Cool and dry tonight then isolated light showers developing west of the mountains Thursday morning. The next low pressure system from the northwest will bring more widespread showers late Thursday afternoon into Friday. With snow levels lowering to around 6000 feet late Thursday night, there could be moderate to heavy snowfall in the mountains above 6500 feet. Some showers could linger into Saturday. Drier and warmer next week with high temperatures warming to within a few degrees of average for Tuesday and Wednesday.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
Evening update, Water vapor imagery shows the center of the upper level low has finally moved east into Arizona with drier air in place over So Cal. As a result, showers have finally come to an end with some clouds lingering across the foothills. With enough clearing tonight, pockets of shallow fog could develop in the valleys. Otherwise cool with lows in the 40s in the valleys and low deserts, mid to upper 30s in the high desert, and low 20s to mid 30s in the mountains.
The next upper level trough is off the Pac NW coast and digs south on Thursday with a closed low developing in base of the trough. Isolated light showers are forecast to develop ahead of this system Thursday morning as moisture and onshore flow increase. Any accumulation with these morning showers will be limited to a few hundredths of an inch.
From previous discussion, The upper level closed low, similar to the last system, will situate right off the coast of Southern California Thursday/Friday before moving inland Saturday. There will be less moisture with this upcoming low compared to the system this past weekend, but we are still looking at widespread rainfall for the end of the week. Showers will begin as early as Thursday morning, becoming widespread and heavier Thursday night and Friday morning, with an associated thunderstorm threat during this time period. Rain may continue for much of the day Friday as the low moves into the CA bight, with increasing winds over the marine waters and the higher elevations. This system looks to only produce moderate winds with limited gradient tightening and relatively weak mid- level flow. Still, gusts in the deserts, mountains, and coastal waters could reach up to 30 mph at times on Friday. The thunderstorm threat lessens Friday afternoon, with only light to moderate rain showers prevailing into Saturday.
The snow level for the beginning of this event looks to hover right around 6000 ft, raising a bit to 6500 ft Thursday night into early Friday morning coinciding with the timing of the heaviest precipitation. With snow levels rising, elevations between 5500-6500 ft will see a mixture of snow/rain, transitioning to more rain by Friday morning. Periods of rain with rates greater 0.50"/hr will fall on already highly saturated soils, which may cause an increased threat of runoff and minor flows on and around recent burn scars. Always monitor the forecast and listen to local emergency officials. Above the snow level, snowfall rates could reach 1"+/hr, primarily in the early Friday morning time frame. Snowfall totals Thursday through Saturday of 2-6 inches for elevations above 6500 ft and around 6-12 inches above 7000 ft, with up to 16 inches for the highest peaks. A Winter Storm Watch is in effect for elevations above 6500 ft for this period, and heavy snow in the early morning hours of Friday may make travel hazardous. Avoid travel during this time if possible, especially along Hwy 38 near Big Bear. If you must travel, ensure you have emergency supplies in your car and have a way to contact emergency services in low cell coverage areas.
This system will follow a similar path to the closed low exiting today, moving into Arizona Saturday into early Sunday. As this system moves across, wrap around moisture will result in lingering light rain and snow showers across the region, best chances in San Diego and Riverside counties. High uncertainty remains on timing and accumulations Saturday due to the model spread in the movement of the low, fairly unusual to have such limited agreement 72 hours out. Snow levels will remain around 6500 ft on Saturday, rising closer to 7000 ft by early Sunday. As the last of the precipitation moves out, a slow warming and drying trend will settle in for the remainder of the forecast period. Highs may return to near normal by Tuesday/Wednesday as weak ridging develops overhead, and dry weather prevails after Sunday.
200430z, FEW clouds remain region-wise, mainly between 2-7 kft MSL. Increasing clouds around 2500-4000 feet MSL throughout the coastal basin after 08Z Thu with ISO -SHRA 09-12Z for San Diego/Orange County coasts and valleys ahead of the next storm system.
CIGS mostly remain above 3 kft through 23z, then lower to 2-3 kft ahead of the frontal passage along with SCT -SHRA and a surge of southerly winds. The frontal passage arrives in northern Orange County around 01z, sweeping through the region from northwest to southeast, reaching KCRQ and KSAN around 04z Fri. Expect 3-5 hours of RA/+RA and briefly strong south, then west winds gusting to 20-25 kts during frontal passage. VIS locally 0-3SM and bases briefly below 1000 ft MSL during heaviest periods of rain, though generally 4-6SM with bases 2-4 kft MSL in -RA/RA.
Mountains shrouded in FG; snow showers above 6,500 feet after 23z, reducing VIS 0-5SM.
Southerly winds will increase late Thursday ahead of the next storm system. Moderate confidence in winds turning westerly and strengthening by early Friday as the front passes, but exact track of the low pressure system may create large differences in both wind speed and direction Thursday evening through Friday. Gusty winds and choppy seas may create hazardous conditions for small craft. See Small Craft Advisory for details. There is also a slight chance of thunderstorms late Thursday through late Friday. Any thunderstorm could bring lightning, gusty winds, choppy seas, and a waterspout.
Ca, Winter Storm Watch from Thursday morning through Saturday morning for Riverside County Mountains-San Bernardino County Mountains.
PZ, Small Craft Advisory from 4 PM Thursday to 1 AM PST Saturday 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.
