Weather Service Report

FXHW60 PHFO 281357

Area Forecast Discussion
National Weather Service Honolulu HI
357 AM HST WED SEP 28 2016

Locally breezy trade winds will maintain a somewhat stable 
rainfall pattern focusing showers across windward slopes into
Friday. Showers are expected to increase during the weekend, and
possibly into early next week, as deeper low level moisture moves
over the islands and a mid to upper level trough strengthens near
the state.


Moderate to locally breezy trade winds and a rather typical
pattern of mainly windward and mauka showers will prevail into
Friday. A surface ridge will hold north of the islands, producing
only minor fluctuations in the trades. Broad mid to upper level
ridging overhead will maintain somewhat stable conditions, with
the inversion hovering around 5000 to 8000 ft for most of this
time. The GFS and the ECMWF are showing a weak mid level trough
passing over the islands tonight, likely bringing an uptick in
showers. Otherwise, expect modest showers to be largely confined
to windward slopes through Friday.

Shower chances will increase heading into the weekend. The GFS
and ECMWF both show a nearly east to west oriented upper level
trough (referred to as a tropical upper tropospheric trough, or
TUTT) setting up on Friday just north of the islands and gradually
strengthening through the weekend. This should gradually erode the
mid level ridge over the islands and weaken the inversion.
Meanwhile, the models are showing deeper low level moisture moving
in from the east. This should result in an increase in rainfall,
especially across the eastern end of the island chain. 

Differences in the GFS and ECMWF solutions appear on Sunday,
leading to increased uncertainty in the potential for heavy
rainfall. The models are showing a mid to upper level low
developing along the western end of the TUTT, but they differ
significantly on the location. The GFS places the low over 500
miles west of Kauai, keeping the greatest amount of instability
just west of the state. The ECMWF develops the upper low just west
of Kauai, which would produce quite a bit of instability. During
this time, moisture from the remnants of tropical cyclone Ulika
will be carried near or over the islands on the trade wind flow.
This could create the potential for heavy rainfall if the ECMWF
solution verifies. Given the relatively high uncertainty at this
time, we continue to hold off on adding heavy showers in the
forecast but will maintain wet conditions.


High pressure north of the state will keep a moderate to breezy
trade wind flow in place through tonight. Predominantly VFR
conditions are expected at the TAF sites. Brief MVFR cigs/vsbys
will be possible in passing -SHRA across windward and mauka
areas, mainly early this morning and again tonight.

AIRMET Tango is in effect for low level turbulence over and
downwind of the mountains. Conditions are expected to remain in
place through the day.


Fresh to strong trade winds associated with high pressure north of
the region will continue through the week. The highest winds and
seas are forecast across the typically windier locations between
Molokai and the Big Island each day. A recent ASCAT pass around
midnight reflected this and showed the highest winds around 25 knots
over the Alenuihaha Channel and the waters south of the Big Island.
The small craft advisory for these locations will remain in place
for these windier zones and has been extended through Thursday based
on the latest numerical guidance. The peak seas are forecast across
windier locations each day, however, should remain just below
advisory levels across the Alenuihaha Channel and the waters south
of the Big Island. 

The current small northerly swell will steadily ease today through
Thursday. A small northwest swell will become a possibility early
next week in response to a low pressure system tracking across the
date line Friday and Friday night. Otherwise, fresh trade winds will
continue to generate choppy surf along east facing shores through
much of the week.


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




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