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Hawaiian Islands Synoptic Discussion and Guidance

985
FXHW60 PHFO 300155
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
355 PM HST Sun Mar 29 2020

.SYNOPSIS...
Light to moderate trade winds will continue for the next couple of
days, delivering clouds and showers that will primarily focus over
windward areas. The island atmosphere will remain somewhat unstable
today, so some of the showers could be briefly heavy, with the
potential for a thunderstorm, especially over the Big Island slopes
this afternoon. A bit of a drying trend tonight and Monday is
expected to be short-lived, with windward showers likely increasing
again on Tuesday and Wednesday. An approaching front will bring
light winds Thursday and Friday, which will likely allow showers to
develop over interior and upslope areas in the afternoons.

&&

.DISCUSSION...
Currently, moderate to locally breezy trade winds are being
supported by a surface high pressure cell centered far N of the
islands. Water vapor imagery highlights an E-W oriented trough aloft
just N of the islands along 24N, with a low aloft centered about 600
miles NE of the islands near 27N 150W. Model analyses indicate that
a jet maximum on the S side of the low is just NE of the islands,
with the associated divergence aloft responsible for fueling heavy
showers and thunderstorms over waters E of Maui and the Big Island
today. In fact, particularly strong thunderstorms have periodically
displayed supercell characteristics just E of the Big Island
windward coastal waters through the day today. The jet max also
helped to fuel some locally heavy showers over the islands, but is
departing the area to the E, taking the threat of heavy showers and
thunderstorms (for most areas save the Big Island and adjacent
windward waters this evening) with it. Therefore, the Flash Flood
Watch for Kauai county was cancelled a little while ago.

With the jet max departing, and subtle warming in the mid-levels
tonight and Monday, the atmosphere will stabilize slightly. While
the potential for thunderstorms and widespread heavy rainfall will
diminish, some briefly heavy showers might still occur. Light to
moderate trade winds are expected to deliver otherwise brief
windward showers as the surface high moves to a position NE of the
islands and the corresponding ridge extends N of the islands.
Leeward areas will continue mostly dry.

Another trough aloft is expected to approach Tuesday, move over the
islands Tuesday night and Wednesday, then linger into Thursday,
acting to destabilize the atmosphere once again. A front passing
well N of the islands will force the ridge southward toward the
islands starting Wednesday, leading to a weakening trade wind flow.
Winds are expected to become light and variable Thursday and Friday
as the ridge moves over the islands. Shower chances will be greatest
over windward areas while trades persist, but as winds weaken, the
greatest rain chances will likely shift to interior and upslope
areas. Some showers could be heavy due to the trough aloft, with the
potential for thunderstorms also increasing. Confidence is not
overwhelmingly high, but these elements have been added to the
forecast for Wednesday.

Deep-layer low pressure developing N of the islands Friday into next
weekend introduces a significant amount of uncertainty to the
forecast as an associated convergence band may be near the islands.

&&

.AVIATION...
The airmass over the state remains unstable, but mid-level ridging
is expected to result in an inversion aloft by late evening.
Isolated heavy showers and thunderstorms should end towards evening
but will linger over the Big Island for several hours longer.
Partial clearing expected across Leeward Coasts after midnight.

Meanwhile, moderate to locally breezy trade wind flow will carry
bands of moisture towards the islands. Overnight, clouds and showers
will favor north and east facing slopes and coasts. AIRMET Sierra
for mountain obscuration across Windward Big Island will likely
remain necessary through the remainder of today and possibly
overnight.

AIRMET Tango for low level turbulence has been cancelled due to
insufficient winds and the lack of an inversion at summit level, but
favorable conditions could redevelop Monday afternoon.

&&

.MARINE...
Moderate to locally fresh trade winds will continue across the
island chain through tonight. A Small Craft Advisory (SCA) is in
effect for the typically windy waters around Maui and the Big
Island, but winds are expected to taper off Monday through midweek
as the driving high pressure system to the northeast moves away from
the area. The High Surf Advisory that was in effect for east facing
shores today has been cancelled as local wind waves have lowered.

A small west northwest swell is expected to give surf a boost along
north and west facing shores Tuesday through Thursday. Small south
and south southwest swells will continue to keep some surf in place
through the week along south facing shores, with a slightly larger
south southeast swell expected boost surf heights a bit Wednesday
night through Friday night.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Monday for Maalaea Bay-Pailolo
Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

&&

$$

DISCUSSION...Birchard
AVIATION...Bedal
MARINE...Foster

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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