Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

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FXUS66 KMTR 250530

Area Forecast Discussion
National Weather Service San Francisco Bay Area
930 PM PST Fri Nov 24 2017

.SYNOPSIS...Dry conditions, along with above average temperatures
and passing high clouds, will continue through Saturday. A storm
system is forecast to spread rain across the entire forecast area
from late Saturday evening through Sunday. Moderate amounts of
rain are expected along with gusty winds. Showers will continue on
Sunday night, mainly across the southern portion of the region.
Dry weather will return by midday Monday. Dry and mild weather is
then forecast for most of the remainder of next week.


.DISCUSSION...As of 9:05 PM PST Friday...Today was a day much
like yesterday with warm temperatures and widespread mid and high
level clouds. Afternoon highs were anywhere from 5 to 15 degrees
warmer than normal. However, no records were set.

KMUX radar has been showing weak returns offshore and across the
North Bay this evening. These returns are from mid level clouds
at or above 14K feet and so any light precipitation that might be
falling from these clouds is likely evaporating prior to reaching
the surface. Expect mild overnight lows once again tonight due to
cloud cover and the persistent warm airmass under the upper ridge
that is centered to our southeast. Although there has been some
drying in the low levels since last evening, there likely is still
enough low level moisture for patchy fog development later tonight
and early Saturday morning, particularly in the North Bay Valleys.

The upper ridge over the southwestern U.S. is expected to maintain
dry and warm conditions across our region for one more day. The
ridge will finally begin to move eastward late on Saturday as an
upper trough over the eastern Pacific advances towards the West
Coast. These developments will result in a significant change in
our weather by Sunday.

Evening IR satellite imagery shows a cold upper trough dropping
south out of the Gulf of Alaska. Well to the south, near 30N/140W,
is another developing system drawing a plume of very moist air
(atmospheric river or AR) towards the northeast. These two
systems are forecast to merge offshore tomorrow and then track
eastward and impact our region from Saturday night through Sunday
night. Latest models show little change in timing of precipitation
onset, with rain expected to spread across the North Bay on
Saturday night, reach San Francisco during the pre-dawn hours of
Sunday, develop across the rest of the SF Bay Area by late Sunday
morning, and then across the Monterey Bay Area by midday Sunday.
Over the past few days the models have been trending wetter with
this system, which makes sense given that the incoming AR is
forecast to have precipitable water values in excess of 1.5 inches
and Integrated Water Vapor Transport (IVT) values in excess of
500 kg/s/m. However, the 00Z models have not continued the trend
towards heavier precipitation and continue to forecast mostly
moderate amounts of rainfall on Saturday night and Sunday. Even
so, there could be at least a brief period of heavy rain as the
frontal boundary moves through late Saturday night and Sunday. The
period of heaviest rain in the North Bay will likely occur from
just before sunrise Sunday until mid morning, and then across the
rest of our forecast area from mid morning through mid afternoon
on Sunday. Given forecast rainfall amounts and the expected
relatively short duration of heavier rainfall, the threat of
debris flows on the burn scars in the North Bay appear rather low.
However, will need to monitor closely for any signs of more
prolonged and intense rainfall.

It appears there will be somewhat of a lull in the precipitation
from late Sunday afternoon through early Sunday evening before a
cold upper low approaches the coast on Sunday night and triggers
widespread shower activity. The model trend on the track of this
upper low has been farther to the south, and it now appears that
the bulk of Sunday night`s shower activity will occur south of the
Golden Gate. The 00Z NAM is forecasting mucape values in excess
of 500 J/kg across the southern two-thirds of our area on Sunday
night and early Monday. Will need to consider adding thunderstorms
to the forecast for Sunday night and early Monday, particularly
for areas from San Francisco southward.

Rainfall totals from Saturday night through Monday morning are
forecast to range from 0.75-1.75" in the North Bay, with local
amounts of up to 2 inches in the hills. Elsewhere, rain totals
are forecast to range from 0.25-0.75" at lower elevations and from
0.75-1.50 inches in the hills.

The weekend system is also expected to generate gusty winds across
our area Saturday night and Sunday. Southerly winds will increase
on Saturday night and peak on Sunday morning. Winds will then turn
to the west on Sunday afternoon and gradually decrease. Wind gusts
on Sunday morning could reach 40 mph at the windiest higher
elevation locations as well as locally near the coast. But for the
most part, winds are expected to remain below advisory levels.

Showers are forecast to taper off on Monday morning and end by
midday as the upper low moves quickly off to the east. Dry weather
is then expected for most of the rest of next week. Expect a
couple of cool nights, especially in the inland valleys, beginning
on Monday night.


.AVIATION...As of 9:30 PM PST Friday...For 06z Tafs. Mid to high
level clouds remain in place across the area. KMUX Radar is
showing some returns just off the coast, but no precipitation is
expected to reach the surface. Guidance has begun to back off a
bit on fog at KSTS, so pushed back timing a few hours in the 06z
taf. Elsewhere, VFR conditions are expected to prevail with CIGS of
around 10000 to 20000 ft. Generally light and variable winds

Moderate confidence.

Vicinity of KSFO...VFR with high CIGS around 15000 ft. Winds have
begun to decrease and are expected to continue to do so

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR with CIGS around 15000 ft for tonight.
Light winds are expected overnight.

&& of 08:28 PM PST Friday...Moderate northerly winds
will continue over the waters through early tonight before winds
turn southerly early tomorrow ahead of an approaching system.
Southerly winds will increase throughout  Saturday resulting in
hazardous conditions by the evening. A  second and more potent
system will then arrive to the waters  Sunday evening. Winds may
approach Gale Force for parts of the  coastal waters. Northwest
swell will also increase by Sunday  morning and peak on Monday. A
brief decrease in swell is then  expected on Tuesday before
increasing again mid-week.






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