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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 211941
Area Forecast Discussion
National Weather Service Honolulu HI
941 AM HST Sun Apr 21 2019
Locally breezy trade winds will continue through Friday, bringing
periods of clouds and showers to windward areas, while leeward areas
remain mostly dry. Lighter winds are in the forecast for next
weekend, which may allow afternoon clouds and showers to form over
interior and leeward areas.
A surface high will remain nearly stationary far NE of the islands
through Tuesday before weakening and moving E on Wednesday. A new
high pressure cell will pass far N of the area around midweek, and
the pressure gradient over the islands will change little, with
locally breezy trade winds persisting into Friday. As this high
moves into the NE Pacific by the end of the week, its associated
ridge will be shunted toward the islands by an approaching front.
Winds will weaken and shift to the SE or S, potentially bringing an
increase in low-level moisture next weekend.
A mid-level ridge will be near or over the islands through the
upcoming week, then shift E by next weekend as height falls over the
islands are supported by the development of a closed low aloft to
the NW. A subsidence inversion based near 6000-7000 feet will change
little as the ridge persists, thus maintaining a stable island
atmosphere. The inversion will be high enough however to allow trade
wind showers to occasionally dampen windward areas, most likely
during nights and mornings. Little in the way of leeward rainfall is
expected, except for leeward Big Island, where a few afternoon and
evening showers will be possible.
Low cloud coverage upstream of the islands has decreased over the
past 6 to 12 hours, and fewer windward showers are expected today as
compared to yesterday, with rain chances expected to be quite low
over windward Big Island, where upstream low cloudiness is minimal.
The short term forecast will be updated to account for the latest
A ridge of high pressure will remain far northeast of Hawaii and
drive fresh trade winds over the next few days. AIRMET Tango remains
in effect for moderate turbulence below 060 over and downwind of the
terrain due to the trade winds. These conditions are expected to
hold in place today and possibly through Monday.
Trade winds will carry in periodic clouds and showers that will
focus over windward and mauka sections. Brief MVFR ceilings and
visibilities, and mountain obscurations may occur within the showers.
For the most part, a ridge north of the state will maintain breezy
to occasionally strong trade winds through Thursday. A front passing
well north of the state today will weaken the ridge slightly and
will cause the winds to ease across select windward zones and
channels later this afternoon. A Small Craft Advisory (SCA) is in
effect for most of the waters near Maui County and the Big Island
through today, and for the typically windy areas around Maui and the
Big Island through Monday. The SCA for these latter areas will
likely be extended due to the strong trades.
Surf along all shores is expected to remain below advisory levels
for this week. A small west-northwest swell will provide small surf
along north and west facing shores for the first half of the week. A
series of small south swells will provide small surf starting Monday
and will likely continue through most of the week and into the
weekend. Surf along east facing shores will hold steady in the small
to moderate range through Thursday due to the breezy to locally
Small Craft Advisory until 6 PM HST Monday for Maalaea Bay-Pailolo
Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island
Small Craft Advisory until 6 PM HST this evening for Kaiwi Channel-
Maui County Windward Waters-Big Island Windward Waters.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office