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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 070644

Area Forecast Discussion
National Weather Service Honolulu HI
844 PM HST Sat Mar 6 2021

A strong surface high far to the north of the islands will
maintain breezy to locally strong trade winds for the next several
days. Passing showers, briefly heavy at times, will continue
through Sunday. More unsettled weather is on the way as the new
week begins. Deep moisture located southeast of the Big Island
will spread northwestward across the main Hawaiian Islands between
Sunday night and Tuesday. An upper level disturbance will enhance
this moist air mass leading to the potential for heavy rainfall
that will remain into next week.


The air mass is rather unstable across the smaller islands this
evening, more so across Kauai where the remnants of an old front
was stalled over the Garden Isle for the past 2 days. In 48
hours, rainfall amounts were roughly between 7 and 11 inches over
a couple of rainier locations across the windward and interior
sections of the island. Current radar imagery shows showery
conditions over interior Kauai, and scattered small pockets of
showers upwind of the island.

Satellite and radar imagery show a little less coverage of widely
scattered showers upwind of the remaining islands. The clouds and
showers, however, are expected to increase as the evening
progresses and favor the windward and mountain areas. With the
breezy trades, these showers will be carried well downwind into
the lee side of the smaller islands.

A broad 1038 mb surface high is located 1240 miles north of Kauai
this evening. It is forecast to drift southeast through Tuesday,
holding at a 1037 mb central pressure. By Tuesday evening, the
high is forecast to be 1030 miles northeast of the islands with a
pressure of 1037 mb. This scenario translates to a veering of the
winds from the current northeast direction, to east- southeast by
Tuesday night. It will still be rather breezy on Tuesday night
into Wednesday across the windward areas and especially the
exposed windward waters of Maui County and the Big Island, with
slightly lighter winds across the sheltered islands downwind of
Maui and the Big Island.

The islands are once again under the influence of an upper level
disturbance. There is currently an east to west oriented upper
trough just north of the islands. It is projected to drift south
to over Kauai in the next 24 hours, with an upper low forming west
of Kauai. As this low forms, it will be drawing mid level
moisture up from the south and southeast of the Big Island Sunday
afternoon and Sunday night. First to be affected by enhanced
showers will be the Big Island, and then these showers will spread
northwestward across the smaller islands by Monday night. This
upper low or deep trough then drifts east and over the islands by
Wednesday night and stalls through Friday. Should the forecast
models hold this course, the islands will be in an extended period
of heavy rains and possible thunderstorms. So more info and
details will be forthcoming in the coming days, including thoughts
of a statewide Flash Flood Watch and Winter Weather Advisory for
the Big Island summits.


VFR conditions prevail at all TAF sites with some brief periods of
MVFR conditions at PHLI in low clouds and shra. Expect these
conditions to continue into the overnight hours. AIRMET Sierra is
currently in effect for Kauai due to mountain obscuration. They
will be brief periods of MVFR conditions elsewhere along windward
slopes through into the morning hours as low clouds interact with
island terrain.

AIRMET Tango is currently in effect over and immediately south
thru west of mountain ranges on Kauai due to low level mechanical


The center of a strong surface high pressure system is located
more than 1000 nautical miles north of Honolulu early this
evening. The forecast guidance continues to indicate the local
pressure gradient will gradually tighten south of this feature as
it moves toward the southeast during the next few days. This will
maintain strong and gusty trade winds across most waters into
early next week. The trade winds may increase to near gale force
over some of the typically windier waters, such as the Alenuihaha
Channel, starting Sunday or Monday.

The strong trade winds are causing the local wind waves to
increase over most windward waters and channels. These wind waves
combined with a north swell are also producing elevated seas
over most waters. As a result of the winds and seas, a Small
Craft Advisory (SCA) remains in effect through early Monday
morning for all Hawaiian waters, except the Maui County and Oahu
Leeward Waters. By Sunday morning, the SCA has been expanded to
also include the Maui County and Oahu Leeward Waters. Expect the
SCA to be extended beyond Monday for all Hawaiian waters, since
the trade winds and elevated seas will likely persist into the
middle of next week.

The strengthening trade winds will likely cause surf to increase
along east facing shores from late Sunday through mid-week. As a
result, expect a High Surf Advisory (HSA) to be issued for most
east facing shores starting Sunday night or Monday. Once this HSA
is issued, it will likely remain in effect during most of next
week due to the strong trade winds.

The current moderate north swell, which was mentioned above, is
forecast to slowly decrease through Sunday. A small north-
northwest swell and a reinforcing north swell are expected to
spread across the area late Sunday and Monday. These swells will
likely maintain medium surf heights along most north and west
facing shores through Monday. As these north-northwest and north
swells gradually diminish, expect small surf heights along north
and west facing shores from Tuesday into next weekend. Finally,
small, long-period south-southwest swells will maintain small
background surf along most south facing shores through Friday.


Flash Flood Watch until 6 AM HST Sunday for Kauai.

Small Craft Advisory until 6 AM HST Monday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters-
Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island
Windward Waters-Big Island Leeward Waters-Big Island Southeast

Small Craft Advisory from 6 AM Sunday to 6 AM HST Monday for
Oahu Leeward Waters-Maui County Leeward Waters.




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

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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
Send comments to: Ryan Lyman