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Area forecast discussion 
National Weather Service San Diego California
138 PM PDT Tuesday Apr 21 2015

Synopsis...
a couple upper level disturbances moving through the region today
through Thursday...in addition to an unstable atmosphere...will
result in the chances for showers and thunderstorms...mainly during
the afternoon and early evening hours each day over the mountain and
desert areas. Best chances and most widespread shower activity
expected on Thursday...when scattered showers and possible
thunderstorms may occur over the coast and valley areas as well. A
third disturbance moving through on Friday could bring some light
showers mainly to areas over and west of the mountains. Night and
morning stratus will persist over the coast...valley and coastal
slope areas...with only partial clearing at best during the day-time
hours. Day-time high temperatures will fall to several degrees below
normal. Onshore flow will result in continued gusty west winds in
the mountains and deserts. Increasing high pressure aloft Saturday
through Monday will bring warming...drying conditions...and a
shallow marine layer.

&&

Discussion...for extreme southwestern California
including Orange... San Diego...western Riverside and southwestern
San Bernardino counties...

Visible satellite indicates that marine layer stratus has not
cleared out of most coast/valley/coastal slope areas this afternoon.
12z Miramar sounding shows a marine layer inversion at 4000 feet.
Visible satellite also shows some towering cumulus/cumulonimbus
over the Riverside and San Bernardino County mountains...and the
high deserts...with radar showing some light showers in these areas.
Radar was a little more active in the late morning hours...with
numerous lightning strikes over the Big Bear Lake area...and some
over the Riverside Mountains and Coachella Valley. There was also a
report of pea size hail in big bear. The showers and thunderstorms
were the result of a vorticity maximum moving up from the south around
the upper low circulation. However...barely any measurable rain was
reported from these showers and thunderstorms...with only some 0.01
to 0.02 inch reports.

According to the nam12...there is a slight influx of middle-level
moisture that occurs this afternoon over western Riverside and
southwest San Bernardino counties...with surface based cape
increasing to 400-600 j/kg...lifted index of -2 to -5 degree c...moving
through these areas as the upper low sits over extreme southwest
California. Surface heating may be limited over the high deserts and
San Bernardino Mountains due to the cloudy conditions in that area
at the moment. Thus...the Riverside and San Diego County mountains
and deserts may be more favored for afternoon and evening showers
and thunderstorms...and the 19z hrrr depicts this well. Chances for
showers diminish overnight as instability from day-time surface
heating shuts off.

For Wednesday...another...deeper trough develops over the
region...with some additional middle-level moisture advection under
southwest flow. Dynamics look weaker...with not a lot of positive vorticity advection...
upper level divergence or low level convergence of winds indicated.
Instability also appears to be a little less...with surface based
cape of 100-500 j/kg and lifted index of -1 to -3 degree c.
However...with the day-time heating...some showers and thunderstorms
could develop over the mountains and portions of the deserts during
the afternoon and early evening hours. The hi-res models do show
mountain and High Desert convection being generated generally in the
same locations as today...and the Wednesday probability of precipitation reflect that.

On Thursday...the trough axis moves inland...bringing deeper
moisture than Tuesday/Wednesday...slightly colder air aloft...and a
continued unstable atmosphere. Dynamics appear to be stronger as
well...with better positive vorticity advection and upper level divergence indicated. High-
res models...particularly the cansac-WRF and local WRF...show the
most widespread showers and thunderstorms occurring on this day as
opposed to Tuesday/Wednesday. As the trough moves inland...the low-
level moist layer looks to deepen to near 7000-10000
feet...which...in addition to the dynamics...could produce scattered
showers and possible thunderstorms over the coast and valley areas.

For today through Thursday...snow levels look to be around 7000-8000
feet...with a couple inches of snow possible above that elevation.
Overall rainfall amounts will be light...except underneath any heavy
showers or thunderstorms. Since winds aloft will generally be
weak...the showers and thunderstorms will likely be slow
moving...and some localized minor flooding could occur. As far as
the marine layer GOES...stratus will continue to be very deep...with
night and morning stratus moving deep into the valleys and the
coastal slopes...and only partial clearing at best during the days.
Onshore flow at the surface will result in gusty west winds in the
mountains and deserts each day...strongest on Thursday with local
gusts to 55 miles per hour possible that day. Finally...day-time high
temperatures will fall to 5-10 degrees below normal...and locally 15
degrees below normal.

The trough moves off to the east late Thursday night...with
diminishing shower chances going into Friday morning. However...some
showers may continue to occur during the day Friday as a new short-
wave trough moves through. These showers are most likely to develop
within the low-level moist layer over and west of the mountains. A
lack of instability will likely not result in thunderstorms on this
day. Any precipitation amounts will be fairly light.

Saturday into next Monday...high pressure will gradually develop
over the region...resulting in warmer...drier weather. Onshore flow
will continue for the most part...with a shallow marine layer. Day-
time highs likely to reach above normal by Monday.

&&

Aviation...
212000z...coast/valleys...sct/bkn low clouds...with bases 4000-5000
feet mean sea level and tops to 6000 feet mean sea level...will continue throughout the
day and locally obscure higher terrain at times. Stratus will
quickly become overcast this evening from the coast to the coastal
mountain slopes...with bases from 3000-4000 feet mean sea level and tops near
5000 feet mean sea level. Low clouds Wednesday will slowly become scattered through
Wednesday afternoon.

Mountains and deserts...clouds over mountain crests and high deserts
through this evening. -Shra/-tsra currently over the high
deserts...with development possible over the mountain crests through
this afternoon. Storm bases are near 10 kft with tops to 25 kft
..and will be capable of producing gusty surface winds and small hail.
Showers will end by 02z...with residual higher debris clouds
overnight. Isolated areas of breezy west winds through mountain
passes...with gusts to 40 knots...will be possible tonight.

&&

Marine...
130 PM...no hazardous marine weather is forecast through Saturday.

&&

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

&&

Sgx watches/warnings/advisories...
California...none.
Pz...none.
&&

$$

Public...Harrison
aviation/marine...jjt

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