Swell Matrix

Area Forecast Discussion

National Weather Service San Diego CA

923 pm PDT Mon Mar 19 2018


Dry weather will continue through much of Tuesday. Light showers will be possible as early as Tuesday afternoon as a strong atmospheric river begins to take shape along the coast. Shower activity should then slowly increase into Wednesday, especially north of San Diego County. The main rain event is anticipated Wednesday night into Thursday, with flash flooding and debris flows near recent burn scars possible. Snow will not be a factor with this storm, with rain likely impacting even area ski resorts through Thursday. Quieter weather is expected over the weekend.


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

From previous discussion, A ridge of high pressure will dominate SOCAL weather through Tuesday morning, though high clouds will be increasing throughout the day. Afternoon highs will be near seasonal averages Tuesday.

The atmospheric river (AR) that will impact California late Tuesday through Friday continues to evolve over the east central Pacific. An initial push of sub-topical moisture is expected Tuesday, with and ahead of a warm frontal passage. This may result in some light shower activity over the region (particularly north and west of San Diego County), along with more persistent cloud cover. Light showers should remain possible through Tuesday night, with areas of orographic enhancement along the coastal slopes.

Some uncertainty remains for Wednesday, with model differences of a couple hundred miles in the north-south placement of the deepest moisture (AR). The area with the highest probability of excessive rainfall looks to be focused on a region near Point Conception (see WPC Excessive Rainfall Outlook). However slight changes could shift this region closer to L.A. Or Orange Counties during this period. In any case rain remains in the forecast for this period with rain chances and accumulations decreasing rapidly from north to south. Snow is not expected, with the sub-tropical influences on this system pushing snow levels well above resort level (Snow levels 9,000 ft or higher).

The main period of concern for flash flooding and/or debris flows near recent burn scars is expected for Orange, San Bernardino, Riverside, and San Diego Counties during the Wednesday Night- Thursday time period. This is when the low off the coast opens up and is absorbed into the broader westerly flow, shifting the core of the strong AR south and east. It will also drag a cold front through the region. Though timing differences still exist in the guidance, forecaster confidence is high that a 6-12 hour period of moderate-heavy rain will occur. The depth of the saturated layer (up to 200 mb!) will result in a heavy rain threat that could extend into the deserts (especially the upper deserts). Highest totals should be found along the coastal slopes, where 850 mb upslope flow may approach 35-45 knots at times. Those with activities that are flood sensitive should be focused on this period! A detailed rain forecast can be found in the Hydrology section below. Gusty winds are expected along the desert slopes, but they should occur in areas that generally see minimal impact. Snow will remain a non factor with snow levels at or above 9,000 ft through Thursday.

Light rainfall may linger into Friday with the northwesterly flow behind the cold front. Snow levels will finally begin to dip during this period, though rapid decreases in available moisture will limit any snow accumulations to just a few inches.

The weekend should see below average temperatures as troughing lingers along the West Coast. A period of light showers can't be ruled out for Sunday.

Rain Forecast

A low pressure system and associated with a strong atmospheric river will impact California Tuesday through Friday. The greatest rainfall and impacts are anticipated in SoCal Wednesday night into Thursday, though light showers are possible as early as Tuesday. Uncertainty is higher for Orange and San Bernardino Counties, where a slight southward shift in storm track could result in an earlier onset of moderate to heavy rainfall. The following provides details on expected storm total rainfall and rainfall rates. In general rain totals should decrease from north to south.

Forecast Rain Totals:

Orange County: 1-3 inches Inland Empire: 1-4 inches Coastal Slopes: 3-6 inches with local amounts to 8 inches San Diego County (coast & valleys): 0.5-2 inches Upper Deserts: 0.75-2 inches Coachella Valley: 0.25-1.00 inches San Diego County Desert: 0.25-0.75 inches

Hourly Rainfall Rates:

Tuesday - Wednesday Afternoon 0.25 inches or less locally higher along the coastal slopes.

Wednesday Night - Thursday Rates near 0.50 inches per hour probable, with the potential for rates as high as 0.75-1.00 per hour for brief periods.

Snow is not expected to be an issue, with snow levels remaining at or above 9,000 ft for most of the storm.


200345z, SCT-BKN clouds above 12000 ft MSL will continue through Tuesday morning with unrestricted vis. Clouds will thicken with bases lowering to 5000-8000 ft MSL Tuesday afternoon. -SHRA and some mountain obscurations possible in Orange County and southwestern San Bernardino County after 20Z.


No hazardous marine weather is forecast through Thursday. A storm system will likely bring rain at times Wednesday and through early Friday, while higher winds and swell are likely Friday and Saturday, possibly reaching small craft advisory criteria.

Watches, Warnings, Advisories

Ca, Flash Flood Watch from Wednesday evening through late Thursday night for Orange County Coastal Areas-Orange County Inland Areas-San Bernardino County Mountains-San Bernardino and Riverside County Valleys-The Inland Empire-Santa Ana Mountains and Foothills.


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