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

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FXUS66 KMTR 301807
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1107 AM PDT FRI SEP 30 2016

.SYNOPSIS...A cooling trend will continue today and persist
through the weekend as an upper level trough approaches the
region. In addition, a dry cold front will push across the region
today and result in breezy onshore winds. As the main upper level
system pushes inland on Sunday, chances for rain showers will
increase with the potential for thunderstorms, especially across
the San Francisco Bay Area. Drier conditions and a slight warming
trend is then forecast by the middle of next week.

&&

.DISCUSSION...as of 09:05 AM PDT Friday... Similar albeit
slightly cooler conditions this morning as an upper trough
continues to dig southward from the Gulf of Alaska and towards our
region. The trough is modeled to dig into the northern half of the
state through the next 24 hours as a pair of vorticity maxima
traverse the base of the trough. Meanwhile at the surface, a dry
cold front will push through the region today, primarily
introducing breezy to gusty conditions, especially along the coast
and over higher terrain, as well as introducing cooler
temperatures running around 3 to 10 degrees lower than yesterday.
The upper trough will weaken slightly in our region on Saturday as
the area aloft becomes void of significant vorticity. The upper
low is expected to restrengthen over Oregon and Northern/Central
California by Sunday as the core of the upper low shifts aloft. As
a result, much cooler temperatures are then to be expected as
500MB temperatures shift from -12C today to up to -24C by Sunday.
Low maximum temperatures should be expected, and given that 500MB
temperatures are anticipated to be around 3.5 standard deviations
from the climatological mean, a few record low maximum
temperatures could be set on Sunday or possibly even Monday.
Additionally, the first measurable rainfall for the season may
fall for many locations, especially in the North Bay, where a
tenth to up to a quarter of an inch may be possible. Further south
in the immediate Bay Area, amounts will range from a few
hundredths up to around a tenth, and further south, only a few
hundredths, if any rainfall, should be expected.

The upper low lifts into the intermountain west by Monday, leaving
behind a cool, transitionary period with northwest flow aloft for
Tuesday. Eastern Pacific high pressure will then build into the
region by mid week next week, leading to gradually warming and
drying conditions for the area. Long term models show another
upper trough descending into the region by late next week.

.PREVIOUS DISCUSSION...as of 03:32 AM PDT Friday...A deepening
mid/upper level trough off of the Pacific Northwest coast
continues to advect cooler air into northern and central
California. As a result, a cooling trend will continue today with
most locations 5-10 degrees below seasonal averages as a dry cold
front pushes through the region. Thus, expecting upper 50s to
lower 60s at the coast this afternoon with widespread 70s inland.
In addition, northwest winds will increase this afternoon with
breezy conditions forecast through the daytime hours. Similar
conditions will persist into Saturday as the upper level trough
deepens and remains off of the Pacific Northwest coast.

On Sunday, the main mid/upper level trough will drop southward and
push inland across northern California. This will advect even cooler
conditions into the region with daytime temperatures only warming
into the 60s for most of the region. Many locations in the higher
elevations and along the coast may only warm into the 50s on both
Sunday and Monday. As the system approaches from the northwest,
chances for rain showers will develop over the North Bay Sunday
morning and will spread southward in to the San Francisco Bay Area
by late morning and early afternoon. In addition, with increased
instability aloft, there remains a slight chance of thunderstorms
Sunday afternoon and early evening across this region. Rainfall
amounts will likely range from a few hundredths of an inch to around
0.20" in the North Bay Mountains. Rain showers may push as far south
as the Monterey Bay region on Sunday, yet widespread measurable
rainfall appears less likely over this region. On Monday, lingering
post-frontal showers will remain possible. The forecast models
differ slightly from Monday night into Tuesday with the ECMWF
showing another weak disturbance dropping into northern California
with another chance of rain showers across the San Francisco Bay
Area. However, the GFS keeps dry conditions across our region once
the main front pushes through on Sunday. At this time, will keep a
slight chance of showers in the forecast from Monday night into
Tuesday morning.

Temperatures will begin to moderate Tuesday into Wednesday as a flat
ridge rebuilds over much of the West Coast. As a result, dry weather
conditions are forecast from Tuesday through Thursday with
temperatures warming back to near seasonal averages. For late next
week, the models again differ with the ECMWF showing another system
pushing into the Pacific Northwest with chances for precipitation as
far south as the San Francisco Bay Area. Given low confidence this
far out, have kept the forecast dry for late next week. Stay tuned
as we continue to refine the forecast in the coming days.

&&

.AVIATION...as of 10:56 AM PDT Friday...Visible satellite shows
stratus clearing over the Monterey Bay Peninsula, where IFR CIGs
still persist. VFR at all terminals by 1830Z, perhaps even
sooner. Winds are already beginning to increase, especially around
the SF Bay Area. With a dry frontal passage expected this
afternoon, winds will be gusty out of the west/northwest at
times. Have CIGs returning for all terminals later this evening,
but confidence is low on timing and duration due to some questions
on how well the frontal passage will mix out the marine layer.

Vicinity of KSFO...VFR. Sustained onshore winds of 20 to 25 kt
with gusts to 30 kt expected this afternoon and evening. Forecast
models disagree when/if the marine layer will recover by tonight.
If it can, low clouds could move in by 08-10Z. Low confidence.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...IFR CIGs will quickly scatter out between
18-1830Z. Winds increase this afternoon, out of the west, and will
be sustained between 12-15 KT. Still some uncertainty with timing
of CIGs returning tonight, but have IFR CIGs at MRY around 02Z and
by 06Z for SNS. Low confidence.

&&

.FIRE WEATHER...as of 3:32 AM PDT Friday...Cooler and wetter
conditions are forecast today through early next week as an upper
level system pushes into the West Coast. This approaching upper
level trough will help maintain a deepening marine layer and
increased onshore flow. The main concern is an increase in
northwest winds this afternoon and evening as a dry frontal
boundary pushes through the region. This will result in breezy and
locally gusty winds near the coast and in the higher elevations.
Northwest winds of 10 to 20 mph with gusts greater than 30 mph
will be possible. As the main system pushes inland on Sunday, rain
showers will be possible from the North Bay down into the Santa
Cruz Mountains, potentially as far south as the Monterey Bay
region. In addition, increased instability will bring a slight
chance of thunderstorms to the aforementioned region Sunday
afternoon.

&&

.MARINE...as of 2:56 AM PDT Friday...An upper level low
approaching from the north will strengthen winds across the
coastal waters today. Strongest winds will be south of Point Sur
through Saturday. Winds will ease Sunday as high pressure builds
back in over the region. A small mixed swell with a long period
southerly swell will continue through the period.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...SF Bay from 12 PM
             SCA...Mry Bay from 12 PM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm

&&

$$

PUBLIC FORECAST: DRP
AVIATION: BAM
MARINE: BAM
FIRE WEATHER: RGass


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