Hail in California, March 17, 2018

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1041 AM PDT Sat Mar 17 2018

.SYNOPSIS...Showers will continue to move across the region today
with the potential for a few thunderstorms this afternoon. Look
for showers to diminish into this evening. Dry and slightly
warmer conditions are likely Sunday into Monday ahead of our next
storm system that will bring widespread rainfall to the region
Tuesday through Thursday of the upcoming week.

&&

.DISCUSSION...as of 08:50 AM PDT Saturday... Local radar imagery
shows scattered showers across the coastal waters and Monterey
Bay region early this morning. Brief heavy downpours have been
reported with these cells as they've come ashore. So far, the
strongest of these cells have been about 10 to 30 miles offshore,
strengthening as they approach the coast, before decaying as they
push inland. These showers are forecast to continue to move across
the Santa Cruz, Monterey, southern Santa Clara, and San Benito
through the remainder of the morning and into the afternoon. An
isolated thunderstorm or two may also develop by late morning.

The broad parent trough that has brought several days of wet,
unsettled weather to the region has begun to transition downstream
and out of the region. The upper core of this 540dm 500mb trough
will shift into Idaho while the base of the trough digs southeast
into southern California through the next 24 hours. In the wake of
this trough, a cold, mostly dry air mass will take hold over the
greater San Francisco and Monterey Bay areas. The main forecast
challenge of the day is tonights forecast, given that clearing
skies and lighter winds will allow temperatures to plummet, but
just how much is the question. In a drier air mass, they would
cool quicker promoting more of a freeze/frost risk, but with
lingering low level moisture, it is also possible we could see
very dense fog, similar to what Santa Rosa is experiencing already
this morning, across the region. With denser fog, temperature will
not cool as quickly, so frost/freeze would be less of a risk. That
said, confidence is still low on fog/frost/freeze for tonight, but
watching satellite trends (amount of clearing on backside of
trough) and low level humidity values should provide the clues
needed.

Dry weather with clearer skies are forecast for Sunday and Monday,
meaning warming daytime temps and cooler nights.

The other forecast challenge of the day will be with the upcoming
storm system that will impact California from Tuesday into
Friday. Forecast models show an upper low coupled with a deep
tropical moisture tap impacting the state mid week next week.
Models still disagree on the positioning and amplitude of this
moisture plume, however, it is clear that the main impacts will be
felt in the locations at the center of this moisture hose. Most
model runs indicate the bulk of the moisture will be aimed at Big
Sur southward to the Santa Barbara area in southern California.
Areas farther northward, ie the North Bay, are increasingly
unlikely to be hit by the heaviest precipitation located in the
concentrated moisture plume. Given the tropical nature of this
moist air mass, temperatures will be relatively warm, causing snow
levels to rise much higher than they have been with our recent
colder storm systems. The Euro model suggests localized amounts of
around 8 inches are possible over the higher Big Sur peaks
assuming the plume hits there. The GFS on the other hand, is a bit
lower...

No significant changes expected to the current (overnight)
forecast package. See additional information on the current
forecast in the previous forecast discussion.

.PREVIOUS DISCUSSION...as of 04:00 AM PDT Saturday...Scattered
showers continue to impact the region this morning and will
persist through much of the day as a short-wave disturbance pushes
southward along the back side of the exiting trough. In addition,
snow levels are estimated to be around 3,000 feet this morning
based off of area profilers likely resulting in some snow
accumulations in the highest peaks across the region. By this
afternoon, forecast models show the atmosphere becoming more
unstable with surface-based CAPE values exceeding 400 J/kg. Thus,
have added thunderstorms to the forecast with the potential for
small hail within any convective cells that do develop. Showers
are then forecast to diminish in coverage late in the evening and
then end by midnight. Sky conditions will begin to clear late in
the night through Sunday morning as a colder air mass settles into
the region. With this, look for much cooler overnight
temperatures with may inland areas dropping into the middle 30s.
If winds weaken as forecast, frost will be a concern for many
inland valley locations. Will need to monitor for the potential
for a Frost Advisory from late tonight into Sunday morning.

A warming and drying trend is then forecast for Sunday and Monday
as a weak ridge of high pressure develops over California. After
a cool start to the day, temperatures will warm into the 60s on
Sunday and into the middle/upper 60s to near 70 on Monday ahead of
our next storm system. Mid/high level clouds will also be on the
increase by late Monday afternoon and evening.

Wet weather conditions are forecast to return to the California
coast by Tuesday as a much deeper plume of moisture advects
inland. The latest forecast models show the potential for an
atmospheric river to set aim somewhere along the central or
southern California coast during the middle half of next week.
This would likely bring widespread rain to the region with periods
of moderate to locally heavy rainfall as a mid/upper level trough
drops southward and approaches the cost Wednesday into Thursday.
At this time, it still appears the heaviest rainfall will be
across the Central Coast and into Southern California. However,
widespread rain is also likely to impact the San Francisco Bay
Area from Tuesday into Thursday. While it remains difficult to
nail down the details at this time, do expect most urban areas to
potentially pick up 1 to 2 inches of rainfall with 3 to 5 inches
along the coastal ranges. Please be sure to stay tuned to the
latest forecast information in the coming days.

&&

.AVIATION...as of 10:38 AM PDT Saturday...Hit or miss showers will
be the story today with convective clouds 3-6k feet. Haven't seen
much lightning this morning and conf on thunderstorms impacting
terminals is less. Therefore, removed VCTS and converted it to
VCSH through the day. Showers diminish afternoon sunset. Poss cigs
in SF Bay tomorrow AM.

Vicinity of KSFO...Light NE wind this morning eventually switching
to NW with cigs 3-5k feet. Some CB are possible given the
convective scenario. Decided to leave the BKN010 in for tomorrow
morning as latest models show some low level moisture lingering. A
lot will depend on clearing this evening for low cig develop
tomorrow. Conf is low to very low for BKN010

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Generally VFR with some passing showers.

&&

.MARINE...as of 10:37 AM PDT Saturday...Light to moderate winds
across the coastal waters today. A passing storm system will
produce scattered showers and possible thunderstorms. Developed
thunderstorms will be capable of producing small hail, gusty
winds, lightning and possibly a waterspout. High pressure will
build in late tonight into Monday  bringing a return of dry
weather. Increasing winds with next  week's low pressure system
will generate fresh mixed shorter  period swell.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...None.

PUBLIC FORECAST: DRP
AVIATION: MM
MARINE: MM

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PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
1148 AM PDT SAT MAR 17 2018

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1148 AM     HAIL             SAN CARLOS              37.50N 122.27W
03/17/2018  E0.25 INCH       SAN MATEO          CA   PUBLIC

            SOCIAL MEDIA PHOTOS SHOWING PEA-SIZED HAIL IN SAN CARLOS.

&&

ASCHNEID