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

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FXUS66 KMTR 181734
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
934 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.

&&

.DISCUSSION...as of 8:30 AM PST Wednesday...Based off of radar did
a minor forecast update this morning to mention at least a slight
chance of rain for our entire CWA. Have been surprised at the
lacks of tips from many of our buckets since radar might suggest
otherwise. Could be a case of bright banding. Latest run of the
HRRR continues to show the heaviest rainfall amounts occuring this
afternoon and evening.

Anyway, CNRFC QPF values are now in so will be issuing an update
with the latest numbers in the next few minutes. Looks like values
are close to 12 hours ago, so the main story remains three systems
will go through our region. Thankfully new CNRFC numbers continue
to keep river forecast points below monitor stage even through
Sunday morning. Will be interesting to see what the numbers show
tomorrow as the impacts from the third system are felt.

.PREVIOUS DISCUSSION...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 9:28 AM PST Wednesday...Not the best day
weatherwise for aviation as a potent cold front sweeps through the
region. The leading edge of the system is spreading light rain
over the region this morning with IFR to VFR conditions. The CAT
reduction is mainly due to vsby currently. For the rest of
today...expect precip to increase in coverage and intensity. The
heaviest will likely be later this afternoon. In addition to the
rainfall...winds will increase. LLWS will be poss with a 50-60 kt
wind about 2k ft. OAK and SFO have LLWS mentioned. Did not include
tsra, but a thunderstorm is poss but location/timing is low conf.
Rain will transition to more showers overnight into early Thurs.

Vicinity of KSFO....VFR this morning with MVFR to poss IFR later
today. LLWS from around 21-22Z through 04Z. Wind shift from S to
SW more likely after 03Z.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR this morning becoming MVFR to IFR
this afternoon.

&&

.MARINE...as of 08:30 AM PST Wednesday...Potent storm system
moving through the region will result in hazardous conditions and
rough seas along the central coast today and tonight. mariners are
urged to use caution if venturing to open waters. 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.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...Wind Advisory...CAZ006-508>513-516>518-528>530
             Wind Advisory...CAZ505>507
             SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm from 1 PM
             GLW...Pt Arena to Pt Reyes 0-10 nm
             GLW...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 3 PM
             GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm from 3 PM
             SCA...Pt Reyes to Pigeon Pt 0-10 nm 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...Mry Bay
             SCA...SF Bay from 1 PM

&&

$$

PUBLIC FORECAST: Bell/RWW
AVIATION: MM
MARINE: MM


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