textproduct: San Diego

SYNOPSIS

Widespread heavy rain is expected Wednesday across Southern California. Scattered locally heavy showers are expected to continue across much of the area Wednesday night into Christmas day. Snow levels will remain high through the duration of the event increasing the flood threat for the mountains, particularly in San Bernardino County. Gusty southerly winds are expected for the coast, valleys, San Bernardino mountains, and High Desert. Another round of more widespread moderate to heavy rain is expected sometime on Friday. Drier conditions are looking more likely for Saturday and Sunday.

DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE

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

Key Points:

* Widespread moderate to heavy rainfall Christmas Eve with the potential for roadway flooding and localized flash flooding from the coast to deserts. Rapid rises in small streams and increased flow in main stem rivers expected.

* The highest risk for flash flooding on Christmas Eve is for the San Bernardino County mountains with a high risk for excessive rainfall that could result in damaging and potentially life- threatening flash flooding, mud slides, and debris flow. The threat will be highest during the day on Wednesday.

* Gusty south winds will develop tonight and continue through Wednesday evening with gusts 30 to 40 mph, locally up to 45 mph near the coast and to 55 to 75 mph along and below the desert slopes of the San Bernardino County mountains. Make sure to secure outdoor furniture. Tree damage is possible.

* Scattered showers will continue on Christmas Day with another round of more widespread showers on Friday. Snow levels will lower to around 7000 feet for Thursday into Thursday evening, then lower to around 6000-6500 feet by late Friday creating slick conditions on mountain highways.

* There is the potential for another storm system around New Years, but confidence is extremely low with regards to timing.

Evening update...

A weak short wave is beginning to lift north across the region this evening, producing scattered light showers across San Diego County northeastward into the Coachella Valley. Rainfall rates with these showers are expected to remain below 0.10"/hr tonight with no impacts expected. More moderate and steady rainfall will begin across the San Gabriel Mountains late tonight into early Wednesday morning as orographic lift strengthens and deeper moisture begins to move in from the west. Main impact tonight will be gusty south winds strengthen across the San Bernardino County Mountains and adjacent desert foothills with gusts increasing to around 50-60 mph, locally higher where any mountain waves surface just north of the mountains, potentially including the Lucerne Valley and Phelan areas. Only thing to note this evening is that the timing of heaviest precipitation on Wednesday has shifted a couple hours later, beginning around 8 AM near the LA County border and spreading southeast through the afternoon and evening, beginning to taper off after about 8 PM. Timing for the next band of heavier precipitation may be slightly delayed as well, potentially holding off until Friday morning.

Previous discussion issued 151 PM Tuesday...

An Atmospheric River is still on track to impact Southern California on Wednesday. The heaviest and most widespread rain is expected through the day Wednesday, with the heavy rain moving northwest to southeast from Orange County to San Diego County deserts. After the rain band passes, scattered showers are expected to continue with locally heavy rain into Thursday (Christmas Day). Latest guidance shows a significant decrease in IVT (atmospheric moisture) during the day Thursday, bringing the potential for periods no precipitation for some areas. Snow levels are expected to remain quite high through the duration of the heaviest precipitation, but are expected to lower later in the week. Please see the Hydrology section below for more information on expected rainfall rates, totals, and snow level information.

In addition to periods of heavy rain, gusty south winds are expected. Strongest winds expected during the day Wednesday. Peak wind gusts of 60-70 mph, locally up to 75-80 mph possible on the desert slopes of the San Bernardino County mountains with gusts 45- 60 mph into the High Desert. Winds will stay elevated in San Bernardino county mountains and High Desert into Thursday. For the coasts and valleys gust of 30-45 mph are expected, with the strongest winds expected with the main band of rain.

Models are coming into better agreement that the next round of widespread moderate to heavy rain. Guidance is indicating a more organized band of rain will move across the area sometime late Thursday into Friday, with some lingering scattered showers through the day Friday eventually tapering off some time Saturday. Snow levels remain near 7000 ft Thursday, falling to 6000-6500 ft Friday evening. In addition to rain and mountain snow, winds are expected to increase over the mountains and into the deserts, with timing of the strongest winds shaping up to be sometime Thursday night into Friday. Overall, rainfall rates and amounts are starting to shape up to look less than what is expected for Wednesday, but additional rainfall on already saturated soils may result in additional areas of flooding.

AVIATION

230430Z...Low clouds with bases 1500-5000ft MSL are expected to push in along the coast with inland overnight, with some areas getting as low as 300-700ft MSL after 12-14Z Wednesday. Rain chances will increase overnight. Vis reductions 1-4 SM are expected as rain chances increase, especially between 21Z Thur and 06Z Fri, when the heaviest band of precipitation is anticipated to move through. The main rain band will be moving in from the northwest to the southeast, and with moderate to heavy precipitation with vis reductions 1-3 SM along with strong south to SE winds during this time. Rain showers will become more intermittent after 06Z Fri, and throughout the rest of the forecast period. Embedded thunderstorms will also be possible, with vis restrictions of less than 1SM, and occasional cloud-to-ground lightning, if this occurs. Small hail and gusty winds will also be possible with some of these storms, as well as an isolated waterspout or two as they move in towards the coastline.

MARINE

The approaching storm system has increased winds out of the south to southeast and will build seas this afternoon. Winds initially around 15-20 kts gusting to 25 kts will increase early Wednesday to gale force, with wind gusts upwards of 35 kts. This will bring steep combined seas of 6-9 feet. Additionally, periods of moderate to heavy rain Wednesday will lead to areas of poor visibility. Winds diminish slightly late Wednesday through early Thursday, but winds and seas will remain hazardous to small craft. Another round of stronger southwest winds expected late Thursday into early Friday, with gusts around 25 kts. Winds weaken and turn northwesterly Friday. Thunderstorms will be possible out over the open waters, especially for tomorrow between 21Z Thur and 06Z Fri. Some of these may contain small hail, gusty winds, and even a waterspout or two will be possible.

BEACHES

Strengthening south winds tonight into Wednesday will lead to elevated surf 5 to 8 feet with sets to 9 feet, highest for south- facing beaches. This will lead to hazardous swimming conditions, as well as possibly minor tidal overflow for south-facing beaches from a combination of winds and elevated surf through Wednesday, especially during high tides. Winds diminish along with surf late Wednesday, but more breezy winds along with large fresh swell will lead to surf building again Friday-Saturday. A Surf Advisory is in effect from tonight to Saturday afternoon. Thunderstorms will be possible, especially for tomorrow between 21Z Thur and 06Z Fri. Some of these may contain small hail, gusty winds, and even a waterspout or two will be possible for some of these storms just offshore.

HYDROLOGY

Isolated light showers will redevelop overnight, with locally moderate and steady rainfall along the coastal slopes of the San Gabriel Mountains. Rain is expected to become heavy at times Wednesday morning along the south slopes of the San Bernardino County mountains with heavy rainfall spreading southward across the area Wednesday morning through Wednesday evening. Showers will continue on Thursday with another round of more widespread showers for Thursday night into Friday. The snow level will rise to above 9000 feet for early Wednesday morning, fall to around 7000 feet on Thursday and Thursday night, then fall to around 5000 feet for early Saturday morning.

Through Wednesday night, for Orange and southwestern San Bernardino Counties, rainfall could range from 2.5 to 3 inches near the coast to 6 to 8 inches on the coastal slopes of the San Bernardino County mountains with isolated totals to around 12 inches on the coastal slopes to the west of the Cajon Pass. Rainfall will decrease from north to south with rainfall across far southern San Diego County around 1 inch near the coast to 2 inches in the mountains with local amounts near 4" near Palomar. For the high desert, 1.5 inch to 3.5 inches near the foothills of the San Gabriel Mountains is expected with 0.50 to 1.5 inch for the lower deserts.

The San Bernardino County mountains are part of a high risk area for excessive rainfall for Wednesday morning through Wednesday night, areas with a heightened risk for damaging and potentially life-threatening flash flooding. Hourly rainfall rates of around 0.50 inch are likely for several hours on the coastal slopes of the eastern San Gabriel Mountains from before dawn on Wednesday through Wednesday afternoon, locally to around 1 inch per hour or more from thunderstorms or where stronger orographic precipitation occurs.

Additional rainfall during the day on Thursday from showers will range from around 0.50 inch north to 0.15 inch south, locally to around 0.50 inch on the coastal slopes of the San Bernardino County mountains.

The Santa Margarita River at Ysidora is currently expected to reach Action/Monitor stage, while the latest forecast for the San Diego River at Fashion Valley now forecast to remain below Monitor Stage. See the RVSSGX for details. If heavier rain develops over either of those two river basins, the rivers may exceed their currently forecast peaks.

For Thursday night and Friday, a weaker atmospheric river will bring another round of more widespread showers with a slight chance of thunderstorms. For Orange and southwestern San Bernardino Counties, rainfall could range from around 0.75 inch near the coast to 1 to 2 inches on the coastal slopes of the San Bernardino County mountains with isolated totals to around 3 inches on the coastal slopes to the west of the Cajon Pass. Rainfall will decrease from north to south with rainfall across far southern San Diego County around 0.25 to 0.50 inch. For the high desert, 0.25 to 0.50 inch is expected with less than 0.25 inch for the lower deserts.

SKYWARN

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

SGX WATCHES/WARNINGS/ADVISORIES

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

Wind Advisory from 3 AM to 10 PM PST Wednesday for Orange County Coastal Areas-Orange County Inland Areas-San Bernardino and Riverside County Valleys-The Inland Empire-San Diego County Coastal Areas-San Diego County Valleys-Santa Ana Mountains and Foothills.

Flood Watch from 4 AM PST Wednesday through late Wednesday night for San Diego County Coastal Areas-San Diego County Deserts- San Diego County Mountains-San Diego County Valleys.

Flood Watch through Wednesday evening for Apple and Lucerne Valleys-Coachella Valley-Orange County Coastal Areas-Orange County Inland Areas-Riverside County Mountains-San Bernardino County Mountains-San Bernardino and Riverside County Valleys-The Inland Empire-San Gorgonio Pass near Banning-Santa Ana Mountains and Foothills.

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

PZ...Gale Warning until 4 PM PST Wednesday 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.