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

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FXUS66 KMTR 182056
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
156 PM PDT Wed Oct 18 2017

.SYNOPSIS...A cold front remains on track to push trough the
region on Thursday bringing our first widespread rainfall event
and cooler temperatures. Look for the greatest rainfall to be
along the coastal ranges and in the North Bay Thursday afternoon
through early Friday morning. Along with cooler temperatures,
breezy onshore winds can be expected through Friday. Dry
conditions return along with a warming trend through the weekend
with temperatures warming well above seasonal averages early next
week.

&&

.DISCUSSION...as of 01:55 PM PDT Wednesday...In the short-term,
have lowered temperatures slightly through the remainder of the
afternoon as the marine layer across the region is sitting at
around 1000 feet in depth. This allowed for low clouds to hang on
through the morning for many locations with the cloud deck
currently hugging the coastline. Look for low clouds to push back
inland overnight with patchy fog along the coast and locally into
the Sonoma Valley.

The much anticipated cold front remains on track to push through the
region Thursday into early Friday morning. With this, light
precipitation is forecast to spread into the North Bay as early as
late Thursday morning through the early afternoon. The rain is then
expected to increase in coverage and intensity by Thursday afternoon
before spreading southward through the evening down into the Santa
Cruz Mountains. Rainfall will then move across the Central Coast
late Thursday night into early Friday morning, generally before
sunrise. Have increased rainfall amounts over the North Bay,
especially in the coastal ranges and inland mountains based off of
the latest short-rage, high resolution models. Cannot rule out some
of these higher locations picking up between 0.25" to 0.50" with
isolated amounts nearing 0.75" with PWAT values forecast to range
from 1.00" to 1.25". Meanwhile, North Bay Valley locations can
expect generally between 0.10" to 0.20" of rainfall from this
system. Heading southward, portions of the Santa Cruz Mountains can
expect between 0.10" to 0.25" with isolated amounts upwards of
0.33". Lesser amounts are forecast along the Big Sur Coast at
generally less than 0.25". Inland valley locations outside of the
North Bay can expect amounts generally around or less than 0.10"
with your typically dry spots such as the Santa Clara and Salinas
Vallies receiving just a few hundredths of an inch or less.

Along with the rainfall, temperatures will turn cooler with
afternoon readings only in the 60s for much of the northern two-
thirds of the region on Thursday. Onshore winds will also become
breezy at times, yet widespread strong winds are not likely with
this frontal passage. Showers may also linger over the Central Coast
into Friday morning as cold air advection sets up in wake of the
frontal passage. By Friday afternoon, region-wide temperatures are
only likely to warm into the 60s with on going breezy onshore winds
and any lingering rain showers diminishing.

High pressure will then develop in wake of the exiting trough
heading into the weekend with a slight warming trend and a return to
dry weather conditions. The ridge will strengthen to our south over
southern California through the first part of next week and allow
for additional warming. By Monday and Tuesday, forecast models are
showing 500 MB heights of around 595 DAM over the region. Thus, look
for afternoon temperatures both days to be in the 80s for most
inland ares with middle/upper 70s near the coast and in the City of
San Francisco.  Cannot even rule out a few of the warmer inland
spots reaching the 90 degree mark by Tuesday. This pattern would
also support dry weather conditions heading into the middle part of
next week as well.

&&

.AVIATION...As of 10:38 AM PDT Thursday...For 18z tafs.
Widespread stratus along the immediate coast, but is beginning to
retreat from Bay Area terminals. Stratus over the San Francisco
Bay is beginning to dissipate with KSFO having gone SCT just
recently. Can also see stratus eroding over KMRY and KSNS. Once
all the stratus clears over land this morning expecting VFR vis
and cigs through this evening. Wildfires are still burning across
parts of the area, but conditions have improved. There`s still a
chance that HZ or FU could temporarily reduce visibilities to MVFR
for taf sites in the vicinity of the fires. Current model
guidance indicates an early return of widespread stratus tonight.
Onshore flow this afternoon with winds 10 to 15 kt possibly
gusting to just over 20 kt at KSFO.

Vicinity of KSFO...Stratus retreating away from the terminal. VFR
conditions to prevail today with possible periods of MVFR
conditions due to HZ from the wildfires. Stratus is expected to
return early tonight although exact timing is uncertain, later
clearing is also expected for tomorrow. West to northwest winds
this afternoon 10 to 15 kt possibly gusting to 20 kt.

SFO Bridge Approach...Stratus is currently burning off and
visibility as improving. Once stratus clears expect similar
conditions to the terminal.

Monterey Bay Terminals...LIFR to IFR conditions currently, but
stratus is burning off. Expecting clearing shortly. VFR
conditions to prevail once stratus has cleared. Some HZ or FU may
reduce vis to MVFR at times due to the wildfire in the Santa Cruz
mountains. IFR cigs due to stratus and fog are likely to return
early this evening.

&&

.FIRE WEATHER...As of 3:35 AM PDT Wednesday...Relative humidity
values have been increasing in the valleys and along the lower
coastal slopes as light onshore flow begins to develop. However,
the airmass in the hills above 1000 feet remains relatively warm
and very dry. Poor humidity recoveries are occurring in the hills
this morning. Fortunately, winds will remain light. More
widespread cooling will take place today as onshore winds increase
in the afternoon. This will result in better humidity recoveries
tonight into Thursday morning and more significant cooling by
Thursday. A weak early season weather system is forecast to bring
light rain to the North Bay from Thursday afternoon through
Thursday evening and to much of the rest of the district on
Thursday night. The models have been trending drier with this
system. Latest forecast rainfall amounts for Thursday and Thursday
night range from a tenth to a quarter of an inch across northwest
Sonoma County and northern Marin County, and generally less than
a tenth of an inch in all other areas. Rainfall is expected to end
by Friday.

A warming trend will begin over the upcoming weekend as a high
pressure ridge begins to build over California. This ridge will
strengthen into early next week and result in warm and dry
conditions districtwide on Monday and Tuesday.

&&

.MARINE...as of 09:07 AM PDT Wednesday...Areas of fog this
morning across the coastal waters and into the bays. Winds will
increase over the bays this afternoon into tonight. A cold front
will move across the waters late tomorrow bringing chances of
light rain. A moderate to long period mixed northwesterly and
southerly swell has arrived with buoys reporting periods of 17 to
18 seconds across the waters. Late Thursday into Friday a large
northwesterly swell train will move across he area. This large
swell will create hazardous conditions.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...SF Bay until 4 AM

&&

$$

PUBLIC FORECAST: RGass
AVIATION: AS
MARINE: AS
FIRE WEATHER: Sims

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