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Temp-4.1 C
RH96 %
WindSW 12 mph
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Hawaiian Islands Synoptic Discussion and Guidance

323
FXHW60 PHFO 121929
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
929 AM HST Mon Apr 12 2021

.SYNOPSIS...
The combination of a surface trough just northeast of the state
and a passing upper level disturbance will maintain light winds
and unsettled conditions over the Big Island and Maui, where
afternoon sea breezes will trigger spotty heavy showers and a few
thunderstorms. On Oahu and Kauai, northeasterly trade winds and
more stable weather will focus showers on windward slopes.
Moderate northeasterly trade winds and a typical pattern of
windward and mauka showers will prevail Tuesday through Thursday,
with an increase in showers possible over some areas Friday and
Saturday.

&&

.DISCUSSION...
We are starting out rather dry this morning, but expect spotty
heavy showers and a few thunderstorms over the eastern half of the
island chain this afternoon. A weak surface trough just northeast
of the state is keeping winds suppressed around the Big Island
and Maui, which allowed land breezes to clear out most areas
overnight. This trough is also causing moderate winds to be
northeasterly across Oahu and Kauai, where a few showers are
confined to windward slopes. A slow moving upper-level trough is
centered over the Big Island this morning and will maintain
unstable conditions through the day east of Oahu. With winds
suppressed on the Big Island and Maui, expect daytime sea breezes
to trigger spotty afternoon showers and a few thunderstorms. On
Oahu and Kauai, a stabilizing and somewhat dry (precipitable water
below an inch) northeasterly trade wind flow will focus modest
showers over windward areas, but a brief afternoon shower over
sheltered leeward areas cannot be ruled out.

Conditions will stabilize as the upper trough moves off to the
east tonight, and the nearby surface trough will weaken, allowing
northeasterly trade winds to strengthen state-wide. We could see a
lingering thunderstorm around the Big Island this evening, though
chances will be falling dramatically. Otherwise, expect modest
showers over windward areas.

A stable northeasterly trade wind flow will settle over the state
on Tuesday. A building ridge aloft will bring stability as the
upper-level trough moves to the east. Surface low pressure will
linger between Hawaii and California as surface high pressure
strengthens far northwest of the state. As a result, trades will
remain from a more northeasterly than usual, and winds will be
strongest over the western end of the state. Precipitable water
values will be near to slightly below normal, and dew points will
remain in the upper 50s to around 60, suggesting modest rainfall
over windward areas and a continued cool feel.

Winds may back off on Friday and the weekend as the ridge to the
north weakens. The GFS and ECMWF are showing a decayed front
dropping near or over the northern half of the state, which favors
Kauai and Oahu for wetter trade wind weather.

&&

.AVIATION...
Light to moderate northeast trades will prevail through tonight,
with a gradual strengthening trend. An upper-level disturbance
will keep some unsettled weather in place over Maui and the Big
Island today, with locally heavy showers and thunderstorm chances,
mainly during the afternoon and evening. Elsewhere, more typical
trade wind weather can be expected, with clouds and showers
favoring windward coasts and slopes and an occasional shower
reaching leeward areas.

No AIRMETs are currently in effect, though AIRMET Sierra may be
needed later today for mountain obscuration across portions of the
state. AIRMET Tango is in effect for moderate upper level
turbulence over the western half of the state between FL280 and
FL380.

&&

.MARINE...
Early week gentle northeast breezes across the central and eastern
Hawaiian coastal waters have been in response to a trough of low
pressure located northeast of the islands. A relatively tighter
pressure gradient over the western half of the state from an area
of high pressure due north of Kauai is producing more moderate to
locally fresh northeasterlies. There will be little change in both
winds and seas through tonight with stronger winds occurring
around Oahu and Kauai. A disturbance rotating through the base of
the upper level trough will keep moderate chances for showers and
isolated thunderstorms alive over northeastern waters...or Maui
and the Big Island's windward coastal and offshore zones. The
threat for locally high, visibility-reducing rainfall within
stronger showers will exist through today. As high pressure builds
in northwest of the state, more areawide moderate to locally
strong channel trades will fill in Tuesday and Wednesday. Winds
will back off later in the week in response to an approaching cold
front northwest of the island chain.

The Waimea PacIOOS buoy is picking up on higher wave period
spectrum energy from the latest west northwest swell this morning.
This small 3 to 4 foot swell traveling around the islands will
subtly trend north and west facing shore surf up through Tuesday.
A fetch region generated by stronger northerlies produced from a
tighter north pressure gradient between strong high pressure
northwest of the state and lower pressure to the east will
generate a slightly higher, lower period north northeast swell
timed to arrive late this week. This swell will peak over the
weekend and lift surf along northern and some eastern facing
shores due to wrap by another foot or two. Incoming small, long
period south southwest swell energy will boost southern shore surf
to seasonal levels through Tuesday with a slow middle of the week
decline.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Wroe
AVIATION...TS
MARINE...Blood

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