NWS Forecast Area Discussion - San Francisco Area

FXUS66 KMTR 180523

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1023 PM PDT Sat Mar 17 2018

.SYNOPSIS...Showers will end in all areas by late evening. Expect
a cool night tonight with low temperatures in the 30s in most
valleys. Patchy dense fog may develop late tonight and persist
into Sunday morning. Dry weather, along with a warming trend, is
forecast from Sunday into Monday ahead of the next storm system.
The next system will bring widespread rainfall to our region from
Tuesday through Thursday, along with the potential for heavy rain
and locally strong southerly winds.


.DISCUSSION...As of 9:05 PM PDT Saturday...Our weather has
settled down this evening after an active day that included
isolated thunderstorms, rotating storms over the coastal waters,
and numerous reports of small hail. Shower activity rapidly
decreased late in the day as surface heating diminished. Current
radar shows almost no returns and expect precipitation to end in
all areas by late evening.

Skies have been clearing since sunset and temperatures have
already cooled into the mid 40s across inland areas. Primary
concern in the short term is how low temperatures will drop
overnight and whether freezing temperatures may occur by sunrise
Sunday. Surface dewpoints are currently in the lower 40s in most
areas and the WRF model indicates only marginal low level drying
overnight. This low level moisture should help keep temperatures
from dropping to freezing in most areas and it`s likely that we
will see patchy dense fog develop by late tonight. Forecast
overnight lows tonight mostly range from the mid 30s to mid 40s,
although a few of the colder inland locations may drop briefly to
near freezing around sunrise. In these areas we may see some
patchy freezing fog which could make for slippery roadways early
on Sunday.

The upper trough that brought our region several days of showery
weather will move well off to our east by tomorrow and a shortwave
ridge will develop over California. This ridge will result in
modest warming on Sunday along with dry conditions. Continued dry
weather along with more significant warming is expected on Monday
when the ridge over California amplifies downstream of a
deepening storm system well offshore.

The storm that will develop offshore early next week is expected
to tap into subtropical moisture. A plume of very moist air with
precipitable water values in excess of 1.5 inches is forecast to
begin arriving over California by Tuesday. Warm advection along
with orographic forcing is expected to begin generating widespread
rainfall across California on Tuesday. The GFS has been rather
consistent in initially focusing the moisture plume over south-
central California which would mean heaviest rain on Tuesday would
be from Monterey County southward. However, the 00Z NAM shows
rather uniform precipitation intensity across our forecast area on
Tuesday, which is similar to the 12Z ECMWF. Thus, there continues
to be uncertainty regarding the location of heaviest rainfall.
The upper trough is forecast to approach the coast on Wednesday
and Wednesday evening and this is when heavier rain rates will
likely be more widespread across our area, and also when southerly
winds will be strongest. Widespread rain is expected to change to
showers by Thursday morning (GFS) or Thursday evening (ECMWF) as
the trough moves inland. Needless to say there are still a lot of
details with this system that need to be sorted out, but the
overall message here is that most of California should be
prepared for potentially heavy precipitation and windy conditions
during the middle portion of next week.

&& of 10:35 PM PDT Saturday...Showers have diminished
rapidly leaving just a few clouds this evening. Low level
moisture remains near the surface which will allow for MVFR cigs
to develop around the SFO Bay Area after 12Z.

Vicinity of KSFO...Possible MVFR cigs after 12Z. Any cigs that
develop will be patchy in nature and should burn off after 16Z.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Some low clouds remain along the coastal
range which will keep brief MVFR cigs to MRY through 08Z. Drainage
wind will clear out the clouds after 08Z. Otherwise VFR.

&& of 10:15 PM PDT Saturday...Weak high pressure off the
California coast will bring light northwest winds through Sunday.
Winds will shift to southerly on Sunday night and increase Monday
and Tuesday as a deep low tracks northeast across the offshore
waters. Gale force winds or gusts is possible Tuesday with large
mixed swells.





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NWS MTR Office Area Forecast Discussion