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

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FXUS66 KMTR 231613
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
913 AM PDT Sun Apr 23 2017

.SYNOPSIS...Dry and seasonably cool conditions are forecast today.
Unsettled weather is expected during the first half of the work week,
with periodic rain chances across mainly the northern portion of
our area. Dry weather will return to all areas by Thursday, with
warmer temperatures anticipated from Friday into next weekend.

&&

.DISCUSSION...As of 9:13 AM PDT Sunday...Skies are quickly
turning mostly sunny this morning with temperatures to warm into
the 60s and lower 70s for this afternoon. Onshore gradients are
ramping up with 2.1 mb from sfo to sac. This will keep a fresh
breeze blowing off the ocean today keeping any nearshore warming
in check this afternoon.

The next late season front will approach far northern California
by late tonight. The incoming cold front will bring light rain into
northern Sonoma County as early as 15z Monday morning. Then by
midday and through the afternoon some light rain will be possible
from the Golden Gate southward to Santa Cruz, mainly near the
coast/coastal hills. Have gone ahead and done a slight forecast
update to account for the slight chance of light rain on Monday.
Light rain chances will shift as far south as the Monterey
Peninsula by late Monday afternoon and early evening. Have
adjusted the PoP and weather grids to show these light rain
chances later Monday afternoon and evening as well, but ended all
light rain chances by 03z Tuesday. Rain total will be light, only
a few hundredths but perhaps enough to turn on the wiper blades
and wet the roads for the afternoon commute.

Yet another cool trough will approach Norcal on Tuesday keeping
slight rain chances going in the North Bay. However most areas
will remain dry with the main theme being seasonably cool weather
with brisk onshore breezes off the ocean. The seasonably active
pattern stays in place at least through midweek with North Bay
light rain chances still in play.

The forecast will turn dry by Thursday as a longwave trough digs
over the Great Basin. This will set the stage for a strong
surface pressure gradient with potential for some strong north
winds Thursday night into Friday, especially in the hills. Will
fine tune those details as the week wears on. Then the ecmwf shows
a strong ridge from the east Pacific beginning to nudge towards
the coast implying a warming and drying trend as we approach the
end of the month and into May.

&&

.PREV DISCUSSION...As of 2:45 AM PDT Sunday...An upper level
trough is currently tracking across northern and central
California. There is no precipitation associated with this trough,
only widespread mid and high level clouds which should mostly
clear by sunrise. This trough has also brought a cooler airmass
into our region which has produced mixing in the low levels and
helped clear most of the patchy low clouds that formed late
yesterday. Thus, we should see mostly sunny skies today along with
seasonably cool temperatures and brisk northwest afternoon winds
in coastal areas.

The weather pattern during the first half of the work week will
consist of cool northwest flow across our region. Embedded within
this flow will be a series of weak upper level disturbances which
may produce periods of light rain across the northern portion of
our forecast area (mainly North Bay) from Monday through
Wednesday.

The models agree that by Thursday the synoptic pattern will become
much more meridional as a deep trough develops to our east over
the Intermountain West and an upper ridge amplifies offshore.
This will set up a dry north flow across our region with the
potential for locally strong northerly winds in our hills on
Thursday night and Friday morning. Temperatures will warm late in
the week, and especially over the weekend as the upper ridge
offshore shifts eastward and over California. By Saturday high
temperatures could be as much as 15 degrees warmer than what we
will see during the first half of this week.

&&

.AVIATION...as of 4:44 AM PDT Sunday...For 12z Tafs. Generally
VFR across the district as of writing this discussion. Could be
brief MVFR cigs early this morning, especially for Monterey Bay
terminals. VFR this afternoon, with breezy and gusty onshore winds
by late morning. Another tough cig forecast for tonight. Local
WRF model suggests a relatively moist boundary layer through the
day today, enough to keep FEW-SCT low clouds across the region.
It increases RH values later this evening. Likewise, latest LAMP
guidance has MVFR cigs at most terminals by 08-09z Mon., earlier
for Monterey Bay terminals. Needless to say, confidence is low on
cig coverage and timing tonight.

Vicinity of KSFO...Generally VFR through this afternoon. Cannot
rule out a couple of hours of MVFR cigs before sunrise, but not
enough confidence to include in tafs. W-NW winds 15-20 knots
Sunday with afternoon and evening gusts to 25-30 knots possible.
Could be MVFR cigs late tonight, into the overnight hours, though
confidence is low.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals..Patchy MVFR cigs possible through about
15z or so, then VFR. Onshore winds 10-15 knots by late morning,
with gusts in excess of 20 kts at times. Potential for MVFR cigs
by 05z Monday.

&&

.MARINE...as of 08:59 AM PDT Sunday...Look for moderate
northwesterly winds to continue through the forecast period with
the highest speeds near the coast. winds and seas will
dramatically increase by thursday as the surface  pressure
gradient tightens along the california coast.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...SF Bay from 3 PM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm from 3 PM
             SCA...Mry Bay from 12 PM

&&

$$

PUBLIC FORECAST: RWW/Dykema
AVIATION: BAM
MARINE: Bell


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