Forecast Discussions mentioning any of
"ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 09/28/17
Please see the 00Z aviation forecast discussion below.
Light to moderate rain continues over a good portion of south central
Texas this evening, with the most persistent activity from SAT/SSF
west toward DRT. We will continue to mention light to occasional
moderate rain in the forecast for the evening and overnight hours.
Otherwise, look for a gradual lowering of ceilings overnight with
MVFR expected at AUS and upper end IFR for our remaining TAF sites.
Gradual improvement is expected tomorrow afternoon.
PREV DISCUSSION... /ISSUED 248 PM CDT Wed Sep 27 2017/
SHORT TERM (Tonight through Thursday Night)...
A complex rainfall situation is setting up across the western half of
the CWA this afternoon and evening. A combination of continued rich
low level moisture advection, an approaching frontal boundary, and a
mesoscale convective vortex will all come into play to continue the
flash flood threat through tomorrow.
Through the rest of the afternoon, while the majority of the heavy
rainfall continues to develop well west of the I-35 corridor.
Increased instability and another surge of low level moisture is
pushing north from South Texas and resulting in isolated thunderstorm
development. While this activity is mostly isolated in nature, CG
lightning observed in the updrafts and significantly colder cloud
tops indicate the possibility for intense downpours with the added
low to mid level CAPE available. While these should weaken as they
approach and are currently moving north rather quickly, they will
still have the capability of dropping up to 2 inches of rain in an
hour or two, which could exceed the very low flash flood guidance in
Atascosa County, and to a lesser extent Bexar County. With this short
term concern for localized flash flooding, it was decided to expand
the flash flood watch to include both Atascosa and Bexar counties.
After midnight, the MCV is progged to continue to shift
northwestward, as evidenced in both hi-resolution guidance and in the
GFS/ECMWF. This should focus the heavier rainfall potential overnight
to Del Rio and Val Verde County. Thus, QPF was increased in these
regions along the Rio Grande and for Val Verde. PoPs will begin to
decrease overnight for the eastern areas as the aforementioned
isolated activity wanes due to a lack of diurnal heating.
Through Thursday, models continue to generate moderate to heavy
rainfall over the extreme western areas of the CWA, with lighter
rates from Uvalde and east. While there may be a drop off in precip
coverage tomorrow over the eastern half of the flash flood watch, it
may be worth maintaining through Thursday evening as currently issued
due to rainfall amounts unofficially exceeding 20 inches in some
spots in Zavala and Dimmit counties.
LONG TERM (Friday through Wednesday)...
Precip chances begin to decline beginning overnight Thursday and
through Friday. Over the weekend, the parent low over the four
corners currently will lift northward and take the upper level
support with it, dramatically decreasing our chances for rain. A more
typical late September, early October pattern will then take its
place by early next week.
By mid week, an elongated ridge axis will broaden over the state
according to the ECMWF resulting in slight chances for rain, mainly
over the southern and southeastern areas where better moisture
resides. However, no hazards are expected.
.PRELIMINARY POINT TEMPS/POPS...
Austin Camp Mabry 71 84 69 82 65 / 40 50 50 30 20
Austin Bergstrom Intl Airport 71 84 69 82 64 / 40 50 50 30 20
New Braunfels Muni Airport 71 84 69 84 65 / 40 50 50 30 20
Burnet Muni Airport 67 80 65 77 61 / 50 60 60 40 20
Del Rio Intl Airport 68 77 69 80 66 / 70 70 60 60 20
Georgetown Muni Airport 69 83 66 80 62 / 50 50 50 30 10
Hondo Muni Airport 69 85 70 86 65 / 60 50 50 40 20
San Marcos Muni Airport 70 84 69 83 64 / 40 50 50 30 20
La Grange - Fayette Regional 72 87 70 85 66 / 20 40 50 30 10
San Antonio Intl Airport 70 84 70 84 66 / 40 50 50 30 20
Stinson Muni Airport 71 84 71 84 67 / 40 50 50 30 20
Flash Flood Watch through Thursday evening for the following
Public Service/Data Collection...YB
Area Forecast Discussion
National Weather Service Honolulu HI
318 PM HST Wed Sep 27 2017
Moderate trade winds will briefly increase to locally breezy
levels on Thursday then ease on Friday. A stable rainfall pattern
will focus clouds and showers over windward slopes. Winds will
weaken and shift out of the southeast on Saturday then out of the
south over most of the island chain on Sunday as a front
approaches. A humid land and sea breeze pattern will develop over
most areas, with clouds and showers favoring interior portions of
the islands during the afternoon hours, and volcanic haze will
likely spread over all islands.
A stable, moderate trade wind flow has been carrying an area of
slightly enhanced low-level moisture across the island chain
today. A 1021 mb surface high far northeast of the state and an
associated surface ridge extending to about 350 miles north of
Kauai are driving the trade winds. A mid- to upper-level ridge
overhead is producing stable conditions, with an inversion running
between 6,000 and 8,000 ft according to afternoon soundings and
aircraft data. An area of low-level moisture has provided a boost
in trade wind showers today, mainly for windward slopes. Rainfall
has been modest, roughly one quarter of an inch or less. Expect
this area of moisture to clear the islands by midnight.
A short-lived increase in the trades is due for Thursday. A front
currently passing 375 miles north of the state will dissipate,
and the surface ridge currently 350 miles north of Kauai will
lift northward and strengthen. As a result, trade winds will
temporarily build into the locally breezy range. A gradual
decrease in winds is expected on Friday as another approaching
front forces the ridge to retreat southward again. Conditions will
remain stable with pockets of moisture producing modest rainfall
over windward slopes.
Humid and hazy conditions are expected to develop during the
weekend. The advancing front will cause the local background winds
to veer out of the southeast on Saturday, then out of the south
across most islands by Sunday. Expect a land and sea breeze
pattern to dominate, causing clouds and showers to favor interior
area during the afternoon hours. By Saturday night, developing
southerly flow will likely draw volcanic haze over the much of the
state along with a humid tropical airmass with dew point temps in
the lower 70s.
Heading into Monday and Tuesday, uncertainty remains rather high.
Both the GFS and ECMWF are cutting off an area of low pressure
northwest of the state, but they differ greatly on how this
feature effects the front. The models are also inconsistent from
run to run as to whether or not the front will reach the western
end of the island chain before retreating westward by midweek. We
continue to paint a broad southeasterly flow over the islands
during this time, but confidence is low.
VFR conditions are expected to prevail, however increasing trade
winds will focus clouds and showers over windward and mountain
areas. Some brief MVFR conditions are possible in these areas.
Leeward areas that clouded up today are expected to clear
overnight. No AIRMETs are in effect at this time, however as the
trades build tonight and tomorrow, and AIRMET for mechanical
turbulence may be needed.
The trailing end of a front northwest of the area weakened the
subtropical ridge and has kept the trade winds moderate to
locally fresh. The ridge is forecast to strengthen tonight with
winds reaching Small Craft Advisory speeds tomorrow and tomorrow
night. Winds will weaken again starting Friday and will shift out
of the southeast over the weekend as a new front approaches from
Surf will be below average along all shores through early next
week. There will be a small north northwest swell tomorrow and
Friday. A moderate northwest swell is expected to arrive Sunday
night and peak Monday well below the advisory threshold. Surf may
rise along the south and east facing shores by the middle of next
Small Craft Advisory from 6 AM Thursday to 6 AM HST Friday for
Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island
Leeward Waters-Big Island Southeast Waters.
AREA FORECAST DISCUSSION
NWS LOS ANGELES/OXNARD CA
821 PM PDT Wed Sep 27 2017
An onshore flow will bring coastal cooling and a overnight marine
layer into the weekend. A high along with offshore flow should
keep inland temperatures above normal into Friday. A low may
approach on Saturday and combine with onshore flow into next week.
This combination will increase the coastal marine layer and start
a cooling trend.
Weak low level offshore flow brought another very warm day across
inland areas today, with many valley areas climbing well into the
90s. However, slight onshore trends and some patchy fog this
morning led to some cooling for coastal areas today. Satellite
imagery showing an area of low clouds and fog across the inner
coastal waters, locally spreading into the LA County coast. There
have already been a couple reports of low visibilities in the 1/4
to 1/2 mile range with this fog. Do expect low clouds and fog to
expand some overnight into Thursday morning across the LA/Ventura
County coastal areas. ACARS data continues to shallow and strong
marine inversion, with marine layer depth currently around 800
feet. This in combination with weak flow near the surface (LAX-
Daggett currently at -0.1 mb) will bring a strong potential for
patchy dense fog through Thursday morning, and may sufficiently
expand to warrant a dense fog advisory for LA/Ventura county
coastal areas. Similar temperature contrast expected for tomorrow,
with cooler conditions near the coast due to onshore marine
influence, and much warmer temperatures inland due to persistent
weak offshore flow (with LAX-Daggett gradient expected to be
around -2 mb on Thursday morning).
*** From previous discussion ***
Aside from the low cloud uncertainty the conditions the next
couple days aren`t expected to change much. Light offshore flow in
the morning and weak onshore in the afternoon will continue and
daytime highs should be more or less what we had today, possibly
slightly cooler for the coast with a very minor onshore trend.
Perhaps some increasing Sundowner winds in the SB area Thu/Fri
evenings but mostly below advisory levels.
On Saturday gradients trend noticeably onshore as another trough
moves into the Pac NW and there should be a 4-8 degree drop in
daytime highs on average as well as better marine layer coverage.
Very minimal changes in the weather conditions Sun/Mon, then
another downward trend in temperatures as models show a deeper
trough developing along the west coast. Still some significant
differences between the medium range models regarding how deep
this trough will get, with the ECMWF continuing to develop more of
a closed circulation and much cooler air while the GFS shows more
of an open trough and not nearly as cold aloft. Since both
solutions would result in additional cooling across the area and
both have been pretty consistent with it confidence is pretty high
that highs will drop below normal by the middle of next week with
a deeper marine layer. How much below normal and how deep the
marine layer gets is still in question, and there is the
possibility of some morning drizzle with this pattern as well.
At 23Z, the marine layer depth was around 600 feet deep at KLAX.
The top of the inversion was around 1900 feet with a temperature
near 27 degrees Celsius.
Satellite imagery showing an areas of low clouds/fog over the
coastal waters advancing quickly towards the LA/Ventura county
coastal areas. As a result, increasing confidence of LIFR/IFR
cigs for KLAX, KSMO, KLGB, KOXR, and KCMA this evening,
potentially lowering to LIFR/VLIFR conditions at each coastal site
overnight into early Thursday morning. There is a 20 percent
chance of LIFR cigs at KSBA and KSMX tonight.
KLAX...VFR conditions are expected through at least 03Z. With low
clouds advancing quickly across the coastal waters this
afternoon, increasing confidence of LIFR cigs at KLAX this
evening, potentially lowering to VLIFR conditions overnight into
early Thursday morning. VFR conditions should redevelop between
16Z and 18Z.
KBUR...VFR conditions are expected throughout the period.
Moderate-to-high confidence in the current forecast.
For the Outer Waters, winds and seas will remain below Small
Craft Advisory levels through Thursday. There is a 50 percent
chance of Small Craft Advisory level winds developing for
Thursday night, then increasing to 80 percent Friday through
Sunday. There is a 40 percent chance of gale force gusts on
For the waters north of Point Sal, winds and seas will remain
below Small Craft Advisory levels through Thursday. There is a 70
percent chance of Small Craft Advisory each afternoon and evening
between Friday and Sunday.
For the waters south of Point Conception, winds and seas will
generally remain below Small Craft Advisory levels through Sunday.
There is a 40 percent chance of Small Craft Advisory level winds
across the Santa Barbara Channel Saturday afternoon and evening.
.HAZARD POTENTIAL OUTLOOK...(SAT-WED)
No significant hazards expected.