FXUS66 KMTR 220545
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
945 PM PST Sat Dec 21 2024

...New AVIATION...

.SYNOPSIS...
Issued at 225 PM PST Sat Dec 21 2024

Our active weather pattern will continue through this week
as several upper weather disturbances continue to slide across 
the west coast over the next 7 to 10 days. The most impactful 
system appears that it'll arrive through the day on Tuesday when 
there will be the potential for widespread shower and scattered 
thunderstorm activity. In addition to the rain and thunder threat,
very hazardous marine conditions today and into at least the 
first half of the week.

&&

.UPDATE...
Issued at 845 PM PST Sat Dec 21 2024

The forecast remains on track this evening. Expect cloudy and cool
conditions with scattered light rain that should not impact travel
plans tonight or tomorrow. Rain chances begin to increase once 
again across the North Bay tomorrow evening, with up to a 0.50"
possible by early Monday morning, with much lesser amounts across
the remainder of our area to the south. A stronger, more dynamic
Atmospheric River system will begin to impact the coastal range of
the North Bay as early as Monday night, spreading across the
remainder of our area during the day Tuesday.

&&

.SHORT TERM...
Issued at 225 PM PST Sat Dec 21 2024 
(This evening through Sunday Morning)

Largely zonal flow will be in place tonight and into early 
Sunday. Gradual ascent atop a diffuse low level frontal boundary 
(more evident in 850mb theta-e fields) will encourage the re- 
development of rain showers near and along this boundary. At 
current time, it's anticipated that this boundary will be draped 
from along the Golden Gate eastward toward the Delta. Locations 
near and south of this boundary have the best opportunity for 
measurable rainfall, especially in areas of higher terrain (Santa 
Cruz Mountains, eastern Santa Clara and East Bay Hills, as well as
the Santa Lucias). Overall rain amounts tonight into midday 
Sunday are anticipated to range between a few hundredths of an 
inch, except across the higher terrain. There is some bust 
potential overnight depending on the location of the stalled 
boundary. If the boundary shifts north or south, then the axis of 
"heaviest" rainfall will shift with it. In addition, if the depth 
of moisture is lower than currently anticipated, we'll likely be 
dealing with more in the way of drizzle and potentially fog. At 
present time, the chances for widespread visibility below 3 miles 
is around 20-30% and may be more tied to brief bouts of heavier 
precipitation. For now, will defer to the evening shift for any 
near-term refinements to the Wx grids, but widespread dense fog 
does not appear probable at this time. 

By day break, the greatest coverage of rain showers is anticipated
somewhere between San Francisco Bay and Monterey Bay regions. PoPs
will be on the order of 30-70% as modest ascent continues along
the frontal boundary. Instability appears paltry and the greatest
MUCAPE values are forecast to reside across extreme southern
portions of the Big Sur Coastline. With the greatest lift
remaining to the north, I believe the the thunderstorm threat will
be quite low on Sunday, but something we'll monitor. There will be
a break in rain shower activity and a good portion of the day on 
Sunday may be rain-free (with perhaps some lingering drizzle in 
higher terrain).

&&

.LONG TERM...
Issued at 225 PM PST Sat Dec 21 2024
(Sunday Afternoon through next Friday)

Key Messages:
*Parade of storm systems through the end of the week
*Most impactful storm appears to be Tuesday
*Rain amounts thru next Saturday 2-6" North Bay | 1/2-3" Bay
 Area/Central Coast | Up to 1/2" Southern Salinas Valley
*Wind gusts of 25 to 45 mph on Tuesday

Late Sunday afternoon, a +150 knot 300mb jet noses into the area.
In response, surface pressure along the coast will fall. These 
surface pressure falls will promote low level convergence along a 
slightly sharper frontal boundary and this will give the area our 
next chance for rain Sunday evening and into Monday. There's some
uncertainty, of course, and with a bulk of the stronger forcing 
for ascent likely to remain far to the north, there will be a 
sharp PoP gradient from the north to the south. In addition, the 
recent wet weather likely has a strong influence on the National 
Blend of Models via it's statistical methods to bias correct PoPs.
Given the location of the strongest forcing being displaced to 
the north and it appears that some of the more dynamical, though 
still ensemble based, NWP paint more of a reasonable picture for 
what is anticipated Sunday and into Monday. Areas across the North
Bay will have the greatest chance for late Sunday and into Monday
afternoon with PoPs still around 80%. PoPs taper downward 
dramatically south of the San Francisco Bay with PoPs ranging from
15% to 50% elsewhere (except far southern Monterey and San Benito
counties where PoPs are around 10%). Rain amounts between 11am 
Sunday and 11am Monday (with a bulk of this falling late Sunday 
into early Monday) range between a few hundredths of an inch to 
maybe 1/2 inch (mainly across the North Bay). There's around a 50%
chance that rain amounts as great as 3/4 inch transpire across 
the extreme northern portions of the North Bay. All in all, the 
Sunday and Monday round of rain is anticipated to be less 
impactful than Tuesday's anticipated rainfall. Checking in on some
of the local hydrology, it appears that 1 and even 6 hour 
rainfall amounts will remain below any sort of Flash Flood 
guidance through Monday, though quick rises in smaller creeks, 
rivers, and streams may still be in play. Special thanks to EKA 
and STO for their coordination on this portion of the short term 
forecast. 

Starting late Monday and into Tuesday, our higher impact system
arrives. Global NWP is in good agreement that a deeper trough 
will amplify and sweep through the area on Tuesday. Taking a look 
at some more of the traditional synoptic meteorology indicates a 
deep trough characterized by 130 meter 500mb height falls. The 
upper trough does exhibit somewhat of a positive tilt and this 
could minimize the threat for this feature stalling. It'll still 
pack a punch, however, by early morning Tuesday, as most of the 
Bay Area and Central Coast will be within the favorable exit 
region of the upper level jet. The resultant cyclogenesis will 
help to enhance low level flow which may translate to 25 to 45 mph
wind gusts as well as help encourage a larger area of 
precipitation. Chances of a peak gust or two above 35 mph is 
around 60-80% mainly across the higher terrain of the North Bay, 
East Bay, SF Peninsula, Santa Cruz Mountains, and the Big Sur 
Coastline/Santa Lucia's. At this time, I don't anticipate the need
for a High Wind Watches given even our local WRF and 90th
percentile NBM has peak wind gusts of around 45 mph along the 
ridgelines (which is where most of the grid edits were focused). 
If the system intensifies, stronger winds will be realized at
lower elevations (including the larger populations areas). 
Regardless, those with outdoor decorations will want to make sure 
they are secure by Tuesday, else they may become airborne with 
this next system.

850mb theta-e progs show a ribbon of more unstable air along the 
surface/850mb fronts. The probability of surface-based CAPE at or
above 200 J/kg is maximized at around 60-70% across the Bay Area 
on Tuesday. Given the spatial extent of higher theta-e air, I'll 
advertise a broad area of isolated thunderstorms that encompasses 
all of the North Bay, East Bay, SF Peninsula and parts of the 
South Bay. It's possible that additional areas of thunder will be 
needed in subsequent forecasts. We will also keep tabs on the 
potential for any sort of severe weather as 500mb winds will 
exceed 50 knots and likely support at least loosely organized 
convective modes. Rainfall amounts late Monday and into Tuesday 
will range from 1/4" across southern Monterey/San Benito counties 
up to around 2 1/2" of rainfall across the North Bay. There's 
around a 40% chance that 24 hour precipitation amounts exceed 3" 
across the North Bay. Additional rises on area creeks, rivers, and
streams are most likely during this time. While deterministic and
50-60th percentile hydrologic output keep most of our mainstem 
river points in monitor/action stages, we'll need to examine the 
potential for any more robust/vigorous showers and storms. If 
instability is a bit greater and/or training activity sets up, the
potential for river and flash flooding will increase. For now, 
we'll forgo issuance of a Flood Watch on this shift, but something
to consider, particularly if Sunday into Monday's round of rain 
overachieves. 

Most of Christmas Day will be rain/free with partly sunny skies.
Hazardous boating and beach conditions are probable, however, so
if headed to the beach, make sure you're aware of any beach hazard
or high surf concerns. Christmas Night is when our next storm
system arrives as northwesterly flow envelops the area. PoPs are
on the higher end (60-80%), but they are rather broad-brushed as
the NBM solution likely represents a rather broad range of
possibilities. In fact, some models in the days 7-8 time period 
show higher amplitude ridging over the west coast (keeping storm 
systems to the north of the area), while others have a slightly 
flatter ridge/more zonal flow (keeping the storm door at least 
slightly ajar). For now, we'll continue to keep rain chances in
the forecast given projections of IVT still warrant mentionable
PoPs in the extended. Additional rainfall amounts of 1-3" are
projected across the North Bay, with up to 1 1/2" of rain
elsewhere (except across the Southern Salinas Valley). Stay tune
for refinements to the forecast.

&&

.AVIATION...
(06Z TAFS)
Issued at 945 PM PST Sat Dec 21 2024

KMUX radar shows light rain arriving from the west. The terminals
are currently reporting MVFR-LIFR. Patchy fog is also likely tonight
and Sunday morning. Otherwise wet weather tonight and Sunday morning
with lingering showers Sunday, greater areal coverage of wet weather
returning Sunday night and early Monday morning.

Vicinity of SFO...IFR tonight and Sunday morning with periodic
light rain. Light rain returning Sunday night and early Monday
morning. Light northerly to easterly wind during the period.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...MVFR-IFR, vicinity showers tonight and
Sunday morning. Mainly light easterly winds.

&&

.MARINE...
(Tonight through next Friday)
Issued at 845 PM PST Sat Dec 21 2024

Large, building westerly swell continue tonight and Sunday morning
with a temporary decrease in swell later Sunday prior to large to
very large westerly swell Monday into Tuesday. Wave heights will
remain elevated through late next week with 15 to 20 foot waves
Monday through Wednesday. Expect dangerous conditions for small
crafts through the weekend through early next week. Elevated seas
will continue to produce enhanced shoaling at harbor entrances and
bars.

&&

.BEACHES...
Issued at 111 PM PST Sat Dec 21 2024

High surf advisory remains in effect through Sunday afternoon for 
elevated surf of 20-25 feet, with breakers up to 30 feet. A long 
period, westerly swell will continue to create hazardous beach 
conditions along the coast into Tuesday. Global wave models are 
forecasting a more intense energetic swell with 20+ feet heights and 
swell periods of 20+ seconds, this will lead to much higher breakers 
(possibly 30-35 feet) including localized coastal flooding impacts.
These conditions will result in dangerous and life-threatening bay, 
ocean, and beach conditions lingering into next week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...High Surf Advisory until 4 PM PST Sunday for CAZ006-505-509-529-
     530.

PZ...Small Craft Advisory until 3 PM PST Monday 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...Bain
LONG TERM....Bain
AVIATION...Canepa
MARINE...Canepa

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Prepared by Boulder-Creek.com Weather at: Sun Dec 22 00:30:02 PST 2024