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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

                            
000
FXUS66 KMTR 162213
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
213 PM PST Wed Jan 16 2019

.SYNOPSIS...A strong cold front will pass over the district this
evening with gusty winds and periods of heavy rain that will taper
off after midnight. Showers and isolated thunderstorms will
continue on Thursday. Dry Friday and Saturday with a chance of
some light rain on Sunday as a weak front passes through. Dry
weather returns Monday through most of next week.

&&

.DISCUSSION...as of 2:13 PM PST Wednesday...Forecast cold frontal
passage is still on track. We've had several lightning strikes 
offshore with a few over San Mateo and around Pt Reyes. The 
heaviest rain is currently falling across the North Bay with rates
around 0.30 per hour for Sonoma and Marin where an urban and 
small stream flood advisory has been issued to deal with the 
evening commute and rising creeks (Willow Brook at Penngrove 
Park). The wind advisory remains in effect for the interior valley
with high wind warnings for the coast and hills through 3 am 
Thursday. The buoys are still getting southeast winds in excess of
40 kts as the 970 mb low to our north continues to deepen. The 
boundary will pass through the district this evening with strong 
wind gusts in excess of 50 mph and several hours of moderate to 
heavy rainfall. In sum the forecast remains on track with no major
changes. The bulk of the storm will impact the region from now 
(North Bay) through around midnight (Central Coast).

Will be watching the rises on the main stem rivers but the Napa
and Russian are forecast to stay comfortably below flood stages 
even though they will see very impressive rises over the next 24 
hours. The Carmel River at Robles Del Rio is still forecast to 
reach monitor stage around sunrise. Due to recent and forecast 
heavy rains in the Santa Lucia range the Big Sur and San Antonio 
rivers are forecast to reach action stages as heavy runoff comes 
off the hills the next 24 hours.

For Thursday expect numerous showers and thunderstorms in post-
frontal environment. Even though there will be plenty of sunny
breaks convective showers will bring brief heavy downpours, small
hail and possible isolated lightning on Thursday. However, the
main storm impacts will be confined to the Weds night frontal
passage.

Please read the beaches section below regarding high surf warning
and coastal flood advisory that goes into effect Thursday for
large incoming surf.

Dry conditions Friday and Saturday. Weak front still progged for
Sunday with a chance of light rain.  

High pressure returns Monday through next week as high pressure
builds. Long range models keep things dry through the end of the
month but show some signs of a return to wet weather by the
beginning of Feb as a trough tries to undercut the ridge.

&&

.AVIATION...as 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.  

&&

.BEACHES...as 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.

&&

.MARINE...as of 01:37 PM PST Wednesday...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.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...High Wind Warning...CAZ006-505-507-509-511-512-517-518-529-
                530 til 3 am Thu
             Wind Advisory...CAZ506-508-510-513-516-528 til 3 am Thu
             Flash Flood Watch...til 3 am Thursday
             GLW...SF Bay until 3 AM
             GLW...Pt Arena to Pt Reyes 0-10 nm until 3 AM
             GLW...Pt Reyes to Pigeon Pt 0-10 nm until 3 AM
             GLW...Pigeon Pt to Pt Pinos 0-10 nm until 3 AM
             GLW...Pt Pinos to Pt Piedras Blancas 0-10 nm until 3 AM
             GLW...Pt Arena to Pigeon Pt 10-60 nm until 3 AM
             GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 3 AM
             GLW...Mry Bay until 3 AM

&&

$$

PUBLIC FORECAST: RWW
AVIATION: DRP
MARINE: DRP

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