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

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FXUS66 KMTR 181214

Area Forecast Discussion
National Weather Service San Francisco Bay Area
414 AM PST Wed Jan 18 2017

.SYNOPSIS...Rain spreads from north to south today with a period
of moderate to heavy rain as the front passes through the region
along with gusty southerly winds. Showers linger on Thursday but
in general there will be a break in the weather. Next front
arrives Thursday night into Friday with another round of wind and
rain. Lingering showers Saturday before the final front brings
more wind and rain Sunday before tapering to showers Sunday night
and Monday. Dry but cool Tuesday.

&& of 4:14 AM PST Wednesday...Light warm advection
rains are already breaking out across the north bay with numerous
stations reporting light rain and a few hundredths of rainfall
overnight. The radar is bright banding right now with freezing
levels around 10,000 feet so returns look more impressive than the
rainfall that is occurring at this time. However the trend is
definitely faster and wetter with really none of the mesoscale
models indicating that rainfall should already be occurring (nam,
hrrr, rap). So the old rule of when your in a wet pattern it
doesnt take much to keep things going seems to be holding true
once again. Expect mainly light rain this morning that will
increase in intensity by midday and then drift southward through
the district as the surface cold front and cold air aloft shifts
southward. Still expect the afternoon and evening commute to be
most impacted with moderate/heavy rain, ponding on roadways and
gusty south winds that are accounted for with the wind advisory.
Only other change for todays forecast was to introduce t-storm
chances over much of the region as the front moves through. Spc
day 1 outlook shows this with spc lightning probability showing
high probability of lightning strikes so wont be surprised to get
reports of embedded t-storms today as the front rolls through.
Rain turns to showers tonight with lingering showers into Thursday
morning. Have also introduced some isolated t-storms for Thursday
afternoon over the hills as the cold/unstable core of the upper
low passes through the region Thursday. Rain totals with the first
storm still show 0.50-1.50 inches for the urban areas/valley with
1-3 inches for the north bay valleys and all coastal hills. A
healthy shot of rain in a fairly short window.

There wont be much break before the next storm barrels onshore
Thursday night into Friday morning. Given the current system
timing and the propensity for these nw flow systems to arrive
faster wont be surprised to see the second feature speed up with
the period of heaviest rain likely Thursday night into Friday
morning. Plenty of left over moisture and instability to keep
showers going for much of the day Friday into Friday night and
even early Saturday morning. Its these cold post-frontal nw flow
regimes where some of the coastal sites and hills can really over
achieve on forecast rainfall totals. In general, the Thu
Night/Friday system should produce similar rainfall totals. If
anything the models and associated moisture plume suggest it may
be a tad drier but post-frontal showers may overproduce in terms
of qpf.

There should be a break on Saturday before the third and final
system arrives Saturday night into Sunday with another round of
steady rainfall. The Sunday system could be the wettest as the
trajectory and associated moisture plume could bring a longer
period of steady rainfall. Its important to remember that last
weeks storms were associated with warm AR stretching back towards
Hawaii. The storms this week are coming from the NW/W and more
progressive. Nonetheless the 5-6 day rainfall totals will still be
impressive no matter how you slice it, especially on top of the
rain that fell last week. Our wettest spots could easily see 6-8
inches of rain by Monday with widespread 3-5 inch amounts in the
coastal hills and 1-3 inches for the valleys/urban areas.
Hillsides, trees and roads that are in a precarious situation will
be stressed over the next 5-6 days.

Noted drying trend starts Tuesday and likely lasts through much of
next week if not the end of the month.


.AVIATION...As of 10:47 PM PST Tuesday...VFR with patchy MVFR
visibilities in lingering hazy conditions or thin fog reported
this evening over the Bay Area. Periodic spotty light rain tonight
mainly over the northern third of the forecast area. Rain with
strong and gusty winds spreads over the entire area from north to
south through Wednesday, cold front passes Wednesday night.

Vicinity of KSFO....VFR/MVFR, light winds tonight becoming strong
and gusty SE Wednesday with cold frontal passage near 00z Thursday
(4 pm Wednesday). Cold front is a fast mover, moderate to heavy
showers leading up to frontal passage should taper off quickly by
this time tomorrow evening.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR. Light winds tonight becoming SE and
increasingly gusty on Wednesday. Rain may be slow to develop at
first Wednesday morning and afternoon due to initial downsloping
drying effects off the nearby coastal terrain. Expect strong and
gusty SE winds to 30-35 knots Wednesday over the northern Salinas
Valley prior to cold frontal passage.

&& of 3:25 AM PST Wednesday...Southerly winds will
increase by wednesday ahead of a frontal system. winds will
decrease late wednesday night as the low lifts northeast but will
increase again by friday as another storm approaches. very large
west swells will arrive later this week with swells building as
high as 26 feet friday and saturday.


     .Tday...Wind Advisory...CAZ006-508>513-517-528>530
             Wind Advisory...CAZ505>507
             SCA...Pigeon Pt to Pt Pinos 0-10 nm from 7 AM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm from 1 PM
             SCA...Pt Arena to Pt Reyes 0-10 nm until 9 AM
             SCA...Pt Arena to Pigeon Pt 10-60 nm until 9 AM
             GLW...Pt Arena to Pt Reyes 0-10 nm from 9 AM
             GLW...Pt Arena to Pigeon Pt 10-60 nm from 9 AM
             SCA...Pt Reyes to Pigeon Pt 0-10 nm from 7 AM until 1 PM
             SCA...Rough Bar Advisory for SF Bar from 1 PM
             GLW...Pt Reyes to Pigeon Pt 0-10 nm from 1 PM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm from 7 AM until 3
             GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm from 3 PM
             SCA...Mry Bay from 7 AM
             SCA...SF Bay from 1 PM




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