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Hawaiian Islands Synoptic Discussion and Guidance

874
FXHW60 PHFO 190636
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
836 PM HST Tue Jun 18 2019

.SYNOPSIS...
Moderate to locally breezy trade winds will deliver just a few brief
windward showers through Thursday, but then winds will weaken and
shift to the southeast from Friday through the weekend. The lighter
winds will lead to daytime sea breezes, spotty showers and muggy
conditions.

&&

.DISCUSSION...
As a surface trough to the NW distanced itself from the islands
today, the trade wind supporting high centered to the NE was able to
build over the area, thus strengthening the trade winds while
backing them to a more typical E direction. These locally breezy
trade winds will continue through Thursday, delivering just a few
brief windward showers that will favor nights and mornings as the
island atmosphere will be stable, and no organized moisture sources
are anticipated. Leeward areas will be mostly dry. Some relatively
thin high cloudiness is possible the next couple of days, but the
high clouds may briefly thicken on Thursday due to the passage of a
weak trough aloft.

While little significant change is expected with the high to the NE
into next week, the trough to the NW will become reinvigorated by a
developing closed low aloft beginning Thursday. This cutoff low and
associated trough aloft is expected to linger NW of the islands well
into next week, supporting the development of several weak surface
lows along the persistent surface trough, while also causing the
trough to drift closer to the islands. The ridge extending SW from
the high will move over the islands in response to the sharpening
trough, causing winds to weaken and veer to the S and SE along the W
end of the island chain, with light to moderate E to SE winds near
the Big Island.

Initially, this flow looks to support only limited showers, although
increasing surface dew points will combine with the light winds to
bring increasing afternoon mugginess. As the deep-layer trough axis
nears Kauai Friday night and Saturday, models indicate increasing
moisture advecting northward over waters W of Kauai, which could
lead to a period of showery weather over Kauai county as well.
Meanwhile, a mostly dry weather pattern is expected over the
remainder of the state through Saturday.

Later in the weekend into early next week, there is increased
uncertainty as to where clouds and showers will occur (and when),
due to the veered wind flow and gradually increasing low-level
moisture. While mostly dry weather (outside of brief windward
showers), will continue over the Big Island, the other islands could
see spotty showers from Sunday into Wednesday. Some showers could
form in the afternoons over interior areas due to daytime heating
and sea breeze convergence, with others forming within eddies and
cloud plumes peeling off other islands, potentially bringing much
needed showers to leeward areas. Additionally, conditions will be
warm and muggy.

Long range outlook presented by latest GFS/ECMWF guidance has
delayed the return of trade winds until the end of next week, as low
pressure lurking NW of the islands continues to veer and weaken the
normally-expected trade winds. Low-level convergence associated with
the low could bring spotty showers, some briefly heavy, primarily to
the W end of the island chain.

&&

.AVIATION...
Relatively strong high pressure far to the northeast of Hawaii will
remain stationary through the period. This will keep light to
moderate trade winds from the east/northeast through at least
Wednesday night. There will be fairly stable conditions with drier
air over most parts the islands. Most of the shower activity will be
confined to the windward slopes, especially on Maui and the Big
Island. Winds will be moderate again Wednesday afternoon in the same
areas as Tuesday but they should not be strong enough to produce
strong low level turbulence.

There are currently no AIRMETs in effect.

&&

.MARINE...
A surface high far northeast of the area will maintain locally
locally strong trade winds over the coastal waters through mid-
week. A Small Craft Advisory is in effect for the Pailolo and
Alenuihaha Channels and waters south of the Big Island through
Wednesday afternoon. Winds will weaken and shift out of the
southeast during the second half of the week as a trough deepens
west of the offshore waters. The southeast flow will be partly
blocked by the Big Island, resulting in light and variable winds
around the smaller islands Friday into the weekend.

Existing small surf will fade even more through Wednesday. Surf will
remain well below the advisory threshold along all shores through
the coming weekend, aside from a small boost in east shore surf as a
short-period, northeast swell builds over the weekend.

&&

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

&&

$$

DISCUSSION...Birchard
AVIATION...Chevalier
MARINE...TS

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office

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