Swell Matrix

Area Forecast Discussion

National Weather Service San Diego CA

258 am PST Tue Feb 19 2019

Synopsis

Dry weather with well below average temperatures will continue into Wednesday morning with cold nights and areas of frost in the valleys and lower deserts. For Wednesday afternoon into Friday, another cold low pressure system from the north will bring showers with low snow levels and with strong and gusty west to northwest winds in the mountains and deserts. Heavy snowfall is possible in the higher mountains. Dry and a little warmer for the weekend. Small chances for light precipitation could return for Sunday night and Monday.

Discussion

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

Short Term

(today through Thursday), The low pressure system that brought showers on Sunday into Sunday night and again Monday afternoon is over Arizona and moving eastward. Skies are mostly clear across southern California with weak northerly offshore flow. The rest of tonight into early this morning is expected to be the coldest morning for many locations with low temperatures in much of the valleys in the upper 20s to mid 30s. With the offshore flow, gusty north to northeast winds will provide some mixing in portions of the Inland Empire and near the coastal foothills of the Santa Ana Mountains. When and where mixing occurs in the Inland Empire, temperatures will rise to around 40 degrees. Tonight is expected to be the coldest night for the lower deserts with a frost advisory extended to portions of the lower deserts for tonight into Wednesday morning. Well below average high temperatures will continue today.

Another cold low pressure system from the north will bring more showers with low snow levels for Wednesday afternoon into Friday along with a slight chance for thunderstorms for Thursday afternoon and early evening. The slight chance of thunderstorms has been added to the forecast for Thursday afternoon and early evening as very cold air aloft will help to destabilize the atmosphere with 500 mb temperatures colder than -30 C.

In Orange and southwestern San Bernardino Counties, rainfall could range from one tenth to one quarter of an inch near the coast, to one half to one inch in the mountains. For San Diego and western Riverside Counties, rainfall could range from one third to one half inch near the coast to 1.00 to 1.50 inches in the mountains with local amounts to around 2 inches. Amounts in the deserts will mostly be less than one tenth inch.

Snow levels will rise to around 4000 feet for Wednesday afternoon and evening, then fall to 2500 to 3000 feet for Thursday, and 2000 to 2500 feet for Thursday night into Friday with local snow levels around 1500 feet. Winter weather advisories and warnings may be needed for the mountains for Wednesday night into Thursday night and possibly also for the high desert and portions of the Inland Empire.

Long Term

(Thursday night through Monday), Showers will continue into Thursday, then decrease from the north overnight into early Friday morning. There are small chances of showers continuing into Friday afternoon with cold air aloft and residual moisture. Saturday into Sunday is expected to be dry and a little warmer.

For Sunday and Monday, there has been a slight westward shift with the next low pressure system moving southward into the western states. This might allow for some entrainment of Pacific moisture with small chances for light precipitation returning for Sunday night and Monday.

Aviation

191000z, Mostly clear skies today and tonight. Local offshore winds northeast gusting 25-35 kt in mountain foothills through 18Z today.

Marine

Onshore winds and seas will increase Wednesday, leading to hazardous conditions as a storm system arrives. Strong winds and high seas will continue into Thursday night before diminishing. There is also a slight chance of thunderstorms Thursday afternoon.

Watches, Warnings, Advisories

Ca, Frost Advisory until 9 AM PST this morning for San Bernardino and Riverside County Valleys-The Inland Empire-San Diego County Valleys.

Frost Advisory from 10 PM this evening to 9 AM PST Wednesday for San Bernardino and Riverside County Valleys-The Inland Empire-San Diego County Valleys.

Frost Advisory from midnight tonight to 9 AM PST Wednesday for Coachella Valley-San Diego County Deserts.

PZ, NONE.

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