Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Diego California
907 PM PST sun Jan 25 2015
a weak low pressure area from the southwest will bring showers to
the region on Monday...with the greatest rainfall expected to occur
over the mountains and lower deserts. Another weak low pressure
system from the south may bring more light showers late in the week.
Otherwise...expect partly to mostly cloudy skies with temperatures
close to average through next weekend.
Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
Clouds continue to stream northward around an area of low pressure
centered off the Baja California coastline near 120w. These interesting and
contrasting layers of clouds helped to bring a spectacular sunset to
the region...and should begin to leak some light rain 0n Monday. In
the meantime...after a windy and dry weekend...winds have dropped
off in many areas...but were still gusty from the east along the
foothills and surface dewpoints very low. Surface pressure gradients are
trending onshore...but at 05z were still running 9 mbs offshore from
Nevada. Looking aloft...the 00z Miramar sounding still had remarkable
50-70kt east winds above 15k feet...but weakened steadily toward the
surface with light and variable winds below 3k feet. Not
surprisingly...a deep dry layer was in place below 15k feet.
At 8 PM PST...satellite imagery showed a well-defined Rex block
along the West Coast of north.A. The closed low portion of The Block was
centered near 28n/120w and was slinging subtropical moisture
northward across northwest Mexico and into socal in the form of high
clouds. The low was drifting slowly to the west...and models show it
continuing to drift to the northwest and eventually opening as a trough
with the axis passing over socal Monday evening. The continued moist
southerly flow east of the low center will eventually overwhelm the
dry Continental air feeding over the region from the east...and
result in some light rain showers Monday/Monday night. As the trough lifts
northward Monday night/Tue...the clouds should part and the showers
end. Models are not developing much precipitation over the region
despite precipitable water values between 0.75 and one inch.
The abundance of clouds and expected precipitation on Monday will hold maximum
temperatures in check...and prop up overnight minimums a bit after nights
of strong radiational cooling. Winds may be gusty again from the
east over the mountains on Monday...but not as strong as this past
weekend. Thunderstorms are not in the forecast...but there is some
instability and good positive vorticity advection west of the mts and over the coastal waters
late Monday...so depending on subsequent model runs...thunderstorms may need to
be introduced for the evening period. Latest lightning detection
shows at least one strike well off the coast of southern Baja California this
evening ahead of an area of vorticity that is heading for the California
bight Monday evening.
Typical with blocks....they don't move much...so through midweek...
The Block remains over the west...while another low pressure area
picks up where the one earlier in the week left off. And there is
fair model agreement on the development and movement of this cut-off
late in the week. It will not have as much moisture to work with
initially...and it appears as though it will not be in a good
position to bring socal much precipitation...so for now chances for rain
260330z...sct-bkn clouds between 10000-20000 feet with no visible
restrictions through tonight...with skies becoming overcast after
14z Monday. Light to moderate rain showers/virga will begin to
impact the region after 16z Monday. Rain showers will be most
widespread over the mountains and deserts from 20z Monday to 04z
Tuesday. Dry air in the low levels will limit precipitation chances west of
the mountains...and keep ceilings in the 3000 to 6000 foot range. Ceilings
could briefly drop below these levels at psp or trm with heavier
showers Monday afternoon/evening. Showers will decrease from north
to south after 00z Tuesday. Freezing levels will hover between 9000
and 8000 feet through 06z Tuesday. Winds will continue to
weaken...but remain east to southeast over inland areas through Monday. Weak
to moderate turbulence remains possible over the mountains and
valleys through 06z Tuesday.
730 PM...no hazardous marine weather is expected through Friday
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.