Area Forecast Discussion
Issued by NWS Los Angeles, CA

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FXUS66 KLOX 221801

1100 AM PDT Fri Sep 22 2017

Updated Aviation Discussion

A low will bring cold air this week with breezy winds and major
temperature drops inland. Next week, a high, an offshore flow
should bring a significant warming trend into Thursday.



Still quite moist at the lower levels but inversion still way too
weak to support much in the way of a marine layer. So skies
started out mostly clear. However, with some heating now we`re
seeing clouds developing especially up along the south facing
slopes and I expect that will continue through the afternoon and
include some of the valleys and coast as well. So partly cloudy
seems a good forecast for today south of Pt Conception. Temps will
remain on the cool side for this time of year but should be a few
degrees warmer than Thu.

Looking ahead, 12z models today were a little more robust with the
offshore gradients Sunday and Monday and the NAM is now showing
30-35kt at 950mb which is also 10-15kt stronger than what it and
the GFS showed yesterday. So will have to keep a close eye on
these trends and adjust as necessary. Current forecast still
holding onto some low clouds for Sunday morning along the LA Coast
which seems reasonable for now but might have to re-evaluate this
if models continue to favor a stronger offshore gradient and upper
support. Still looks like an extended period of warm/dry
conditions through next week, the question is how warm and windy?
Still a fair amount of uncertainty there that likely will remain
for a few days at least until models settle down with their
respective solutions.

***From previous discussion***

The elongated upper low across Idaho and Nevada will move only
very slowly eastward tonight and Saturday. Expect skies to be
mostly clear tonight/Sat, except for coastal sections of L.A.
County, where there may still be a marine inversion, and some low
clouds may develop late tonight and linger through mid morning
Sat. Max temps on Sat should be about the same as those reached
today, except there should be a few degrees of warming in the
interior valleys, mountains and the Antelope Valley.

The upper low will eastward Sat night and Sunday, while an
upper ridge amplifies in the eastern Pacific and moves
toward the West Coast. Low level gradients are forecast to turn
offshore Sunday morning, and could be strong enough to produce
some locally gusty northeast winds in the mtns and valleys of
L.A. and VTU Counties. Heights will rise significantly on Sunday,
so expect significant warming, especially west of the mountains
where downsloping northeast flow will add to the warming. Max
temps will likely get close to 90 degrees across the warmer
locations in the valleys on Sunday.


The upper ridge will continue to approach the West Coast Mon, and
heights/thicknesses will rise. Offshore flow will be a bit stronger
Mon, so there should be a bit more in the way of gusty northeast
winds across L.A./VTU Counties, especially in the mtns and
valleys. Max temps should be up a few degrees in most areas Mon.

The models now diverge a bit beyond Mon. The GFS shows the ridge
axis moving into the Pac NW and northern CA by late Tue, then into
the West Coast Wed and Thu, while a very weak upper low develops
west of the forecast area on Thu. Low level offshore flow would
peak on Tue, but would remain weakly offshore through Thu. This
would result in very warm to hot weather in most areas Tue through
Thu with well above normal temps. The EC shows shortwave energy
dropping southward into Great Basin Tue, with an upper low
developing in southern Nevada, then this upper low will linger
through early Thu before moving east. Low level flow is also
forecast by the EC to be weakly offshore, at least Tue and Wed.
This scenario has somewhat lower heights across the region Tue
through Thu than does the GFS, but interestingly enough, the
forecast max temps on the EC numerical guidance are close to
those of the GFS, though slightly lower. Overall, still have
fairly high confidence that temps will be well above normal Tue
through Thu, and skies will be mostly clear through the period.



At 17z at KLAX...There was no marine layer or inversion.

Moderate confidence in VFR SKC-SCT030 prevailing through
Saturday. 30 percent chance of CIG 20-02Z today at KBUR KVNY KLGB.
30 percent chance of bkn002-008 at KSMX KSBP 03-15Z tonight, but
confidence is low. 20 percent chance of bkn020 at KSMO KLAX
KLGB 12-18Z Saturday.

KLAX...Moderate confidence in VFR SKC-SCT035 through tonight,
with a 20 percent chance of an hour or two of BKN030 20-02Z.
Moderate confidence in VFR on Saturday, with a 20 percent chance
of BKN015-025 12-17Z. Gusty west winds likely today, but 5-10KT
weaker than yesterday. SE winds likely after 10Z tonight, with a
20 percent chance of exceeding 8KT.

KBUR...Moderate confidence in VFR SKC-SCT030 through Saturday,
with a 30 percent chance of BKN030 20-02Z today.


.MARINE...22/900 AM.

The gusty northwest winds from yesterday will be weaker today, but
will still be close enough to Small Craft Advisory (SCA) levels by
this afternoon to warrant extension into tonight. There is a 60
percent chance that some of the outer waters will need an SCA
through Saturday night, but those winds should weaken
significantly below SCA by Sunday. Short period choppy sea
conditions will persist everywhere through at least Saturday.


PZ...Small Craft Advisory in effect from 3 PM this afternoon to 11
      PM PDT this evening for zones 645-650-655. (See LAXMWWLOX).
     Small Craft Advisory in effect from 3 PM this afternoon to 6
      AM PDT Saturday for zone 670. (See LAXMWWLOX).
     Small Craft Advisory in effect until 6 AM PDT Saturday for
      zones 673-676. (See LAXMWWLOX).


It will be hot Monday through Thursday, especially away from the
coast. There will be gusty northeast winds at times, especially
across Los Angeles and Ventura Counties Monday and Tuesday. The
combination of heat, gusty winds and low relative humidity values
will bring heightened fire danger, especially to the mountains and



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