Rip Current in California, May 28, 2020

Area Forecast Discussion
National Weather Service San Francisco Bay Area
450 PM PDT Thu May 28 2020

.SYNOPSIS...Interior locations will remain very warm to hot on
Thursday where a Heat Advisory remains in effect. Cooler
temperatures return to the coast on Thursday and region-wide on
Friday as onshore flow increases. There is a chance of rain
showers late Friday night into Saturday when isolated
thunderstorms will also be possible. Near seasonable weather is
expected through much of next week.

&&

.DISCUSSION...as of 02:30 PM PDT Thursday... The high pressure
system responsible for several days of excessively hot
inland temperatures is nearing its exit from the Bay Area early
this afternoon. The ridge axis of this 592dm 500mb ridge has now
shifted into the intermountain west and is centered roughly over
the Grand Canyon in AZ. Noontime temperatures were down roughly 5
to 15 degrees region wide (most cooling inland where it had been
warmer), though, some of the deepest inland areas still under the
waning influence of the ridge have once again soared into the
90s. A cool, moist marine air mass surged inland overnight,
persisting through the morning, and continues to hold on along the
immediate coastline early this afternoon. This marine air mass
has helped to usher out the warm, dry air that had been dominating
the region over the previous days and marked a tangible shift in
the weather today. The heat advisory for inland areas will expire
this evening.

Satellite imagery shows a 565dm 500mb cutoff low 625 miles
southwest of Point Conception this afternoon. This feature has
been fairly stagnant over the last week, initially riding on the
southern flank of the aforementioned ridge prior to the ridge
moving ashore over the Memorial Day weekend... then transitioning
to the southwestern flank of the ridge after it moved ashore.
Forecast models are in high agreement that this feature will
finally approach the Pacific coastline and move ashore directly
over the San Francisco Bay Area after looming offshore for roughly
a week. As this storm system shifts closer to home on a
northeastward trajectory, southerly winds will increase across the
usual spots (coast, higher terrain) and become rather gusty there
meanwhile generating breezy conditions for most urban inland areas
as well. Peak gusts along the coast and higher terrain could range
from 35 to 50 mph as the storm nears.

IR satellite imagery indicates an area of increased convective
enhancement on the divergent north/northeastern segment of the
upper low. Lightning detection networks have also signaled a
steady amount of lightning strikes in this outer band as well as
in the core of the upper low. These two hotspots of moisture and
instability will pass over the region from late Friday through
midday Saturday and bring with them a chance of late season
convective precipitation but also the potential for lightning.
Model cross sections of this storm suggest that the initial bands
associated with the upper low arriving on Friday will be higher
based and may have more difficulty forcing precipitation to the
surface given the drier air mass in the mid levels beneath the
cloudbase. Nearer to the core of the upper low, increased
instability and potential for surface based convection will
increase the probability that convective showers could produce
measurable rainfall as there will be no dry air lingering beneath
the base of the showers. Showers may also be stronger and more
widespread in coverage on Saturday with the arrival of the core
of the low over the San Francisco Bay Area. Rainfall accumulations
will generally be light, less than a tenth of an inch for
locations south of the Golden Gate. For the North Bay where most
of the shower activity is forecast, amounts closer to a quarter
of an inch are possible.

The best chance of thunderstorm development will be throughout
Saturday morning through early afternoon, primarily over the
immediate San Francisco Bay Area before spreading north/eastward
as the upper low shifts inland and merges with a broader parent
trough. Lightning may be offset from any of the sporadic showery
precipitation from this storm system and result in dry lightning,
however, fuels currently are not as dry as they could be, thus no
fire weather products (watches/warnings) are planned at this
time. That said, now is the time to begin preparing for the
upcoming fire season as other western states deeper inland
(NV/UT) are already seeing conditions warranting Fire Weather
Watches/Red Flag Warnings.

A much broader but unimpressive upper trough will sweep in behind
the cut off low and essentially reintegrate it into the mean
westerly flow later Saturday and into Sunday. Forecast models are
generally in agreement that this upper trough will sag over the
Pacific coast from Sunday throughout much of the next week,
promoting seasonable weather with a bias towards out typical May
Gray/June Gloom onshore flow regime. That said, minor to moderate
fluctuations may still occur as vorticity lobes within the
broader parent trough ebb and flow aloft.

&&

.AVIATION...as of 4:50 PM PDT Thursday...For 00z TAFs. VFR
conditions at present with the stratus deck sitting along the
coastline with fingers of clouds sticking through the Golden Gate
and the Salinas Valley. Increasing confidence in an earlier
return of low clouds this evening. MVFR/IFR cigs expected
overnight with patchy fog along the coast and in the North Bay.
Cigs will improve Friday afternoon, but cloud cover lingers ahead
of the next weather system approaching from the SW. Winds increase
Friday afternoon and come from the SW.

Vicinity of KSFO...Winds remain onshore and strong this evening,
but will slowly ease overnight. Conditions are VFR at present with
the stratus deck remaining along the coastline. With the deepening
marine layer, MVFR cigs are possible Friday morning, but with the
winds switching to a more southerly direction, low confidence in
impacting the KSFO terminal. MVFR/IFR possible at surrounding
terminals. Cigs will improve Friday afternoon, but cloud cover
lingers ahead of the next SW weather system. Winds will increase
in the afternoon and be out of the south-southwest.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...The stratus sitting in Monterey Bay has
already started to creep into the Salinas Valley and over the
peninsula. MVFR trending to IFR conditions are forecast overnight,
with patch fog possible. Winds remain onshore and breezy, but will
ease overnight.

&&

.MARINE...as of 02:04 PM PDT Thursday...Light to locally moderate
northwest winds will persist into tonight before gradually
shifting out of the south overnight and tomorrow as a low pressure
system approaches from the southwest. Southerly winds along the
coast will increase by tomorrow afternoon as well as over the San
Francisco Bays and Monterey Bay. These winds will generate steep
fresh swell resulting in hazardous seas conditions for small
crafts. This system will also bring a slight chance of
thunderstorms over the northern waters tomorrow night into
Saturday. Northwest winds will return into Sunday. Mixed seas will
persist with a short period northwest swell and a longer period
southwest swell. A longer period northwest swell will arrive
tomorrow.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...NONE.

&&

PUBLIC FORECAST: DRP
AVIATION: DK
MARINE: AS

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