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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 281331
Area Forecast Discussion
National Weather Service Honolulu HI
331 AM HST Sun Feb 28 2021
A trough aloft will keep the island atmosphere unstable through
much of the week, keeping a wetter than normal trade wind pattern
in place. Showers will favor windward and mauka areas with a few
showers spreading leeward from time to time. Locally heavy
rainfall and thunderstorms will be possible across portions of
Maui and the Big Island today. In addition to the showery pattern,
trade winds will strengthen today, with windy conditions expected
to persist through Tuesday. The trades will then ease to moderate
to locally breezy levels during the middle to latter part of the
Currently at the surface, a 1035 mb high is centered around 1150
miles northeast of Honolulu, while another 1044 mb high is
centered around 1300 miles north of Kauai. The resulting gradient
is producing breezy trade winds across the island chain early this
morning. Infrared satellite imagery shows mostly cloudy
conditions in windward areas, with partly cloudy skies prevailing
in leeward locales. Radar imagery shows scattered to numerous
showers moving into windward and mauka areas, with a few showers
making it into leeward sections of the smaller islands from time
to time. Main short term concerns continue to revolve around the
ongoing Flash Flood and Winter Weather headlines, as well as the
potential need for Wind Advisories.
High pressure northeast of the state will become absorbed into a
building surface high north of the state today. This new high
will then settle slowly southward while weakening slightly Monday
through Tuesday. A new strong surface high will build northwest
of the state Wednesday through Friday, then shift eastward far to
the north of the island chain next weekend. Overall, breezy to
windy trades will continue through the first half of the week,
with the trades peaking this afternoon through early Tuesday. The
strengthening trade are expected to boost wind speeds to advisory
levels across portions of Maui County this afternoon through at
least Monday, so a Wind Advisory has been posted for these areas.
As the trades peak on Monday, the Wind Advisory may need to be
expanded to include much, if not all of the island chain.
Additionally, the Wind Advisory may need to be extended in time
through Tuesday for some of the typically windier areas of Maui
County and the Big Island. The trades should ease a bit for the
middle and latter part of week as the gradient relaxes, but will
likely hold in the moderate to locally breezy range.
As for the remaining weather details, a trough aloft will keep a
moist and unstable airmass over the state today and tonight. The
deepest moisture and most unstable conditions will reside over the
eastern islands of Maui and the Big Island however, where the
threat for locally heavy rainfall and thunderstorms will continue,
mainly over windward areas. The Flash Flood Watch remains in
effect until 6 PM this evening, but this may eventually need to
be extended through early Monday morning. Across the remainder of
the state, a wet trade wind pattern is expected with showers
favoring windward and mauka areas and a few showers spreading
leeward from time to time.
Due to the moisture depth extending above the high elevation
summits of Mauna Kea and Mauna Loa, a Winter Weather Advisory
remains in effect for a wintry mix of freezing rain/drizzle and
snow through 6 AM Monday.
The upper trough axis will shift south of the state on Monday,
then linger just south of the islands through mid week. A series
of shortwave troughs may then rotate through the region Thursday
through next weekend. Overall, with troughing aloft more or less
being maintained over the state through much of the week, we
should see a wetter than normal trade wind pattern prevailing with
showers favoring windward and mauka areas and a few showers
reaching leeward communities from time to time.
An east-west oriented upper trough remains over the island chain
this morning. This is causing widespread MVFR ceilings and showers
for eastern sections of the Big Island, where AIRMET Sierra
remains in effect for tempo mountain obscuration from Upolu Point
to Cape Kumukahi to South Cape. Early morning satellite and radar
imagery also shows an increase in shower activity and clouds
along east sections of the smaller islands, so AIRMET Sierra also
accounts for tempo mountain obscuration in these areas as well.
These conditions should begin improving later this morning by
The wet pattern will continue throughout the day, especially for
the Big Island. Daytime heating and the unstable air mass could
result in a few thunderstorms on the leeward side of the Big
Island this afternoon and early evening. Elsewhere, scattered
showers will favor windward and mountain areas, causing tempo MVFR
conditions at times.
Breezy to locally strong easterly trade winds will continue today
as a strengthening surface high builds north of the state. AIRMET
Tango remains in effect to account for low-level mechanical
turbulence over and immediately south through west of the
mountains for all islands. As the winds strengthen, it is
possible that AIRMET Tango will also be needed for sustained
surface winds of 30kts or greater by tonight, particularly for the
Alenuihaha Channel and waters south of the Big Island.
Strong trade winds will continue into the middle of the week,
resulting in a Small Craft Advisory (SCA), Gale Warning, and High
Surf Advisory for east facing shores.
A high pressure system to the far northeast will be reinforced by
a high pressure far to the north. The high to the north is
expected to becoming the dominant of the two systems over the next
24 hours. The pressure gradient over the region will strengthen,
resulting in an uptick in winds over the coastal waters. The
typical windier areas have been placed under a Gale Warning
starting at 6pm tonight as a result. A SCA is in effect for all
waters today, and then will be in place for the remaining waters
not under the Gale Warning starting tonight. Seas will also remain
above the 10 ft SCA levels. Winds are forecast to start to back
off some on Wednesday, so will likely see some changes to the
Gale/SCA areas at that point.
The persistent trade wind pattern we have been experiencing over
the last several days has already been generating advisory level
surf along east facing shores. The current High Surf Advisory for
east facing shores continues through Tuesday, although its
possible that it may need to be extended.
A small to moderate northwest swell is expected to begin to
decline today. The next northwest swell is expected to arrive
Wednesday night and currently is expected to run higher than the
current swell. This swell could raise surf to near advisory levels
as it peaks Thursday. A small bump is likely along south facing
shores Monday night into Wednesday, and again at the end of the
High Surf Advisory until 6 PM HST Tuesday for Kauai Windward-
Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.
Flash Flood Watch through this afternoon for Maui Windward West-
Windward Haleakala-Haleakala Summit-South Big Island-Big Island
North and East-Big Island Interior.
Wind Advisory from noon today to 6 PM HST Monday for Molokai-
Lanai-Kahoolawe-Maui Windward West-Maui Leeward West-Maui
Central Valley-Windward Haleakala-Leeward Haleakala-South Big
Island-Big Island North and East-Kohala.
Winter Weather Advisory until 6 AM HST Monday for Big Island
Small Craft Advisory until 6 PM HST this evening for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.
Gale Warning from 6 PM this evening to 6 PM HST Tuesday for
Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island
Leeward Waters-Big Island Southeast Waters.
Small Craft Advisory until 6 PM HST Tuesday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maui County Leeward Waters-Big Island
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office