Scientific Forecaster Discussion
fxus66 klox 180957
Area forecast discussion
National Weather Service Los Angeles/Oxnard California
257 am PDT Wed Sep 18 2019
Relatively cool temperatures will persist across the area through
Friday as low pressure moves across the West Coast. Skies will
remain generally clear although some stratus and fog will be
likely tonight through Friday morning with the most widespread
stratus likely tonight and Thursday morning. Gusty northerly winds
will continue across southern Santa Barbara County. For Saturday
and Sunday, warmer conditions are expected.
Short term (tdy-fri)...18/256 am.
Overall, 00z models in good synoptic agreement through the
short-term period. At upper levels, low will develop over the
Pacific northwest today then will move through northern California and
the Great Basin on Thursday and Friday. Near the surface, there
will be some decent west to northwest flow across the area today
and Thursday with weak offshore flow by Friday.
Forecast-wise, main issues will be the extent of the marine layer
stratus and winds. Current satellite and surface obs indicate
clear skies across the district. There may be some patchy stratus
and fog by daybreak along the coast, but will keep the forecast
clear for today. For tonight and Thursday, expect a deeper marine
inversion and relatively widespread stratus and fog (reaching into
the coastal valleys) due to the development of the upper low and
stronger onshore gradients. However, northwesterly flow across the
Santa Ynez range should keep the sba South Coast relatively clear.
For Thursday night and Friday, the weak northeasterly flow should
limit any potential stratus to the coastal plain of lax County
(and possibly the immediate coast of Ventura county). Other than
the potential stratus, skies are expected to be clear through the
As for winds, expect some gusty southwesterly winds across
interior sections this afternoon. For tonight and Thursday night,
increasing northwesterly gradients across sba County will generate
some gusty sundowner winds with the strongest winds expected from
Refugio westward (with advisory-level gusts likely in some
areas). By Friday, weak northeast flow will develop with some
locally gusty northeast winds likely across Ventura and lax
counties (but winds should be below advisory levels due to weak
gradients and limited upper level support).
As for temperatures, expect some slight cooling today for most
areas with strongest onshore gradients. For Thursday, more
noticeable cooling for all areas due to increased marine influence
and lowering thicknesses/500 mb heights. For Friday, temperatures
should rebound several degrees as upper low moves eastward and
weak offshore flow develops in the morning.
Long term (sat-tue)...18/256 am.
For the extended, models have the same general synoptic idea, but
differ noticeably in the details. So, confidence in the forecast
wanes noticeably after Saturday.
On Saturday, models indicate some slight increases in thicknesses
and 500 mb heights as well as weak gradients. So, temperatures should
increase a few degrees across the area. Skies will remain mostly
clear although there is a chance of some patchy stratus/fog
Saturday morning across the Ventura/lax coasts.
On Sunday, models begin to develop an upper low just off the
Pacific northwest. This will result in some lowering of
thicknesses and 500 mb heights and increasing onshore gradients. So,
temperatures will be a couple degrees cooler with the potential
for a bit more extensive coastal stratus across the Ventura and
lax coastal plain. Additionally, southwesterly winds will increase
Sunday afternoon across interior sections.
For Monday and Tuesday, models drop the upper low south across
California with the low settling over far Southern California by Tuesday.
The GFS is further east with the track of the upper low than the
European model (ecmwf) which leads to the greatly reduced forecast confidence. The
GFS solution would mean cool and unsettled weather for the area
with the possibility of some showers on Tuesday while the European model (ecmwf)
forecast would indicate warmer conditions with some weak offshore
winds. Given the difficulty of the models handling this feature,
will keep with idea of previous shifts, indicating cooling on
Monday and warming on Tuesday with no pops. No doubt the details
of the forecast for Monday/Tuesday will change from model run to
model run over the next few days.
At 0500z at klax, the marine layer was 900 feet deep. The top of
the inversion was near 1200 feet with a temperature of 26 c.
Low confidence in the 06z tafs for coast and valley sites, and
high confidence for inland sites. Restrictions in cigs and vsby
tonight could be periodic and broken for ksmx and the l.A. And
Ventura coastal sites, and timing is lower confidence. There is a
30% chance these sites could remain VFR overnight.
For Wednesday evening and overnight, more widespread low cloud
coverage is expected, with cigs likely IFR or MVFR. Onset of cigs
along the coast expected between 03z-06z, but timing could differ
by +/- 2 hrs from taf times. Valley sites kbur and kvny expected
to see cigs after 06z.
Klax...low confidence in 06z taf. There is a 40% chance that MVFR
cigs/vsby will not develop overnight. There is a 30 percent
chance of east wind component greater than 6 kt after 12z. MVFR
cigs onset Wednesday evening expected between 03z-06z.
Kbur...moderate confidence in 18z taf. A 30 percent of MVFR haze
or cigs from 08z-15z. VFR through 02z Thursday with IFR-MVFR cigs
likely after 06z.
Moderate to high confidence in the current forecast.
For the outer waters, Small Craft Advisory (sca) level winds are
likely to persist through early Friday morning. Winds will
subside some on Friday, with a 40% chance of Small Craft Advisory gusts continuing
through Friday evening. The latest models now show winds near or
above Small Craft Advisory levels Sunday and Monday, strongest on Sunday.
Across the inner waters north of pt sal, there is a 60-70%
chance for (sca) for northwest wind gusts Wednesday and Thursday
afternoon and evening hours. Lighter winds expected Friday and
through the weekend.
Across the Santa Barbara Channel, there is a 60 percent chance of
Small Craft Advisory level winds across western portions late this afternoon and
evening hours. Small Craft Advisory conditions are likely Thursday afternoon and
evening in the West Channel. There is a 40% chance for Small Craft Advisory
conditions across the Santa Barbara Channel both Friday and
Saturday, with stronger Small Craft Advisory level gusts expected on Monday.
Otherwise, the remainder of the southern inner waters should
remain below Small Craft Advisory conditions through Monday.
Pz...Small Craft Advisory in effect from 3 PM this afternoon to 3
am PDT Thursday for zones 645-650. (See laxmwwlox).
Small Craft Advisory in effect until 3 am PDT Friday for
zones 670-673-676. (See laxmwwlox).
Hazard potential outlook (fri-tue).
No significant hazards expected.