Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 181714

Area Forecast Discussion
National Weather Service San Diego CA
1000 AM PDT Sun Mar 18 2018

Fair weather will prevail through Monday with another cool day
today, then slightly warmer Monday. Occasional clouds will drift
over through Tuesday, then rain will likely develop around Wednesday
in Orange and southwestern San Bernardino Counties and gradually
spread south to San Diego County by Thursday. Dry weather will
likely return by the weekend some clouds with slightly below normal



Skies were partly cloudy at mid-morning with a mix of thin high
clouds and a few inland low clouds. Early morning NKX sounding
showed the freezing level around 4500 feet, and it will be another
cool day across the region with highs mostly in the lower 60s west
of the mountains with a few lower 70s in warmest lower desert
locations. Little or no marine stratus is expected tonight due to
lack of a marine inversion, though high-resolution model profiles
indicate a weak inversion setting up Monday and Monday night.
Stratus will be minimal even then, especially if high clouds develop.

The important thing this week will be the storm system around the
Wed-Thu time frame. System is currently a deep closed low around
35N/148W and will very slowly move east and attract a lot of
moisture on its south side, and already there is about 200 percent
of normal total precipitable water over a fairly large area south of
the system. 12Z GFS has slowed the pace and is more in line with
previous versions of the ECMWF/Canadian models with light precip
developing Tue night over our northern areas, mainly SW San
Bernardino County with precip rates increasing Wed. Precip likely
will not spread south into San Diego County until Thu. There will
likely be an atmospheric river with this system, and the 06Z run of
the GFS had the integrated vapor transport up to around 800 kg/m/s
over the So-Cal coastal waters, along with a rather warm atmosphere
as 850 MB temps will be in the 8-11 deg C range and 700 MB temps
will be 0-3 deg C when the best moisture is aloft. Thus, snow levels
will be very high, possibly near the highest peaks. There could be
substantial orographic precip amounts on southwest facing mountain
slopes due to the southwest flow, likely to be 20-30 knots at 850 MB
combined with the deep moisture, which could bring 4-6 inches of
rainfall on southwest facing slopes in San Bernardino County and
possibly Orange County as well. There will be some wind in the
mountains and deserts with this system, but with dynamics mostly
well to the north, winds likely will be less than with most storms.

The storm should depart to the east by Friday and end the precip.
Fair weather, except for some clouds, will prevail this weekend with
temperatures near normal.


181520Z...SCT clouds based 3000-6000 ft MSL and local mountain
obscurations though midday otherwise clear with unrestricted
visibilities through Monday morning. The risk of a BKN cloud deck is
minimal through Monday morning so the confidence in only SCT clouds
in the KSAN TAF is high.


No hazardous marine weather is expected through Tuesday. A storm
will bring moderate to heavy rain Wednesday and Thursday.


Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.





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