Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 12/03/19

Area Forecast Discussion

National Weather Service Honolulu HI

 340 PM HST Mon Dec 2 2019

.SYNOPSIS...
High pressure north and northwest of the state will maintain
moderate trade winds and a somewhat stable pattern of mainly
windward showers through Thursday. High clouds will spread over
the islands from the west of Wednesday, and cloud cover and
rainfall chances will briefly increase Thursday night and Friday
as a disturbance aloft passes overhead. Trade winds will likely be
disrupted during the weekend as a front approaches.

&&

.DISCUSSION...
Locally breezy trade winds continue to focus showers over windward
slopes, while a shield of high clouds is poised to move east of
the islands. The trades are being driven by a weakening high
located about 1000 miles north of the state. A field of shower-
bearing open-celled cumulus clouds moving along the trade wind
flow continues to produce modest rainfall along windward slopes,
where rainfall totals of generally 1/4 of an inch or less
occurred today. A weak mid level ridge overhead is maintaining
somewhat stable conditions, with a poorly defined inversion
observed between 5,500 to 7,500 ft on the afternoon soundings and
recent aircraft data. However, a couple of heavy showers have
briefly flared along leeward Big Island slopes this afternoon. The
shield of high clouds that was overhead this morning has cleared
the western half of the island chain.

Though still locally breezy in the evening, trades will be on a
slow decline tonight, and high clouds clear the entire state. The
high currently north of the state will erode, leading to a
gradual weakening of the trade winds. A disturbance aloft
generating the high clouds will lift off to the northeast tonight,
causing high clouds to clear. Showers will continue to favor
windward slopes, though a few may briefly blow over to leeward
areas on the trades.

Moderate trade winds and mainly windward showers will prevail
Tuesday through Thursday. By Tuesday, surface high pressure far
northwest of the state will be the primary feature driving the
trades. This rather strong high will drift to about 1100 miles
north of the state by Thursday, and a weakening surface front
between the high and the islands will modify the local pressure
gradient, allowing only moderate 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 likely 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. No
such pockets can be seen at this time, and short term fine tuning
will be needed. In addition, high clouds will likely increase as
early as Wednesday.

An increase in cloud cover and shower chances is expected
Thursday night into Friday, as another disturbance aloft passes
from west to east over the islands, but details are uncertain. The
GFS and ECMWF both stall the above mentioned dissipating front
over or just northeast of the islands during this time. With
moderate trades remaining in place, low level moisture from this
feature will likely provide an increase in showers for at least
windward areas. The disturbance aloft should destabilize the
atmosphere enough to warrant an increase in shower chances for
leeward areas as well, though confidence is not very high at this
time. There is high confidence that mid to high level clouds will
thicken as the disturbance aloft moves over the region, and these
clouds may be deep enough to produce some snow over the high
summits of the Big Island.

Heading into the weekend, cloud cover will decrease and trades
will likely be disrupted as a front approaches from the
northwest. A stable land and sea breeze pattern is the most likely
outcome, featuring clouds and a few interior showers each
afternoon.

&&

.AVIATION...
The mid and high level clouds that stretched across the entire
state this morning have cleared the western half of the island
chain. Beneath that cloud deck, lower clouds and showers continue
to impact mainly windward and mauka areas as they move across the
area with the breezy trades. Additionally, the Big Island is
still seeing showers across the Kona coast this afternoon. Brief
periods of MVFR conditions will be possible in these areas.

A high to the north of the state that has been driving our breezy
trades over the last few days is expected to slowly weaken. This
will allow wind speeds to diminish some overnight, but until
then, AIRMET Tango for tempo moderate turbulence remains in
effect.

&&

.MARINE...
Moderate to locally strong trade winds will continue for the next
several days, as surface high pressure prevails to the distant N
and NW of the islands. Winds will likely remain strong enough to
warrant a Small Craft Advisory (SCA) for waters around Maui and
the Big Island for the next couple days. Trade winds are expected
to diminish significantly by the weekend as a front approaches
from the NW. In the meantime, a SCA posted for all waters remains
in effect through tonight, as observed seas continue to run higher
than wave model guidance, with nearshore PacIOOS buoys indicating
seas between 9 and 11 feet. Seas are expected to generally trend
down the next couple of days, allowing the SCA for seas to be
dropped by Tuesday morning.

Although gradually diminishing, wave energy from the NE is providing
elevated surf to N and E facing shores of most islands, and a
High Surf Advisory (HSA) remains in effect for E facing shores.
While surf along N facing shores has diminished below HSA levels,
surf along E facing shores (where criteria are lower) may remain
above HSA levels beyond Tuesday. Generally decreasing surf is
expected along all shores until a long-lived long-period N swell
builds Thursday/Friday, turns to the NE and gets reinforced over
the weekend, and diminishes thereafter. Peak surf heights will
approach HSA levels along N facing shores, and High Surf Warning
levels along E facing shores, especially when the swell turns NE.
A long-period NW swell is possible late in the weekend as well.

The recently updated Oahu Surf Discussion (SRDHFO) contains
details on the sources of the swells arriving over the next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Tuesday for east facing shores
of Kauai, Oahu, Molokai, Maui, and the Big Island.

Small Craft Advisory until 6 AM HST Wednesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

Small Craft Advisory until 4 AM HST Tuesday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maui County Leeward Waters-Big Island
Windward Waters.

&&

$$

DISCUSSION...Wroe
AVIATION...Dye
MARINE...Birchard