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RH47 %
WindSSW 6 mph
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Hawaiian Islands Synoptic Discussion and Guidance

073
FXHW60 PHFO 071404
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
404 AM HST Sun Mar 7 2021

.SYNOPSIS...
Unsettled weather is likely for the upcoming week as an upper
level disturbance interacts with tropical moisture. Breezy trade
winds will give way to light and variable winds by mid week as a
surface trough forms over the state. Strong northeast winds will
then follow Friday over Kauai, and to the remaining islands over
next weekend.

&&

.DISCUSSION...
Although the evening has been rather tame, there are a couple of
spots that we are watching closely. The two areas are windward
Kauai and windward Haleakala above Hana, where heavy showers are
occurring.

There has been a significant shift in the wind forecast during the
second half of the week. Instead of strong and gusty trade winds
through the week, models are suggesting a surface trough forming
over the islands Tuesday and remaining in the vicinity of the
state through Friday. Strong and gusty northeast winds are
expected to build back in to the area next weekend as high
pressure passes north of the islands.

Another addition to the forecast is a slight chance of
thunderstorms has been introduced for the Big Island and windward
Maui starting this afternoon and remaining in forecast through at
least Monday. The rest of the forecast package is pretty much the
same, that is it will be showery for most of the week. This
prolonged period of unsettled weather will likely warrant a Flash
Flood Watch for most if not all of the islands and a Winter
Weather Advisory for the Big Island Summits.

An east to west oriented upper level trough with cold temperatures
of around minus 13 degrees C is located just north of the islands
this morning. An upper low is forecast to form later today west
of Kauai as enhanced low level moisture moves toward the Big
Island from the southeast. This moisture will then spread up the
island chain through Monday night. The upper low then drifts over
the islands by Wednesday night and stalls through Friday. The
combination of the cold temperatures aloft and the moist air mass
near the surface will likely result in an extended period of heavy
rains and possible thunderstorms.

&&

.AVIATION...
FR conditions prevail at all TAF sites with some brief periods of
MVFR conditions at PHLI and PHTO in low clouds and shra. Expect
these conditions to continue into the morning hours. AIRMET
Sierra is currently in effect for Kauai, Maui and the Big Island
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 due to low level mechanical
turbulence.

&&

MARINE...
The center of a strong surface high pressure system is located
more than 1000 nautical miles north of Honolulu early this
morning. The forecast guidance continues to indicate the pressure
gradient will gradually tighten south of this feature as it moves
toward the southeast during the next couple of days. This will
produce strong and gusty trade winds across most waters into
Tuesday.

The strong trade winds are causing the local wind waves to
increase over exposed waters. These wind waves combined with a
modest north swell are also producing elevated seas over most
waters. As a result, a Small Craft Advisory (SCA) is now in
effect through early Monday morning for all Hawaiian waters.
Expect the SCA to be extended beyond Monday morning for all
Hawaiian waters, since the trade winds and elevated seas will
likely persist through Monday night. Also, there is a slight
chance of thunderstorms in the vicinity of the Big Island from
this afternoon through Monday.

The strengthening trade winds will likely cause surf to increase
along east facing shores from later today and tonight through
Monday. As a result, expect a High Surf Advisory (HSA) to be
issued for most east facing shores starting tonight or Monday.
Once this HSA is issued, it will likely remain in effect for the
first few days of the new work week.

The latest guidance from the various forecast models is starting
to reach some agreement that a surface trough may develop in the
vicinity of the islands around mid-week. Assuming this feature
does develop, it would likely cause the winds to trend down over
some areas by late Tuesday or Wednesday. Stay tuned for updates
to future coastal waters forecasts as this scenario continues to
become less ambiguous.

The current north swell, which was mentioned above, is forecast
to slowly decrease today. A small north-northwest swell and a
reinforcing north swell are expected to spread across the area
later today and tonight. 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 relatively small surf heights along most north
and west facing shores from Tuesday into next weekend. Small,
long-period south-southwest swells will maintain small background
surf along most south facing shores through Friday. Note that some
of the trade wind wrap may produce a localized boost in surf
heights along some exposed south and north facing shorelines later
this week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Monday for all Hawaiian
waters.

&&

$$


DISCUSSION...H Lau
AVIATION...Morrison
MARINE...Houston

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