Flash Flooding and Flooding in California, December 15, 2016

Area Forecast Discussion
National Weather Service San Francisco Bay Area
954 AM PST Thu Dec 15 2016

...Locally heavy rainfall along with strong and gusty winds to
impact the region today...

.SYNOPSIS...Rain will become heavy later this morning over the
North Bay then heavy rain will gradually spread southeastward
across the rest of the Bay Area during the late afternoon, and
reach the southern counties of the CWA during the evening and
overnight. Expect the afternoon and early evening commute to be
very wet along with gusty south winds. Rain ends from north to
south by early Friday. It'll be mainly dry and much cooler Friday
except some lingering showers are possible over the coastal hills.
It'll be dry with unseasonably cold night-time and morning low
temperatures this weekend. Dry weather continues next week.

&&

.DISCUSSION...as of 09:29 AM PST Thursday...Widespread rainfall
beginning to increase in coverage and intensity across the region
this morning ahead of an approaching frontal system. Currently,
the heaviest rainfall is occuring over the North Bay and will then
spread down into the greater San Francisco Bay Area this
afternoon, mainly during the afternoon/evening rush hour. In
looking at the latest model guidance, have increased forecast
rainfall amounts region-wide, especially over the Santa Cruz and
Santa Lucia Mountains. These locations along the southwest facing
slopes could pick up 4 to 6 inches of rainfall with most of that
occuring in the late afternoon and evening during a 3 to 6 hour
window of time. Thus, the threat for debris flows, mud/rock slides
and localized flash flooding over the burn scar areas remains
through late this evening. Updated forecast products have been
sent.

&&

.PREV DISCUSSION...as of 3:26 AM PST Thursday...The area of heavy
rains has stalled somewhat north of our CWA, although light to
moderate rains have been falling all night across the northern
portions of the district. So far the Venado gage in northern
Sonoma County has picked up the most rainfall with a 12-hour total
of over 2 inches. Amounts drop off from there as you head south.
Latest model guidance has slowed the progression of the frontal
band by a few hours with the heaviest rains now reaching the
northern portions of the North Bay by late this morning, and
spreading south through the San Francisco Bay Area by early
evening, and then into the Monterey Bay area and Big Sur Coast by
late evening. An Urban and Small Stream Flood Advisory is still in
effect for the North Bay counties through 6 am. The heavier rains
won't arrive until later this morning, so another one will likely
have to be issued later as the the current will expire at 6 am.

Strong and gusty southerly winds will accompany the frontal
passage and the High Wind Watch will be converted to a WInd
Advisory for mostly the same areas, although slight timing
differences have been made to the segmented product. Expect the
new NPWMTR product to be issued shortly.

Rainfall totals of 1-2 inches on average for the valleys still
looks reasonable with much of it falling in a fairly short window
of time. The hills will of course receive more with 2-5 inch
totals common from the North Bay hills southward through the
Santa Cruz and Santa Lucia mountains.

Rain should mostly end by daybreak on Friday, although a few residual
showers are possible mainly across the southern half of the
district during the day.

Dry but cool conditions are on tap for the remainder of the
weekend. Overnight low temperatures Friday and Saturday nights are
projected to become very chilly, especially in sheltered, inland
areas where lows in the 20s and 30s are possible. May need to
address this with advisories once the current storm system moves
through.

&&

.AVIATION...As of 9:45 AM PST Thursday...Challenging package with
widespread rainfall across our region with RA to +RA possible
later in the day for all spots. CIGs will likely bounce up and
down between categories. In addition, gusty southerly winds will
pick up. Big question is when the higher speeds will make it down
to the terminals. Conditions expected to improve tonight as the
front goes through. Overall only low confidence on the specifics.

Vicinity of KSFO...Generally looking at rainy MVFR conditions
through the day. Winds of 140 to 150 to 20 KT will switch to 160
to 180 by 19Z with gusts to 25 KT. Winds become post-frontal and
switch to 260 to 280 after 05Z. Low confidence.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Conditions expected to switch to MVFR
later this morning with winds generally 150 to 170. Winds will
gust to over 20 KT as the front approaches with the front. CIGs
will remains MVFR generally. Winds go more 240 to 260 after 10Z.
Low to moderate confidence.

&&

.MARINE...as of 09:41 AM PST Thursday...Mariners are urged to use
caution today as a passing cold front will produce hazardous
conditions. southerly winds will continue to increase through the
morning hours with local gale force winds up to 40 kt. the strong
winds will produce steep wind waves and fresh swell. as the front
passes this afternoon and evening winds will decrease and become
northwesterly beginning in the northern waters and progressing
southward.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...Wind Advisory...CAZ508-509-511-512 10 AM TO 7 PM PST
             Wind Advisory...CAZ517-518-530 1 PM TO 10 PM PST
             Wind Advisory...CAZ505-507 4 PM TO 1 AM PST FRIDAY
             Flash Flood Watch...CAZ512-517 For the Loma...Soberanes
               and Chimney Fire Burn Scar Areas Through Late Tonight
             GLW...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             GLW...Pt Reyes to Pigeon Pt 0-10 nm until 3 PM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm until 12 PM
             GLW...Pt Pinos to Pt Piedras Blancas 0-10 nm from 12 PM
             SCA...Mry Bay until 1 PM
             GLW...Mry Bay from 1 PM
             SCA...SF Bay

&&

PUBLIC FORECAST: RGass
AVIATION: Bell
MARINE: Bell

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