Weather Service Report

FXUS66 KSGX 200423

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.



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.


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.


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.


Skywarn activation will not be needed today.


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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