Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 09/18/19

Area Forecast Discussion

National Weather Service Honolulu HI

 348 PM HST Tue Sep 17 2019

.SYNOPSIS...
Increasingly stable easterly trade winds will build over much of
the state tonight. Typical trade wind weather will prevail on
Wednesday and Thursday, with showers focused over windward terrain
on all islands and across leeward Big Island slopes each
afternoon. Unstable conditions will develop on Friday and hold
through the weekend, bringing increased chances for heavy showers.

&&

.DISCUSSION...
A stable trade wind flow is established from the Big Island to
Oahu, while a slightly unstable east-southeast flow is lingering
near Kauai. Surface high pressure far north of the state is
driving the moderate trade winds, and a broad and complex area of
low pressure west and southwest of the islands is causing the
easterly trade winds to veer out of the east-southeast near Kauai.
A mid to upper level ridge continues to build over the state from
the south and bring increasingly stable conditions to most islands.
As a result, the afternoon Hilo sounding and recent aircraft data
showed a well-defined inversion between 5,000 to 7,000 ft, and
scant rainfall has been observed along windward slopes from the
Big Island to Oahu today. Though not impressive, showers have
remained active across Kauai today due to lingering deep moisture
(PW near 1.8 inches) and the destabilizing effect of an upper
level trough to the west. This feature is also sending high clouds
over mainly the western end of the state.

The stable trade wind flow will gradually spread westward
overnight. The mid to upper level ridge will continue to slowly
build over the state from the south, and drier air will spread
over the island chain from the east. Showers will continue to be
somewhat active near Kauai, mainly during the evening, but
elsewhere expect modest showers to be confined to windward
slopes.

Stable trade wind conditions are due on Wednesday and Thursday.
Moderate to locally breezy east to east-southeast winds will
persist, and deep mid to upper level ridging will produce stable
conditions. This will focus showers over windward slopes and will
inhibit heavy showers over all areas except the leeward Big Island
slopes in the afternoon hours.

A return of unsettled conditions is looking increasingly likely
heading into the weekend. An upper level trough will drop from
the north on Thursday night and Friday, wiping out the inversion
and setting the stage for spotty heavy showers in the trade wind
flow. During the weekend, the upper trough will develop into a low
as it moves over the state, and a surface trough could drift over
the islands from the south. The GFS and ECMWF are in fairly good
agreement in keeping the bulk of the deep moisture just east of
the state, while the islands are under a convective regime.
Expect lighter winds and humid conditions along with the
increased chances of heavy downpours.

&&

.AVIATION...
A 1026 mb surface high pressure cell centered well north of the
state will allow for moderate easterly or east southeasterly flow
to persist through the remainder of the forecast period.
Relatively deep low level moisture remains in place over the
western most islands tonight. Thus, expect scattered SHRA/+SHRA to
affect Kauai, which may bring brief periods of MVFR conditions.
Otherwise, isolated SHRA mainly along the windward slopes and
coasts of the remaining islands.

No AIRMETs are currently in effect.

&&

.MARINE...
Moderate to locally fresh, east to east-southeast winds will hold
through Wednesday as a tropical disturbance to the southwest of
the islands lifts northwestward. A shift back to a more typical
east to northeast direction is expected Thursday through Friday,
possibly strengthening to Small Craft Advisory speeds for a short
time around Maui and the Big Island. Guidance shows a return of
light to moderate east-southeast flow late Saturday through Sunday
due to an approaching trough from the east-southeast, which would
support localized land and sea breeze conditions near the coasts.

A series of small swells from the southwest, south, and southeast
is expected this week. The largest of these looks to be towards
the end of the week as a long-period south-southwest swell and
potential short-period west-southwest swell combine across the
area. Surf heights are expected to be below advisory levels with
these swells.

Small surf is expected along north, east, and west facing shores
through the week. Expect a small swell from Tropical Cyclone Kiko
around Thursday along exposed east facing shores before dropping
over the weekend. Exposed west shores may see some of the
aforementioned west-southwest swell late Thursday into Friday. A
small northwest swell is possible late in the weekend into early
next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Wroe
AVIATION...Thomas
MARINE...TS