High Surf in California, May 19 to 20, 2020

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1027 AM PDT Tue May 19 2020

.SYNOPSIS...Drier conditions will develop today, although shower
chances will remain, especially in the hills. Dry weather is
forecast from Wednesday through the Memorial Day weekend.
Temperatures will warm to near seasonal averages by Thursday, and
then likely warm above average over the long holiday weekend.

&&

.DISCUSSION...as of 08:30 AM PDT Tuesday...Isolated to scattered
rain showers moved over portions of the South Bay and adjacent
hills as well as the Central Coast earlier this morning. During
the past hour or so, most of this activity has diminished in
coverage with rain showers now mainly confined to the southern
coastal waters off of the Big Sur Coast. As mentioned below, there
will likely be another renewed chance of showers this afternoon
given increased instability associated with daytime heating.
However, most of the region will remain dry today. For additional
details, please see the previous forecast discussion below.

&&

.PREV DISCUSSION...As of 3:05 AM PDT Tuesday...The upper low that
brought unsettled weather conditions to our region over the past
few days is currently centered near Sacramento. A shortwave trough
rotating down the back side of the low is producing scattered
showers early this morning, mainly over our southern coastal
waters. But a few showers are making their way inland around the
Monterey Bay Area, some of which are producing brief heavy rain.
Based on short-term model output, scattered showers will mostly
remain over our southern coastal waters before dissipating around
sunrise. The remainder of the morning looks to be mostly dry.
Although the upper low will continue to drift off to our east
today, models indicate that enough residual moisture and
instability will remain to result in isolated showers once
sufficient surface heating develops in the afternoon. Afternoon
showers are expected to mostly develop over the inland hills. The
NBM's afternoon shower chances are primarily across our southern
higher elevation areas. But some short-term models, most notably
the HRRR and to a lesser extent the NAM, also develop afternoon
showers over Napa County. Have therefore added a slight chance of
afternoon showers to much of Napa County, mostly over the hills.
Model consensus is that all shower activity will end by this
evening, although the NAM maintains isolated shower activity
through tonight as one last shortwave trough drops from north to
south over California.

The upper low brought a cool airmass into our area and inland
temperatures will remain well below seasonal averages today while
coastal temps will be only a few degrees cooler than normal. A
modest inland warming trend is forecast for Wednesday and
Thursday as a warmer and drier airmass develops under a weak upper
ridge. The warming trend will stall temporarily on Friday when an
upper trough over the Pacific Northwest moves to the southeast
and clips northern California.

Dry and warmer weather is forecast for the Memorial Day weekend,
but just how much temperatures warm is the difficult part of the
extended forecast. The ECMWF builds a strong upper ridge over
California and warms temperatures well above normal by Sunday and
Monday, especially inland. The GFS develops much less robust
ridging and thus forecasts more modest weekend warming. Until the
models come into better agreement, the best forecast approach is
to split the difference and forecast warmer than average
temperatures, but not as much warming as forecast by the European
model.

&&

.AVIATION...as of 10:27 AM PDT Tuesday...For 18z tafs. VFR
conditions to prevail through this evening before more widespread
low clouds are anticipated overnight as the boundary layer forms.
Patchy convective cigs into this afternoon 2,000 to 6,000 ft AGL.
SW/W winds this morning becoming more W/NW this afternoon around
10-15 kt with locally higher gusts anticipated.

Vicinity of KSFO...VFR through this evening with SCT to BKN cigs
2,000 to 6,000 ft. Cig conditions expected to gradually improve
this afternoon. MVFR cigs forecast to develop overnight. W/NW
winds increasing this afternoon to around 15 kt with gusts 20-25
kt possible.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR conditions through this evening
before low clouds expected to develop this evening and last
overnight. SCT to BKN cigs around 3,000 to 5,000 ft. W/NW winds
around 10-15 kt with occasional higher gusts possible.

&&

.MARINE...as of 09:30 AM PDT Tuesday...North to northwest winds
will gradually increase across the coastal waters today and
tonight as an upper low remains over California. Isolated showers
remain possible this morning over the southern waters. Breezy
afternoon and evening winds are also expected over both of the San
Francisco Bays as well as the Monterey Bay. The strongest winds
today are anticipated north of Point Reyes. Winds will then remain
elevated through the rest of the work week. Mixed seas will
persist with a short to moderate period northwest swell and a
longer period southerly swell.

&&

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

&&

PUBLIC FORECAST: RGass
AVIATION: AS
MARINE: AS

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