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Hawaiian Islands Synoptic Discussion and Guidance

088
FXHW60 PHFO 101930
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
930 AM HST Fri Apr 10 2020

.SYNOPSIS...
An approaching front will bring scattered showers and light to
moderate west to southwest winds to most areas through Saturday.
The front will bring a wind shift to the north, and some increase
in rainfall as it stalls over the islands from Saturday night into
Sunday. A brief period of trade winds is expected late Sunday and
Sunday night, with showers favoring windward areas. Lighter winds
and an increase in showers are expected statewide Monday afternoon
through Tuesday night, as a trough of low pressure develops over
the islands. Moderate to locally breezy trade winds are expected
to return during the middle to latter part of next week, bringing
more typical trade wind weather back to the island chain.

&&

.DISCUSSION...
Currently at the surface, an east to west oriented ridge of high
pressure is located around 100 miles south of the Big Island, with
a cold front around 550 miles northwest of Kauai. The pressure
gradient remains quite weak this morning, with land breezes
present in most areas. Visible satellite imagery shows partly to
mostly cloudy conditions across the state. Radar imagery shows
a few showers over the coastal waters and along the southeast
coast of the Big Island, with very little shower activity
elsewhere. Main short term concern revolves around rain chances
the next few days.

A light wind regime is expected to remain in place this morning,
with sea breezes developing in some areas. As the day progresses,
we should begin to see an increase in southwesterly winds,
particularly over the western islands as the cold front moves
closer to the state. Light to moderate southwest to west winds
will spread eastward across the central and eastern islands
tonight and continue through the day Saturday. The cold front
will slide southeastward across through Kauai and Oahu Saturday
night, then stall out over the central islands on Sunday. Behind
the front we expect a brief period of moderate northerly winds
Saturday night, with winds shifting around to northeasterly
trades statewide on Sunday. High pressure will slide by to the
north of the islands Sunday night, with light to moderate trades
prevailing. The trades will diminish Monday with winds shifting
around to the southeast as surface troughing develops over the
central islands. This will lead to a sea breeze / land breeze
pattern taking hold across the smaller islands through Tuesday,
while light to moderate southeasterly winds prevail near the Big
Island. The trades will then begin to return from west to east
across the state Tuesday night and Wednesday as the surface trough
shifts west of the state and high pressure builds northeast of
the islands. Moderate to locally breezy trade winds are expected
to develop statewide by Wednesday night and continue through the
remainder of the week.

A Wind Advisory has been issued for the Big Islands summits
through noon Saturday due to strong winds associated with an upper
level disturbance.

As for weather details, broad convergent southwesterly boundary
layer flow in advance of the cold front will keep scattered
showers in the forecast today through Saturday. A shortwave trough
moving overhead this afternoon into this evening, may even de-
stabilize things enough for a thunderstorm to pop over the Big
Island, so will mention a slight chance of thunderstorms here.
A few showers will accompany the front as it slides southeastward
through the western islands Saturday night and into the central
islands on Sunday, but no significant rainfall is expected. We
should begin to see an increase in trade wind showers Sunday night
and Monday, favoring the typical windward areas. Shower coverage
and intensity may then increase Monday afternoon through Tuesday
night as surface troughing develops over the islands and some
shortwave energy moves overhead. A more typical trade wind pattern
should then return by the middle and latter part of next week
featuring mainly windward showers and the occasional leeward
spillover.

&&

.AVIATION...
Another day of light winds is on tap for the island chain,
although an approaching front will bring increasing southwesterly
winds late today and tonight. Moisture will also increase,
leading to an uptick in showers overnight into Saturday. Tempo
MVFR conditions are possible with these showers. Otherwise, expect
VFR conditions to prevail through the day.

There are no AIRMETs currently in effect and none are anticipated.

&&

.MARINE...
A remnant frontal boundary lies east of the Big Island with a
ridge just south of the state. The wind flow around the islands
remains light and variable with land breezes over near shore
waters this morning and sea breezes developing near the islands by
this afternoon.

A weak front approaching from the NW will turn winds around to the
southwest later today. The front is expected to move across the
area Saturday night and Sunday with north to northeast winds
filling in behind it. An area of high pressure passing north of
the area Sunday night will bring light to moderate trade winds. A
trough of low pressure developing over the state will bring
southeasterly winds to the waters Monday afternoon through
Tuesday, with trade winds then returning from east to west across
the island chain Tuesday night and Wednesday.

A moderate, short period northwest swell has arrived slightly
larger than expected, increasing surf heights to advisory levels
for north facing shores of the smaller islands. This swell will
peak today, then lower Saturday and Sunday. A series of small,
short period north swells will move through the area Saturday
through Tuesday, before a larger northwest swell arrives Tuesday
night. This swell is expected to peak Wednesday, likely reaching
advisory levels. A series of small south swells will continue
through the weekend and into next week, with peak surf heights
likely Saturday, but still below advisory level. Surf will remain
small along east facing shores into early next week due to the
lack of trades, but should increase Tuesday and Wednesday, as the
trades pick up.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST this evening for Niihau-Kauai
Windward-Kauai Leeward-Oahu North Shore-Oahu Koolau-Olomana-
Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

Wind Advisory until noon HST Saturday for Big Island Summits.

&&

$$

DISCUSSION...Jelsema
AVIATION...Dye
MARINE...TS

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



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