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fxus66 kmtr 181258 

Area forecast discussion
National Weather Service San Francisco Bay area
458 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 Arkansas stretching back towards
Hawaii. The storms this week are coming from the northwest/west 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.

&& of 4:45 am PST Wednesday...light rain is falling in
the North Bay and will spread into the northern sfo Bay area by
18z. Cigs remain high enough to keep VFR conditions until the main
front arrives in the sfo Bay area after 22z with more moderate
rain and vsbys down to 3-5 miles. Rain turns to showers after 06z
after the front passes. Southeast winds increase to 10 kt after
15z and 15-20 kt with gusts to 30 kt after 21z. A wind shift to
the southwest occurs in the late afternoon or early evening but
will Switch Back to the south late tonight as the trough axis

Vicinity of ksfo....VFR with high cigs becoming MVFR with more
moderate rain after 22z. Light east to northeast winds becoming
southeast to 10 kt after 15z and 15-20 kt with gusts to 30 kt
after 22z. Cold front is a fast mover so rain will change to
showers after 06z and taper off overnight.

Sfo bridge approach...similar to sfo.

Monterey Bay terminals...a few low clouds in the area could bring
brief MVFR cigs through 15z. Rain may be slow to develop due to
initial downsloping drying effects off the nearby coastal terrain
so rain should hold off until 21z and be mostly on the light side.
Cigs not lowering to MVFR until after frontal passage when the
winds above the surface turn more westerly and there is upslope
effects off the hills. Expect gusty southeast winds to 30 knots
in the afternoon 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.


Mtr watches/warnings/advisories...
... Advisory...caz006-508>513-517-528>530
Wind Advisory...caz505>507
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm from 7 am
Small Craft pinos to pt Piedras Blancas 0-10 nm from 1 PM
Small Craft Arena to pt Reyes 0-10 nm until 9 am
Small Craft Arena to Pigeon pt 10-60 nm until 9 am Arena to pt Reyes 0-10 nm from 9 am Arena to Pigeon pt 10-60 nm from 9 am
Small Craft Reyes to Pigeon pt 0-10 nm from 7 am until 1 PM
Small Craft Advisory...rough bar advisory for sf bar from 1 PM Reyes to Pigeon pt 0-10 nm from 1 PM
Small Craft Advisory...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
Small Craft Advisory...mry Bay from 7 am
Small Craft Advisory...sf Bay from 1 PM



Public forecast: rww
aviation: west pi
marine: west pi

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