Hail in California, January 19, 2018

Area Forecast Discussion
National Weather Service San Francisco Bay Area
942 AM PST Fri Jan 19 2018

.SYNOPSIS...Chillier, unsettled weather is here for the weekend
and will last into next week. Unstable weather conditions will
cause scattered showers and a few thunderstorms to develop today.
Snow showers are likely along the highest ridge tops. A band of
rain, possibly heavy at times, moves into the North Bay Sunday
evening spreading southeastward Sunday night into Monday morning.
Rain returns during the middle of next week with showery weather
lingering into late next week.

&&

.DISCUSSION...as of 09:08 AM PST Friday...Current forecast
remains on track. Simply put, cold unstable airmass and lingering
moisture will lead to scattered showers, possibly thunderstorms
and lowered snow levels.

The system that brought rain to the region continues to exit to
the east. KMUX radar imagery shows scattered showers moving NW to
SE across the region behind the departing system. Farther to the
west, about 200 miles or so, is a cluster of lightning associated
with more organized open-cell cumulus clouds. Utilizing NUCAPS
soundings shows CAPE values greater than 400 J/kg near this
cluster. That being said, latest models bring this instability
over the Bay Area through the day. SPC updated the Day 1 outlook
to include Central California for a general mention of
thunderstorms. Given the lowered snow levels, any developed storm
will be capable of producing small hail.

Dry, but cool weather for Saturday. Rain chances return Sunday
evening into Monday.

No update planned.

&&

.PREV DISCUSSION...As of 5:34 AM PST Friday...An ongoing large scale
pattern change over the west continues to bring in chillier,
modified maritime polar air on NW winds into California early this
morning. IR satellite imagery shows a fair amount of upstream
instability with open cellular cumulus clouds with a positive tilt
500 mb trough axis accompanied by mid-upper level temperatures as
cold as -33C. KMUX radar shows scattered showers in the NW wind
flow. Several lightning strikes were detected approx 500 miles
west of the Central Coast at 5:10 am.

Hit or miss showers today will be the theme in a convectively driven
environment. There's also a slight chance of thunderstorms. As the
mid-upper levels cool off today this will greatly steepen lapse rates
allowing air parcels to freely move upwards. A little strange to see
differences between the HRRR model indicating instability offset by
some convective inhibition while the WRF indicates similar instability,
but virtually no convective inhibition. The 500 mb trough is elongated
and vorticity spread thin perhaps, but relative and absolute vorticity
values do tend to tighten up as the trough approaches and passes over
the forecast area later this morning and afternoon. Convection that
develops over the coastal waters could have tops to 50 thousand feet
thus indicating how vertically extensive the cold air is with this
trough. Cells that develop may have some tilt to them, possibly
generating some with hail as well. Will need to keep a close watch
on the radar. Cold air advection also has the area ridge tops in the
mid 30s this morning thus snow showers are possible on the highest
peaks this morning onward today.

A dry break Saturday into Sunday is then followed by a much shallower,
but more intense cold frontal boundary with stronger confluence and
shear along the boundary late Sunday evening; this air mass is the
next in line originating from the Bering Sea and Alaska. The front
will be accompanying the tail end of a weak atmospheric river so it'll
have some juice with it to some extent. Cross sections of omega values
show a strong pocket of upward vertical motion at lower levels
nearing 40 microbars/second over the North Bay Sunday evening,
lesser amounts as the frontal boundary moves southeast Sunday night.
Leaning toward little if any hydro hazards based on the pace of
frontal movement and 00z ECMWF, 00z/06z GFS qpf between moderate and
heavy rain rates. However, the 06z NAM goes wetter specifically over
coastal Sonoma county which may extend to Marin county as well
before matching qpf output from the ECMWF and GFS and before the
rain band reaches a SFO-OAK line. 925 mb SW winds, warm air
advection also ramp upwards for a 3-4 hour period prior to cold
frontal passage; 925 winds 40-50 knots are indicated. Best guess at
the moment is that strong upward vertical motion along the front
should preclude downward momentum transport enough to preclude a
wind advisory for now, but will monitor subsequent model output on
this. Rain rates/amounts will be of primary concern with wind a
secondary concern in this particular case, but will keep a close
watch on it nonetheless.

Wet, chilly weather continues next week. Rain returns focusing in our
area during the middle of next week.

&&

.AVIATION...as of 9:42 AM PST Friday...for 18Z TAFs. VFR
conditions to prevail for most sites aside from patchy morning fog
at KSTS and in the North Bay. Scattered showers will continue to
stream over the area through late this afternoon. Passing showers
may briefly lower cigs and reduce visibilities to MVFR/IFR. Main
concern today will be the chances for some scattered thunderstorms
along the coast. This would mainly affect sites closer to the
Pacific. Thus far the nearest lightning strike has been over 150
miles offshore, but the potential for convection through this
afternoon still exists. Confidence is low enough to keep TS out of
the tafs. Onshore winds this afternoon 10-15 kt.

Vicinity of KSFO...VFR conditions will prevail across area
terminals as showers continue to move across the region. Passing
showers may briefly reduce conditions to MVFR. Chance of
scattered thunderstorms into this afternoon. So far, no lightning
strikes have been reported anywhere near the coast. Radar will be
closely monitored. Onshore winds this afternoon 10-15 kt.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR cigs and vsbys will prevail with
passing scattered showers. Brief vis and cig reductions possible
with a heavier shower. Slight chance of thunderstorms through
this afternoon along the coast and over the waters, main concern
would be for KMRY. No lightning has been observed near the airport
thus far, but radar will be closely monitored. Light winds.

&&

.MARINE...as of 09:27 AM PST Friday...Large west to northwest
swell will continue to impact the coastal waters through today,
causing large breaking waves and high surf along the Central
Coast. Scattered showers with possible thunderstorms will continue
through the day. Slight increase in northwest winds tonight. Winds
will increase and turn southerly on Sunday morning ahead of the
next system. Swells will gradually decrease over the weekend.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...High Surf Warning...CAZ006-505-509-529-530
             SCA...Mry Bay
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Rough Bar Advisory for SF Bar

&&

PUBLIC FORECAST: MM/Canepa
AVIATION: AS
MARINE: AS

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PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
136 PM PST FRI JAN 19 2018

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

0136 PM     HAIL             CUPERTINO               37.31N 122.05W
01/19/2018  E0.25 INCH       SANTA CLARA        CA   TRAINED SPOTTER

&&

STANDERS