Scientific Forecaster Discussion
fxus66 kmtr 190524
Area forecast discussion
National Weather Service San Francisco Bay area
1024 PM PDT sun Mar 18 2018
Synopsis...dry weather conditions and a slight warming trend can be
expected through Monday ahead of our next storm system. This
system will bring widespread rainfall to our region from Tuesday
into Friday, along with the potential for periods of heavy rain
and locally strong southerly winds. The heaviest rains are
expected over the Big Sur region.
Discussion...as of 9:00 PM PDT Sunday...a cool start to the day,
plus widespread thick high clouds through much of the day, kept
afternoon highs mostly in the 50s today, and about 5 to 8 degrees
cooler than normal. High clouds have begun to thin this evening,
which will allow for better radiational cooling overnight.
However, do not expect low temperatures tonight to be as cool as
last night. More sunshine is expected on Monday under a shortwave
ridge, so high temperatures will likely warm back close to
Evening satellite imagery shows an upper low centered about 1100
miles west of the northern California coast. To the south of this
low a plume of subtropical moisture, with precipitable water
values between 1.5 and 2 inches, is streaming eastward. The low is
forecast to begin drifting eastward Monday and Monday night and
circulation around the low will direct the plume of moisture
towards central California. Precipitation will begin in our area
late Monday night or Tuesday morning, well ahead of the
approaching upper low, as warm advection and orographic lift begin
to generate rainfall from the incoming moisture plume. The GFS
has been consistent in focusing this moisture plume over south-
central California, at least initially from Tuesday into
Wednesday, and heaviest rainfall is projected to occur between
Monterey and Los Angeles during this period of time. The 00z NAM
has the same general idea as the GFS, and the European model (ecmwf) seems to be
trending towards the GFS solution as well, especially by
Wednesday. By Wednesday night and Thursday, as the upper trough
finally approaches the coast, the moisture plume will shift into
Southern California. However, increasing dynamics from the
approaching trough by then could generate heavy precipitation
nearly anywhere in California. The upper trough is forecast to
move inland by late Thursday and rain is expected to change to
showers by then. Showers will then continue into Friday, and
perhaps well into next weekend too, as a second trough drops in
from the north.
The bulk of the precipitation this week is expected to fall
between Tuesday and Thursday. During these three days, rainfall
totals of 0.75 and 1.5 inches are expected in most urban areas,
while 2 to 5 inches could fall in the hills, with locally up to 8
inches possible in the Santa Lucia mountains above the Big Sur
Southerly winds will begin to increase on Tuesday and Tuesday
evening and periods of strong and gusty winds will then continue
through Thursday. Strongest winds with this system are expected
Wednesday afternoon and evening when a Wind Advisory may be needed
for at least coastal and higher elevation locations.
A hazardous weather outlook will be issued shortly to highlight
the expected weather hazards during the week ahead.
Aviation...as of 10:30 PM PDT Sunday...lower humidities will
make for VFR conditions through Monday.
Vicinity of ksfo...VFR.
Sfo bridge approach...VFR.
Monterey Bay terminals...VFR.
Marine...as of 10:23 PM PDT Sunday...high pressure over the
coastal waters will weaken as a deep low tracks northeast across
the offshore waters. Winds will shift to southerly on Monday and
increase Tuesday as a deep low tracks northeast across the
offshore waters. Along with gusty winds seas will become mixed
Tuesday through Thursday.
Public forecast: dykema
aviation: west pi
marine: west pi
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