Swell Matrix

Area Forecast Discussion

National Weather Service San Diego CA

854 pm PST Fri Jan 27 2023

Synopsis

Onshore flow will increase on Saturday, with noticeable cooling, increasing humidity, and some marine clouds to coastal areas. The marine layer and associated clouds will deepen further this weekend, with some light rain breaking out on Sunday. The rain will become more widespread on Monday, and as snow levels fall, significant snow will accumulate over the mountains. The showers are expected to wind down on Tuesday, followed by dry and warmer weather for the latter half of next week.

Discussion

For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,

Skies are clear over Southern California this evening, but low clouds were expanding off the Central Coast and could make it here by Saturday morning. Dewpoints have already recovered with the onshore flow this afternoon. The sfc pressure gradients have weakened and wind reports were light at 8 PM PST. It will be much cooler, especially in the valleys on Saturday, then all areas on Sunday with more clouds, and some light precipitation breaking out. No forecast updates are planned, but a Hydrology section has been added below to highlight uncertainty regarding the coming event.

From previous discussion,

A low pressure system will move over Southern California Sunday and Monday, bringing a blast of cold air, gusty winds, and precipitation to the area. Scattered showers and winds will begin on Sunday. Westerly winds will peak at 50 to 60 mph Sunday into early Monday over the mountains and into portions of the deserts. Snow levels on Sunday will be 4500 to 5000 feet, dropping to closer to 4500 feet Sunday afternoon, falling even farther to 3000 to 3500 feet by Monday morning in northern areas and afternoon/evening in southern areas. Impactful snowfall is expected in the mountains with 6-12 inches likely above 5000 feet. An inch or two of snow is even possible along I-15 and I-8 through the mountain passes and in portions of the High Desert. Major travel impacts are expected in the mountains.

West of the mountains, rounds of scattered showers are possible as early as Sunday morning. Rainfall totals reaching 0.20 to 0.50 inches expected for San Diego County coast and valleys and Inland Empire. Totals closer to 0.15 to 0.25 inches expected in Orange county. Highest totals expected in the eastern valleys of San Diego County. Under a tenth of an inch of rain is expected for desert locations. This system is not tapping into tropical moisture, like the storms we saw earlier this month so rainfall totals are expected to be less than what we have been seeing.

The system will move to our east on Tuesday, but cold air will remain in place. A few showers could linger over San Diego County mountains Tuesday but most of the region will be dry. A cold morning is expected on Wednesday, with the potential for widespread frost in the inland valleys. An upper ridge looks to begin to move in from the west Wednesday through late next week, leading to dry and warmer conditions, with periods of offshore winds.

Aviation

280430z, Clear skies will prevail through this evening with unrestricted VIS. Areas of stratus will develop near the immediate coast after 08Z with bases most likely between 1500 and 2000 ft MSL. Stratus spreading up to 15 mi inland by 14Z then clearing back to the coast 18-20Z. Local vis 3-5 miles will be possible over higher coastal terrain 10Z-18Z Sat. Widespread low clouds with bases near 1500-2000 ft MSL and tops to 2500 ft MSL spreading back inland by 29/00Z. Tops rising to over 40000 ft MSL Saturday night with coastal mountain slopes becoming obscured. ISO DZ/-SHRA beginning near the coast as early as 07Z Sunday.

Marine

No hazardous marine weather conditions are expected through next Wednesday, except for some gusts from the west over 20 knots possible Sunday afternoon and evening over the coastal waters.

Watches, Warnings, Advisories

Ca, Winter Storm Watch from Sunday morning through late Monday night for San Bernardino County Mountains.

Winter Storm Watch from Sunday afternoon through late Monday night for Riverside County Mountains.

Winter Storm Watch from Sunday afternoon through Tuesday morning for San Diego County Mountains.

Wind Advisory from 10 AM Sunday to midnight PST Sunday night for Apple and Lucerne Valleys.

Wind Advisory from 1 PM Sunday to 7 AM PST Monday for Coachella Valley-San Diego County Deserts-San Gorgonio Pass Near Banning.

PZ, None.

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