Hail in California, March 22, 2017

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
930 AM PDT Wed Mar 22 2017

.SYNOPSIS...

Possible precipitation with isolated thunderstorms will occur into
tonight. A dry and breezy northwest flow may bring a warming trend
from Thursday into Friday. But from Friday night through Saturday
precipitation is again possible with cooler temperatures. Then
Sunday will be a dry day before a low arrives that night with
increasing clouds for next week.

&&

.UPDATE...

Scattered precipitation across the region this morning as the tail
end of the system begins to move across Southern California. The
primary front remains off shore for SoCal but was moving through
northern San Luis Obispo County as of 16z/9am. From there the
front stretched out to the low center located about 300 miles
southwest of Point Conception.

There has been steady lightning associated with the low center
as it drops south and east but it is likely to move onshore well
south of our service area and perhaps south of the California/Mexico
border. However there was also vertical development along the
front itself and though no lightning has been noted thus far
there is decent instability indicated along the coastline. Even
stronger CAPE and negative LIs develop in the mountains of Ventura
and Los Angeles Counties by this afternoon. So though the steady
rain amounts ahead of the front look to be on the lighter side
there continues to be an expectation of thunderstorm development
this afternoon as the front moves through the area of instability
that could bring short periods of heavier precipitation.

Not anticipating many changes to the general forecast at this
time. Afternoon temperatures may be a bit warmer if the clouds are
more broken than overcast later today... but a colder airmass
behind the front would make for chillier morning temperatures
tomorrow. Breezy northerly winds may pick up tonight across Santa
Barbara but they are not expected to reach advisory criteria at
this time.

&&

.SHORT TERM...(TDY-FRI)

Latest satellite imagery showed enhanced vertical development of
clouds aloft with a few lighting strikes along an approaching
cold front stretching from the San Francisco area southwestward
200 to 300 miles offshore of Southwest California. Remnant
moisture near the surface with broad cyclonic southwesterly flow
aloft continues to support spotty drizzle or light showers mainly
to the south of Point Conception. The spotty shower activity has
been mostly undetected by the radar. To the north of Point
Conception, patchy fog will continue through mid morning and may
be locally dense with visibilities under one quarter of a mile.

The main story today continues to be the threat of isolated
thunderstorms. A cold core low aloft will pass overhead today
bringing good instability to the area 500 mb temperature down to
-26 Celsius, widespread CAPE values 400 to 800 j/kg and strong
lapse rates near the surface. While a thunderstorm is possible
anywhere, there are slightly better chances across the mountains,
especially across Ventura County, as well as anywhere that sees
prolonged sunny skies sometime late morning to the afternoon
hours. Scattered showers are expected in addition to the
thunderstorm threat. Rainfall amounts will be highly variable with
some areas likely seeing little or no rain while areas in heavier
showers or thunderstorms may see local amounts to one inch. Small
hail and gusty winds are possible in thunderstorms due to the
cold unstable air aloft. Mud and debris flows are unlikely with
any thunderstorms that do form with relatively low precipitable
waters up to 0.75 inches and strong mean enough mean flow which
should help to avoid storms stalling over a single area.

The colder air aloft and cloudy skies will bring several degrees
of cooling to many areas with the exception of Los Angeles and
Ventura County coasts and valleys where similar if not slight
warmer high temperatures are expected.

The thunderstorm threat will diminish from north to south tonight
as we lose daytime heating near the surface and the atmosphere
continues to dry out as northwest flow commences behind the cold
front. Showers are also expected to end in a similar manner,
expect for north facing mountain slopes where showers will likely
continue overnight as winds turn to the north behind the exiting
trough of low pressure. Locally gusty northwest to north winds
will set up through the I-5 corridor during this time, but will
likely remain below advisory level.

Snow levels will be around 6000 feet for most of the
precipitation today, dropping as low as around 4000 feet overnight
with showers continuing across north mountain slopes. Snowfall
amounts will be highly variable from little to no snow, with local
amounts up to 6 inches generally above 6000 feet in areas affect
by heavier showers or thunderstorms. A light dusting of snow is
possible as low as 4000 feet across the north facing mountain
slopes, possibly affecting the I-5 corridor through the Grapevine.

Mostly sunny skies along with a shortwave ridge of high pressure
and northerly gradients will provide a few degrees of warming for
daytime highs Thursday and Friday for most areas. Increasing
onshore flow Friday may limit warming for coastal areas due to
increasing marine influences. While the onshore flow increases the
downsloping flow across interior areas with 5 to 10 degrees of
warming for these areas Friday, with the greatest warming for
interior areas to the south of Point Conception. Increasing mid to
high level clouds ahead of a frontal system approaching from the
northwest will likely stunt the warming trend to the North of
Point Conception.

Model guidance begins to diverge late Friday through the extended
period with the GFS trending slower and more amplified with large
scale weather features than the ECMWF. The ECMWF was preferred
over this period as it as performed the best of the two recently.
Thus, the current forecast package brings rain to Santa Barbara
city and points north, with a chance of rain to the south by
Friday night.

.LONG TERM...(SAT-TUE)

The cold front is expected to push to the south and east of the
forecast area by Saturday afternoon, pushing most of the shower
activity out of the area, save for some lingering showers across
north facing slopes. Rainfall amounts currently look to be on the
light side with preliminary estimates of under a quarter of an
inch for Santa Barbara City and south and up to one half of an
inch to the north. Light snow is possible with this system but may
be confined to resort levels.

Drier and warmer conditions are expected Sunday as ridging
develops aloft and northerly gradients strengthen.

Model guidance continues to show significant differences for
early next week with each maintaining their respective solutions.
Both indicate a weak trough passage by Monday, possibly bringing
light showers to the north of Point Conception. By Tuesday the GFS
follows this weak trough with an inside slider, while the ECMWF
features a ridge. Have leaned slightly towards the warmer and
drier solution of the ECMWF.

&&

.AVIATION...22/1150Z

At 1130Z, there was no marine inversion at KLAX.

Low confidence in 12Z TAF package through 06Z. There will be
frequent flight cat changes as both cigs and vis vary up and down.
There is a 40 percent chc of a shower for the LA county TAF
sites. There is a 20 percent chc of a TSTM in the morning N of Pt
Conception and a 20 percent chc in the afternoon S of Pt
Conception.

Good confidence in TAFs after 06Z with the transition to VFR
conds.

KLAX...Low confidence in 12Z TAF package through 06Z. There will be
frequent flight cat changes as both cigs and vis vary up and down. There
is a 40 percent chc of a shower today and a 20 percent chc of a
TSTM in the afternoon. Good confidence in mid evening VFR
transition. East winds should remain below 8 kt through the
period.

KBUR...Low confidence in 12Z TAF package through 06Z. There will be
frequent flight cat changes as both cigs and vis vary up and down. There
is a 40 percent chc of a shower today and a 20 percent chc of a
TSTM in the afternoon. Good confidence in mid evening VFR transition.

&&

.MARINE...22/900 AM

Isolated thunderstorms are possible early today across the
northern waters initially, then across all coastal waters late
morning through the early evening hours. Any thunderstorm that
forms will be capable of producing locally gusty winds and rough
seas, dangerous lightning, heavy rainfall with reduced visibility,
and waterspouts.

Gusty NW winds will reach SCA levels tonight through Thursday.
There will be a few gale force gusts between San Nicolas Island
and Santa Rosa Island during this time. The winds will weaken late
Thursday night into Friday.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement in effect through Thursday morning
      for zones 39>41-87. (See LAXCFWLOX).
PZ...Small Craft Advisory in effect from 6 PM this evening to 3 AM
      PDT Thursday for zones 645-650-670. (See LAXMWWLOX).
     Small Craft Advisory in effect from 6 PM this evening to 4 PM
      PDT Thursday for zone 655. (See LAXMWWLOX).
     Small Craft Advisory in effect from 6 PM this evening to 9 PM
      PDT Thursday for zones 673-676. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK...(FRI-TUE)
A weak rain event is expected Friday night and Saturday with
minimal impacts. High surf and strong rip currents are likely
Friday into Sunday.

&&

PUBLIC...Kj/Munroe
AVIATION...ASR
MARINE...Sweet
SYNOPSIS...STu

weather.gov/losangeles