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Hawaiian Islands Synoptic Discussion and Guidance

695
FXHW60 PHFO 250659 CCA
AFDHFO

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
859 PM HST Tue Nov 24 2020

.SYNOPSIS...
A moist and unstable airmass will bring the potential for a few
lingering downpours across the state through Wednesday, with a
thunderstorm or two possible over the Big Island Wednesday
afternoon. A more stable trade wind weather pattern will return
Wednesday night and hold through the Thanksgiving Day weekend.
Showers will favor windward and mauka areas during the long
weekend, with a stray shower reaching leeward areas from time to
time.

&&

.DISCUSSION...
Currently at the surface, a trough of low pressure is located
around 150 miles west of Kauai and is moving westward at around 15
mph. Meanwhile further from the islands, a 1034 mb high is
centered around 1600 miles northeast of Honolulu, while a cold
front is located around 1150 northwest of Kauai. The resulting
gradient is producing moderate trades across the island chain this
evening. Infrared satellite imagery shows a mix of high and low
clouds across the state producing partly to mostly cloudy
conditions. Radar imagery shows scattered showers moving into
windward areas, with a few of the more substantial showers making
it into leeward locales from time to time. The greatest
instability has shifted south and west of the islands where the
only thunderstorm activity is seen at the moment. Main short term
concerns focus on the potential for heavy rainfall and
thunderstorms overnight, as well as the Flash Flood headline.

High pressure northeast of the state will slide eastward and
weaken over the next couple days, as a cold front approaches from
the northwest. This will allow the trades to ease slightly, but
they should hold at moderate levels through Thursday. The cold
front will fizzle out north of the islands Thursday night and
Friday, as a new high builds north of the state, and this should
strengthen the trades back up to locally breezy levels. Trades
should then hold at locally breezy levels through the weekend,
before easing into the light to moderate range as a new front
approaches from the northwest.

As for the remaining weather details, the greatest instability and
deepest moisture has exited to the west of the islands, and this
should lead to a decrease in the potential for heavy rainfall and
thunderstorms. A few brief downpours will be possible over the
next 24 hours or so, but the threat for widespread heavy rainfall
and flooding has diminished considerably. Therefore the Flash
Flood Watch for Maui and the Big Island has been cancelled with
the evening package. The threat for thunder has diminished as
well, with linger thunder chances expected to remain confined to
the waters south and west of the state. The one exception is over
the Big Island Wednesday afternoon, where daytime heating and
terrain effects may lead to a thunderstorm or two. Elsewhere, we
should see a return to a more typical trade wind shower regime.

Fairly typical trade wind weather should then prevail Wednesday
night through the Thanksgiving Day weekend and into early next
week. Showers will favor windward and mauka areas with a stray
shower reaching leeward areas from time to time. There is the
potential for some old frontal remnants and some cooler weather to
move into the islands over the weekend, but details differ
considerably between the GFS and ECWMF at the moment. Given the
uncertainty will follow a blended forecast approach until detail
become more clear.

&&

.AVIATION...
Although most of the heavy showers and thunderstorms should
remain out over the adjacent coastal waters overnight into
Wednesday with the upper trough over the region, a few moving
ashore and impacting the islands will remain possible. MVFR CIGS
and VSBYS will be expected in and around any showers that develop
and move through. Although most of this activity will favor
windward and mountain locations, breezy trades will carry some
showers into interior and leeward locations periodically.

AIRMET Sierra remains in effect for north through east sections of
all islands.

AIRMET Tango for low-level mechanical turbulence remains in
effect over and downwind of terrain for the entire area.

&&

.MARINE...
A slight weakening of northeasterly high pressure will allow for
the subtle lowering of trades through mid week. The Small Craft
Advisory (SCA) has been scaled back to the typical windier waters
and channels surrounding Maui County and those waters south of
the Big Island through Wednesday afternoon. After a brief lull in
winds late Wednesday through early Thursday, trades are expected
to restrengthen and back a bit northeast from Thanksgiving into
the holiday weekend in response to the passage of a late week
decaying front.

Northwest swells will continue to subside through Wednesday. Low,
moderate period northwest swells will arrive Wednesday and
Thursday. Slightly larger northwest swells will build Friday and
peak out late Saturday into Sunday. As these weekend swells
subside early next week, longer period northwesterly swells
emanating from a strong weekend North Pacific gale will begin
arriving late Sunday. North and west-facing shore surf will again
lift to double digit heights by early next week but is forecast to
remain below advisory levels.

Weakening easterly trades will have eastern-facing shore surf
declining and then leveling out the next few days. Other than a
small bit of wrap from the declining east wind wave swell, surf
along south-facing shores will remain small through the Holiday
weekend.

&&

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

&&

$$

DISCUSSION...Jelsema
AVIATION...Gibbs
MARINE...Blood

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