains, resulting in widespread flash flooding and numerous instances of debris flow in the mountains and surrounding drainages, a Flood Watch is now in effect for the San Bernardino Mountains, High Desert, and portions of the northern Inland Empire adjacent to the mountains from midnight tonight through 4PM Friday afternoon.

Rainfall will trend down in intensity Friday afternoon, but additional light precipitation remains possible into Saturday morning before clearing out completely late Saturday morning. Snow levels fell below 7000 feet briefly today, but rise back to 7500- 8000 feet for most of the duration of the precipitation before falling back to near 5000 feet by Saturday morning. 2-3 inches of snow is possible near 7000ft with 8-12 inches possible near mountain peaks.

SGX WATCHES/WARNINGS/ADVISORIES

CA...High Surf Advisory until 11 AM PST Saturday for Orange County Coastal Areas-San Diego County Coastal Areas.

Flood Watch from midnight PST tonight through Friday afternoon for Apple and Lucerne Valleys-San Bernardino County Mountains-San Bernardino and Riverside County Valleys-The Inland Empire.

High Wind Warning until 3 PM PST this afternoon for Apple and Lucerne Valleys-San Bernardino County Mountains.

PZ...None.