569
FXUS66 KMTR 031202
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
502 AM PDT Fri May 3 2024

...New AVIATION, MARINE...

.SYNOPSIS...
Issued at 1243 AM PDT Fri May 3 2024

A cold front will bring rain, gusty winds, and colder
temperatures on Saturday. Dry weather returns Sunday as a gradual
warming trend kicks off.

&&

.SHORT TERM...
(Today and tonight)
Issued at 108 AM PDT Fri May 3 2024

A shallow marine layer will bring low stratus to coastal areas and
adjacent valleys Friday morning. High temperatures will depend on
whether a location is in or out of the marine layer. For example,
Pt. Reyes is only expected to reach 56 degrees today, while Santa
Rosa will flirt with 80, and enjoy ample sunshine. By the
afternoon, an approaching cold front will cause the winds to
increase, with rain reaching the North Bay early Saturday morning.

&&

.LONG TERM...
(Saturday through Thursday)
Issued at 108 AM PDT Fri May 3 2024

A gale force, 997 mb, vertically stacked low pressure system is
currently near the Gulf of Alaska. This system will be driven
southeast by a 150 kt jet stream through Saturday. This strong jet
max will also create divergence aloft, allowing the surface
pressure to remain steady or even decrease slightly as the system
loses latitude. The low center is expected to come ashore near
Northern California or Southern Oregon early Saturday morning. An
associated cold front will cross the Bay Area through the day
Saturday. This frontal passage will bring periods of rain and
gusty winds as it ushers in a new, colder air mass.

Precipitable water values are expected to be above 1" on Saturday,
with some guidance as high as 1.25". As the cold, dense air lifts
this moisture, rain clouds will develop. The total QPF is holding
pretty steady with most places expected to receive around 0.5" on
Saturday. While not a total wash-out, this is a significant
amount for May, which has monthly normal rainfall pretty close to
the expected amount from this one system.

During the afternoon OAK balloon launch Thursday, the 1000-700 mb
thickness was measured at 2967 m. This new air mass will drop the
that to around 2815 m, according to the NAM point sounding. By
Sunday morning, NAM has the 700 mb temperature down to -12.7C,
which is well below the 10th percentile (-5.2 C) and even the
daily minimum (-9.9 C) from May 5, 1988. Note the 1000-500 mb
thickness drop isn`t as impressive due to more flat mid-level
lapse rates. This suggests that any convection that takes
advantage of the cold air aloft will be low-topped, even by Bay
Area standards. Additionally, the coldest mid-level air arrives
overnight Saturday/early Sunday during the diurnal minimum. These
two factors should limit the potential for thunderstorms, although
a slight chance remains.

Finally, this system will bring a noticeable drop in temperature,
especially for inland areas. Friday will feature inland temps in
the mid to upper 70s. 24 hours later, these same places will be
stuck in the upper 50s or low 60s.

Beyond Saturday, the forecast is more straight forward. Ridging
from subtropical high pressure builds in Sunday which will
gradually transition to the standard coastal trough set-up by mid
next week. This pattern will bring the return of dry weather, lots
of sunshine, and a gradual warming trend. Coastal areas may reach
the low 70s and some inland areas will be in the 80s by late
week.

&&

.AVIATION...
(12Z TAFS)
Issued at 457 AM PDT Fri May 3 2024

Currently seeing between LIFR and VFR at the terminals. Widespread
VFR to prevail by this afternoon. Winds will be breezy to gusty out
of the west through this evening. Mid-to-low end MVFR ceilings make
a return tonight ahead of the approaching cold front. This cold
front will bring rain and breezy to gusty southwesterly winds to
the terminals tonight into tomorrow with LVK, MRY, SJC, and SNS
not slated to see rain until beyond the 24-hour TAF period.

Vicinity of SFO...Currently IFR with breezy to gusty westerly flow.
Confidence is highest in the terminal returning to VFR between 15Z
and 18Z this morning. Westerly winds will only continue to ramp up
as the day goes on. An Airport Weather Warning is in effect from 22Z
through 03Z for gusts of up to 40 knots. Winds will remain gusty
while becoming southwesterly ahead and through the cold front. Rain
chances return to the terminal after 13Z tomorrow morning.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Currently LIFR with breezy southwesterly
flow at MRY and IFR with breezy northwesterly flow at SNS. Highest
confidence is in both terminals returning to VFR by 18Z. MVFR
stratus returns to both terminals this evening with MRY further
deteriorating to IFR as the night progresses. Winds will be breezy
out of the west through the TAF period. Rain does not reach either
terminal until beyond the 24-hour TAF period.

&&

.MARINE...
(Today through Wednesday)
Issued at 457 AM PDT Fri May 3 2024

Strong and gusty northwesterly winds will persist for yet another
day as the region remains in between high pressure over the
Eastern  Pacific Ocean and low pressure over the Southwest. Gale
force gusts  will be possible, especially over the coastal waters
and  southernmost zones. Tonight, a cold front will drop down from
the  Pacific Northwest bringing rain and moderate to fresh breezes
across  the waters. As the low pressure system exits to the east
and the  pressure gradient is allowed to relax, winds will
diminish going  into next week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 9 AM PDT this morning for Mry Bay.

     Small Craft Advisory from 3 PM this afternoon to 9 PM PDT this
     evening for Mry Bay.

     Small Craft Advisory until 9 PM PDT this evening for 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 Reyes to Pigeon Pt 0-10 nm.

     Gale Warning until 3 AM PDT Saturday for Pt Pinos to Pt Piedras
     Blancas 0-10 nm.

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

     Small Craft Advisory from 3 AM to 9 AM PDT Saturday for Pt Arena
     to Pt Reyes 10-60 NM.

&&

$$

SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...Sarment
MARINE...Sarment

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