Area Forecast Discussion Issued by NWS San Francisco Bay Area, CA
000
FXUS66 KMTR 110445
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
945 PM PDT Wed Apr 10 2024
...New UPDATE...
.SYNOPSIS...
Issued at 212 PM PDT Wed Apr 10 2024
Our warming trend is well underway across the Bay Area and
Central Coast and will continue tomorrow and potentially into
Friday. The latest forecast has a delayed onset of rain showers
with locations across the North Bay being impacted first late
Friday afternoon and evening. Rain and embedded thunderstorm
chances spill southward late Friday and into the day on Saturday.
The better thunderstorm risk will be confined to the offshore
waters, though coastal areas of Santa Cruz and Monterey have a low
potential for a rumble of thunder or two. It`ll turn sharply
colder this weekend with rebounding temperatures next week.
&&
.UPDATE...
Issued at 917 PM PDT Wed Apr 10 2024
Temperatures are dropping, though not as fast as last night due
to the increased high cloud cover inhibiting radiational cooling.
While this reduced cooling should help the inland areas stay out
of the fog in the morning, the dew point is also a few degrees
higher, essentially balancing out. With light winds, patchy
radiation fog is possible, particularly in the North Bay. Coastal
areas along the North Bay, SF Peninsula, and Monterey Bay also
have a chance for fog due to a shallow marine layer. The Ft. Ord
profiler was showing the top of the marine layer around 250 ft
above sea level most of the day, but has since risen to near 1,000
ft, possibly due to a coastal trough developing and the surface
pressure dropping a couple mb. If this deepening trend lasts
through the night, it would be more supportive of low clouds than
fog. With all of these competing factors in the fog forecast, it`s
not unreasonable to go with a persistence forecast and I`ve
nudged the official forecast in that direction.
&&
.SHORT TERM...
(This evening through Thursday)
Issued at 212 PM PDT Wed Apr 10 2024
As discussed earlier, the potential for fog/low stratus across
the coastal areas has increased a bit more tonight into Thursday
morning. As T-Td spreads diminish, mostly clear skies (except some
passing high clouds) and reduced surface-925mb flow will
encourage a bit more in the way of fog. If high clouds are really
dense, then fog may struggle to develop. Speaking of struggling,
HREF guidance yesterday didn`t appear to capture the evolution of
this morning`s fog and appears to offer low probabilities for
tonight. However, forecast soundings from afternoon RAP guidance,
along with some of the NBM guidance support inclusion of patchy
fog with visibilities of 2-4 miles across the North Bay, parts of
the SF Peninsula, and across a good portion of the Central Coast.
Guidance indicates that areas across Monterey Bay will stand the
best chance (around a 30-40% chance) to see visibility slip below
1 mile. Visibility reductions may be observed as far inland as the
Salinas Valley, but given the shallow nature of saturation (less
than 1000 feet), it`s probable that any nocturnal drainage flows
will keep the boundary layer sufficiently mixed to curb the dense
fog potential. Given that the chance for visibility to fall below
1 mile is confined to a small area, dense fog headlines are not
anticipated. Motorists are still urged to exercise caution when
commuting late tonight into early Thursday morning as sudden
changes in visibility may occur. With the shallow nature of the
marine layer, fog and low clouds should dissipate by mid-morning
Thursday, though will linger through the early afternoon hours
offshore.
As advertised earlier this week, Thursday will be one of the
warmest days we`ve seen since last Fall. Record high temperatures
are safe, but we`ll continue to see a minor heat risk for most of
the area. This means that those sensitive to the heat (elderly
and infants) may succumb to heat-related illnesses. We encourage
individuals across the area (especially interior locales such as
the Santa Clara Valley, Salinas Valley, and San Benito County) to
hydrate adequately with water and take breaks in the shade if
working outside. Never leave children or pets unattended in
vehicles. High temperatures are forecast to range between the mid
to upper 50s along the coast to upper 70s to even mid-80s across
interior sections. There`s still a bit more spread in the max
temperature forecast along coastal sections. The marine layer
isn`t overly deep and so I do anticipate mixing will erode any
low clouds/fog prior to peak heating. However, if areas stay
socked in with clouds/fog, highs along the coast may struggle to
get out of the mid 50s.
&&
.LONG TERM...
(Friday through next Tuesday)
Issued at 212 PM PDT Wed Apr 10 2024
Friday Onward....Some noteworthy alterations were made to the
PoP/QPF grids. Thanks to EKA, STO, HNX, and EKA for the
coordination. This entire week, model guidance has slowed the
progression of the upper trough that will be responsible for our
wetter and cooler conditions this weekend. Blended guidance
rain amount probabilities have trended lower and with some of the
higher resolution NWP now available, it seems prudent to update
PoP/QPF to match our IDSS messaging. These alterations result in
rain chances being confined to areas across extreme northern
portions of the North Bay during the daylight hours on Friday.
While a stray shower or two (or sprinkles/drizzle) cannot be ruled
out for locations farther south, however, confidence is
increasing that most locales will remain rain-free. While cloud
cover will inhibit heating, I did adjust temperatures upward
closer to the 75th percentile of the NBM on Friday afternoon which
means most areas away from the coast will see high temperatures
in the upper 60s to low 70s.
Forcing for ascent increase late Friday into Saturday as our main
upper storm system arrives. Upslope flow across the higher
terrain (Santa Cruz Mountains, Santa Lucia Range, and Mayacamas)
will promote some orographically enhanced precipitation, but the
primary band of showers may not enter parts of the North Bay until
closer to 10pm Friday night. Rain chances peak during the pre-
dawn hours on Saturday with values averaging between 60 and 80
percent. As the upper low attempts to scoot eastward through the
day on Saturday, this feature will slow down and the 500mb low
forecast may reside across the northern portions of our marine
zones during the afternoon hours on Saturday. This will encourage
additional onshore flow on Saturday and thus additional rounds of
precipitation. With widespread cloud cover and rain, Saturday
will not be an ideal day for outdoor activities and temperatures
will remain in the 40s and 50s which will be quite the change from
the middle portion of the week. The unidirectional flow through
the troposphere will promote training of convective cells and rain
bands over a given area, thereby increasing the potential for
minor flooding. The greatest potential for excessive rainfall, as
highlighted by the Weather Prediction Center, appears that it`ll
reside across the Big Sur coastline, however, any robust
convective cell over a more urbanized area may promote minor
flooding.
As it pertains to the potential for 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.
With the slower progression of the upper low, 20-40 PoPs linger
through the day on Sunday. With the upper low traversing the region,
the cooler pocket of air aloft will shift overhead during peak
surface heating across southern Monterey/San Benito counties. As
such, lapse rates steepen Sunday afternoon across southern
Monterey and San Benito counties and I`ve inserted just a 20%
chance for isolated storms here. Severe weather is not expected,
but storms may be efficient at producing larger quantities of
hail.
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...
(00Z TAFS)
Issued at 520 PM PDT Wed Apr 10 2024
VFR with some high clouds across the area. Winds vary in direction
but mostly onshore flow with moderate to breezy speeds. Expect winds
to become light overnight. With the strong ridge and compressed
marine layer, expect another round of some low visibility and low
clouds early Thursday morning for most terminals. Moderate
confidence on ceiling heights but expect a mix of MVFR, IFR, and
LIFR conditions starting near 12-14Z. VFR will return after 18Z with
mostly clear skies.
Vicinity of SFO...Currently VFR with breezy N/NW flow before
diminishing overnight. Confidence is increasing that the terminal
will see a reduction of flight category tomorrow morning to MVFR.
Winds will be breezy with speeds up to 15-16 knots out of the west
again tomorrow afternoon.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...Currently VFR with moderate northwesterly
flow. Winds will diminish overnight which will allow for some
lower visibility and ceilings causing a mix of IFR to LIFR
conditions overnight with KMRY seeing low ceilings near 07Z and
13Z for KSNS.
&&
.MARINE...
(Tonight through next Tuesday)
Issued at 520 PM PDT Wed Apr 10 2024
High pressure over the Eastern Pacific Ocean will dominate the
forecast through tomorrow, allowing for fair weather and fresh to
strong northerly breezes to prevail over the coastal waters. Long
period northwesterly swell will continue to build through the
waters through today before beginning its abating trend tomorrow.
Friday, a deep upper-level low will bring renewed chances for
rain, gusty winds, thunderstorms, and hazardous seas to the
coastal waters through the weekend.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 3 PM Thursday 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....Bain
AVIATION...SO
MARINE...SO
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