Hail and Lightning Strike in California, March 14, 2018

Area Forecast Discussion
National Weather Service San Francisco Bay Area
846 AM PDT Wed Mar 14 2018

.SYNOPSIS...An active weather pattern will continue for the rest
of the week with cool temperatures and periods of rainfall along
with breezy and gusty winds at times. Dry conditions develop for
the weekend with an unsettled weather potentially returning
during the first half of next week.

&&

.DISCUSSION...as of 08:45 AM PDT Wednesday...Active weather day
around the Bay Area as showers and possibly thunderstorms will
impact the region.

Showers have been rather consistent since last night bringing
additional rainfall to the Bay Area. Over the last 12-18 hrs a few
tenths to nearly a half inch of rain has fallen. This highest
amounts have been generally across the North Bay, Santa Cruz Mts
and in the vicinity of Concord.

Bay Area radars continue to show scattered showers around the Bay
Area early this morning. Some of these showers may be heavy at
times and contain small hail. Freezing levels are about 4500-5000
feet so any developed cell/strong shower has a good shot at hail.
Additionally, peaks above 4000 feet may even some some snow mixed
in this morning.

For the rest of today, shower activity will become more numerous
as an approaching shortwave trough moves overhead. Latest HRRR/WRF
models both indicate an increase in coverage for shower activity.
Additionally, the atmosphere will become more destabilized with
daytime heating and the approaching shortwave. Therefore, a slight
chance for afternoon thunderstorms seems plausible. This thinking
aligns with the general mention of thunder on the SPC Day 1
Outlook, which has much of the state in a chance for thunder.
Similar to yesterday, the Central Valley has a better chance of
seeing more developed storms. The Bay Area is more likely to see
showers with hail.

Current forecast remains valid and no major update is planned.

&&

.PREV DISCUSSION...as of 3:30 AM PDT Wednesday...An upper level low is
off the coast of British Columbia with a trough along the west
coast. Radar indicates isolated showers moving through the area
this morning mostly in the North Bay. Shower activity will
increase over the entire area late this morning and this afternoon
as a shortwave disturbance moves through the area. There is a
chance that the North Bay will receive less of the activity as the
NAM and the hi-res models show rain mainly confined from the SFO
Bay Area south. There is a slight chance of thunderstorms this
afternoon as the disturbance destabilizes the atmosphere.

There will be a short break from the precip Wednesday night as
the upper low starts to drop south. A new frontal system will
bring rain to the North Bay early Thursday morning and over the
rest of the district during the morning. The front will move
rapidly towards the southeast and will be through the southern end
of the district by evening. In addition this is a cold system so
moisture content is not very high. As a result only light to
moderate rain is expected with this front. No hydro issues are
anticipated aside from some ponding of water on streets.

Behind the system there will be plenty of shower activity Thursday
night and Friday as the upper low moves towards northern
California and brings cold air aloft into the district. Snow
levels could potentially drop below 3,000 feet so there could be
snow showers in the highest elevations. The airmass will be
unstable all day Friday so there is the threat for thunderstorms
containing small hail and locally heavy showers. Highs Thursday
and Friday will only be in the 50s.

Latest models show shower activity decreasing Friday night except
along the Monterey and Big Sur coast. All shower activity will be
done Saturday afternoon. Dry weather is then expected through the
rest of the weekend. Clearing skies will allow lows to drop to the
30s in the inland valleys Saturday night. Gradual warming is
expected Sunday and Monday as the flow becomes more zonal.

Medium range models show another system approaching the area from
the west with rain beginning Monday night or Tuesday. Will have to
see if this system will be able to tap into subtropical moisture
which would mean potentially higher rainfall amounts.

&&

.AVIATION...as of 4:59 AM Wednesday...For 12z Tafs. Latest
observations across the region show mainly VFR conditions, with
patchy MVFR and IFR CIGs mainly in valley locations. KMUX shows
isolated showers passing through the greater San Francisco Bay
Area, ahead of a cold front that is forecast to pass through this
morning. Expect intermittent VFR/MVFR CIGs through the day today.

More shower activity will develop behind the cold front later this
morning/early afternoon, as diurnal heating occurs and
destabilizes the atmosphere. Showers may be heavy at times and an
isolated thunderstorm cannot be ruled out. A break in showers
will occur by late afternoon or early evening.

Southeast winds will turn southwesterly/westerly and increase
once the cold front passes. Winds will decrease after sunset this
evening.

Vicinity of KSFO...A cold front will drop CIGs to MVFR over the
next few hours, with rain and showers along and behind the front.
VFR conditions expected by this afternoon. SE winds will turn W/SW
with FROPA, forecast to pass through KSFO by 18-20z. Sustained
winds 15-20 kts behind the front.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Patchy dense fog has developed around
KSNS. Fog should lift over the next couple of hour. Will monitor
accordingly. Otherwise, MVFR CIGs will develop later this morning
as another round of steady showers move through. Southeasterly to
southwesterly winds this afternoon at around 10-15 kt.

&&

.MARINE...as of 2:22 AM PDT Wednesday...2 AM PDT surface analysis
depicts a surface cold front draped across the northern coastal
waters, with showers along the front. This cold front will
continue to sweep southward through the morning. Winds behind the
front will switch out of the west or northwest, and will remain
light to moderate. Southerly winds will return and become gusty
again by Thursday morning, ahead of another cold front. Unsettled,
wet weather will persist through the remainder of the week.

&&

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

PUBLIC FORECAST: MM/W Pi
AVIATION: BAM
MARINE: BAM

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