A low pressure system will impact the region through Monday, bringing moderate rainfall, high elevation snow and a slight chance of thunderstorms west of the mountains. This low pressure system may linger over the area into Monday, bringing additional light showers. Drier and slightly warmer weather is expected by the middle and end of next week.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
Light and intermittent rain shower activity pick up tonight as a large upper level low moves into Northern California. Heights continue to fall both today and tomorrow with 850mb temps dropping to around 5C by Sunday, ushering in surface high temps in the mid-60s for most locations, and 30s-40s for the mountains. Ample mid and low level moisture will translate to light to moderate rain showers early Sunday morning as a cold front pushes through, with rain rates around 0.2-0.4"/hr, locally higher for the mountains. The front looks to arrive around sunrise on Sunday for Orange and San Bernardino counties, moving southward throughout the day with the heaviest rain expected late morning into early afternoon. While the threat is low, there remains a chance for thunderstorms associated with the frontal passage on Sunday, primarily late Sunday morning into early afternoon. Should storms develop, brief but heavy rainfall with rain rates exceeding 0.50"/hr will be possible. Snow levels will be falling throughout the weekend, from around 6500 ft today closer to 5000-5500 ft Sunday afternoon. Elevations above 6000 ft may see light accumulating snow on Sunday. The coldest point of the system will be occurring as the rain tappers off, so snow accumulations will likely be 1" or less for populous areas. Rain chances decrease on the backside of the front by early Monday, although a shortwave embedded in the deeper low will quickly move through Monday afternoon before the system departs. With this shortwave, additional rain/snow in San Diego County and up along the spine of the mountains to the San Bernardino Mountains will be possible Monday afternoon and evening, but the lower levels will be quickly drying out and little in the way of precipitation is expected.
Ultimately, rain totals will be low with this event, likely at or below 0.50" for the coastal basin and inland areas, less than 0.25" for the immediate coast, and around 0.50-1.00" for the mountains. Higher elevations will perform better where upslope flow may assist in thunderstorm development on Sunday, allowing for spots of higher rain amounts.
Winds,
Increasing winds tonight with the tightening gradient as the low approaches, providing sustained surface winds 35+ mph and gusts as high as 55 mph through mountain passes and on the desert slopes of the mountains. Closer to mountain peaks, even stronger winds may make for dangerous conditions for low-flying aircraft and an increase in turbulence. While models project the strongest winds with this system to begin after 1 PM today and last through midnight/2 AM Sunday, the push of this cold front may maintain stronger wind gusts into Sunday afternoon, with no significant drop off between 2 AM and sunrise. Along with the gusty winds, blowing dust through the High Desert and Coachella Valley will be a threat today through Monday, before winds settle a bit on Tuesday.
Beyond Monday,
Increasing heights and drier air will settle in by Tuesday, with northwest flow and lighter winds. Weak upper level ridging attempts to build in and temperatures should rebound closer to near normal for mid April by Wednesday. A late week pattern change could potentially bring additional rainfall, although there are equal chances for an inside slider to bring about strong offshore winds. Currently, the forecast remains dry into next weekend with high temperatures increasing each day, but should this system be able to swing further south than currently projected, another rainy weekend may be in store. For now, plan on dry and warm with highs returning to the 80s for many inland areas, and gustier offshore winds.
120400z, Coasts/Valleys, Increasing clouds based 3-6 kft MSL to this evening throughout the coastal basin. ISO -SHRA develop after 08z becoming more numerous overnight into early Sun. Mostly VFR vis/cigs overnight until main frontal band pushes into the region from the northwest, reaching Orange County around 16-18z. This band is expected to bring a few hours of SHRA/RA and periods of MVFR vis/cigs (1-3 kft bases, 4-6SM) with the occasional +SHRA/+RA, yielding brief IFR cigs/vis. The front reaches SD Co. around 19z, sweeping through the county during the afternoon and yielding similar conditions. Behind the front, winds switch from SW to W.
Forecast window for RA/+RA and IFR cigs/vis:
KSNA, 17-20z Sun KONT, 17-21z Sun KSBD, 17-22z Sun KCRQ, 20-23z Sun KSAN, 20-24z Sun
.Mountains/Deserts, West to southwest winds gusts 35-50 kt to persist throughout the TAF period in mountain passes, along deserts slopes and locally into the deserts. Local areas BLDU (vis 3-5SM) expected. Moderate up/downdrafts in lee of mtns. VIS down to 0-5SM in mtns with FG where clouds and terrain meet; SNSH to reduce vis locally above 6500 ft MSL.
There is a slight (~10%) chance of thunderstorms Sunday afternoon. Additionally there will be periods of increased northwest winds with gusts 15 to 20 kt and choppy seas through the day Sunday.
Ca, None. PZ, None.
