Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Diego California
121 PM PST Monday Jan 26 2015
a low pressure system from the southwest will bring increasing
showers through the afternoon and particularly this evening over
Southern California. Best rainfall amounts will likely occur in the
mountains...deserts and valleys. A few to several inches of snow is
possible in the mountains above 7500 foot elevation with this
system. Showers dissipating quickly Tuesday...with slightly warmer
weather and better sunshine for Wednesday. Another weak low pressure
system from the southwest will bring increasing clouds again and a
chance of showers Thursday into the weekend.
Discussion...for extreme southwestern California including Orange...
San Diego...western Riverside and southwestern San Bernardino
Current water vapor imagery shows a deformation zone stretching from
off the coast of central California into northern San Bernardino
County and Arizona...with a cut-off upper level low to the
west-southwest of San Diego and an upper ridge to the northeast. It
is evident that the low continues to spin up moisture into the area
from the southeast this afternoon...but a lot of it is in the middle
and high levels of the atmosphere. Thus...despite some decent
looking radar returns moving into the area this afternoon...rainfall
accumulations have been very light...around a trace to locally 0.04
inches. The low is still on track to move north and weaken today and
then develop into an open wave as it moves into Southern California
tonight...before moving out of the area on Tuesday. As more moisture
streams in from the southeast...eventually the lower layers will
saturate as well...resulting in better rainfall accumulations
particularly for the low elevations. There has been a dramatic
decrease in storm-total quantitative precipitation forecast generated by the WRF and nam4...as the
12z runs of those models do not show much at all for this event. The
cansac WRF has backed off significantly as well. Given these model
trends...have lowed the quantitative precipitation forecast for today into late tonight. Rainfall
amounts for the mountains are now expected to range from 0.25-0.75
inches...0.15-0.30 inches in the deserts and inland valleys...with
0.10 inch or less at the coast. The warm nature of the storm will
keep snow levels high...falling to 7500-8000 feet this
evening...with a few to several inches of snow possible above those
elevations. A Winter Weather Advisory has been issued for the San
Bernardino and Riverside County mountains above 7500 foot elevation.
Finally...looking at the BUFKIT forecast soundings from local WRF
and nam4...do not see much in the way of instability. However...with
a vorticity maximum moving through this evening in coincidence with some cold
air aloft associated with the low...a lightning strike or two and
some heavier downpours cannot be ruled out...particularly for the
coastal waters and coast.
Showers will dissipate quickly Tuesday morning as the trough moves
north of the area. With the continued cloud cover and lower 850 mb
temperatures...there should be some cooling for Tuesday. Slightly
warmer with better sunshine on Wednesday with some weak high
pressure aloft moving over the region. Then models continue to show
another trough moving inland over Southern California Thursday and
Friday...moving southeast across southeast California/Arizona/baja
Mexico Saturday...and then south into lower Baja California Mexico on Sunday.
Models differ as to the amount of moisture and precipitation...with
the 12z GFS mostly dry...and the 12z European model (ecmwf) somewhat wet. With low
confidence due to these differences...the slight chance to chance of
showers currently in the forecast for that time seem reasonable.
262130z...low level wind shear below 1000 feet at ksan...kcrq and ksna through 01z.
Through 08z...bkn-ovc050-080 with areas of rain...local broken-ovc030-050
in heavier showers. The rain will move from south to north through
10z Tuesday. After 10z...sct-bkn050-080.
130 PM...a slight chance for a thunderstorm over the coastal waters
late this afternoon and evening.
Tuesday through Friday...no hazardous marine weather is expected
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.
California...Winter Weather Advisory from 6 PM this evening to 2 am PST
Tuesday above 7500 feet for the San Bernardino County