Area Forecast Discussion
Issued by NWS Los Angeles, CA

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

FXUS66 KLOX 211709

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA
1009 AM PDT Wed Mar 21 2018

.SYNOPSIS...21/649 AM.

The temperatures will be below normal into Friday. A significant
precipitation storm with isolated thunderstorms is forecast to
persist into early Friday. The storm should make an exit on Friday.
Then skies will began clearing  with a warming trend for the weekend
into Tuesday.


.SHORT TERM (TDY-FRI)...21/1008 AM.

It`s been a tricky forecast getting a handle on the heavier
pockets of rain, one of which moved through SB/SLO counties
earlier this morning and is now moving through Ventura County. And
as expected LA County, especially the eastern and southern
portion, has largely been left out of the action. The western
portion will get some of the heavier rain from the impulse moving
through this morning but the rest of LA County will still need to
wait until Thursday for any significant precip.

For the rest of the area after this morning burst of heavier rain
moves through we`ll see a little lull this afternoon and evening
as the plume shifts north a bit. Still be a lot of showers around
but not quite as steady as this morning. Late tonight into
Thursday morning another surge in moisture coupled with a big
increase in southerly low level flow and lift with the cold front
will likely bring the biggest threat to burn areas from this storm,
especially south facing slopes of Ventura and SB Counties. Rain
rates, even without convection, could easily hit a half inch per
hour and likely higher. With convection those rates could reach an
inch or more per hour. Model soundings actually less unstable
tomorrow as the front comes through than they showed for this
morning. Though there was some lightning this morning most of it
was offshore and cloud tops warmed as they moved over land. Still,
with the frontal boundary can`t rule out an isolated thunderstorm.

Given the slightly lower rainfall amounts last night forecast
rain amounts for the storm have been adjusted down just a tad. Now
expecting 2-4" coast/valleys 4-8" foothills/mtns for
SLO/SB/western Ventura Counties, and 1-2" coast/valleys, 2-5"
foothills/mtns for eastern Ventura and LA Counties.

***From previous discussion***

Right now it looks like the storm will enter its most threatening
period Thursday morning as the AR moves southward. Due to its SW
to NE orientation its flow will be nearly perpendicular to the
transverse ranges of southern SBA county and western VTA county.
It looks like there will be about a 9 hour period of mdt/hvy rain
Thu morning and afternoon. Its during this time when LA county
will begin to see rainfall in earnest.

The event will wrap Thursday evening and overnight into Friday
morning. The rain will end starting over the Central Coast and
ending in LA county.

TSTMs are the latest wrinkle in the forecast. Quite a bit of
lightning has been observed in the plume. The satellite soundings
have picked up on this area of instability as well. The area of
instability will move over the area this morning esp over the
higher terrain. There will be a slight chc of TSTMs across SLO
county and western SBA county today and this afternoon.

Starting around midnight on Thursday the trof associated with
parent low will move into Srn Ca and will add a little more
instability and a lot more dynamics to the area North of LA
county. Added a slight chc of TSTMs to SLO/SBA/VTA counties from
Midnight to late afternoon Thursday. TSTMs will be very
problematic as any convective lift in this moist an atmosphere
could easily produce rainfall rates in excess of an inch per hour.

The current rainfall estimates still look good. The forecast calls
for amounts of 2-5 inches of rain across the coasts and valleys
and 5-10 inches of rain in the foothills and mountains. The higher
amounts should be over SBA county and western VTA County. The LA
area may end up with a little less rain as it will not have the
longer duration of rainfall that SLO/SBA/VTA will. Peak rainfall
intensities of 0.5 to 0.75 inches per hour are expected with
potentially higher rates in the SBA/VTU Co foothills due to
orographic enhancement. As mentioned above any convective cells
could bring rainfall rates in excess of an inch per hour (perhaps
well in excess)

No snow expected as snow levels will be above 8000` for the
duration of this event.

The sub tropical airmass will combine with the persistent clouds
and rain and make for two days with only about an 8 degree swing
between the below normal highs and above normal lows.

There will be some lingering showers on Friday but otherwise look
for clearing skies with max temps 4 to 8 degrees blo normal.

.LONG TERM (SAT-TUE)...21/320 AM.

Long range forecast has changed as both GFS and EC swing a decent
trof through the area Saturday afternoon and evening. This will
bring some more clouds to the area...prevent much of any warming
and will bring a slight chc of light showers to SLO and SBA

Sunday will be and breezy behind the trof.

On Monday and Tuesday a ridge will push into the Pac NW while a
large upper low spins over Las Vegas. This will set up an offshore
event but not as strong as it was looking yesterday. Hgts remain
below normal so despite the offshore flow max temps will remain
blo normal.



At 1200Z, there was no marine inversion present at KLAX.

Low confidence in 12Z TAFs due changing cigs/vsbys in and around
shower activity. Low confidence in the timing and location of
heavier shower activity especially after 00Z/22. Isolated
thunderstorm activity is possible from KOXR/KCMA and north through
the TAF period with slightly higher chances for KSMX and KSBP.
Southeast winds may gust up to around 20 kts ahead of an
approaching cold front after 00Z for KSMX, KSBP, and KPRB.

KLAX...Low to moderate confidence in 12Z TAF. There is a 50
percent chance of east winds of 8 kts or greater after 10Z/22.
There is also a 10 percent chance of a thunderstorm with a cold
front pushing through the area after 12Z/22.

KBUR...Low to moderate confidence in 12Z TAF. There is a 10
percent chance of a thunderstorm with a cold front pushing through
the area after 11Z/22.


.MARINE...21/256 AM.

SCA gusty south to southeast winds will continue into Thursday for
the Outer Waters and Inner Waters north of Point Sal. There is a
20 percent chance of reaching Gale force winds for the Outer
Waters sometime this evening into Thursday morning. Isolated
thunderstorms are possible at times across the waters through
Thursday evening. Brief gusty and erratic winds with an isolated
water spout is possible near any thunderstorm.

SCA winds are likely again late Friday into this weekend across
the Outer Waters with a 30 percent chance of reaching the inner
waters at times. There is also a 20 percent chance of Gale force
winds across the Outer Waters by Sunday.


CA...Flash Flood Watch in effect through Thursday afternoon for
      zones 34-36-39-40-44-51>53. (See LAXFFALOX).
     Flash Flood Watch in effect from this evening through late
      Thursday night for zones 41-46-54-88-547. (See LAXFFALOX).
PZ...Small Craft Advisory in effect until 3 AM PDT Thursday for
      zone 645. (See LAXMWWLOX).
     Small Craft Advisory in effect until 9 AM PDT Thursday for
      zone 670. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 AM PDT Friday for zone
      673. (See LAXMWWLOX).
     Small Craft Advisory in effect until 3 PM PDT Thursday for
      zone 676. (See LAXMWWLOX).



No significant hazards expected.



SYNOPSIS...STu is the U.S. government's official web portal to all federal, state and local government web resources and services.