351
FXUS66 KMTR 041543
AFDMTR

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

...New UPDATE, MARINE...

.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.

&&

.UPDATE...
Issued at 838 AM PDT Sat May 4 2024

Forecast on track with no changes needed this morning as rain has
spread well into the Bay Area. Some evidence of a narrow cold
frontal rain band evident on radar, currently exiting the North
Bay as of writing. Expect gusty winds and brief heavy rain as this
feature passes through, with lingering light to moderate rain
behind it into late morning. Rain totals appear largely on track
with between 0.50-0.80" in the North Bay so far. As far as we can
tell, no major impacts so far, outside of some very minor water on
roadway reports. See previously issued SHORT TERM for more info.

&&

.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...
(12Z TAFS)
Issued at 447 AM PDT Sat May 4 2024

Currently seeing the low pressure system and its associated cold
front moving southeast across the region with STS already
reporting rain. This cold front will continue to traverse across
the region through the morning bringing widespread rain, winds
with gusts of up to 25 knots, and MVFR conditions to all
terminals. Winds will briefly become southerly/southwesterly ahead
and through the cold frontal passage. VFR conditions and westerly
winds will prevail at all terminals once the cold front moves out
from over a terminal.

Vicinity of SFO...Currently MVFR with breezy southwesterly flow.
Winds will become gusty ahead and through the cold frontal passage.
Behind the cold front, VFR conditions and westerly winds prevail.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Currently MVFR with breezy southerly flow
at MRY and IFR with breezy northerly flow at SNS. Winds will become
gusty out of the southwest ahead and through the cold frontal
passage. Behind the cold front, VFR conditions and westerly winds
prevail.

&&

.MARINE...
(Today through Thursday)
Issued at 838 AM PDT Sat May 4 2024

Widespread rain and fresh southwesterly breezes are affecting the
waters. Fresh northwesterly breezes will prevail behind main rain
band with scattered rain showers possible into the evening. Dry
conditions return and winds reduce into Sunday. Gusty winds build
into the middle of the next work week and last into the late week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 3 PM PDT this afternoon for SF Bay N
     of Bay Bridge.

     Small Craft Advisory until 9 PM PDT this evening for Mry Bay-Pt
     Pinos to Pt Piedras Blancas 0-10 nm.

     Small Craft Advisory from 9 AM Sunday to 3 AM PDT Monday for Mry
     Bay-Pt Pinos to Pt Piedras Blancas 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.

     Small Craft Advisory until 3 PM PDT this afternoon for Pigeon Pt
     to Pt Pinos 0-10 nm-Pigeon Pt to Pt Pinos 10-60 NM.

     Small Craft Advisory from 3 PM Sunday to 3 AM PDT Monday for
     Pigeon Pt to Pt Pinos 0-10 nm.

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

     Small Craft Advisory until 9 AM PDT this morning for Pt Arena to
     Pt Reyes 10-60 NM.

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

&&

$$

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

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