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Hawaiian Islands Synoptic Discussion and Guidance

421
FXHW60 PHFO 121309
AFDHFO

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

.SYNOPSIS...
A slow moving trough aloft will maintain unsettled conditions
over the eastern half of the state today. Northeasterly trade
winds will gradually strengthen over Kauai and Oahu today, while
lighter winds and lingering instability maintain chances for
spotty showers and thunderstorms across Maui County and the Big
Island. 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...
Have lowered the PoPs over Kauai and Oahu a little for the first
36 hours or so based on the overall model trend, and the lower
precipitable water (PW) values moving over the western end of the
state. Have boosted the winds a touch in the first couple of days
in keeping with the general trend of the higher resolution
models. Otherwise, the forecast remains largely unchanged.

An upper level trough remains over/near the Big Island early this
morning and will continue to bring some instability to the eastern
half of the state into tonight. The chance for thunderstorms
remains in the forecast for Maui County and the Big Island, with
the Big Island being slightly more likely to experience
thunderstorms in part due to the proximity to the upper trough.
PW from the Lihue sounding at 2am was 0.76, while Hilo was just
over an inch at 1.07. Satellite derived PW shows the PW is just a
bit higher than both soundings over Oahu and Maui County this
morning, however all in all, values are near or below normal for
April. The upper level trough is bringing much colder than normal
temperatures to the upper atmosphere over the islands, which is
helping to fuel the instability that has sparked some of the
heavier showers and thunderstorms. As the trough makes it way to
the east, we do expect warmer temperatures to move in at the upper
levels from the west, which will help to stabilize the atmosphere.

A ridge to the north of the islands has already returned a
moderate to locally breezy trade wind pattern to the western end
of the state. Combined with the drier PW values moving in,
expecting showers over Kauai and Oahu to be somewhat limited
today, and focused over the typical windward and mauka areas. For
Maui County and the Big Island, winds remain on the lighter side,
which will likely lead to some localized land and sea breezes.
This will in turn lead to some afternoon clouds and showers
developing over leeward and interior locations. As the upper
trough moves to the east, expect the trade winds to filter in to
Maui County and the Big Island.

Look for a statewide stable trade wind flow Tuesday. A lingering
low pressure system far to the northeast, combined with the
strengthening high to the far northwest will mean the trades will
be little 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 next weekend as the ridge to the
north weaken. 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 today and
tonight, with a gradual strengthening trend expected. An upper
level disturbance will keep some unsettled weather in place over
Maui and the Big Island today, with some locally heavy showers
and even a thunderstorm or two possible, mainly during the
afternoon and early evening hours. Elsewhere, more typical trade
wind weather can be expected, with clouds and showers favoring
windward and mauka areas, and a stray shower reaching leeward
locales from time to time.

No AIRMETs are currently in effect. AIRMET Sierra may be needed
later today for mountain obscuration across portions of the state.
AIRMET Tango may be needed later this morning for moderate upper
level turbulence over the western half of the state between FL290
and FL370.

&&

.MARINE...
A ridge north of the area will produce moderate northeast trade
winds over Kauai and Oahu waters today and light to gentle northeast
trade winds over Maui County and Big Island waters. A trough aloft
has been producing locally heavy showers and thunderstorms over
the Maui County and Big Island waters. The threat of thunderstorms
will continue through tonight.

Northeast winds will strengthen through mid-week as a high far
northwest of the area moves east. Winds may reach Small Craft
Advisory speeds in some areas. Winds will subside through the
second half of the week as a front approaches from the north.

A small northwest swell will peak tonight, then subside through
Wednesday night. A moderate, shorter-period north swell expected
Thursday may persist through next weekend. Lighter than normal
trade winds will keep surf small along east facing shores through
mid-week. The north swell will wrap in to some east facing shores
later in the week. Swells from distant southern hemisphere storms
will produce small to moderate surf along south facing shores.

&&

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

&&

$$


DISCUSSION...M Ballard
AVIATION...Jelsema
MARINE...Donaldson

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



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