522
FXUS66 KMTR 031827
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1127 AM PDT Fri May 3 2024

...New AVIATION...

.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...
(18Z TAFS)
Issued at 1126 AM PDT Fri May 3 2024

VFR prevails through the daytime today, though stratus makes a
return in the early to mid evening for most terminals, bringing
largely IFR/LIFR conditions as ridging continues to squash cloud
decks. Winds generally W/NW and strong today, with gusts around 20-
25 knots, higher at SFO. As stratus approaches in the evening, winds
ease, but remain close to 10 knots through much of the night, and
slowly turn to become more S/SW ahead of the approaching low
pressure system. By the mid morning hours of Saturday, most
terminals will see winds increase once more to become breezy and
gusty out of the SW, and rain begins, lowering visibilities at
terminals. Cloud decks are expected to improve to become MVFR around
the same time to slightly after when rain approaches as greater
lifting from associated low pressure is enabled. Rain continues
through the end of the TAF period for most terminals, with winds
turning to become N/NW and breezy.

Vicinity of SFO...VFR conditions last through the afternoon today,
with strong and gusty onshore winds. Max gusts up to 35 knots are
expected. Stratus makes an early return in the evening, bringing IFR
conditions. Models suggest a reduction to LIFR conditions in the
overnight hours, but at this time, confidence in that development
is low. Have kept CIGS IFR and then improving to MVFR in the later
morning for now. Rain and gusty SW winds begin just prior to
sunrise Saturday for KSFO. Winds turn to become W/NW in the later
morning, but rain lasts through the afternoon.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Conditions improve to VFR in the early
afternoon today, though only for a few hours before stratus and
IFR/LIFR CIGs make a quick push inland in the early evening. Winds
NW and breezy this afternoon, but easing into the evening to below
10 knots. IFR/LIFR CIGs last through the majority of the overnight
hours. Towards the late morning of Saturday, rain begins and CIGs
lift to become IFR. Some model solutions hint at CIGs lifting enough
to become MVFR by the end of the TAF period. However, confidence in
this outcome is low at this time. For now, have left CIGs as
lifting, but remaining in IFR through the end of the 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 PM PDT this evening 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 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...AC
MARINE...AC

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