413
FXUS66 KMTR 231203
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
403 AM PST Mon Dec 23 2024

...New AVIATION, MARINE...

.SYNOPSIS...
Issued at 119 AM PST Mon Dec 23 2024

Coastal drizzle and North Bay showers are possible today with more
widespread significant rainfall across the region later tonight into
Tuesday morning. Very hazardous beach conditions continue through
the next couple of days. Rain chances continue in the days after
Christmas.

&&

.SHORT TERM...
(Today and Tuesday)
Issued at 119 AM PST Mon Dec 23 2024

Key Messages:
*Train of storm systems through next week
*Most impactful storm appears to be late tonight into Tuesday
*Rainfall totals between today and Sunday: North Bay: 4-10" | Bay
 Area: 1-6" | Central Coast: 1-5" | Salinas Valley: Up to 1"
*Wind gusts up to 40 mph (isolated 45 mph in spots) on Tuesday
*Life-threatening beach and ocean conditions linger into next week

High resolution model output points to scattered coastal drizzle and
some scattered showers in the North Bay, particularly in
orographically favored areas, through the course of the day with
probabilities ticking upwards starting in the late morning and early
afternoon hours. Otherwise most of the region remains dry with high
temperatures reaching the upper 60s to near 70 in the inland valleys
of the Bay Area and Central Coast, the mid 60s near downtown San
Francisco and Oakland, the lower 60s in the North Bay valleys and
along the Pacific coast, and the middle 50s to the lower 60s in the
higher elevations. Very hazardous marine conditions continue through
the day with a Coastal Flood Warning and a High Surf Warning in
effect through noon on Tuesday (see BEACHES for more information).

Tonight into Tuesday morning, the next band of rain is expected to
be the most impactful rainband of this active weather pattern so
far. Intense rain is set to begin impacting the North Bay late
tonight with the band moving through the Bay Area and Central Coast
overnight into the morning hours of Tuesday. Rainfall totals are
expected to reach 0.5-1" across the North Bay valleys and coastal
ranges south of the Golden Gate, up to 2" in the North Bay
mountains, 0.3-0.6" in the Bay Area valleys and Monterey Bay region,
and up to 0.25" across the interior Central Coast. The Weather
Prediction Center has issued a Marginal Risk for Excessive Rainfall
(at least a 5% chance that rainfall totals exceed flash flood
guidance) for tonight through 4 AM Tuesday across the northern and
western halves of Sonoma County. The main threats will be the
possibility for rapid rises in the creeks, streams, and small rivers,
especially as the last couple of days have moistened the soils.

Gusty winds will also spread with the rainband with gusts around 20-
30 mph in the valleys, reaching up to 35-45 mph in the ridgelines
and through the gaps and passes. As for convective probabilities,
model output is showing relatively robust CAPE values of around 200-
400 J/kg on Tuesday, but the 0-6km shear remains low (around 10-20
knots) during the passage of the highest CAPE values, thus
minimizing one of the three key ingredients (instability, shear, and
lift) for convection. The robust instability (for this part of the
world) may still foster some robust updrafts and thus a gusty wind
(up to 45-50 mph) and lapse rates on the order of 7-7.5 C/km per
forecast sounding would support coin-sized hail (pennies or
smaller).

Temperatures are expected to drop on Tuesday with the highs in the
middle 50s to near 60 in the lower elevations and the lower to
middle 50s in the higher elevations.

&&

.LONG TERM...
(Wednesday through Sunday)
Issued at 119 AM PST Mon Dec 23 2024

The Christmas Day forecast remains dry and partially sunny, although
hazardous marine and beach conditions continue to persist. Rain
returns to the forecast on Thursday with some uncertainty in the
exact totals. Analysis of the ensemble model clusters continues to
suggest that groupings that mainly consist of runs from the Canadian
and GFS ensemble models show shortwave ridging and "drier" scenarios
while those drawing from the European EPS model offer "wetter"
solutions.

Regardless of the actual rainfall totals, impacts from these
forthcoming systems could be notable. The Weather Prediction Center
has issued Marginal Risks for Excessive Rainfall across Sonoma
County for Thursday and Friday. In addition, with saturating soils
across the Bay Area, the risk for shallow landslides is increasing,
especially in the North Bay. We`re continuing to use the
deterministic NBM in our forecast, although additional refinements
may be necessary through the next couple of days.

&&

.AVIATION...
(12Z TAFS)
Issued at 401 AM PST Mon Dec 23 2024

Conditions continue to vary widely from MVFR to LIFR early this
morning per surface metar observations. Conditions begin to improve
by late morning or early afternoon to MVFR-VFR. Southerly winds
remain generally light through the day with the exception of the
Monterey Bay terminals which will be light northwesterly this
afternoon. Pre-frontal rain showers will be possible early afternoon
across the North Bay with the main frontal boundary moving across
that area late in the TAF period. Rain and breezy to gusty winds
develop tonight into Tuesday morning with breezy to gusty westerly
winds and lowering visibilities/ceilings.

Vicinity of SFO...IFR to brief LIFR conditions prevail early this
morning due to reduced visibilities and ceilings. Conditions begin
to improve late in the morning and into the early afternoon, but
generally remain MVFR to brief IFR. Cannot rule out VFR conditions
briefly this afternoon before conditions begin to deteriorate early
evening ahead of a frontal boundary. This boundary is anticipated
around 12Z Tuesday and will bring with it rain and gusty westerly
winds.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...MVFR to brief IFR conditions prevail early
this morning due to reduced visibilities and ceilings. Winds turn
light onshore this afternoon with improving visibilities and
ceilings. May see a period of VFR conditions briefly this afternoon
before conditions begin to deteriorate late evening. A frontal
boundary will approach the region late in the TAF period or just
beyond it with gusty westerly winds.

&&

.MARINE...
(Today through Saturday)
Issued at 401 AM PST Mon Dec 23 2024

A large to very large westerly swell arrives today and will
persist into Tuesday. Wave heights will remain elevated through
late week with 15 to 20 foot waves today through Wednesday.
Expect dangerous conditions for small crafts through at least
midweek. Elevated seas will continue to produce enhanced shoaling
at harbor entrances and bars.

&&

.BEACHES...
Issued at 119 AM PST Mon Dec 23 2024

A High Surf Warning is now in effect through noon Tuesday for
breaking waves 30 to 35 feet, with some of the favored spots such
as Mavericks potentially experiencing up to 40 foot breaking
waves. With the duration of this High Surf Warning, a Coastal
Flood Warning is also in effect to address the potential for
significant impacts to coastal locations such as piers, wharfs,
and other structures along the coastline. Flooding/inundation of
roads, parking lots, and coastal trails can also be anticipated.
These conditions will result in dangerous and life-threatening
bay, ocean, and beach conditions through Tuesday. Beyond Tuesday,
swell heights will begin to diminish, but favorable conditions for
hazardous surf conditions remain into the next week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Coastal Flood Warning until noon PST Tuesday for CAZ006-505-509-
     529-530.

     High Surf Warning until noon PST Tuesday for CAZ006-505-509-529-
     530.

PZ...Small Craft Advisory until 3 PM PST Tuesday for Mry Bay-Pigeon
     Pt to Pt Pinos 0-10 nm-Pigeon Pt to Pt Pinos 10-60 NM-Pt
     Arena to Pt Reyes 0-10 nm-Pt Arena to Pt Reyes 10-60 NM-Pt
     Pinos to Pt Piedras Blancas 0-10 nm-Pt Reyes to Pigeon Pt 0-
     10 nm.

&&

$$

SHORT TERM...DialH
LONG TERM....DialH
AVIATION...RGass
MARINE...RGass

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