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Hawaiian Islands Synoptic Discussion and Guidance

184
FXHW60 PHFO 150643
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
843 PM HST Mon Oct 14 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...
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 easterly-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
passing MVFR conditions in these areas and VFR conditions
elsewhere.

No AIRMETs are currently in effect.

&&

.MARINE...
Expect moderate to locally breezy trades through tonight with a
ridge persisting north of the state, and Small Craft Advisory
(SCA) conditions continuing over the typically windier waters near
Maui County and the Big Island. A front north of the ridge 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. Trades will weaken Tuesday
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 Tuesday 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
Wednesday night, 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 will continue along east facing shores
through tonight, before trending down Tuesday through Thursday as
the trades ease. East shore surf will then trend up toward the
end of the week and over the weekend as the trades increase.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Tuesday for Maalaea Bay,
Pailolo and Alenuihaha Channels, and waters south of the
Big Island.


&&

$$


DISCUSSION...H Lau/AVIATION...Chevalier/MARINE...Kinel

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



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