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Hawaiian Islands Synoptic Discussion and Guidance

134
FXHW60 PHFO 201957
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
957 AM HST Fri Apr 20 2018

.SYNOPSIS...
Wet weather and cloudy skies will remain through today with some
partial clearing over Kauai and Oahu this afternoon. Some showers
may be heavy at times and a slight chance of thunderstorms remains
through today. A typical trade wind pattern will return by
Saturday, with showers favoring windward and mauka areas. High
pressure north of the state will maintain moderate to breezy trade
winds through the weekend, before strengthening to locally windy
levels next week.

&&

.DISCUSSION...
Wet trade wind weather will continue today with gradual clearing
from west to east across the state through tonight. The wet
trades will linger over windward Maui County and the Big Island
before normal trade wind weather sets in across the state
Saturday. An upper level trough moving across the state has
increased instability, while an old frontal boundary riding the
trades has enhanced low level moisture. The wet weather, along
with the copious amounts of rain in the recent past, warrants
continuation of the flash flood watch through this afternoon.
Mostly cloudy to overcast conditions will prevail this morning
with some clearing over Kauai and Oahu this afternoon as the upper
trough moves slowly east. These layered mid and high clouds are
bringing a wintry mix of weather to the Big Island summits, and
a winter storm warning remains through tonight.

Current radar shows diminishing showers pushing into windward
areas of Kauai, while showers continue over windward areas of
Oahu, Maui County and the Big Island. The forecast calls for
showers diminishing from west to east today with lingering
enhanced moisture over windward Maui and the Big Island into
Saturday.

A surface ridge to the north will maintain moderate to
locally breezy trade winds through the weekend. High pressure
will strengthen Monday bringing trades to locally windy levels
through Wednesday. Overall, a fairly typical trade wind pattern is
expected through the period, with bands of enhanced clouds and
showers pushing through the islands from time to time. Showers
will focus over windward and mauka areas with some showers riding
the strong trades to leeward locations.

&&

.AVIATION...
VFR conditions prevail at most TAF sites with occasional MVFR
conditions being reported at windward terminals such as PHLI and
PHTO in low clouds and shra. Expect these conditions to continue
through the day at PHTO but improving conditions are likely at
PHLI.

AIRMET Sierra is currently posted for mountain obscuration over N
through E sections in low clouds and shra. Conditions may improve
from W to E this afternoon. AIRMET Tango is currently posted for
low level turbulence S through W of mountain ranges due to breezy
trade winds. AIRMET Tango is also posted for upper level
turbulence as a strong upper level jet moves over the area.
AIRMET Zulu is currently posted for moderate rime and ice from
Molokai eastward due to a deep layer of moisture. Conditions should
gradually end over Maui County later today.

&&

.MARINE...
A surface high north of the area will move northeast through
Saturday, and will continue to produce locally strong trade winds
over Hawaiian waters. An upper level trough is keeping winds
weaker than might be expected, will continue the possibility for a
slight chance of thunderstorms with locally heavy downpours today.

A Small Craft Advisory (SCA) continues through early Saturday
morning for the strong trade winds in the windier areas near Maui
County and the Big Island. A new high building northwest of the
area will cause the trade winds to strengthen starting this
weekend. The SCA will likely be extended and expanded as this
occurs.

Surf is expected to remain below High Surf Advisory (HSA)
thresholds along all shores through this weekend. Rough surf will
persist along east facing shores through Sunday night due to the
trade winds, but heights will remain moderate through the weekend.
As trade winds strengthen early next week, surf may approach the HSA
threshold.

A moderate northwest swell is expected to build today. A slightly
larger swell will peak Sunday and another moderate swell is
possible next Thursday.

A series of small swells from the southern hemisphere will maintain
modest surf along south facing shores through this weekend. Lows
in the southern hemisphere may strengthen and shift east enough to
produce some bigger surf along south facing shores by the end of
next week.

See the latest Oahu Surf Discussion (SRDHFO) for additional
details on surf and swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch until 6 PM HST this evening for Niihau-Kauai-
Oahu-Molokai-Lanai-Kahoolawe-Maui-Kona-South Big Island-Big
Island North and East-Kohala-Big Island Interior.

Winter Storm Warning until 6 AM HST Saturday for Big Island
Summits.

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

&&

$$

Morrison...Discussion
Burke...Aviation
M Ballard...Marine

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