National Weather Service Raw Text Product

Displaying AFOS PIL: AFDMTR Received: 2017-11-14 22:07 UTC


635 
FXUS66 KMTR 142207
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
207 PM PST Tue Nov 14 2017

...Windy and rainy weather returns for Wednesday into Thursday...

.SYNOPSIS...Look for mostly dry conditions today before a storm 
system brings widespread rainfall and breezy to locally gusty 
winds during the day Wednesday to the North Bay and through the 
remainder of the region Wednesday night. After another period of 
dry conditions, a third system is possible late in the upcoming 
weekend.

&&

.DISCUSSION...as of 2:07 PM PST Tuesday...This morning's low 
clouds and fog in the North Bay have since mixed out as we 
transition to mostly clear skies for the afternoon region-wide. 
Visible satellite imagery does show some cumulus clouds popping up
in the higher elevations of the North Bay as well as Santa Cruz 
Mountains. Other than that and a few passing high clouds, expect 
mostly sunny conditions throughout the San Francisco Bay Area and
central California coast through the afternoon hours. Highs are 
expected to top off in the 60s along the coast as well as for 
interior areas. This equates to near or slightly below middle 
November climatological normals.

The big forecast concern will be later on Wednesday and Thursday 
as a moderate-to-high impact atmospheric river is forecast to 
bring moderate-to-heavy rainfall to the region. Forecast models 
over the past 24 hours have slowed the onset of the precipitation 
as the system's parent upper level low approaches the Pacific 
Northwest through the day on Wednesday. A cold front extending 
from the surface low is progged to push onshore sometime on 
Wednesday afternoon or evening and then continue southward 
overnight.

Additionally, this boundary will tap into a rich moisture plume 
(atmospheric river) that will help to enhance locally heavy 
rainfall. The 12Z GFS indicates maximum integrated vapor transport
(IVT) values exceeding 700 kg/m/s off the California coast by 
midday Wednesday and eventually moving onshore by the evening. If 
this moisture plume is slow to push south from the North Bay, it's
quite conceivable areas of the North Bay will approach or exceed 
0.5" per hour rain rates. With the October fires that hit areas of
the North Bay, these rain rates are especially concerning with 
increased potential for debris flows, mud slides, and flash 
flooding to the burn scars and downhill/downstream areas. As a 
result, a Flash Flood Watch has been issued for the North Bay 
Mountains and Valleys beginning 4 pm PST Wednesday with impacted 
areas including the recent Atlas, Tubbs, Nuns, and Pocket Fire 
burn scars, as well as the Fountaingrove neighborhood in northeast
Santa Rosa.

When all is set and done with this system, the North Bay 
Hills/Mountains can expect about 2-3" of rain with locally higher 
amounts possible, particularly for southwest-facing slopes. The 
Santa Cruz Mountains can also expect moderate to heavy rain 
amounts with 2-4+" forecast. For other locations, 1-2" is forecast
for the North Bay Valleys, 0.75-2" for urban areas of the San 
Francisco Bay Area, and 0.50-1" for the Monterey Peninsula.

Increasing southerly winds will be another concern with this 
system. The favored locations in the North Bay Mountains and East 
Bay Hills may see wind gusts exceed 50 mph prior to the frontal 
passage on Wednesday evening/night. As a result a Wind Advisory 
has been issued. The wind concerns will shift south on Thursday to
the higher elevations of San Benito and Monterey Counties.

We'll enter a brief dry period on Friday as the longwave trough 
pushes inland to the North American continent. Weather conditions 
are expected to be pleasant through Saturday and part of the day 
on Sunday before the next system approaches the California coast. 
There remain differences between the GFS and ECMWF model 
placement, timing, and strength, so confidence in specific details
on this storm remain low at this time. Nonetheless, the weather 
pattern is expected to remain active for the West Coast into the 
Thanksgiving holiday. The latest 6-10 day precipitation outlook 
(valid November 20-24) from the Climate Prediction Center gives 
much of California an increased likelihood of experiencing above 
normal precipitation through the period.

&&

.AVIATION...As of 10:15 AM PST Tuesday...For 18z Tafs. CIGS will 
improve through midday and into the early afternoon with 
widespread VFR. An initial shot of moisture will arrive over the 
North Bay late tonight, bringing lower ceilings (BKN TO OVC 
040-060) and vcsh/-shra to some TAF sites through tomorrow 
morning. Brief improvement in conditions late tomorrow morning 
through early afternoon before second main round of precipitation 
and lowering cigs/visbys arrives by Wednesday evening.

Vicinity of KSFO...VFR. Winds generally light today. For tonight,
VFR with BKN-OVC at 050-080 by sunrise Wednesday with a few
vicinity showers to the north. Increasing southerly winds through
the day tomorrow ahead of approaching front. Precipitation from
next frontal band arrives around 03Z THU. 

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...A few clouds around the Monterey Bay will
dissipate by late morning. Otherwise, VFR through the TAF period,
with increasing high clouds tomorrow morning. Next precipitation
arrives around 09Z THU.  

&&

.MARINE...as of 01:57 PM PST Tuesday...Light winds today will
gradually increase and turn southerly tonight and tomorrow ahead
of a frontal boundary arriving later Wednesday. Winds ease and
turn more westerly in the wake of the front. Moderate seas.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Pt Arena to Pt Reyes 0-10 nm from 3 AM
             SCA...Pt Arena to Pigeon Pt 10-60 nm from 3 AM

&&

$$

PUBLIC FORECAST: Rowe
AVIATION: DRP
MARINE: DRP

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