Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 12/04/19

Area Forecast Discussion

National Weather Service Honolulu HI

 346 PM HST Tue Dec 3 2019

.SYNOPSIS...
High pressure northwest of the state will maintain moderate to
locally breezy trade winds and mainly windward showers through
Thursday. High clouds will spread over the islands from the west
on Wednesday, and cloud cover and rainfall will briefly increase
Thursday night and Friday as a disturbance aloft passes overhead.
Trade winds will be disrupted on Saturday, leading to a land and
sea breeze pattern. Trade winds will likely rebuild late Sunday
and Monday as a dissipating front moves over the island chain.

&&

.DISCUSSION...
Moderate to locally breezy trade winds continue to focus clouds
and showers over windward terrain. The trades are being driven by
a strong surface high sitting over 1500 miles northwest of the
state. A front stretching between the high and the islands is
weakening the local pressure gradient slightly and keeping trade
winds at moderate to locally breezy strength. A band of low level
moisture that moved through last night and this morning has
largely dissipated. As a result, shower activity has decreased
considerably this afternoon, and modest rainfall is now focused
along windward slopes, though a few showers remain possible over
leeward Big Island. A mid to upper level ridge building in from
the west is producing somewhat stable conditions, even though the
low level inversion remains weak and elevated according to
afternoon soundings and recent aircraft data.

Moderate trade winds and mainly windward showers will persist
through Thursday, with high clouds increasing on Wednesday. The
surface high will drift to about 1100 miles north of the state by
Thursday, and the weakening surface front between the high and the
islands will continue modify the local pressure gradient, keeping
moderate to locally breezy trade winds over the state. The
islands will lie under a northwesterly flow aloft between an upper
ridge to the west and an upper trough to the northeast. The
inversion will continue to be weak and slightly elevated during
this time. As a result, brief periods of enhanced showers could
occur as pockets of low level moisture move through on the trades.
The next such pocket is expected Wednesday night. In addition,
high clouds will likely increase on Wednesday as a shortwave
aloft approaches.

An increase in cloud cover and rainfall is due Thursday night
into Friday, as a shortwave aloft passes from west to east over
the islands. This rather strong shortwave is expected to bring
thick layered clouds capable of producing periods of steady light
to moderate rain to all areas, and a decreasing trade wind flow
will continue to bring showers to windward slopes. The thick mid
to high level clouds will also bring the potential for snow,
possibly heavy, and elevated winds on the high summits of the Big
Island. Conditions will quickly improve from west to east Friday
afternoon and night as the shortwave moves eastward.

On Saturday, trades will be disrupted as an approaching front
pushes a weakening surface ridge over the state. A stable land and
sea breeze pattern is the most likely outcome, featuring clouds
and a few interior showers during the afternoon.

Uncertainty increases on Sunday, as the GFS and ECMWF solutions
diverge on the handling of the approaching front. The latest
ECMWF drives the front down the island chain Sunday afternoon and
night, ushering the return of northeasterly trade winds earlier
than the GFS, which shows a slower dissipating front reaching the
islands 12 hours later. For now, the wind and rainfall forecast
reflects a broad brushed approach, showing wetter windward
conditions and slowly rebuilding trades. Trade winds should
prevail state-wide by Monday.

&&

.AVIATION...
Shower activity from this morning has decreased along with much of
the lowered ceilings along windward coasts. Big Island remains the
cloudiest, but will likely drop AIRMET Sierra for all other
islands. Moderate trades will continue to focus showers over
typical windward and mauka areas through the forecast period.

An area of high clouds is beginning to encroach on the area from
the west and will fill in over the island chain through Wednesday.

No other AIRMETs are currently in effect.

&&

.MARINE...
Have bumped the winds in the typical windier areas around Maui
County and the Big Island to low end Small Craft Advisory (SCA)
levels. An early ASCAT pass showed some 25 kt winds, and when
combined with land observations holding steady to 24 hours ago and
high resolution models suggesting low end advisory winds, the SCA
has been issued for those areas through Wednesday afternoon.

High pressure north of the state will maintain moderate to
locally fresh trade winds through Thursday night.

A quick moving shortwave trough moving over the islands late
Thursday into Friday will likely bring increasing showers. Winds
are expected to lower over the weekend as a front passes north of
the state, and then increase again with the new work week.

Surf along east facing shores has fallen below advisory levels,
so the High Surf Advisory has been cancelled with the afternoon
package.

A long-period north swell is expected to build Thursday/Friday,
before turning the northeast and getting reinforced over the
weekend. During the peak of the event, surf is expected to reach
advisory levels along north facing shores, and warning levels
along east facing shores. A long-period northwest swell is
possible late in the weekend as well.

The Oahu Surf Discussion (SRDHFO) from Monday afternoon contains
details on the sources of the swells arriving over the next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Wednesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$

DISCUSSION...Wroe
AVIATION...Dye
MARINE...M Ballard