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

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697
FXUS66 KMTR 211554
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
854 AM PDT Mon May 21 2018

.SYNOPSIS...Below normal temperatures will continue as an upper
level low moving across the state maintains a moderately strong
gradient resulting in brisk onshore flow across the region and a
deep marine layer. Another upper level low will approach the
later half of the week and into the weekend with onshore winds
keeping temperatures on the cool side of normal while mainly dry
conditions persist.

&&

.DISCUSSION...as of 08:53 AM PDT Monday...Coastal drizzle
continues to impact the Central Coast this morning along with low
clouds. Further north, the entire North Bay and inland portions of
the San Francisco Bay Area are under clear sky conditions as a
result of northerly flow. Look for clouds over the Central Coast
to burn-off later this morning for inland areas while some cloud
cover may hold along the coast for much of the day. The ongoing
forecast remains on track this morning with no major updates
needed at this time. For additional details, please see the
previous forecast discussion below.

&&

.PREV DISCUSSION...as of 3:25 AM PDT Monday...Nighttime optics
reveal widespread low clouds across much of the district from the
San Francisco/San Mateo county line south. Meanwhile areas to the
north remain relatively clear with the exception of a couple of
patches over portions of Napa and Marin. Latest observations show
cloud decks around 1500 feet. A distinct difference from yesterday
is the gradients. As the low digs farther south the northerly
gradient has strengthened while the westerly gradient has
weakened. ACV to SFO (northerly gradient) strengthened from 3.2 mb
to 5 mb over the last 24 hours. While SFO to WMC (westerly
gradient) weakened from 5.2 mb to 2.1 mb during the same time.
This and the cyclonic flow associated with the upper low currently
centered near Fresno are the reasons why the northern half of the
forecast area remains clear at this hour.

Temperatures at this hour are running in the low to mid 50s, within
a degree or 2 of temperatures at this time yesterday. While
temperatures are forecast to be slightly warmer than yesterdays
highs, temperatures will remain below normal for this time of
year. Warmest temperatures are expected over the North Bay today
and Tuesday as a result of the stronger northerly flow that has
all but cleared out the low clouds and will allow for abundant
sunshine. Santa Rose is forecast to reach 79 degrees today with
mid 80s possible over the warmest inland North Bay locations. With
another upper low on the heels of the low mentioned above there
will be little chance of a warm up. The approaching low will
maintain cool temperatures across the region as it once again
strengthens the onshore gradient and deepens the marine layer
allowing widespread penetration of the cool marine air and low
clouds.

Coastal drizzle remains possible this morning mainly for portions
of the San Mateo, Santa Cruz and Monterey County coastlines. The
NAM and WRF continue to show localized light precipitation along
the immediate coast.

The next upper level low is forecast to arrive Thursday night
into Friday. Anticipate onshore flow to strengthen and daytime
highs to remain below normal. Models continue to fine tune their
solutions with respect to timing as well as the possibility of
precipitation over the area. Will hold off on making any changes
as we are still far enough out. Models do indicate however that
this system will kick out of the region by Sunday and high
pressure will begin building back in. Warmer temperatures appear
to be on tap for at least the second half of the holiday weekend.

&&

.AVIATION...As of 4:56 AM PDT Monday...For 12z TAFs. A very
erratic stratus deck in place across the Bay Area this morning.
Generally MVFR cigs at all but North Bay terminals at this hour.
Anticipate this ill defined/pseudo marine layer to mix out
earlier today than it did yesterday, except for terminals along
the immediate coast, such as KMRY. Onshore winds will continue
through the day, less windy/gusty than yesterday. Stratus will
return again this evening.

Vicinity of KSFO...Currently MVFR cigs affecting the terminal.
However, latest fog product satellite shows a hole in the stratus
over the bridge approach. A little tough to know how this will
affect the terminal and clearing time. TAF may have to be amended
if this hole spreads northward. Breezy, occasionally gusty,
onshore winds this afternoon, not as breezy as previous days.
Stratus expected to return again early this evening.

SFO Bridge Approach...Hole in the stratus is resulting in CLR
conditions, per latest SMB ob. However, intermittent MVFR cigs may
affect bridge approach through 18-19z.

Monterey Bay Terminals...Another day where MVFR cigs are possible
through the TAF period, specifically at KMRY. Partial clearing
may occur late this morning/early afternoon at KSNS. Light to
moderate westerly winds, occasionally gusty, by afternoon. Low
clouds return by late afternoon or early evening.

&&

.MARINE...as of 08:53 AM PDT Monday...Moderate occasionally gusty
northwest winds will gradually decrease later today and tonight
as pressure gradients begin to ease. The strongest winds are
expected from Point Reyes northward. Mixed seas with a long period
southwest swell combined with a shorter period northwest swell
through the coming days.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...SCA...Mry Bay from 10 AM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm from 10 AM
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...SF Bay from 3 PM

&&

$$

PUBLIC FORECAST: RGass/CW
AVIATION: BAM
MARINE: DRP

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