658
FXUS66 KMTR 040807
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
107 AM PDT Sat May 4 2024

...New SYNOPSIS, SHORT TERM, LONG TERM...

.SYNOPSIS...
Issued at 1201 AM PDT Sat May 4 2024

A cold front will bring rain, strong winds, and much colder
temperatures to the area today. A gradual warming trend will start
Monday with dry weather and plenty of sunshine next week.

&&

.SHORT TERM...
(Today and tonight)
Issued at 1201 AM PDT Sat May 4 2024

KMUX is starting to pick up on the main frontal rain band as it
approaches the North Bay early this morning. To add timing
precision, I used a blend of high-resolution short term models to
create 1 hourly PoP grids. Based on the new official forecast,
rain is expected to reach Santa Rosa by 4 AM, San Francisco by 6
AM, San Jose by 8 AM, and Salinas by 10 AM. The main band of rain
should last 6-8 hours before tapering off. The expected total
rainfall is continuing to tick up slightly, with 0.5-1.0" in most
lowlands and 1.0-1.5" in the coastal mountains. Prefrontal SW
winds are moderate this morning, but will increase to a strong
breeze as the front approaches and switch to a NW direction after
it passes.

The confidence in the prefrontal stratiform rain described above is
quite high, with very little ensemble spread. The bigger question
today is if thunderstorms will develop in the cold, post frontal
environment. The expected 700 mb temperature is near the daily
minimum, which would suggest some instability, but there`s a few
complications. First, the coldest mid-level air doesn`t settle in
until Saturday night, after the surface temperature is on the
down swing. Second, the 500 mb temperature is not as impressive
as the 700 mb temperature with a more stable 3.5 C/km lapse rate
between those two levels. This indicates any convection that
initiates tomorrow will have a vertical limit of about 10,000 ft.
With the top end of the hail growth zone (-30C) at over 22,000
ft, this limits the amount of ice that will be generated. The most
likely scenario is that after the front moves through some
isolated, low-topped convective showers will pop up through the
evening. There is a scenario, however, where the skies clear
after the front passes and the surface temperature increases more
than expected. This could lead to some evening thunderstorms,
particularly in the North Bay where the front will clear first
and the potential surface heating is higher. To demonstrate this,
I adjusted the NAM model sounding at Santa Rosa, increasing the
surface temperature a few degrees to line up with our forecast of
13C/55.4F. This alone increased the CAPE from 130 J/kg to over 500
J/kg. If the surface temperature exceeds our expectations, the
instability will be even higher. In other words, if the sun comes
out in the afternoon, the chance for evening thunderstorms
increases significantly.

&&

.LONG TERM...
(Sunday through Friday)
Issued at 1201 AM PDT Sat May 4 2024

The cold front and associated low pressure system will clear the
area by Sunday, leaving cold air in place. Sunday morning will be
on the chilly side, with many inland areas dropping into the upper
30s or low 40s. There is a slight chance for some lingering
showers on Sunday, but they should be isolated and on a decreasing
trend through the day (if anything develops in the first place).
Otherwise a gradual warming trend will kick off next week with
Sunday highs in the 50s and 60s and Friday highs in the 70s and
80s. Expect lots of sunshine with breezy winds along the coast,
especially during the first half of the week.

&&

.AVIATION...
(06Z TAFS)
Issued at 942 PM PDT Fri May 3 2024

MVFR/IFR level clouds are moving inland. Winds are reducing and
are expected to become light to moderate, then slowly turn to
become more S/SW ahead of the approaching low pressure system. By
the mid morning, 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 lift slightly as the rain approaches. Rain continues
into Saturday afternoon before dissipating, with winds turning to
W/NW and becoming breezy. Expect these breezy winds to last into
Saturday night.

Vicinity of SFO...IFR and breezy into the night. Winds reduce
further into the night and turn southwesterly. These winds become
gusty as the rain arrives early Saturday morning with MVFR CIGs
and slight reductions in visibility. Winds turn to become W/NW in
the later morning, but rain lasts through the afternoon. VFR
returns behind the rain band, with breezy westerly winds.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...IFR/LIFR CIGs last through the majority
of the overnight hours with lighter winds. As the rain band
arrives, expect CIGs lift to become IFR/MVFR late Saturday
morning. Gusty westerly winds build along the rain band. VFR
returns behind the main rain band with breezy winds easing into
the late night.

&&

.MARINE...
(Tonight through next Thursday)
Issued at 907 PM PDT Fri May 3 2024

Expect breezy to gusty northwest winds to continue with gale
force gusts possible over the southernmost zones. This evening, a
low pressure system and associated cold front will arrive from the
north, bringing rain and moderate to fresh breezes across the
waters. Winds will turn briefly southwesterly ahead of the system
early Saturday morning, but turn to become northwesterly once more
in the late part of Saturday morning. Rain ceases late in the day
Saturday, and winds ease to become moderate into the new
workweek.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 9 PM PDT this evening for Pigeon Pt
     to Pt Pinos 10-60 NM-Pt Arena to Pt Reyes 10-60 NM-SF Bay N
     of Bay Bridge.

     Small Craft Advisory from 9 AM this morning to 9 PM PDT this
     evening for Mry Bay-Pigeon Pt to Pt Pinos 0-10 nm.

     Small Craft Advisory from 3 PM this afternoon to 9 PM PDT this
     evening for Pt Arena to Pt Reyes 0-10 nm.

     Gale Warning until 9 AM PDT this morning for Pt Pinos to Pt
     Piedras Blancas 0-10 nm.

&&

$$

SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...Murdock
MARINE...Murdock

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