Technical NOAA Weather Discussion

FXUS66 KMTR 211814

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1113 AM PDT Tue Mar 21 2023

.SYNOPSIS...A strong Pacific storm system will spread high winds
and widespread rain across the central California coast today,
with isolated thunderstorms possible from around the Monterey Bay
region southward. High Wind Warnings remain in place for all of 
Monterey and San Benito counties and now also northward along the 
coastal strips of Santa Cruz and San Mateo counties as well as 
over the Santa Cruz Mountains through 9 PM PDT. San Francisco and
all of the East and South Bay are under Wind Advisories. Showers 
will continue through tonight along with the continuing 
possibility of a thunderstorm, then taper off from the north 
Wednesday afternoon and evening. Cool and mostly dry conditions 
then appear on tap for the latter part of the week and into the 


...Very strong Pacific storm system to bring wind and rain impacts

.DISCUSSION...As of 08:50 AM PDT Tuesday...Water vapor imagery 
shows the next powerful and vertically stacked Pacific storm 
poised off the Central California this morning. The atmospheric 
river portion of this storm is currently focused over San Diego 
County, with the 12z KNKX sounding showing a relatively saturated 
column and precipitable water values of 1.07 inches. In 
comparison, the 12z KOAK sounding shows precipitable water values 
of 0.58 inches with a decent dry layer between approximately 825 
mbs and 700 mbs. That said, composite radar imagery shows the warm
front lifting north across the Central Coast at this time, with 
the circulation center approximately 60 miles west southwest of 
the Monterey Peninsula. For the most part, the rain rates at this 
time are at or below a tenth of an inch. However, there are some 
heavier embedded showers with rain rates upwards of a quarter inch
per hour. With this rain hitting at the core of the morning rush,
we are seeing some reports of minor street flooding on the chp 
incident page. Therefore, Urban and Small Stream Flood Advisories 
have been issued for the nuisance flooding. 

That said, the bigger concern for the day will be with the winds. At
this time, winds are starting to increase across Monterey County,
especially in the Santa Lucia Mountains. Several sites are 
already gusting between 45 and 60 mph, with Highlands Peak being 
the winner so far with a gust to 75 mph. Meanwhile, the Salinas 
Valley and the Monterey Bay are starting to see gusts in the 30 to
40 mph. These winds will continue to increase across the region 
today, as the low deepens off the coast. Models are progging the 
low to rapidly deepen today, and we are already seeing places 
like Watsonville reporting their pressure falling rapidly. 
In fact, Watsonville is currently reporting their SLP as 994.1 
mbs, with Monterey's SLP currently at 993.3 mbs. Needless to say, 
it system is intensifying quickly, and expect the winds to 
continue to strengthen through the morning hours. As a result, a 
High Wind Warning and Wind Advisories remain in place for San 
Francisco southward today. Although it shouldn't take much wind to
knock down trees given our saturated soils, expect more more 
downed trees and power outages possible today. Now is the time to 
charge your electronics and back up battery packs. 

Isolated thunderstorms will be possible today, mainly in the
vicinity of the Monterey Bay and areas southward, where the
greatest instability. Any thunderstorms that do develop will be 
capable of producing small hail and gusty winds. Cannot rule out
an isolated water spout either, as helicity values are nearly
200 m^2s^2. The current forecast looks on track, so no updates 
are planned at this time.  Palmer


...Very strong Pacific storm system to bring wind and rain impacts

.PREVIOUS DISCUSSION...As of 03:00 AM PDT Tuesday...Latest model 
guidance is in very good agreement on rapid cyclogenesis underway 
off our coast. Really wish there was something by way of surface 
obs out there, but presently/recently no ship of opportunity 
reports (and likely for good reason given the developing weather 

High-resolution consensus has now shifted southward a bit with 
the track of the surface low, bringing it into the coast in the 
vicinity of Pigeon Pt late Tuesday afternoon rather than up near 
the San Francisco Peninsula. The projected central pressure has 
also moderated just a tad, but still way down in the 985-990 mb 
range and if bears out would be among the lowest ever here for the
month of March. 

It appears two bands of especially strong winds, extending out 
like the arms of a pinwheel, will rapidly move through from off 
the Big Sur coast: the first (essentially the warm front) Tuesday
morning and the second (the cold front) Tuesday afternoon. 
Highest wind speeds in both cases look to be focused over the 
southern half of our district, with highest gusts in windiest 
locations potentially reaching up into the 65-75 mph range. 
Impacts could be substantial, with yet a lot more downed trees, 
power outages and blocked roads.

Accompanying rain looks to be heavy at times, especially over the
Santa Lucia Mountains and Santa Cruz Mountains where maximum  
rainfall totals by 12Z Wednesday are forecast to get up to around
6-7 inches and 4-5 inches, respectively. Corresponding lower 
elevation rainfall totals look to be in the 0.5-1.0 inches range 
in the North Bay, 0.75-1.25 inches heart of the San Francisco Bay 
Area, and 1.0-2.0 inches in the Monterey Bay region. Do not 
presently expect any rivers or streams to reach flood stage, but 
can't entirely rule out the possibility of flash flooding and or 
mudslides/debris flows, with primary concern over the coastal 
slopes of Big Sur which WPC has placed in the Slight Risk category
in its latest Excessive Rainfall Forecast for 12Z Tues - 12Z Wed.
Will also note that USGS research instrumentation indicates that 
across the CWA shallow soils on steep hillsides are approaching 
saturation -- and therefore the possibility of shallow landslides 
exists if and where rainfall rates and intensities become 
sufficiently large.

Also want to note that thunderstorm chances develop Tuesday 
afternoon and evening, primarily from the Monterey Bay region 
southward. With 500 mb temps then projected to lower to around 30 
deg C, surface based CAPES look to locally get up into the 
500-700 J/kg range. Given the projected accompanying strong lower 
tropospheric vertical wind shear, can't rule out the possibility 
of a waterspout, or local spinup over land, and will need to 
carefully monitor the radar. Small hail could also well occur.

Showers look to taper off from the north Wednesday afternoon and 
evening with mostly dry conditions prevailing districtwide by 
Thursday morning. Cool and mostly dry weather then looks to 
prevail through the end of the week and into the weekend, though 
can't entirely rule out the possibility of a few showers at times 
as large-scale flow remains moist and out of the northwest.

Friday night and then Saturday night especially, overnight 
minimum temperatures are forecast to get unseasonably chilly. This
is of particular concern in the Wine Country of the North Bay 
where "bud break" has now begun in the vineyards and lows look to 
then potentially reach to freezing or below. Will continue to 
monitor as the time frame approaches as freeze/frost products 
could well end up being needed.

In the extended, new 00Z GFS, ECMWF and Canadian Ensemble
solutions indicate yet another unseasonably deep upper level
trough developing off the California coast the first part of next
week, with another round of rainfall then appearing likely. 


.AVIATION...As of 11:13 AM PDT Tuesday...For the 18z TAFs. Deep 
low pressure system is now located ~ 100 miles west of Monterey, 
the low approaching the Santa Cruz Mountains and the San Francisco
Peninsula through the afternoon. Strong winds and moderate to heavy
rain /IFR-MVFR conditions/ continues to accompany a northward 
moving trough extending eastward from the low pressure center. The
low center is forecast to reach vicinity San Francisco tonight 
then moves across the Central Coast Wednesday while in a weakening
stage tonight and Wednesday. Expect additional showers /IFR-MVFR/
and breezy to gusty winds tonight and Wednesday.

Vicinity of KSFO...MVFR, rain and gusty east to southeast wind to 25
to 34 knots (below weather warning wind criteria) through afternoon
and early evening. MVFR with showers tonight. VFR and less wind on 

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Mainly MVFR, rain and gusty east to southeast
winds, winds shifting to south and southwest later today and tonight.
Showers tonight and Wednesday morning.

&& of 10:59 AM PDT Tuesday...A deep low pressure system
located 150 miles west of Monterey this morning will continue to 
move northeastward and reach the San Francisco Peninsula later 
today. The low will bring strong winds and moderate to heavy rain 
across the waters and bays today and tonight. The low will become 
nearly stationary and begin weakening near the San Francisco 
Peninsula tonight and Wednesday morning, the low will then move 
southward across the Central Coast while weakening through 
Wednesday. Northwesterly winds prevailing mid to late week, with a
mainly dry cold front rapidly approaching and passing across the 
coastal waters late Thursday and Thursday night.


     .Tday...Wind Advisory...CAZ006-508-510-513>515
             High Wind Warning...CAZ509-512-516>518-528>530
             SCA...SF Bay
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             GLW...Mry Bay
             GLW...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             GLW...Pt Pinos to Pt Piedras Blancas 0-10 nm
             GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm



MARINE: Canepa

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Prepared by Weather at: Tue Mar 21 12:30:02 PDT 2023
From the National Weather Service