NWS Forecast Area Discussion - San Francisco Area

FXUS66 KMTR 161812

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1012 AM PST Wed Jan 16 2019

.SYNOPSIS...A powerful storm system will bring multiple impacts
this afternoon through early Thursday. Heavy precipitation will
spread north to south this afternoon and winds will become gusty.
Rainfall rates will likely peak late this afternoon and this
evening as a cold front approaches. The Wednesday evening commute
will be severely impacted with urban and small stream flooding
likely along with downed trees and branches. Heavy rain will
continue for the Central Coast through early Thursday morning.
Numerous post frontal showers with possible thunderstorms with
small hail on Thursday. Dry Friday and Saturday with a chance of
light rain Sunday followed by a drying trend next week.

&& of 9:20 AM PST Wednesday...Summation is that
forecast is on track and confidence remains high for timing and
impacts of incoming storm. No changes to ongoing products with
flash flood watches and wind warning/advisories in place along
with high surf warning for tomorrow behind the front.

Only change is probably increased confidence for thunderstorms
later this afternoon and tonight as main cold front barrels
through the Bay Area. SPC has just updated the day 1 severe
weather outlook and put the entire Bay Area into marginal risk for
severe thunderstorms with the biggest risk this evening being from
strong winds associated with convective downdrafts along with
strong synoptic scale winds. Looking at latest satellite and
model imagery expect a well defined cold frontal rain band to move
through the district this evening with torrential rains and
strong winds. Before that occurs steady warm advection rains will
develop across the North Bay and spread southward through the Bay
Area and Central Coast later this afternoon and evening.

Latest river forecast center updates have been issued and show
large rises on our main stem rivers. The Russian and Napa Rivers
will show rapid rises but remain below flood stage on Thursday.
The Carmel River is forecast is to hit monitor stage around 7 am
Thursday morning. Keep in mind some of the other rivers may have
delayed responses due to run-off and not peak until Thursday
afternoon. In addition the Big Sur and San Antonio rivers in
Monterey county are forecast to show big rises into "action stage"
as heavy runoff from the Santa Lucias occurs tomorrow. Small
creaks and streams in the North Bay, Santa Cruz mtns and Alameda
Creek near Niles canyon will also be susceptible to rapid and
dangerous rises.

Repeating that the commute this afternoon and evening will be
severely impacted by rain and strong winds.

We will be behind the front on Thursday with numerous showers
throughout the day. Snow levels will fall to around 4000 feet with
small hail and intense downpours in convective post-frontal
showers on Thursday. Onshore winds and high surf will be of
concern along the coast as well with possible coastal flooding due
to tides running high with heavy runoff.

Clearing out and cooler Thursday night. Friday will finally bring
tranquil weather for post-storm clean-up. Expect dry and sunny
weather Friday into Saturday.

Another weak storm will bring some light rain on Sunday.

Dry Monday through most of next week as strong high pressure


.PREV of 04:05 AM PST Wednesday...Showers continue
across the coastal mountains and the North Bay this morning under
prevailing moist southwesterly flow. Rainfall amounts over the
past few hours have generally been light, but enhanced areas of
rainfall are being observed over the higher terrain with rainfall
rates of 0.1 to 0.2 inches per hour. Temperatures this morning are
mild with most areas in the lower to mid 50s. Southerly winds
continue with speeds 5-15 mph in the lower elevations. Higher wind
speeds are being observed in the higher terrain with gusts up to
30-40 mph. Finally, precipitable water values are 0.8-0.9 inches
over the area.

The main event with a plethora of hazards will arrive on
Wednesday and continue through early Thursday morning. A 518 dm
500 mb trough with a 968 mb surface low centered at 140W 40N will
continue to move north- northeast through the day Wednesday. An
area of high moisture content at the base of this trough will
advect east, with PWAT approaching 1.2-1.3 inches over our region
by midday Wednesday. 925 mb winds from the south-southwest will
increase to 50-60 kts Wednesday afternoon through early Thursday
morning. This strong flow will combine with high moisture content
to produce onshore IVT values of 750-850 kg/m/s, categorizing this
event as a strong atmospheric river.

Warm advection and strong onshore flow will lead to moderate to heavy
rain developing first in the North Bay early Wednesday afternoon
and spreading south through the day. Heavy rain will spread south
into the Bay Area in time to severely impact the afternoon
commute. A strong cold front will then push through from north to
south Wednesday evening. The heaviest rain will likely fall along
and ahead of the cold front late Wednesday afternoon through
evening. A Flash Flood Watch is in effect from Wednesday afternoon
through early Thursday morning due to several hours of heavy rain
with a burst of heavier rain along the cold front. Instability
will develop and a band of convection and thunderstorms along the
cold front are likely. Small hail and heavy rain will be possible
with any thunderstorms that develop. Additionally, very strong
winds aloft will be present and some of these could mix down to
the surface with the convection. After the cold front moves
through, precipitation will become more showery and precip rates
will generally decrease. Showers and isolated thunderstorms in the
unstable postfrontal airmass will continue through Thursday
afternoon before tapering off. Precipitation totals through
Thursday afternoon will range from 1-3 inches in the valleys with
3-6 inches in the mountains, with some of the coastal mountains
seeing locally higher amounts.

Surface winds will increase Wednesday morning and peak Wednesday
afternoon and evening ahead of the front. A High Wind Warning is
in effect Wednesday afternoon through early Thursday morning for
the coastal areas and higher terrain, where gusts of up to 60 mph
are possible. The highest peaks will likely see even higher gusts.
For the lower elevations away from the coast, a Wind Advisory is
in effect Wednesday afternoon through early Thursday morning,
where wind speeds of 25-35 and gusts up to 50 mph are expected. In
combination with wet soils, these strong winds could cause tree
and powerline damage as the front moves through. The combination
of heavy rain, areas of roadway flooding, strong winds, and
tree/powerline damage will create very hazardous driving
conditions Wednesday afternoon and evening.

The weather will quiet down Friday and Saturday with the storm
track shifting north. Isolated showers are possible across the
North Bay with other areas dry. Rain chances will increase on
Sunday as a trough moves through the area. Rainfall amounts with
this system are expected to be light. After Sunday, a return to
dry weather is expected as a high amplitude ridge sets up along
the coast. Highs will warm to slightly above normal with lows
cooling to the upper 30s to mid 40s.

&& of 10:10 AM PST Wednesday...Vigorous storm system
will advance over the terminals through the day and this evening.
This storm system will bring gusty S-SE winds, peaking 35-50mph at
most terminals ahead of frontal passage. LLWS is possible this
morning ahead of the strongest winds aloft in the LLJ mixing to
the surface but the threat will diminish significantly by noon
time. Moderate to heavy rain is expected to arrive late this
afternoon through the evening depending on terminal location, with
IFR visibilities possible during frontal passage. Cigs should
cooperate and remain MVFR through the duration of the event. In
addition, deeper convection has already been observed and will
likely continue, bringing the threat of isolated to scattered
thunderstorms throughout the taf period. This will introduce the
risk of erratic gusty winds, brief intense downpours, lightning,
and hail. See each taf for most likely time frame for VCTS/TSRA at
each terminal.

Vicinity of KSFO... Radar showing rain with embedded deeper
convection advancing towards KSFO as of 10am. Expect to see these
rapidly deteriorating conditions near the terminal through the
remainder of the morning. Then, expect MVFR cigs to persist
throughout the duration of the event. Very strong S-SE winds will
accompany the front with peak gusts of up to 50mph possible late
this afternoon into this evening. In addition, embedded thunderstorm
activity is possible through the day but particularly in a frame
from approx 02z-06z THU. An airport weather warning will likely
be issued for the terminal shortly to address these concerns. Wind
shift expect to SW between 09-12Z THU.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Expect to see winds ramp up
significantly, particularly near KSNS in the coming hours. Prefrontal
rain will arrive shortly after. Short term models suggest KSNS
may be somewhat rain shadowed through the onset of the storm
system but should see steady rain arrive by this evening.
Generally MVFR to borderline VFR ceilings throughout the event.
Moderate to brief heavy rain possible through this evening.

&& of 04:10 AM PST Wednesday...A strong cold front
will push through the region this afternoon and evening bringing
locally heavy rainfall, strong and gusty winds as well as the
potential for isolated to scattered thunderstorms. This will
result in hazardous conditions along area beaches. In wake of the
frontal passage, a large and long period westerly swell will
impact the coast Thursday into Friday. Westerly swells of 16 to 24
feet with a period of 17 to 18 seconds will build through the day
Thursday and result in breaking waves of 25 to 30 feet, favored
locations up to or exceeding 40 feet. Thus, a High Surf Advisory
has been issued. These large breaking waves will lead to
increased wave run- up on beaches with waves topping and washing
over large rocks and jetties. These conditions may also produce
localized coastal flooding of vulnerable locations. Use extreme
caution near the surf zone as these large waves will be capable of
sweeping people into the frigid and turbulent ocean water. Cold
water shock may cause cardiac arrest, and it also can cause an
involuntary gasp reflex causing drowning, even for a good swimmer.
The surf zone will be very dangerous due to strong currents and
powerful breaking waves. The large swell is forecast to gradually
subside on Friday.

&& of 10:10 AM PST Wednesday...A marine weather
statement is in effect. Please view SFOMWSMTR for more

A strong storm system will move across the coastal waters today
and through this evening, bringing significant marine hazards.
Widespread gale force southeast winds ahead of the frontal passage
will bring steep fresh swell. Local storm force gusts are
possible through coastal gaps such as under the Golden Gate
bridge. In addition, deeper convection will spawn isolated to
scattered thunderstorms across the waters today bringing the risk
of erratic gusty winds, locally rougher seas, brief heavy rain,
hail, lightning, and even the potential for weak waterspouts. A
very large and long period west swell will arrive Thursday morning
and last into Friday before subsiding.


     .Tday...High Wind Warning...CAZ006-505-507-509-511-512-517-518-529-
             Wind Advisory...CAZ506-508-510-513-516-528
             Flash Flood Watch...CAZ006-505>513-517-528>530
             GLW...Pt Arena to Pt Reyes 0-10 nm
             GLW...Pt Reyes to Pigeon Pt 0-10 nm
             GLW...Pigeon Pt to Pt Pinos 0-10 nm
             GLW...Pt Pinos to Pt Piedras Blancas 0-10 nm
             GLW...Pt Arena to Pigeon Pt 10-60 nm
             GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             GLW...Mry Bay from 4 PM
             GLW...SF Bay from 1 PM




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NWS MTR Office Area Forecast Discussion