Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

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716
FXUS66 KMTR 070614
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1010 PM PST Tue Dec 6 2016

.SYNOPSIS...Cool overnight temperatures are expected region-wide
with patchy frost through sunrise Wednesday morning. A weather
pattern change will then bring unsettled conditions to the region
from Wednesday afternoon through the upcoming weekend. Periods of
rain can be expected at times across the region, especially late
Wednesday into Thursday morning and again Thursday night into
Friday.

&&

.DISCUSSION...as of 9:00 PM PST Tuesday...An upper level
disturbance passing north of the area brought some stratocumulus
clouds to the area today. Highs today were generally a few degrees
below normal for this time of year.

A few changes will be occurring in rapid succession over the next
24-48 hours. The flow will switch from northwest to an offshore
flow tonight as surface high pressure builds briefly over the
Great Basin. This will bring in colder air from the interior and
will cause cold minimums tonight. Lows could get into the upper
20s and lower 30s in parts of the inland valleys and patchy frost
is possible. Then on Wednesday the flow aloft switches to a more
zonal pattern and the airmass starts to warm. A plume of moisture
originating from near Hawaii will bring rain to the area by
Wednesday night continuing into Thursday. Rain could begin in the
SFO Bay Area as early as Wednesday afternoon but there will be a
lot of dry air to overcome so most likely it will be Wednesday
night before rain starts to fall. Winds will switch to southerly
on Wednesday and increase to 15-20 mph with higher gusts Wednesday
night. This could be a situation where temperatures rise during
the night.

Another forecast challenge will be how far south the initial rain
spreads. The GFS and ECMWF bring rain to pretty much all of the
CWA. The previous runs of the NAM and the current WRF keep rain
from the Santa Cruz mountains north with little or no rain for the
MRY Bay Area south until Friday. The latest NAM now brings precip
to the southern part of the CWA Thursday which is more in line
with the GFS. So there is growing confidence that people in the
MRY Bay Area and areas to the south will see rain Thursday. This
system will have decent orographics to work with so locally heavy
rain is possible over the Santa Cruz Mountains and around the
North Bay Mountains.

Rain is forecast to continue off and on through Friday as more moisture
gets pumped in. Rainfall amounts are expected to range from
1.5-2.5 inches along the coast and in the North Bay with up to 4
inches in the hills...3/4 to 1.25 inch around the SFO Bay
Area...and 1/2 inch or less in the southern zones. Models show
rain decreasing this weekend but we will remain under a moist
westerly flow so there will be still be some chance of precipitation
over the weekend. Another plume of moisture is expected early next
week but will be shifted further north. This would likely bring
precip back to at least the northern half of the CWA with lesser
chances in the south.

&&

.AVIATION...As of 10:09 PM PST Tuesday...VFR except 06z KSTS TAF
indicates IFR in fog 10z-18z Wednesday. Winds becoming mainly light
offshore late tonight and Wednesday, however E-SE wind gusts to 20
knots or stronger are possible over the East Bay hills and mountains
and San Mateo coast Wednesday. Increasing clouds Wednesday with
light rain developing Wednesday evening into early Thursday.

Vicinity of KSFO...VFR. Offshore wind. Light rain possible Wednesday
evening.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR. Offshore wind. Light rain possible
after 06z Thursday.

&&

.MARINE...as of 9:30 PM PST Tuesday...High pressure will build
over northern Nevada tonight and extend over southern Nevada and
southern California later Wednesday through Thursday night. A low
pressure trough over the eastern Pacific will move eastward and
begin weakening later Wednesday through late week. light to moderate
rain is forecast from late Wednesday night into Saturday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm until 3 AM

&&

$$

PUBLIC FORECAST: W Pi
AVIATION/MARINE: Canepa


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