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Hawaiian Islands Synoptic Discussion and Guidance

389
FXHW60 PHFO 292156 CCA
AFDHFO

Area Forecast Discussion...corrected
National Weather Service Honolulu HI
1000 AM HST Sun Mar 29 2020

CORRECTED W/W/A SECTION

.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 on 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 high pressure cell centered far N of the islands. An
E-W oriented trough aloft is just N of the islands, and overnight
soundings show that the island atmosphere is quite unstable as a
result. Latest radar and satellite images show heavy rain cores
embedded within the clouds moving toward the islands with the trade
wind flow. As these clouds grow in response to the instability,
westerly winds above 15 thousand feet can be seen blowing their tops
back toward the E. Although the greatest conglomeration of moisture
is currently over waters E of Maui and the Big Island, and the
chance of a briefly heavy downpour is about the same statewide
today, a Flash Flood Watch (FFA) for Kauai county remains posted due
to the saturated soils, especially in windward Kauai. Most showers
should move steadily W with the trades, thereby reducing the threat
of significant flooding, except that the forward motion of some of
the deeper convection will slow as it interacts with the westerly
flow aloft.

The forecast for today was updated to increase expected rain and
thunderstorm chances over the Big Island this afternoon, and to
include winter weather over the Big Island summits as deep
convection is expected to develop over the slopes. Otherwise, expect
some briefly heavy downpours, mainly over windward areas, but if
trades succumb to leeward sea breezes this afternoon on the smaller
islands, a leeward shower can't be ruled out.

Warming in the mid-levels tonight and Monday will allow the
atmosphere to stabilize somewhat, with diminishing chances for heavy
showers and thunderstorms. Light to moderate trade winds will
deliver brief windward showers as the surface high moves to a
position NE of the islands and the corresponding ridge extends N of
the islands. 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. A front passing well
N of the islands will force the ridge southward over the islands
starting Wednesday, with winds becoming light and variable Thursday
and Friday. 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 nearby trough.

Deep-layer low pressure developing N of the islands next weekend
complicates the forecast and introduces a large amount of
uncertainty as an associated convergence band may be near the
islands.

&&

.AVIATION...
The airmass over the state remains unstable. Expect a slight chance
of thunderstorms or heavy showers for all islands beginning this
afternoon. Across Leeward and Interior Big Island, afternoon sea
breezes will combine with abundant moisture and cold temperatures
aloft to create widespread shower activity with possible embedded
thunderstorms.

Otherwise, strong surface high pressure far to the north and
northeast of the state will continue to promote moderate to locally
strong trade winds across the islands. Multiple bands of showers
will favor east and northeast facing slopes and coast. Brief
spillover showers can be expected across the leeward slopes of the
smaller islands, especially in the morning. AIRMET Sierra for
mountain obscuration across Windward slopes will likely be lifted
around midday in response to daytime heating.

Later today, mid level ridging is expected to build over the state
resulting in an inversion aloft around 8kft by late evening.
A few locally heavy showers will still be possible over Kauai
overnight, but the general trend will be towards increased air mass
stability statewide.

&&

.MARINE...
Moderate to 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. Trade winds
will taper off Monday as the driving high pressure system to the
north weakens. A High Surf Advisory for east facing shores remains
in effect through today due to local and upstream trade winds, with
a gradual trend down in surf heights expected this week as winds
weaken.

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...
High Surf Advisory until 6 PM HST this evening for East facing
shores of Kauai Oahu Molokai Maui and the Big Island.

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

Flash Flood Watch until 6 PM HST this evening Niihau-Kauai.

&&

$$

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

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



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