Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Diego California
900 PM PST Wednesday Jan 28 2015
a warm trough moving in from the southwest will bring cloudy skies
and areas of light rain Thursday afternoon through Saturday morning.
Snow levels will be quite high...between 6500 and 7000 feet. Light
rain will end Saturday with building high pressure bringing mostly
sunny skies Sunday and Monday.
Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
Middle and high clouds are streaming northward into so-cal tonight
ahead of a weak trough west of central Baja California. This trough is tapping
into subtropical moisture that extends almost all the way to the
Equator. Even with this moisture tap...the atmosphere is still quite
dry with the 00z sounding showing a precipitable water of only 0.67". Most that
moisture is above 15000 feet. The low layers are still fairly dry
from the offshore flow of the past few days. Large temperature-dpt spreads
at the coast of 10-15 degrees along with weak offshore gradients
tonight will keep any patchy marine layer clouds confined to the
coastal waters Thursday morning.
Mostly cloudy Thursday with a steady stream of middle level moisture
gradually saturating the atmosphere from the top down. Virga or very
light precipitation could start as early as Thursday afternoon.
Small measurable amounts are possible in the mountains by sunset.
The trough gets a little better organized Thursday night and
develops into a closed low over San Diego County Friday afternoon.
By this time the atmosphere will be nearly saturated in the surface-500
mb layer and the rain will be more widespread. The precipitable
water peaks at 0.80-0.90" Friday. There isn't any real strong
lifting mechanism with this trough...but broad synoptic scale lift
should bring light rain to just about anywhere across Southern
California...punctuated at times by brief periods of moderate rain
with weak shortwaves rotating around the closed low.
Precipitation projections are highest with the GFS model. Both the
18z run this afternoon and the 00z run this evening were consistent
in that regard. Both runs forecast the highest precipitation totals over the
mountains...specifically on the east-northeast facing slopes due to midlevel
upslope flow around the upper low. The higher resolution WRF-ems and
nam4 also offer a similar solution. Given the deep saturated layer
that will be in place Friday into Friday night...quantitative precipitation forecast was raised
across the region with the greatest increase over the mountains.
This southern stream system will be a warm one. Wet bulb zero temperatures
hover between 6500-7000 feet Thursday...then match the 6500-7000
foot freezing level Friday when the atmosphere reaches saturation.
Accumulating snow will be mostly confined above 7000 feet with
several inches possible above 8000 feet.
The precipitation will taper off during the day Saturday and end
from north to south as the upper low moves southeast into Mexico.
High pressure aloft will build over the southwest Saturday night and
Sunday. Sunny skies and weak offshore flow Sunday will push
temperatures 5 to 10 degrees above average west of the mountains weaker
offshore flow and a return of the sea breeze will bring minor
cooling west of the mountains Monday and Tuesday...but temperatures will
still be at or slightly above average.
290430z...scattered high clouds at or above 15000 feet mean sea level will gradually thicken
and lower to 10000 feet mean sea level by Thursday morning. Broken/overcast middle level
clouds will continue to thicken through Thursday afternoon with
light diurnal surface winds. Coastal low clouds with bases 1500 feet
mean sea level will move onshore after 31/0300z. Scattered/broken multiple cloud layers up
to 20000 feet mean sea level. By Thursday evening...light to moderate precipitation will
increase over the entire region.
800 PM....no hazardous marine weather is expected through Sunday
Skywarn activation will not be needed Thursday.