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

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

573
FXUS66 KMTR 201153
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
453 AM PDT Wed Jun 20 2018

.SYNOPSIS...Not much change in weather conditions through
Thursday, then a pronounced warming trend for the end of the week
as high pressure builds in and winds turn more offshore. A return
to cooling and redevelopment of the marine layer appears in store
for Sunday into the first part of next week as flow shifts back
onshore.

&&

.DISCUSSION...as of 3:30 PM PDT Wednesday...GOES-East fog product
reveals another morning of stratus across much of the Pacific
Coast and pushing into parts of the San Francisco Bay Area. The
Fort Ord profiler indicates the marine layer is ~ 1000 ft deep, or
within a few hundred feet of what it was this time yesterday
morning. The shallow marine layer will bring localized fog to
parts of the coast again this morning. If you encounter decreased
visibilities for your morning commute, be mindful of your speed,
following distance, and make sure your headlights are turned on.

Synoptically-speaking, an upper level ridge continues to build
with its center located just off the Baja California coast. Over
the next 24 hours, a shortwave trough will ripple through this
ridge`s outer periphery and clip northern California. While no
precipitation is expected with this shortwave, this should limit
substantial warming for the North Bay this afternoon. Southern
portions of the CWA such as the southern Salinas Valley and
interior San Benito Counties will probably be too far south to see
any influence from this shortwave and would be the most likely
locations to experience 3-6 degrees of warming from yesterday to
today.

The shortwave will quickly push east toward the Northern Rockies
by Thursday. For the San Francisco Bay Area and central California
coast, not expecting much change from Wednesday to Thursday for
afternoon highs as the upper level ridge flattens. The ridge won`t
stay flattened for long as models agree that broad high will
build offshore, pushing 500 mb heights over the Bay Area anywhere
from 590 to 592 DAM by Saturday. At the surface, this will result
in hot conditions, mostly for interior locations, where highs are
expected to soar to the middle/upper 90s to the triple digits.
Locations along the immediate Pacific coast should remain
protected from these hot temperatures with just enough of an
onshore flow. This all changes as you head inland and/or up in
elevation.

The big question at this point in time is exactly how hot it will
get along the SF Bay shoreline for Saturday. One tool forecasters
use for determining the potential outcomes is through ensemble
forecasting. An ensemble is comprised of ensemble members
(anywhere from a dozen to several dozen, depending on the model),
each of which produces a slightly different forecast solution. The
spread of these forecast solutions is valuable to meteorologists
as we can see the potential spread through all the ensemble
members. Now, when looking at the ECMWF Ensemble Prediction System
(EPS) for Saturday`s heat event, some of the hottest ensemble
members -- at SJC, for example -- do exceed 100 deg F. The current
forecast package for Saturday does bump up maximum temperatures a
bit along the SF Bay shoreline and the South Bay, though
confidence still remains on the low side in regard to these exact
values as there remains a rather large spread in possible
outcomes. The potential heat risks based on this forecast does
introduce widespread moderate and isolated high risks across the
North Bay East Bay interior, South Bay, and interior Monterey +
San Benito Counties. Still a bit early to launch any headlines,
but if the current forecast holds, we`d likely be looking at heat
advisories for the warmest locations. Overnight low temperatures
will remain elevated in the thermal belts with some locations
at/above ~ 1000 ft elevation likely to only cool to the mid/upper
60s to 70s.

Another thing to mention for the late Friday to Saturday timeframe
is the potential for gusty winds in the higher elevations. The
GFS does advertise north to northeast winds for the North and East
Bay Hills/Mountains that could produce elevated fire weather
concerns. Will continue to monitor these forecast trends,
especially as we approach the event when the higher resolution
models are able to capture this with finer detail.

Models for Sunday suggest the start of what would be a welcomed
cooling trend across the interior. The latest GFS run even brings
in what could be a southerly surge along the Big Sur coast,
pushing north toward the Santa Cruz and San Mateo coasts through
the day. Additional cooling is expected for much of the region --
especially interior locations -- heading into the beginning of
next week.

&&

.AVIATION...As of 4:53 AM PDT Wednesday...For 12z TAFs. Fort Ord
Profiler shows the marine layer depth around 1000 feet. Surface
obs across the CWA show a mixed bag of flight categories, from VFR
for inland terminals to LIFR at immediate coastal terminals.
Expect VFR at all terminals between 16-18z this morning. Moderate
onshore flow this afternoon will increase winds around 12-15 kts,
with locally higher gusts. Stratus will push back in again
tonight, but low confidence on exact timing and coverage.

Vicinity of KSFO...Borderline MVFR/IFR cigs will gradually lift
and mix out through the morning, becoming VFR by 17-18z. Onshore
winds around 20 kt, with gusts near 30 kt this afternoon and
evening. MVFR/IFR cigs likely again tonight, best guess around
midnight, give/take a couple of hours.

SFO Bridge Approach...IFR through about 16-17z, then VFR.

Monterey Bay Terminals...LIFR cigs to IFR/MVFR by 15-16z, then
VFR by 17-18z. West/northwest winds 10-15 kt this afternoon and
early evening. IFR/LIFR cigs return this evening, around 02-03z.

&&

.MARINE...as of 2:56 AM PDT Wednesday...High pressure off the
California coast will continue to strengthen through the day
resulting in increasing northwest winds, which will prevail
through much of the upcoming weekend. Sustained gale force winds
are expected along the Big Sur coast, south of Point Pinos, this
afternoon and evening. Gale force winds will also be possible for
the northern outer waters, north of Pigeon Point, Friday evening
through Saturday morning. More details will be addressed in future
updates. Steep fresh swell from strong winds will create
hazardous seas into the weekend, particularly for operators of
small crafts.

&&

.FIRE WEATHER...as of 3:30 AM PDT Wednesday...A pronounced
warming and drying trend is forecast for the end of the week as
high pressure builds in and winds turn offshore. Saturday
presently looks to be the hottest day, with poor overnight
relative humidity recoveries for both Friday night and Saturday
night. Locally gusty north to northeast winds are possible in the
North and East Bay hills Friday night and Saturday morning. A
return to cooling and redevelopment of the marine layer appears in
store for Sunday into the first part of next week as flow shifts
back onshore.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...SCA...SF Bay from 12 PM
             SCA...Mry Bay from 12 PM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm from 12 PM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm from 12 PM
             SCA...Pt Arena to Pigeon Pt 10-60 nm from 12 PM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm until 2 PM
             GLW...Pt Pinos to Pt Piedras Blancas 0-10 nm from 2 PM
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm from 2 PM

&&

$$

PUBLIC FORECAST: Rowe
AVIATION: BAM
MARINE: BAM
FIRE WEATHER: Blier/Rowe

Visit us at www.weather.gov/sanfrancisco

Follow us on Facebook, Twitter, and YouTube at:
www.facebook.com/nwsbayarea
www.twitter.com/nwsbayarea
www.youtube.com/nwsbayarea



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.