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000
FXUS66 KMTR 111908
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1208 PM PDT Thu Apr 11 2024

...New SHORT TERM, AVIATION...

.SYNOPSIS...
Issued at 103 AM PDT Thu Apr 11 2024

Another warm day ahead today with temps trending cooler into the
weekend. Much cooler on Saturday with rain chances throughout the
region. Slight thunderstorm chances for coastal areas around Santa
Cruz and Monterey. Warming trend after the weekend into next week.

&&

.SHORT TERM...
(This afternoon through Sunday Evening)
Issued at 1120 AM PDT Thu Apr 11 2024

Our main weather highlight will be in the short term as the next
storm system is forecast to slowly meander southward along the
California coast. After a week of above normal warmth, this
weekend will be cooler, wetter, and windier.

A pleasant afternoon will be on tap for the area with mostly sunny
skies, except along the immediate coastline where marine
stratus/fog persist. Temperatures this afternoon will range
between the 50s along the coast to the low to mid 80s in interior
sections of the Bay Area and Central Coast. The fog forecast is a
little tricky tonight, but at this time, guidance suggests more in
the way of a stratus intrusion for the coast and inland areas of
the North Bay, East Bay, and Monterey Bay regions. Visibility may
still fall down below 5 mile in spots, with chances of this
happening around 20-30%.

Friday will be largely rain-free for the area, with the exception
being perhaps some pockets of drizzle or spotty rain showers as
clouds build with more in the way of onshore flow. The clouds and
onshore flow will translate to temperatures being anywhere from 5
to 15 degrees lower than today (Thursday). Where clouds arrive
after peak heating. Highs will range between the low to mid 50s
along the coast to the 60s and low 70s elsewhere. With the
approach of the upper low, the surface pressure/low level height
gradients will strengthen. This will support a wind field
characterized by 15 to 25 mph southerly flow, with gusts ranging
between 30 and 35 mph. Gusts closer to 35 mph are more probable
across coastal and mountainous areas. For areas between Big Sur
and the Monterey Peninsula, there stands a chance (around 35%)
that a coastal jet capable of wind gusts of over 40 mph will
exist.

As the upper low slowly eases into the region, forcing for ascent
will support an extensive precipitation band that will invade from
the northwest. Model solutions continue to come into better
agreement regarding the timeline in which areas can anticipate
a HIGH probability (>70%) of the onset of rain showers at their
location.

-North Bay (Bodega Bay and Inverness Ridge)- 6-9pm Fri.
-North Bay SF Peninsula/western SC County - Midnight-3am Sat
-Western Monterey County/Big Sur Coast - Midnight-3am Sat
-East and South Bay and San Benito Counties- 3am-6am Sat


Overall, rainfall amounts with this system remain consistent with
this morning`s forecast package with rain amounts largely around
1/2 inch to 1 1/2 inches for most locales. Higher terrain across
Big Sur Country, the Santa Cruz Mountains, and some of the
Mayacamas will average closer to 2 inches. A few spots across the
Santa Lucia Range may get closer to 3 inches when all is said and
done, however, the probabilities of exceeding this remain under
20%. As such, there will be a continued risk for ponding, minor
flooding (especially across poor drainage and low-lying areas).
The threat for slides will also continue, especially down across
the Big Sur coastline where percent of normal rainfall over the
past couple of weeks remains in excess of 150%. Caution is urged
particularly across this area. In addition to the potential for
heavy rainfall, cooler air aloft associated with the main upper
low will support a risk for thunderstorms, especially south of the
San Francisco and East Bay regions where the chances for surface
based CAPE or fuel for storms have increased. The probabilities
for CAPE in excess of 100 J/kg now range between 50-60% (compared
to 20-30%) over the land areas as the upper low may be farther
east during the daylight hours on Saturday. With lapse rates
projected to approach 8 C/km, there will be a risk for showers
and a few storms, capable of mostly copious amounts of small hail
(1/4 inch). Again, this threat is greatest south of SF Bay and
East Bay regions. We`ll keep an eye on the potential for
waterspouts as well, given the presence of a couple of frontal
boundaries aiding in low level vorticity generation.

Not to be outdone, there`s a potential for light snow
accumulations (around 2-3") across the southern end of the Santa
Lucia Range at the highest peaks. At this juncture, significant
impacts are not anticipated.

Temperatures will tumble 10 to almost 15 degrees (more in some
areas) Saturday compared to Friday. Highs on Saturday will
struggle to get out of the 50s and 60s thanks to the cloud cover
and widespread rainfall.


&&

.LONG TERM...
(Friday through Wednesday)
Issued at 103 AM PDT Thu Apr 11 2024

By Friday, the influence of the incoming system begin to be felt
across the area as cloud cover increases, first in the North Bay
in the early morning, the southward across the rest of the area
by late morning to early afternoon. High temperatures Friday still
look to be rather pleasant, albeit cooler...mid-to-upper 60s for
inland valleys, upper 50s to lower 60s along the coast. With minor
adjustments to timing throughout the past few days, it looks like
most of the rainfall will be confined to the North Bay late
Friday, with some light rain beginning ahead of the main event in
the Santa Cruz Mtns as moist SW 925 mb flow increases notably by
about 6pm or so. Expect the main line of showers to spread
southward overnight into Saturday morning, with the heaviest
during the period roughly between sunrise and late morning.

Later on Saturday as the surface low approaches and makes its way
down the coast, the Central Coast will likely see an extended
period of orographically enhanced rainfall as the aforementioned
moist SW low-level flow continues to increase here through
Saturday night. It also appears that some degree of limited
instability will be present over the waters and along the coast
Saturday afternoon into the event, presenting the possibility of a
few thunderstorms. Forecast instability values from deterministic
guidance suggest that the greater risk resides across the
offshore marine areas. Some instability does build on Saturday
afternoon, however, it`s quite meager. In fact, the potential for
CAPE values over 100 J/KG are around 30%. This translates to a low
potential for a rumble of thunder or two on Saturday afternoon.
Chances for CAPE values over 100 J/kg are closer to 60% across the
marine zones. 700-500mb lapse rates do steepen around 7.5 C/km
offshore and so there remains some potential for waterspouts
(beyond 40-50 miles from the Central Coast). Snow levels do appear
that they`ll fall down sufficiently such that a few snow flakes
or very light snow showers unfold across central and southern
portions of the Santa Lucias. At this time, measurable snowfall is
not expected.

In terms of total rainfall, a slight increase with this forecast
package, but overall nothing too significant. Still looking at
about 1-2" for coastal ranges, with the higher amounts favoring
the Santa Cruz Mtns and Santa Lucia. Some favored upslope
locations along the Big Sur coast could see up to 3". Elsewhere,
generally about 0.5-1", with the lesser amounts across
rainshadowed valleys and locations further inland.

Heading into next week, we`ll see mid-level ridging set up across
the region which should limit the opportunities for precipitation.
Temperatures will rebound and we will be well on our way to near or
above normal warmth.

&&

.AVIATION...
(18Z TAFS)
Issued at 1120 AM PDT Thu Apr 11 2024

VFR is the main rule this afternoon and I anticipate that it`ll
prevail until the pre-dawn hours on Friday. With modest onshore
flow, I expect that marine stratus will invade most of the
terminals, with the exception being KSJC and KLVK. Precipitation
chances look to peak just after this TAF cycle and will likely
need to be introduced with the 00Z TAF set.

Vicinity of KSFO...VFR will prevail, however, poor flying
conditions are advertised after midnight tonight. There`s a chance
(less than 20%) that the onset of MVFR cigs could be prior to that
indicated in the TAF set (as early as 03Z), but confidence is too
low to include at this time. Intermittent MVFR cigs are more
probable (around a 40-50% chance) after 09Z with chances being
closer to 80% around 12Z. The potential for IFR cigs exist, but
it`s less than 20% and thus low confidence. Clouds will attempt
to scatter in the extended period of the TAF and I`ve shown a more
optimistic forecast around 21Z Friday. However, confidence in the
timing is low and future updates may opt for MVFR through 00Z
Saturday. Otherwise, variable to NE`ly winds early this afternoon
will yield to a more westerly wind component.

KSFO Approach Bridge...Similar to SFO.

Monterey Bay Terminals...VFR as of 18Z, but MVFR and IFR will
return around 09Z Friday at KMRY. Similar to KSFO, there`s some
potential that MVFR could arrive as early as 03Z, but we`ll
monitor trends to determine whether or not the onset needs to be
adjusted. IFR visibility and ceilings should be a bit more
intermittent and with the approach of our next system, stronger
mixing should help to scour out any MVFR BR and help elevate cigs
to VFR. The latter is not likely until after 18Z Friday.

&&

.MARINE...
(Today through Tuesday)
Issued at 430 AM PDT Thu Apr 11 2024

Persistent northerly flow over the coastal waters today. Coastal
jets will develop this afternoon leading to locally stronger
winds along the immediate coastline north of Point Reyes and near
Point Sur. A moderate northwesterly swell will traverse the
coastal waters too. A passing storm system will bring unsettled
weather Friday and over the weekend with rain, thunderstorms,
stronger winds, and steep fresh swell.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 3 PM this afternoon to 9 AM PDT Friday
     for Pt Arena to Pt Reyes 0-10 nm-Pt Pinos to Pt Piedras
     Blancas 0-10 nm.

     Small Craft Advisory until 9 AM PDT Friday for Pigeon Pt to Pt
     Pinos 10-60 NM-Pt Arena to Pt Reyes 10-60 NM.

&&

$$

SHORT TERM...Bain
LONG TERM....Behringer
AVIATION...Bain
MARINE...MM

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