Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 191727

Area Forecast Discussion
National Weather Service San Diego CA
1027 AM PDT Mon Mar 19 2018

Dry weather will continue through the day today. Light showers
possible as early as Tuesday 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 flow near recent burn
scars possible. Snow will not be a factor with this storm, with
rain likely impacting even the areas ski resorts through Thursday.
Quieter weather is expected over the weekend.



At 1000 AM...The weather was beautiful and the forecaster was
focused on the upcoming storm system that will affect the region
late Tuesday through Friday. Currently the region was dominated by
ridging, with just a few high clouds passing overhead. Though the
storm system that will affect the region later in the week was
evident/churning about 1,200 miles west of San Francisco on
satellite imagery. Temperatures were warming quickly and winds
were light.

Ridging will dominate for the next 36 hours, though increasing
high clouds will be a harbinger of things to come. Afternoon highs
will be near seasonal averages for all today and Tuesday.

The atmospheric river (AR) that will impact California Tuesday
through Friday was taking shape 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 light upslope favoring the coastal slopes.

Some uncertainty begins to creep into the forecast 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 nature of this system pushing snow
levels well above resort level (Snow levels 9,000 ft or higher).

The main period over 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 east west flow, shifting
the core of the strong AR south and east. It will also drag a
cold front through the region. Though timing differences of around
12 hours 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.


191515Z...SCT-BKN clouds above 12000 ft MSL will continue through
Tuesday morning with unrestricted VIS.


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


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
provided 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: 4-8 inches with isolated amounts to 10 inches
San Diego County (coast & valleys): 1-3 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.




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