084 
FXUS66 KLOX 211022
AFDLOX

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

.SYNOPSIS...20/1012 PM.

A strong storm moving into the region will bring waves of significant
precipitation to the region through early Friday. This could result
in debris flows at all recent burn areas as well as periods of urban
and small stream flooding. The heaviest rainfall and a chance of 
isolated thunderstorms will come Wednesday afternoon through Thursday.
Notable snowfall will remain above 9000 feet.

&&

.SHORT TERM (TDY-FRI)...21/304 AM.

Forecast hit a pretty big bump in the road last evening and early
this morning. The atmospheric river (AR) event did not really
start despite the best forecasting efforts of weather forecast
computers across the globe. This is not totally unexpected as the
forecast of systems coming up from the SW is the most difficult to
forecast since they originate over the unpopulated ocean and there
is really no good data going into the forecast. Too late for this
storm but the situation should improve once the latest and
greatest NOAA weather satellite gets set up over the pacific later
this year.

But it looks like the forecast is going to get back on track over
the course of the next few hours. Satellite imagery shows a much
better organized band of clouds approaching the coast. Consider
the last 12 hours just the prelude to the main system and its only
purpose was to get the atmosphere saturated. (This was very
evident last evening as there were all sorts of radar echos and
little to no rainfall on the ground as all of the rain evaporated
in the lower atmosphere before it could reach the ground)

The main event will start around dawn this morning across the
Central Coast. By mid morning the rain will reach across most of
VTA county and moderate to heavy rain will be over SBA county.
Mostly moderate rain should then continue over SLO/VTA/SBA county
through the afternoon.

The system will tilt into more of a SW to NE orientation this
evening and the rain will briefly let up south of Pt Conception
but will really intensify over Western SLO and SBA counties.

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
counties.

Sunday will be dry...cool 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.

&&

.AVIATION...21/0608Z.

At 0530Z, there was no marine inversion present at KLAX.

Moderate confidence overall in the 06Z TAF package. Rain will 
gradually spread south and east across TAF sites tonight with MVFR
conditions likely with steady rain aft about 08Z for KPRB, KSBP, 
KSMX and KSBA, then lower to IFR for these airfields early Wed 
morning with moderate to heavy rain thru Wed evening. For KOXR and
KCMA, VFR cigs are expected tonight then lower to MVFR Wed 
morning thru the afternoon with steady rain before increasing to 
VFR cigs early Wed evening, but there is low confidence in this. 
VFR cigs should prevail at remainder of airfields tonight through 
Wed evening, altho there will still be a 30-40 percent chance of 
MVFR cigs at times Wed into Wed evening. Overall confidence in 
timing of lowering flight cats is low, and could be off +/- 2 
hours or more.

KLAX...Moderate confidence overall in the 06Z TAF. VFR cigs are
expected thru the TAF period with an increasing chance of rain 
on Wed and Wed night which should lead to MVFR vsbys later Wed 
night. There will still be a 30-40 percent chance of MVFR cigs at
times Wed into Wed night. Overall confidence in timing of any 
lowering flight cats is low, and could be off +/- 2 hours or more.

KBUR...Moderate confidence overall in the 06Z TAF. VFR cigs are
expected thru the TAF period with an increasing chance of rain 
on Wed and Wed night. There will still be a 30-40 percent chance 
of MVFR cigs at times Wed into Wed evening. Overall confidence in 
timing of any lowering flight cats is low, and could be off +/- 2 
hours or more.

&&

.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.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
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).

&&

.HAZARD POTENTIAL OUTLOOK (FRI-TUE).

No significant hazards expected.

&&

$$

PUBLIC...Rorke
AVIATION...Sirard
MARINE...Munroe
SYNOPSIS...Kj

weather.gov/losangeles