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Hawaiian Islands Synoptic Discussion and Guidance

066
FXHW60 PHFO 281946
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
946 AM HST Sat Mar 28 2020

.SYNOPSIS...
An approaching upper low will trigger heavy rainfall across
portions of the main Hawaiian Islands through the weekend. Trade
winds will focus showers over windward and mauka areas, with
heavy showers and thunderstorms possible leeward mainly during
the afternoon and evening hours. A brief return to a trade wind
pattern is expected early next week, followed by a land and sea
breeze pattern Wednesday and Thursday. Another switch to trade
wind weather is then possible by the end of the week.

&&

.DISCUSSION...
A surface trough is located just west of Kauai, while a high is
centered far north of the islands. Water vapor imagery shows an
upper low west-northwest of Kauai. Scattered to patchy broken low
clouds are noted across most of the state, but deep convection
over and just east of eastern Kauai continues. Radar shows
heavy rainfall over windward sections of Kauai and Oahu, with
isolated to scattered windward showers across Maui County and the
Big Island.

The trough near Kauai will shift westward through Sunday, while
the high to our north slides eastward. This should keep moderate
to fresh trade winds blowing across most of the state, although
the flow may veer east-southeast enough across the western
islands to allow for localized sea breeze development today. The
high will weaken as it moves, allowing trades to diminish to
light to moderate levels. Trades may become light enough for land
and sea breeze development in more sheltered areas Wednesday and
Thursday. Trades will strengthen late next week as as another high
develops far northwest of the islands.

Instability associated with the upper low will increase heavy
rainfall and thunderstorm potential over the next couple of days.
The greatest threat will remain over Kauai and Oahu today and
tonight, but locally heavy showers and isolated thunderstorms will
remain possible across Maui County and the Big Island as well.
A Flash Flood Watch remains in effect for Niihau, Kauai and Oahu
through 6 AM Sunday, where the flash flood threat is the greatest.
At this time, the greatest threat for flash flooding across Maui
County and the Big Island is expected to hold off until Sunday,
and the Flash Flood Watch may eventually need to be expanded to
include these areas at some point later today or tonight.

By Sunday night, the strongest upper forcing appears to be east
of the islands. This should bring a return of more typical trade
wind weather featuring mainly windward and mauka showers, with
the occasional leeward spillover, through Tuesday. Lighter winds
Wednesday and Thursday may result in a land/sea breeze pattern
across much of the state, with showers favoring windward coasts at
night and interior/mauka areas during the afternoon and early
evening. Models suggest a return to trade flow again by Friday.

&&

.AVIATION...
The threat for heavy showers and thunderstorms will continue
through the weekend as an upper low drifts southward into the area
and a surface trough remains near the western end of the state.
Although much of the shower coverage will focus over windward and
mountain areas with the breezy trades still in place, more
widespread coverage will persist for Kauai and Oahu where better
instability exists. IFR to MVFR CIG/SHRA are expected in and
around the heavy showers and storms that develop.

AIRMET SIERRA remains effect for the entire island of Kauai and
windward areas of Oahu, Maui and the Big Island for mountain
obscuration above 1000 feet for Kauai and 2000 feet elsewhere.
Expect this pattern to continue through tonight.

AIRMET Tango continues for moderate turbulence between FL250 and
FL400 over Maui County and the Big Island, but is likely able to
be cancelled later today as the turb area moves off to the east.
However, some upper level turbulence may develop near and just
west of Kauai later this afternoon or this evening as the upper
low nears.

&&

.MARINE...
A low aloft NW of the islands has induced a surface trough just W
of Kauai, with these features triggering heavy showers and
thunderstorms over waters from Oahu westward. Meanwhile, high
pressure to the distant N continues to support moderate to locally
strong trade winds near and upstream of the islands. The low aloft
is expected to gradually weaken through Sunday, with the potential
for heavy showers gradually easing.

A Small Craft Advisory remains in effect today for many zones due
to a combination of winds and seas, and an extension in time is
likely for at least some areas as winds and seas will only
gradually diminish through the weekend. A High Surf Advisory for
east facing shores posted through today will also likely be
extended through the weekend. No other significant swells are
expected, with a small WNW swell expected Tuesday through
Thursday, and small S to SE swells through the period.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through late tonight for Niihau-Kauai-Oahu.

High Surf Advisory until 6 PM HST this evening for Kauai
Windward-Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-
Windward Haleakala-South Big Island-Big Island North and East.

Small Craft Advisory until 6 PM HST this evening for Kauai
Windward Waters-Kauai Channel-Oahu Windward Waters-Kaiwi Channel-
Maui County Windward Waters-Maalaea Bay-Pailolo Channel-
Alenuihaha Channel-Big Island Windward Waters-Big Island Leeward
Waters-Big Island Southeast Waters.

&&

$$

DISCUSSION...Powell
AVIATION...TS
MARINE...Birchard

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



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