Flash Flood Watch 1, NWS San Francisco Bay Area, CA

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1000 AM PST Tue Jan 15 2019

.SYNOPSIS...An approaching storm system will push inland over the
Central Coast this afternoon resulting in strong gusty winds,
widespread rain and potential for an isolated thunderstorm or
two. Lingering showers will tapper off through the night before a
stronger and more organized cold front impacts the region on
Wednesday with strong winds and heavy rainfall. Lingering showers
continue into Thursday with a drying trend Friday into Saturday
before another chance of light rain Sunday.

&&

.DISCUSSION...as of 9:26 AM PST Tuesday...Surface low pressure is
deepening west of Big Sur this morning which is different than
was expected about 24 hours when this surface low was pegged to
head towards the Santa Barbara region. The end result is a wide
swath of rain currently on radar from the Central Coast and
rotating northward through the Bay Area today. Rain rates with
this feature continue to be light to occasionally moderate with
hourly totals of a tenth or less while a few locations in the
hills are reporting values of quarter to half inch across the
Central Coast. Not expecting major issues with this slug of rain
but it will act to prime the pump as soils saturate and streams
start to get flowing. Given the surface low pressure spinning up
west of Monterey there will be increasing south to southeast winds
across the Central Coast with gusts to 24 kt last hour at
Salinas. The strongest winds will likely be confined to the hills,
coastal waters and along the Big Sur coast but a Wind Advisory
was been issued for the Central Coast through 4 pm to account for
the cyclogenesis off the coast.

The surface low and upper support will shift inland over the Bay
Area by 02-04z this evening with steady rain rapidly ending.
Coastal slopes and portions of the North Bay could hold on to some
light rain all night as moist southwest boundary layer flow
develops in it wake.

Things are still on track for a strong weather system to impact
the Bay Area Weds afternoon and night. Water vapor shows a potent
shortwave out near 38n/150w that will race towards the region over
the next 24 hours and interact with a high tpw tongue of moisture
(atmospheric river).

Expect light rain in the North Bay as early as Weds morning in
warm advection pattern. Rain will fill in and intensify through
the day and reach the greater Bay Area by the afternoon hours. To
be clear the Weds afternoon/evening commute will be severely
impacted. If you dont need to go out, dont. Stay off the roads if
possible. There will be urban and small stream flooding as heavy
rain rates will quickly overwhelm storm drains and culverts. In
addition strong winds of up to 50 mph (even 60-70 mph in the
hills) will accompany the frontal passage. This will most likely
topple trees, branches and lead to power outages across the
region. Urban and small stream flood advisories will likely be
issued for Weds afternoon and evening across the region. Will make
a decision on upgrading the High Wind Watch with afternoon
updates today.

The front will travel north to south Weds night and bring the
heaviest rain to the Central Coast Weds night. On Thursday
numerous post-frontal showers will linger across the region as
snow levels fall to around 4000 feet. Showers or t-storms with
small hail will be likely on Thursday.

Dry Friday and Saturday. Chance of light rain Sunday, then dry for
MLK Monday.

&&

.PREV DISCUSSION...as of 03:30 AM PST Tuesday...Isolated to scattered
showers continue to push inland from the Pacific this morning as
an upper level short-wave disturbance approaches the coast. Have
also seen isolated thunderstorms over the Pacific since late last
evening with some cells even indicating weak rotation at times.
The latest forecast models continue to indicate an increase in
rain showers, both in coverage and intensity through late morning
and into the early afternoon. The ARW, NMM and even the HRRR show
surface low pressure also deepening as the aforementioned system
approaches the Central Coast during the 18-21Z timeframe. These
high-resolution models show 925MB winds associated with this
system exceeding 50 kts (up to 70 kts just off the Big Sur Coast
in the NMM). Thus, do believe there is a potential for a brief
period of strong, gusty winds of up to 45 mph along portions of
the Central Coast late this morning and into the early afternoon.
Thus, have issued a Wind Advisory from 10 am - 4 pm for these
areas. It is worth mentioning that localized gusts in the coastal
ranges may even exceed 45 mph and given the southeast orientation,
could see strong winds channel up the Salinas Valley as well.
Something we will keep a close eye on.

Rainfall amounts through tonight will range from 0.25"-0.50" with
slightly greater totals across the Central Coast where the focus of
heavier rainfall is expected. Meanwhile, could see a period of
locally heavy rainfall in the coastal ranges along the Big Sur Coast
this afternoon where totals of 1.00"-3.00" (isolated amounts up to
4.00") will be possible. This could lead to localized flooding
concerns in the coastal areas south of the Monterey Peninsula.

Showers are forecast to diminish in coverage later in the day as the
surface low pressure pushes inland around the Monterey Bay and then
lifts northward. However, southwest flow is forecast to develop in
wake of the exiting system which would be conducive for light to
occasionally moderate rain to continue along the south/southwest
facing slopes through the night. Meanwhile, inland and valley
locations should see conditions dry out later in the day with only
light rainfall amounts expected through Wednesday morning.

Now for the main event! The forecast models remain in good agreement
with respect to a more potent mid/upper level low developing over
the norther Pacific that will then advance toward the Pacific
Northwest Coast on Thursday. This system will tap into a much deeper
plume of moisture and drive a cold front across the San Francisco
and Monterey Bay Area Wednesday into Wednesday night. IVT values in
the 500 to 750 kg/m/s are forecast to advect inland and result in
widespread moderate to locally heavy rainfall across the region.
Rain is expected to increase in coverage and intensity over the
North Bay around midday Wednesday and then march southward through
late Wednesday night. Given recent rainfall, the threat for urban
and small stream/creek flooding will increase as rainfall rates may
approach 0.75"-1.00" per hour at times ahead of and along the main
frontal band. There will also be the potential for convection to
develop with isolated lightning strikes, small hail and strong gusty
winds from Wednesday afternoon into Wednesday night. In addition,
strong and gusty southerly winds are forecast to develop ahead of
the frontal passage. Thus, will continue a High Wind Watch from
Wednesday afternoon into Wednesday night as wind gusts of 50 to 60
mph will be possible. Will continue to fine tune the details as the
event draws nearer. Rain showers are then forecast to linger into
Thursday morning before drying conditions return late in the day
Thursday.

Additional rainfall amounts are forecast to range from 1.00"-2.00"
in most urban areas and up to 2.50" in the North Bay Valleys.
Coastal ranges will likely see 3.00"-6.00" of additional rainfall
from Wednesday into Thursday. Again, these rainfall amounts will
likely result in rapid rises on area creeks and streams. However,
widespread flooding of the main stem rivers is not expected at this
time. For additional information, please see the Hydrologic Outlook
(ESFMTR).

Dry conditions are likely to continue Friday into Saturday. One
additional frontal system is expected to drop southward across the
region on Sunday. However the latest forecast models have been less
bullish on rainfall amounts and are currently only expected light to
moderate rainfall, especially for the northern half of the region.
Areas along the Central Coast may see little to no measurable
rainfall as the system could potentially dissipate as it reaches
this portion of the state. Dry conditions then appear likely for the
latter half of the month.

&&

.AVIATION...As of 09:48 AM PST Tuesday...For 18z tafs. Widespread
light to moderate rain continues to spread northward across all
terminals. CIGS for the most part are rather high - 4k ft and
above. Latest guidance shows a transition to showers later today
and eventually VCSH overnight. There will be periods of dry
weather overnight and early tomorrow, but nailing timing is low
conf and covered it with VCSH in tafs. Winds will really pick up
Wednesday afternoon with rain possible toward the end of the taf
period - 18-21Z.

Vicinity of KSFO...Breezy SE winds and wet runways will be the
norm through roughly 00-03Z. Cigs will be in the 4-5k ft range.
Hit or miss showers overnight with lingering clouds. Winds
increase tomorrow after 18Z with much strong winds and developing
rain around 21Z. May need an Airport Weather Warning with the next
round of wind.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Steady rain, SE winds, high cigs through
this afternoon. Less rain this evening and overnight, but
lingering cigs. Winds will really pick up at SNS tomorrow
afternoon.

&&

.MARINE...as of 09:39 AM PST Tuesday...Inclement weather will
bring rain, possibly thunderstorms and very gusty winds to the
coastal waters today. A much stronger storm will bring widespread
gale force winds on Wednesday and Thursday  over the waters and
bays. This system will bring heavy rainfall  and scattered
thunderstorms Wednesday night into late Thursday  morning. A very
large west swell is forecast to arrive on  Thursday. This will
make for hazardous conditions across the  coastal waters. Mariners
are urged to monitor weather forecast  closely this week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...Wind Advisory...CAZ516-517-528-530 til 4 pm
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Rough Bar Advisory for SF Bar until 10 AM
             SCA...Mry Bay from 10 AM
             SCA...SF Bay from 1 PM

&&

PUBLIC FORECAST: RWW
AVIATION: MM
MARINE: MM

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