Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 241520 CCA
AFDHFO

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
520 AM HST Tue Apr 24 2018

.SYNOPSIS...
Breezy trades will hold through midweek, then begin to weaken and
shift out of the south by late Thursday as a front approaches
from the northwest. Clouds and showers will continue to focus
mainly over windward and mauka areas through midweek. The front is
forecast to push down the island chain Friday and Friday night,
then potentially stall and bring another round of wet weather
through the weekend over portions of the state.

&&

.DISCUSSION...
The latest surface analysis showed strong high pressure (1033 mb)
centered around 1000 miles north-northeast of the state and a
cold front extending south-southwest across the north-central
Pacific to an area just northwest of Midway. Observations depicted
breezy trades holding over the area with gusts reaching the 30 to
35 mph range over some of the windier locations due to terrain
accelerations. Satellite/radar imagery showed mainly low
clouds/showers riding in on the trades focusing over windward
locations. A larger batch of clouds was just east of Hilo, which
should move into the windward areas of the Big Island and Maui
County through the early morning period today, potentially
supporting a slight uptick in windward shower coverage. The
latest rainfall summary showed minimal accumulations over the past
24 hrs with peak totals coming in over windward Maui County and
the Big Island.

Guidance remains in decent agreement through midweek and
is initializing well with the current pattern and trends.
Consensus supports stable conditions with breezy trades holding as
high pressure passes to the north. Clouds and showers will
continue to focus over windward/mauka locations, especially as
pockets of enhanced moisture ride in on the trades. Model
precipitable water (PW) values reflect this and are forecast to
hold around normal (1.1- 1.3").

Conditions will begin to evolve through the day Thursday and
Thursday night as the aforementioned front over the north-central
Pacific approaches from the northwest. Guidance continues to
advertise a strong shortwave trough digging southeast toward the
region and closing off an upper low Friday through Saturday north of
the state. This upper feature will support strong cyclogenesis
setting up several hundred miles north of the islands with a cold
front advancing down the island chain late Friday through Friday
night. A pre-frontal trough is depicted Thursday night over the
northwest islands, which could correspond to the beginning of
another wet scenario unfolding for the state.

Although uncertainty remains high this far out in the forecast,
the latest guidance depicts this boundary quickly moving through
Kauai late Friday and Friday night with northerly winds and drier
air filtering in. This boundary is forecast to then stall at some
point over the central or eastern half of the chain by the end of
the weekend. The upper low combined with this stalling boundary
will act to draw up 2" plus PWs out of the tropics that will
support a continuation of the wet pattern over the eastern end of
the state through the weekend and possibly into early next week.

&&

.AVIATION...
Fresh to strong trade winds will carry in an area of low clouds
and scattered showers to the islands this morning. The clouds and
showers will focus along windward and mountain areas of all
islands, with some showers tracking to leeward sides of the
smaller islands at times. Brief mountain obscurations will be
likely across windward areas today, with AIRMET Sierra for tempo
mountain obscurations possible.

AIRMET Tango is in effect through today on the leeward side of
the mountains on all islands due to the strong winds.

&&

.MARINE...
Fresh to strong trade winds will pick up slightly and persist
through the day as strong high pressure currently far north of
the state moves off to the northeast. A Small Craft Advisory (SCA)
covers all Hawaiian waters through Tuesday night. Winds will
gradually ease Wednesday and Thursday as the ridge north of the
state weakens. The SCA will likely be trimmed back to the usual
windy areas around the Big Island and Maui, then be dropped
completely Friday as a front moves over the state from the
northwest. Look for northerly winds to build behind the front, but
uncertainty in its eventual position means there is low
confidence in the wind forecast for the weekend.

A pair of moderate northwest swells are due during the next
several days, with considerable uncertainty for the weekend. The
current swell will decrease through today, and fade Wednesday. A
similarly sized swell will build Thursday and peak Friday.
Uncertainty increases in the swell forecast for the weekend. The
GFS and ECMWF are showing a gale low forming north of the state
between 30N and 40N, but the models continue to lack agreement in
the exact position and strength of the low. There is the potential
for a large, short-period north-northwest swell to produce surf
well above advisory levels.

Rough, moderate surf will hold along east facing shores during the
next few days. Seas should increase slightly today, and resulting
surf along east facing shores will approach the high surf
advisory threshold. East shore surf will gradually decline
Thursday and Friday.

Small to occasionally moderate surf is expected along south
facing shores this week. An inconsistent southwest swell will
persist during the next few days. A moderate south-southwest swell
is expected this weekend into early next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Wednesday for all Hawaiian
waters-

&&

$$

AVIATION...Chevalier/Foster
MARINE...Kinel
REST OF DISCUSSION...Gibbs


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