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Hawaiian Islands Synoptic Discussion and Guidance

017
FXHW60 PHFO 151331
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
331 AM HST Tue Oct 15 2019

.SYNOPSIS...
Locally breezy trade winds will be weakening through Tuesday as a
front approaches the islands from the north. This front, combined
with an upper-level disturbance moving into the area, will
support increasing rainfall chances by Wednesday, which could
linger into Thursday. Drier trade wind weather will return Friday.

&&

.DISCUSSION...
No change to the philosophy behind the forecast. The only change
with this package is the inclusion of a slight chance of a
thunderstorm for Kauai, Oahu, and Molokai on Wednesday, along
with the possibility of locally heavy showers. Heavy showers are
expected to be easing off Wednesday evening over Kauai as the
upper low and surface trough drifts west of the Garden Isle.
Still some lingering instability for Kauai on Thursday, but all
together, the air mass should be stabilizing area-wide. Kept the
Big Island, Maui and Lanai, out of this unsettled weather stuff
on Wednesday, but is subject to change. The weekend should be the
normal trade wind pattern. The POPs, sky, for Friday and Saturday,
appears way to wet and cloudy with frequent windward showers.
Thus, they were scaled down to partly cloudy, and at most,
showers likely for the night time hours.

The trade winds, as per models, will continue to trend lighter,
but just a little bit. All the models, including the high resolution,
only went local sea breeze for the west end of Kauai and Oahu on
Wednesday. Trades, otherwise, will be in the light to moderate
range. The near-shore waters of Maui County and south of the Big
Island, will have trades hovering just below Small Craft
Advisory. See marine section below for further details.

&&

.PREVIOUS DISCUSSION...
The trade winds are still a little breezy in some places, but
still pretty pleasant. There are a few light showers coming in
with the trades, but not a whole lot. The POP grids have been toned
down for the overnight hours, and the smaller islands for Tuesday.

The trades will be trending down through Tuesday night, becoming
light to moderate speeds on Wednesday. This comes about as the
surface ridge falls apart north of the islands, due to a southward
moving front advancing towards the islands. The front will be
weakening as enters the northern offshore waters Tuesday night.
The front will then stall and dissipate near our northern or
windward coastal waters Wednesday. The front will have some upper
level support, by way of a low with a pool of cold air of about
minus 8 degrees C. As the front falls apart, a north to south
oriented surface trough, perhaps the reflection of an upper level
low, forms near Kauai on Wednesday. The trough then pushes west
through Wednesday night. Both the GFS and ECMWF, and the NAM, are
on board with this. The added moisture from the front, in
combination of the upper low, will make Wednesday and Wednesday
evening to be pretty showery, especially the islands from Molokai
westward. The Big Island could be left out of any significant
weather. The three models points to a slight chance of a
thunderstorm, over the windward waters, both near- shore and
offshore early Wednesday. This threat of thunderstorms comes
closer to both Oahu and Kauai County Wednesday afternoon, before
shifting west of Kauai County by midnight, in tandem with the
movement of the upper low. Lingering instability, however, may
keep Kauai and Oahu, a bit unstable through Thursday afternoon,
although moderate trades will be returning at this time. The
slight chance of thunderstorms only applies to the near-shore
waters of Oahu and Kauai County for now, but is likely to be
included on land with the upcoming forecast packages.

With a 1027 MB surface high situated 960 miles northeast of Oahu
Friday evening, expect locally breezy trades to be in place by
Friday, or may be sooner like Thursday night. This high will move
eastward for a couple hundred miles more before stalling off the
northern California coast early next week. A front pushing this
high eastward will stall some 550 miles west of Kauai. In doing
so, our trade winds will bend to the east-southeast over the
upcoming weekend.

&&

AVIATION...
We will continue to see moderate northeasterly trade wind flow
across the state through this forecast period. We have not had any
reports of low-level turbulence and winds may not become strong
enough to warrant an AIRMET for this, but we will continue to
monitor that situation and issue an AIRMET if necessary. Otherwise
we will continue to see windward mauka showers which probably
won't remain persistent enough to have to issue an AIRMET for
mountain obscuration. Therefore, expect brief passing MVFR
conditions in these areas and VFR conditions elsewhere.

No AIRMETs are currently in effect.

&&

.MARINE...
With trade winds backing off this morning in response to a front
approaching from the north, the Small Craft Advisory (SCA) has
been cancelled over the typically windier waters near Maui County
and the Big Island. The aforementioned front is expected to push
south, stalling in the northern coastal waters and dissipating by
midweek. A remnant trough is then forecast to form and move west
through Thursday. As indicated earlier, trades will weaken today
with the advancing front, with weaker trades extending through
Thursday. At the same time, an upper low is forecast to develop
northeast of the islands and trek over the state to the southwest.
This feature will likely enhance showers, and possibly induce
isolated thunderstorms, over offshore and some coastal waters.

Trade winds will strengthen again toward the end of the week as high
pressure settles in north of the islands behind the surface front.

Surf is expected to remain below advisory levels along all shores
into the weekend. A series of south and south-southwest swells
will continue to move through the islands. A small long-period
west swell generated by former Tropical Cyclone Hagibis is
expected to peak today then drop off Wednesday. Some of the swell
energy could wrap into some northern exposures.

A couple of small long-period northwest swells will build over the
next day or two, peak Thursday, then gradually lower through the
end of the work week. A smaller long-period northwest swell is
expected over the weekend.

Rough and choppy surf along east facing shores will be trending
down today and tonight, hold steady on Wednesday, then bump back
up slightly Thursday night and Friday as breezy trade winds
return.

&&

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

&&

$$


H Lau/Chevalier/Kinel

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