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Hawaiian Islands Synoptic Discussion and Guidance

418
FXHW60 PHFO 290137
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
337 PM HST Fri Feb 28 2020

.SYNOPSIS...
Wet and windy conditions will remain through the weekend. Strong
trade winds will focus the majority of showers along windward
slopes, but downpours will also occur over leeward locations. An
upper low will bring a slight chance of thunderstorms and the
possibility of winter weather to Haleakala and the Big Island
summits through the weekend. Unsettled conditions may persist into
early next week.

&&

.DISCUSSION...
A strong high pressure located far northeast of the state is
expected to strengthen further over the next few days, boosting
our already strong trade winds to near advisory levels. However,
an upper level low now east of Hawaii will edge over the eastern
portion of the state tonight and drift overhead this weekend,
bringing increasing instability. This feature will likely weaken
the local surface pressure gradient and keep the strongest trade
winds to the northeast of the islands. Nonetheless, island winds
are expected to be near advisory levels through the weekend before
abating early next week as the high weakens.

The aforementioned upper low will cool temperatures aloft and
remove our normal stabilizing temperature inversion. Models have
been consistently forecasting 500 mb temperatures near -16 C which
is about as cold as it gets over Hawaii. These cold temperatures
and associated instability will bring the chance for thunderstorms
and locally heavy downpours. The strong winds near the surface
will keep most of the heavy rain moving along at a fast pace, but
windward areas will have a steady diet of moderate to heavy
showers and isolated thunderstorms. The chances of flood
advisories are likely, and a flash flood warning is not out of the
question for windward areas, even with the strong trades. Highest
rainfall totals are expected over the eastern half of the state
as the coldest temperatures aloft and higher boundary layer
moisture remains to the east. For leeward areas of the smaller
islands, expect strong, gusty trade winds with quick hitting heavy
showers.

Winter weather at the higher elevations of the Big Island and
Haleakala are a possibility as well. The upper low will drop
freezing levels to 9000 feet or lower, so any moisture falling at
the summits will be in the form of snow or freezing rain. The
highest probability for winter weather on the summits appears to
be on Saturday night into Sunday and a Winter Storm Watch has been
issued for that time frame.

&&

.AVIATION...
A strong surface high located far northeast of the main Hawaiian
Islands will maintain gusty northeast to east trade winds through
Saturday. As a result, AIRMET TANGO remains in effect for moderate
low-level turbulence below 090 immediately south through west of
the mountains on all islands.

The strong trades will also continue to transport low clouds with
embedded showers into northeast facing sections of the islands
this evening, producing brief periods of MVFR conditions due to
lower ceilings and reduced visibilities. Otherwise, expect VFR
conditions to prevail across most of the state. Low cloud and
shower coverage will likely increase later tonight through
Saturday due to the proximity of an upper level low, along with a
slight chance of thunderstorms over the eastern half of the state.
This could result in more widespread MVFR conditions over the
higher terrain and northeast facing slopes and the need for AIRMET
SIERRA for mountain obscuration over some islands.

Strong winds on the west side of the aforementioned low could
also result in an area of moderate upper-level turbulence tonight
through Saturday. Will monitor the situation for possible issuance
of AIRMET TANGO later tonight.

&&

.MARINE...
Strong high pressure northeast of the area is resulting in strong
to near gale force winds across the coastal waters. A Small Craft
Advisory (SCA) is in effect for all coastal waters through the
weekend. At this time, it does not look like winds will reach gale
force. An upper level low is expected to move over the state from
the northeast tonight, introducing periods of locally heavy
showers and a slight chance or thunderstorms through the weekend.

The current northwest swell has peaked and the High Surf Advisory
has been cancelled. This swell will then subside into the
weekend, with no significant northwest swells expected into next
week.

Surf will also continue build along east facing shores as the
trade winds strengthen. A High Surf Advisory is posted for east
facing shores through the weekend. Surf may reach warning levels
later this weekend into the beginning of next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Sunday for Kauai Windward-Oahu
Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

Winter Storm Watch from Saturday evening through Sunday
afternoon for Haleakala Summit-Big Island Summits.

Small Craft Advisory until 6 PM HST Sunday for all Hawaiian
waters-


&&

$$


DISCUSSION...Morrison
AVIATION...TS
MARINE...EATON

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
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