742
FXUS66 KMTR 311059
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
359 AM PDT Fri May 31 2024

...New SYNOPSIS, SHORT TERM, LONG TERM...

.SYNOPSIS...
Issued at 339 AM PDT Fri May 31 2024

Thermal ridging will bring a minor warming trend to the region
through today. A trough will move into the Pacific Northwest this
weekend though, which will help strengthen the onshore flow and
deepen the marine layer, bringing a slight reprieve to the warmer
conditions. However, high pressure will build back across the
region by the middle of next week, bringing a much more pronounced
warming trend to the region, especially across the inland areas.
Inland areas could be upwards of 10 to 20 degrees above normal,
with a moderate heat risk by Wednesday.

&&

.SHORT TERM...
(Today and tonight)
Issued at 359 AM PDT Fri May 31 2024

There`s been a slight decrease in 500 mb heights since last
evening, however 500 mb geopotential heights are still near 580
decameters, a little greater than late May Oakland upper air
climatology (578 decameters). Despite weak cyclonic curvature at
the 500 mb level this morning there is still plenty of thermal
ridging evident on recent upper air sounding(s) and the Bodega Bay
and Fort Ord profiler data. The marine layer temperature inversion
is compressed to near sea level, the marine layer depth between
300 and 600 feet per latest profiler data. Dry air through a great
depth of the troposphere is allowing for nocturnal radiative
cooling, helping to bring about a narrow channel of stratus & fog
along the Big Sur coast to the Monterey Peninsula. Recent wind
swept and lofting of sea salt is also helping to form stratus/fog.
A 2.2 millibar SMX-SFO pressure gradient (light southerly wind)
is helping to slowly transport stratus/fog northward along the
immediate coast.

Otherwise the sky is clear, leading to another sunny day most
places today. Highs today will warm up to similar levels compared
to Thursday, 60s/70s coastside, 80s/90 inland. Lows tonight
cooling to the 50s, mixing with upper 40s especially across the
southern two thirds of the forecast area where late night cool air
advection is forecast to develop. Coastal stratus redeveloping
tonight may also produce spotty light drizzle.

&&

.LONG TERM...
(Saturday through Thursday)
Issued at 359 AM PDT Fri May 31 2024

Expect coastal stratus and fog Saturday morning, possibly leading
to additional spotty light drizzle, and a repeat of coastal stratus,
fog and patchy light drizzle redeveloping Saturday night and Sunday
morning. Saturday-Monday daytime highs cooling a few to several
degrees mainly inland, while near and along the coast daytime highs
will be about the same as today.

The rate of warming next week will then depend at least on
several synoptic factors, including what occurs upstream across
the mid-high latitudes of the North Pacific basin. A deep long
wave trough (cold air) north of a strong long wave ridge (warm
air), plenty of strong zonal jet stream wind including water
vapor transporting eastward from the western Pacific (also running
across positive anomaly sea surface temps with the Japan Current
in the central North Pacific) will bring about late season low
pressure system development, an eastward progression and zonal
pattern to southwestern Canada/the Pacific NW. Even as far south
as our forecast area the long wave trough off the Pacific NW may
briefly pierce/lower broad ~ 580 decameter 500 mb heights early in
the week.

Disagreement exists in the global model forecasts as to the
earliest stages of 500 mb high pressure system development, more
specifically the geographic location of building air mass, will it
initially be west (or east) of our forecast area? There`s good
global model agreement on the large scale with the exception of this,
and this is within the usually reliable 120 hour (5 day forecast).
But, it matters and it`ll influence whether we quickly or slowly
warm up next week, the depth of the marine layer, inland extent of
maritime influence, how hot it`ll get inland in our forecast area.
By early next week we`ll also see much greater water vapor move
across our forecast area with precipitable waters near early June
max moving average 1.25". Not enough dynamics for late season
rain, however with water vapor riding over chilly sea surface
temps expect good chance of coastal stratus and fog. Otherwise
remember we`re nearing peak sun angle and heating, we are about
three weeks from the summer solstice, continental surface heating
over the CONUS almost always couples to a warm core high pressure
system.

With subsequent global model output we should see whether or not
the warm up will be quicker or slower to develop next week. In the
meantime, it`s a good idea to plan on daytime highs in the 90s to
lower 100s inland, at least a moderate heat risk next week. Highs
becoming well above early June normal, and so far looks below record
highs at the long term stations. We`re going into this warm up
next week with good potential for immediate coastal cooling sea-
breeze even if limited as a shallow marine layer. The strength of
the northerly pressure gradient ACV-SFO will matter too, which
goes back to what was discussed in the previous paragraph in the
global model handling in the early stage of high pressure development.

&&

.AVIATION...
(06Z TAFS)
Issued at 1107 PM PDT Thu May 30 2024

VFR for the majority of terminals through the TAF period, with the
exception of Monterey Bay. Winds have eased for most locations, and
are expected to remain light through the overnight. Onshore winds
will increase in the afternoon tomorrow to become breezy. In the
later night of Friday, stratus will begin to make an appearance at
coastal terminals as it surges back along the coast.

Vicinity of SFO...VFR through the TAF period. Models appear to be in
good agreement that the SF bay should remain largely cloud free
through the TAF period. Winds will increase out of the W/NW in the
afternoon to become breezy once more, but ease into the nighttime.
Low clouds will begin to advect inland in the late night, though
model agreement appears uncertain as to whether CIGs will form.
Current thoughts are that an approaching trough will help to deepen
the marine layer and elevate cloud formation, allowing FEW-SCT low
clouds to develop around area terminals. However, confidence is
currently low in a CIG developing. This will be something to watch
in further updates.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Currently IFR at KSNS where the metar is
reporting OVC006. Models did not originally show CIGs forming at
this hour, however thoughts are that current CIG formation is a
result of micro-scale effects: sea salt whipped up into the air from
stronger winds the past few days may be contributing as cloud
droplet nuclei, allowing clouds to begin forming as the air cools
and condenses in the night. Either way, thoughts are that IFR CIGs
will prevail through the night for KSNS, and develop in the early
morning for KMRY. It is not out of the question that LIFR CIGs could
form, however, confidence is currently low on this so have left out
of the TAF. Expect a return to VFR conditions after sunrise.

&&

.MARINE...
(Tonight through next Wednesday)
Issued at 900 PM PDT Thu May 30 2024

Breezy to gusty winds will continue over the outer waters into
the weekend with gale force winds expected late Friday night into
Saturday. Significant wave heights offshore are currently up to
12 feet, but will begin to abate tomorrow through the remainder
of the weekend.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 3 AM PDT early this morning for Pt
     Arena to Pt Reyes 0-10 nm.

     Small Craft Advisory from 3 PM this afternoon to 3 AM PDT
     Saturday for Pt Arena to Pt Reyes 0-10 nm-Pt Reyes to
     Pigeon Pt 0-10 nm.

     Small Craft Advisory until 3 PM PDT this afternoon for Pt Arena
     to Pt Reyes 10-60 NM.

     Gale Warning from 3 PM this afternoon to 3 AM PDT Saturday for
     Pt Arena to Pt Reyes 10-60 NM.

     Small Craft Advisory until 3 AM PDT Saturday for Pigeon Pt to Pt
     Pinos 10-60 NM.

&&

$$

SHORT TERM...Canepa
LONG TERM....Canepa
AVIATION...AC
MARINE...JM

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