Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Diego California
845 PM PDT Wednesday Aug 20 2014
an upper level low pressure system will slowly move east over
extreme Southern California Thursday. There is a very small chance
of an isolated shower or thunderstorm tonight and early Thursday
over most areas...then the chance of thunderstorms will be primarily
over the mountains and deserts by Thursday afternoon. Temperatures
will continue to be below normal Thursday. After the low exits to
the east Thursday night...there will be dry and warmer weather
Friday followed by slight cooling over the weekend. High pressure
aloft will build next week and bring a warming trend.
Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
Thunderstorms brought some decent rainfall to mainly the inland
Empire and a few adjacent areas this afternoon into early
evening...including 0.90 inches at oat flats...southeast of
Temecula...and over 0.40 inches at several spots around Riverside
and San Bernardino. Skies were mostly clear this evening...with
patchy stratus over the coastal waters. Stratus will be rather
sparse this evening as the marine inversion has weakened to around
3-4 degree c based on Lindbergh Field/John Wayne Airport acars
soundings. Thunderstorm chances look rather sparse overnight as the
dynamic forcing is weak...but with the upper low just to the west
and some elevated instability and precipitable water values around
1.2 inches at the coast and in the deserts...an isolated storm is
not out of the question. I have removed the slight chances for the
coastal waters and Orange County though where instability/moisture
will be lowest.
The upper low will start to move east along the California/Baja California
California border with good moisture in the 750-550 mb layers and
little above that for relatively few high clouds. That should result
in scattered thunderstorms in the mountains and deserts. The dry
layer above 550 mb could help with hail generation...and with
lighter winds...there will be some risk of flash flooding due to
slower movement of the storms. Since precipitable water will mostly
stay below 1.3 inches...any flooding will tend to be more isolated.
We have coordinated with Phoenix and Las Vegas...and our offices
will make decisions for any flash flood watches for Thursday in our
forecast areas on the midnight shift.
After the low moves east Thursday night...we get into dry west
flow...and temperatures aloft increase...which will be reflected at the
surface in most areas. However...mean troughing in the west will
keep the warming trend from extending too far...and Saturday/Sunday
could actually be a little cooler. There will be some redevelopment
of the marine layer as well due to the onshore flow Friday through
the weekend...with local WRF showing a stratus in coastal and some
valleys areas in the nights and mornings. Models are coming into
somewhat better agreement for ridging next week and the monsoonal
moisture staying south and east of our area. Temperatures should mostly be
near normal...but if the upper high becomes centered over California
as some ensemble solutions are showing...temperatures will be somewhat
above normal by mid-week.
210300z...low clouds will move ashore and into coastal airports
between 04-08z tonight with medium confidence they will reach kont
by 12z or so. Bases will be around 1800 feet mean sea level. Scatter out
Thursday will take place by 16-17z. Scattered debris clouds above
10000 feet mean sea level will continue through Thursday. Good confidence that
some thunderstorms will erupt along mountain crests after 18z
Thursday. Bases will be around 8000 feet mean sea level with tops around 35000
feet. When/where thunderstorms occur...variable winds to 25 knots can be
expected. Storms will diminish by sunset Thursday.
800 PM...no marine weather hazards are expected through Monday.
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.