Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 05/03/19

Area Forecast Discussion

National Weather Service Honolulu HI

 351 PM HST Thu May 2 2019

.SYNOPSIS...
Trade winds will weaken and shift southerly tonight as an
advancing upper level trough gradually destabilizes the
atmosphere. Clouds and showers will favor windward slopes this
evening, with most showers moving offshore late. An unseasonably
strong front will approach the state on Friday, leading to an
unstable southerly flow that will produce spotty showers over
most islands. The front will move down the island chain Saturday
into early Sunday, likely bringing a period of heavy showers,
gusty winds, and possibly thunderstorms. Cooler northwesterly
winds are expected behind the front into the middle of next week.

&&

.DISCUSSION...
Trade winds are weakening, and the atmosphere is destabilizing
this afternoon. Trades are on the decline as a cold front
centered 600 miles northwest of Kauai weakens the subtropical
ridge north of the state. The upper level trough driving the cold
front is eroding the inversion, which has lifted up to as much as
10,000 ft according to afternoon soundings and recent aircraft
data. This has led to isolated showers across leeward terrain on
most islands, while a narrow band of low level moisture continues
to fuel showers over windward slopes.

The trades will continue to weaken and shift out of the southeast
tonight as the atmosphere slowly destabilizes. The afternoon
leeward showers will diminish around sunset, while windward
showers persist, especially from Maui to Oahu. In fact, we could
even see a brief flare up on windward Oahu this evening under the
veering wind flow. Windward showers should push offshore late
tonight as the shifting background flow causes land breezes to
develop. However, Oahu and Kauai could see a few showers moving up
from the south late.

A somewhat unstable southerly flow will develop on Friday as the
unseasonably strong front approaches the state. The upper trough
supporting the front will continue to produce cooling aloft that
will wipe out the inversion. Models show a developing prefrontal
trough focusing moisture just west of Kauai, leaving the state
under an unstable land and sea breeze regime with near normal
precipitable water. Though there could be an isolated downpour at
any time, showers over land will favor the afternoon hours.

The front will reach Kauai early Saturday, then bring unsettled
weather down the island chain through Saturday night. We expect
the front will be near Kauai in the predawn hours on Saturday,
reach Oahu midday, move to Maui County Saturday afternoon and
evening, then possibly advance to the Big Island by predawn hours
on Sunday. During this time, a potent upper low will drop
southward into the upper trough and approach the state from the
north. A jet stream along the southern flank of the upper low
should move over the islands and produce forcing that will
generate heavy showers along the front. The eventual position and
strength of the compact upper low will be critical, but at this
time, the greatest amount of forcing and coldest air aloft look to
remain just northeast of the state. However, a line of heavy
showers or thunderstorms along the front could be capable of
producing strong, or even damaging, winds. Another item of note is
the potential for gusty downsloping southwest to west winds along
the front. It does not appear that a wind advisory will be need,
though localized wind gusts of around 40 mph are a strong
possibility on Saturday, especially on Oahu and parts of Maui
County. At higher elevations, the high summits on the Big Island
will almost certainly experience warning level winds, while
advisory level conditions are expected on summit of Haleakala.

On Sunday, the front will likely weaken near the Big Island as the
compact upper level low passes north of the state. Latest runs of
the GFS and ECMWF are hinting at a surface low developing below the
upper level feature, potentially driving another band of showers
and gusty northwest winds down the island chain.

Stable and rather cool conditions are expected to fill in on the
northwesterly flow later Sunday and Monday. The latest guidance
drops dew points into the upper 50s, which will provide rather
cool conditions for early May. As the upper level low weakens and
lifts off to the northeast, stable and rather dry northerly winds
will likely lead to dry conditions through the middle of next
week.

&&

.AVIATION...
Expect scattered showers through tonight and into Friday. The
showers through the day so far have included lots of areas for
mountain obscuration along windward slopes. Therefore, AIRMET
Sierra for mountain obscuration is in effect for all islands from
Kauai to Maui and will probably stay in place through this
evening. We are seeing some interior island showers this
afternoon.

Overall, the winds have been gradually weakening through today and
should continue to do so into this evening and for Friday with a
cold front beginning to close in from the northwest, loosening the
pressure gradient. The winds may be light enough by tonight that
we have a nighttime land breeze scenario.

&&

.MARINE...
Southeast flow is already evident across the coastal waters ahead
of a front that is expected to reach the islands this weekend. Winds
are expected to weaken some tonight with the high pressure north
of the islands moving to the east. The high will be replaced by a
developing low that will bring a front to the islands this
weekend. The front will bring the potential for heavy showers,
possibly a few thunderstorms, and gusty winds. The greatest focus
will be to the northeast of the islands.

Behind the front, moderate to locally strong northerly winds are
expected which could prompt a Small Craft Advisory (SCA) for
Saturday into Sunday night for some coastal waters. A small
northwest swell at that time could also contribute to an uptick in
seas, bringing seas near SCA levels mainly near Kauai. The latest
model run from the NWPS show seas remaining below the 10 foot
level, but will continue to monitor.

The current small northwest swell is expected to persist through
Saturday. The developing low referenced above will send a moderate
sized short-period northwest swell toward the islands Saturday
night through Monday, with surf expected to remain below advisory
levels as the swell peaks late Sunday and Sunday night. A new
small but longer period northwest swell will build Tuesday night,
and persist through Thursday. Storm systems in the South Pacific
will send small south and southwest swells our way through the
middle of next week, with an uptick in swell energy expected over
the weekend, and maybe again early next week.

See the latest Oahu Surf Discussion (SRDHFO) for additional
details on surf and swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Wind Watch from Friday evening through late Sunday night
for Big Island Summits.

&&

$$

DISCUSSION...Wroe
AVIATION...Chevalier
MARINE...M Ballard




Area Forecast Discussion

National Weather Service Los Angeles/Oxnard CA

 812 PM PDT Thu May 2 2019

.SYNOPSIS...02/739 PM.

Low clouds are expected to develop along the coasts and valleys tonight
into Friday morning with strong onshore flow. High pressure will
remain in control on Friday and temperatures may warm slightly. An
upper level low will approach the region on Saturday bringing
cooler temperatures through the weekend and a chance of light
showers Sunday and Monday.

&&

.SHORT TERM (THU-SUN)...02/811 PM.

A weak upper level ridge of high pressure will remain over the
area through Friday. Near the surface, onshore flow was a bit
stronger this afternoon compared to yesterday, and is expected to
further strengthen on Friday afternoon, when LAX-Daggett gradient
is expected to exceed +6 mb onshore. Also, a weak eddy
circulation is expected to spin up the marine layer coverage for
areas south of Point Conception overnight into Friday morning.

Current satellite imagery showing low clouds across portions of
the LA county coast. Current ACARS data showing the marine layer
depth around 1300 feet across the LA basin this evening. Slight
deepening of the marine layer is expected by Friday morning due to
the eddy circulation. Low clouds are going to be most widespread
overnight into Friday morning across LA county with coverage
expected to reach the San Fernando and San Gabriel valleys.
However due to the eddy circulation, low clouds and fog should
spread northwest up the coast of Ventura and SBA counties. Little
change in temperatures expected across the coast on Friday, with
slight warming across inland areas due to increasing boundary
layer temperatures.

*** From previous discussion ***

A cutoff upper level low pressure system, currently 1300 miles
west of Los Angeles, will move over central California on Sunday.
As it approaches the coast on Saturday, it will deepen the marine
layer while strengthening the onshore pressure gradients. This
should add up to a healthy inland push of low clouds into the
coastal mountain slopes and maybe slow clearing at the coast, with
even more clouds on Sunday. Cannot discount a little drizzle in
some spots over the weekend, with the best chances on Sunday. By
Sunday, some projections show a week and small frontal boundary
moving through the area. While there is little moisture to work
with, it could interact with the deep marine layer and result in
some benefit showers, with the best chances over SBA and SLO
Counties. If any showers result, amounts will be light. Although
the low will be right over the area on Sunday, since the system
has been spinning off the coast for awhile, the airmass is not
terrible cold aloft. The thunderstorm threat is fairly minimal as
a result, and snow levels should stay above 7000 feet.

Temperatures will drop by a few degrees on Saturday, and even more
on Sunday...especially over the interior. The stronger pressure
gradients will result in gusty southwest winds over the interior
sections, but looks to stay just under advisory strength at this
time.

.LONG TERM (MON-THU)...02/214 PM.

The weekend low mentioned in the short term will still be in play
on Monday. With the cutoff nature of this system, the latest
computer projections have to be taken with a grain of salt, as
these type of systems tend to outwit computers and forecasters
alike. With that said, the latest guidance has the low wobbling
over southern California Sunday night and Monday, before exiting
the region by Tuesday. If this track holds, ample low level clouds
and light showers will remain possible, with the better chances
shifting to Ventura and LA Counties. Conditions will remain cool
like Sunday.

The latest projections call for some clearing and warming on Tuesday
and Wednesday, but not a whole lot as onshore pressure gradients
strengthen. Some moist northerly flow aloft could keep some
clouds around, especially in the mountains where a stray shower to
two is possible. Computer solutions diverge for the later half of
next week, so went closer to climatology.

&&

.AVIATION...03/0019Z.

At 0000Z, the marine layer was around 1200 ft at KLAX. A weak
inversion was in place. The top of the inversion was around 2000
ft at 16 degrees C.

Moderate confidence overall in the 00Z TAFs. Low clouds and fog
are expected to redevelop across most coastal and valley
locations overnight into Friday morning, with cigs expected to
move into KLAX by around 06z, spreading northwestward to KSBA by
around 14Z Fri. The timing of the onset of the low clouds and
changes in flight cats may be off +/- an hour or two. There is a
20%-30% chance of IFR cigs affecting KSBP at times late tonight
into Fri morning.

KLAX...High confidence overall in the 00Z TAF in regards to
cigs returning overnight. Low clouds with MVFR cigs are expected
to move into the airfield around 06Z then persist until around 19Z
before scattering to VFR thru Fri afternoon. The timing of the
onset of the low clouds may be off +/- an hour or two.

KBUR...Moderate confidence overall in the 00Z TAF. Low clouds
with MVFR cigs are expected to move into the airfield around 11Z
late tonight then persist thru 17Z Fri. There is a 20 percent
chance of IFR cigs late tonight into early Friday morning.

&&

.MARINE...02/752 PM.

For the Outer Waters...Moderate to high confidence in the current
forecast. A low end Small Craft Advisory (SCA) will continue for
gusty northwest winds through late tonight. Otherwise, winds and
seas should remain under SCA thresholds through Tuesday.

For the Inner Waters N of Point Conception...Moderate to high
confidence that winds and seas should remain under SCA thresholds
through Tuesday.

For the Inner Waters S of Point Conception...Generally high
confidence in the current forecast. Winds and seas are expected to
remain under SCA thresholds through Tuesday.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...Small Craft Advisory in effect until 3 AM PDT Friday for
      zones 670-673-676. (See LAXMWWLOX).

&&

.HAZARD POTENTIAL OUTLOOK (SUN-THU).

No significant hazards expected.

&&

$$

PUBLIC...Gomberg/Kittell
AVIATION...Gomberg
MARINE...Gomberg/Sirard
SYNOPSIS...Stewart

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