Scientific Forecaster Discussion
fxus66 kmtr 190959
Area forecast discussion
National Weather Service San Francisco Bay area
259 am PDT Tue Mar 19 2019
Synopsis...a cooling trend will begin today as cloud cover
increases over the region ahead of an approaching weather system.
Rain showers will develop over the region tonight into Wednesday
with an unsettled pattern likely to continue through late week.
While dry conditions are possible this weekend, unsettled weather
will return heading into next week.
Discussion...as of 02:58 am PDT Tuesday...no major changes to
the forecast this morning with low clouds near the coast and
widespread mid/high level clouds advecting inland across the
region. Increased onshore flow and cloud cover will make
temperatures notably cooler this afternoon with highs in the
Lower/Middle 60s at the coast to upper 60s/near 70 deg f inland.
Rain showers will likely hold off through much of the day before
spreading inland late this evening and into the overnight. This
will be the result of an approaching mid/upper level trough and
surface low that will approach the coast through Wednesday
afternoon. Breezy to locally gusty winds will be possible with
scattered showers producing periods of light to moderate rainfall.
Cannot rule out isolated thunderstorms as well late in the night
into Wednesday morning, yet widespread coverage is unlikely.
Rainfall totals through Wednesday will likely range between
0.25"-0.50" for most urban areas. Some of the region's drier
locations may see lesser amounts however. Meanwhile, the Santa
Cruz Mountains and Santa Lucia Range could see amounts of
0.75"-1.50" with isolated amounts upwards of 2.00". Showers are
forecast to diminish late Wednesday night into Thursday morning
and persist through Thursday night.
Dry conditions are looking likely heading into the Friday morning
commute ahead of the next system approaching from the northwest
late Friday into early Saturday morning. This system will move
through more quickly and produce less rainfall than the previous
system, especially south of the Golden Gate. Temperatures through
late week will generally be near to slightly below seasonal
averages given the progressive pattern and cooler troughs passing
The upcoming weekend looks mainly dry with a weak short-wave
ridge building over the region in between weather systems. The
ensembles continue to point toward the return of yet another
unsettled pattern early next with periods of rainfall through
midweek. While details remain limited this far out; there is the
potential for heavier rainfall during this time frame.
Aviation...as of 10:55 PM PDT Monday...it's VFR except areas of
IFR-LIFR in stratus/fog along the coastline locally moving inland
on onshore breezes. A surface ridge is located along the coastline;
the smx-sfo pressure gradient is 1.5 mb and the sfo-SAC pressure
gradient is 0.9 mb. Model forecast lower level cool air advection
near the 925 mb level has helped deepen the marine layer to 1,000
feet and up to 2,000 feet in spots, and an increase in water vapor
with higher dewpoint temperatures over chilly sea surface temps has
caused more stratus and fog to develop. Spotty drizzle may form
along the coast overnight, not presently in 06z tafs. Rapid
weather transitions (in this case stratus and fog) are difficult
to Pin Point, the inland extent of stratus and fog intrusion is a
lower confidence forecast for late tonight and Tuesday morning.
Ongoing cool air advection and drier air aoa 925 mb Tuesday should
mix out the stratus and fog fairly quickly in the morning. As an
upper level trough approaches the coast from the west later Tuesday
and Tuesday night conditions will be VFR-MVFR and light rain is
forecast to develop.
Vicinity of ksfo...low confidence VFR forecast holding through
Tuesday morning. Northwest-west wind near 10 knots shifting to east-southeast early
to mid Tuesday morning. Cloud ceilings with the incoming upper
level trough gradually lower through the day with MVFR and light
rain arriving by early Tuesday evening.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...stratus and fog is a bit more extensive
and developed more rapidly than thought, made adjustments to tafs
to go IFR tonight into Tuesday morning. Ceilings lifting Tuesday
morning, VFR returns. MVFR with light rain developing Tuesday
Marine...as of 02:58 am PDT Tuesday...southerly winds expected
ahead of an approaching low pressure system and cold front moving
in from the west. Winds will veer increasing to the west to
northwest Wednesday. A long period westerly swell will arrive
today, increasing shoaling on bars and Harbor entrances.
... Pt to pt pinos 0-10 nm from 3 PM
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm from 3 PM
Small Craft Advisory...mry Bay from 3 PM
Public forecast: rgass
Visit US at www.Weather.Gov/sanfrancisco