Area Forecast Discussion
Issued by NWS Los Angeles, CA
FXUS66 KLOX 060338
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
838 PM PDT THU MAY 5 2016
A slow-moving low pressure system will bring a chance of showers
and a slight chance of thunderstorms to the region through the
weekend. High pressure will rebuild next week bringing mostly
clear skies with a warming trend to the area Monday through the
middle of next week.
Cold upper level low pressure system currently centered near Point
Conception this evening will slowly track eastward and become
centered across the eastern deserts of California by this time
tomorrow evening. The cold air aloft combined with daytime surface
heating this afternoon to generate scattered showers and isolated
thunderstorms across interior sections of Ventura, Santa Barbara,
and San Luis Obispo counties this afternoon. The strongest
thunderstorm produced flash flooding and closure of Highway 166
between Cuyama and the Kern county line. In addition, there were
a few reports of hail with storms earlier this afternoon.
As of this evening, still some scattered showers across the
forecast area, with a new batch of showers approaching the
LA county coast. With the upper level cold core (500 mb
temperatures of -22 degrees celsius) passing over forecast area
tonight into Friday morning, there will be a chance of showers
and a slight chance of thunderstorms just about anywhere across
the forecast area, with the highest probability focused towards
LA county. By Friday afternoon, the upper level low pressure
system is forecasted to be just east of the forecast area,
somewhere in western San Bernardino county.
There is still ample moisture (precipitable water vaules in
excess of .80 inches) on the back side of this upper low that is
expected to focus afternoon convection over the interior sections
of the forecast area where there will be optimal daytime surface
heating. Of particular concern, will be the threat of slow moving
thunderstorms forming over the mountains tomorrow afternoon.
Latest 00Z nam model shows fairly impressive convective parameters
on Friday afternoon across the local mountains, with lifed index
values ranging between -4 and -8, along with CAPE values locally
exceeding 1000 J/kg. With steering flow winds expected to be 10
knots or less, this has the potential to set up flash flooding
concerns for the local mountains. Later shifts will need to
consider issuing a flash flood watch for the mountains of Santa
Barbara, Ventura, and Los Angeles counties for Friday afternoon
into early evening. In addition to the heavy rain and flash flood
potential, thunderstorms that develop on Friday will have the
potential to produce hail, gusty winds, and dangerous cloud to
ground lightning. With the upper flow winds from the northwest
during this period, any storms that do develop over the mountains
will have a greater probability to drift into adjacent valley and
*** From previous discussion ***
A slight chance of showers will linger across the forecast area
Fri night and Sat as the upper low passes just to the s and e of
the region. A slight chance of thunderstorms can also be expected
Sat afternoon mainly over the mtns and Antelope Vly thanks to
lingering moisture and instability.
Rainfall amounts thru Sat will be highly variable due to the
convective nature of this weather system. Most areas across the
coast and vlys should receive 0.15 inch or less, with up to 0.30
inch possible for L.A. County. In the mtns, about 0.25 to 0.50 inch
is expected with local higher amounts of 0.75 inch in the VTU County
mtns. However, where any thunderstorms occur, local precip amounts up
to an inch or more cannot be ruled out.
Snow levels will lower to 6000 to 6500 feet by Fri, otherwise snow
levels of 6500 to 7000 feet are expected thru Sat. Accumulating snow
is possible where showers and thunderstorms occur in the higher
mtns, with local totals of 6 to 9 inches possible thru Sat above
7000 feet for VTU County, and locally up to 4 to 6 inches possible
above 7000 feet for L.A. County.
A chance of showers will linger Sat evening in all areas, with a
slight chance of thunderstorms mainly in the mtns and Antelope Vly.
A slight chance of showers will then linger later Sat night thru Sun
afternoon for the mtns and deserts as residual moisture from the
departing weather system persists. Instability is marginal at best
for Sun afternoon, so no thunderstorms are expected to develop.
Otherwise, plenty of clouds should prevail across the forecast area
thru Sun with perhaps a few more breaks in the clouds especially s
of the mtns.
Needless to say, with varying amounts of clouds and unsettled
weather conditions, temps will be much cooler than normal across the
area Fri thru Sun, with highs ranging from 7 to 16 deg below normal
for Fri, from about 3 to 13 deg below normal overall for Sat, and 2
to 12 deg below normal for Sun. Hi temps for the cst and vlys will
be generally in the 60s Fri, then warm slightly into the mid 60s to
low 70s for Sat, and upper 60s to lower 70s for Sun.
EC/GFS in generally good agreement Mon thru Thu. Upper level ridging
will continue to build off the CA cst Mon thru Tue, then move into
srn CA for Wed and Thu. Dry weather with warmer temps will prevail
across the forecast area during the period. Some night and morning
marine layer low clouds and fog should affect some of the coast s of
Pt Conception Tue thru Thu, otherwise clear to partly cloudy skies
can be expected across the region for Mon thru Thu. High temps will
still be several degrees below normal for Mon, then warm to near
normal for Tue and Wed, and to several degrees above normal for many
areas on Thu, with the warmest vlys reaching into the mid 80s.
At 0005z at KLAX... there was no marine inversion.
Overall... Low to moderate confidence in the 00z tafs. The
reduced confidence is primarily due to the hit and miss nature of
the showers and thunderstorms in the forecast. Conditions near any
thunderstorms will include variable and gusty winds... brief heavy
rain... and possible hail. Highest probability of showers and
tstms for KLAX, KLGB, KBUR, and KVNY is later this evening through
KLAX and KBUR...Low to moderate confidence in the 00z taf. The
reduced confidence is primarily due to the hit and miss nature of
the showers and thunderstorms in the forecast. Highest probability
of showers and thunderstorms for KLAX and KBUR will be 06z-18z,
where there is a 60 percent chance of showers and a 20 percent
chance of thunderstorms.
There is a chance of showers across the coastal waters through
early Sunday and a slight chance of thunderstorms through Friday.
Any storms that develop will be capable of brief heavy downpours,
gusty winds, small hail, and dangerous cloud to ground lightning.
By Friday, the greater threat of showers and thunderstorms will be
the southern waters. Winds and seas should remain below advisory
criteria during this period except in the vicinity of any
thunderstorms that develop.
The storm system moves out the of region by late Sunday and no
significant wind or sea conditions are expected through the early
half of the week.