956
FXUS66 KMTR 201444 CCA
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
740 AM PDT Tue Aug 20 2019

.SYNOPSIS...A warming trend is forecast through Thursday as high
pressure builds over central and southern California. Inland areas
will see less in the way of night and morning low clouds as the
marine layer compresses. Seasonable temperatures are forecast
through the upcoming weekend, along with continued dry
conditions.

&&

.DISCUSSION...As of 07:40 AM PDT Tuesday... The base of an
advancing 546dm 500mb low pressure trough offshore of the Pacific
Northwest has temporarily lowered heights aloft and allowed the
marine layer to deepen slightly versus yesterday morning (2600 ft
vs 2200 ft on 12Z KOAK sounding). The Fort Ord profiler shows
less of a day over day change as it is farther southward from the
trough. As a result, the marine layer is inundating most inland
areas approx 2500 ft and below. Above the marine layer, a
warm/dry air mass looms in response to a retrograding ridge
backing westward over the southern half of California. This high
pressure ridge will become the dominate synoptic scale feature
midweek and result in tomorrow (Wednesday) being the warmest day
of the next few. Temperatures tomorrow will range from near normal
along the shorelines to up to 10 degrees above normal for the
inland areas. Afternoon highs of 60s to low 70s are forecast along
the coast, upper 70s to upper 80s for areas in some proximity to a
coast/shoreline, and into the low to mid 90s for the extreme
inland areas far from the coast.

A few reports of coastal drizzle came in this morning, but
otherwise, the current (overnight) forecast looks on track and no
updates are planned until the afternoon package. The main forecast
challenges today are determining if/how much onshore flow will cool
temperatures Thursday and what impacts, if any, the remnants of a
tropical system may have across our area early next week. See
previous discussion for more information on the current forecast.

.PREVIOUS DISCUSSION...As of 3:35 AM PDT Tuesday...Our weather
during the past few days was primarily influenced by a deep marine
layer, resulting in widespread night and morning low clouds,
local drizzle, and cooler than normal inland temperatures. The
first sign that the pattern is changing is evident early this
morning with the Fort Ord Profiler indicating a decrease in the
depth of the marine layer as an upper ridge over the Southern
Plains and Desert Southwest begins to build to the west and across
southern California. In addition, light southerly low level flow
over the weekend is changing to a more typical west and northwest
direction. The building ridge and more typical wind patterns will
result in a warming trend across our region through Thursday.

Satellite currently shows an upper trough offshore along 140W.
The models track this trough inland across the Pacific Northwest
on Wednesday, but the trough is expected to remain too far north
of our area to have an impact on our weather. In fact, the models
agree that the biggest increase in temperatures during the
upcoming warming trend will occur on Wednesday. Temperatures are
forecast to warm slightly above normal by Wednesday when low to
mid 90s will be common in the inland valleys. Persistent light
onshore flow will keep coastal temperatures seasonably mild.

As the marine layer compresses over the next few days, expect
decreasing night and morning low clouds across inland areas, while
patchy fog is likely to develop near the ocean.

Models forecast a continued gradual increase in H5 heights late
in the week and through the weekend as the upper ridge continues
to build over California. However, both the GFS and ECMWF indicate
the warming trend will stall on Friday, and even reverse in some
areas with slight cooling forecast from Friday into the weekend.
The likely reason for this counterintuitive temperature trend
during the extended forecast period is the development of a weak
surface trough off the California coast which will turn surface
winds to a southerly direction by Friday. This southerly flow
will more readily transport marine air inland, especially in
places like the North Bay Valleys and Santa Cruz County. Forecast
temperatures from Friday through Sunday have been reduced below
the NBM values and brought more in line with the GFS and ECMWF MOS
guidance.

One wildcard late in the extended period is the possibility of
tropical moisture reaching our area during the first half of next
week. Longer range models track a weak tropical system near the
west coast of Mexico to the northwest and to a position off the
coast of the Baja Peninsula by the upcoming weekend. Moisture
from this system could then be drawn north and across California
sometime early next week. In fact, the 00Z GFS develops
precipitation over our area by next Tuesday night. There is still
a great deal of uncertainty as to how this scenario will play out
and any potential impacts in our area are currently at the end of,
or just beyond, our extended forecast period. But this is
something that will need to be monitored closely over the next few
days.

&&

.AVIATION...As of 4:39 AM PDT Tuesday...For 12z TAFs. Widespread
stratus under a 3000 ft marine layer has brought MVFR to localized
LIFR cigs to area terminals this morning. Low clouds are forecast
to scatter out between 17z-22z today depending on location. See
TAFs for more details. Light onshore flow around 5 to 10 kt will
persist through the morning hours becoming moderate and gusty
around 10 to 15 kt in the afternoon. Gusts to 25 kt possible over
KSFO.

Vicinity of KSFO...MVFR cigs will prevail through the morning
hours with clearing anticipated around 18z-22z today. West winds
of around 10 kt will prevail overnight becoming moderate and
gusty around 10 to 15 kt in the afternoon with gusts to 25 kt
possible over KSFO.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...MVFR to IFR cigs will persist through
the morning with clearing is anticipated around 18z-20z today.
West winds of around 5 kt will prevail through the morning then
increasing to 10 to 15 kt in the afternoon.

&&

.MARINE...as of 3:00 AM PDT Tuesday...High pressure over the
Eastern Pacific will maintain generally light to moderate
northwesterly winds over the coastal waters through mid-week.
Locally gusty coastal jets south of Point Arena, Pigeon Point and
Point Sur will create hazardous conditions for small vessels.
Breezy winds are also forecast for San Francisco Bay around the
Golden Gate and through the Delta, and over the Monterey Bay.
Mixed seas will continue with shorter period northwest waves at
around 8 to 10 seconds, a light longer period west swell, and a
light southerly swell.

&&

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

&&

$$

PUBLIC FORECAST: DRP/Dykema
AVIATION: CW
MARINE: CW

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NWS MTR Office Area Forecast Discussion