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Hawaiian Islands Synoptic Discussion and Guidance

929
FXHW60 PHFO 100632
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
832 PM HST Fri Apr 9 2021

.SYNOPSIS...
Light winds will develop statewide tonight and Saturday, with
limited rainfall anticipated. Moderate northeast trade winds will
redevelop over Kauai and Oahu on Sunday, and gradually strengthen
statewide early next week. A period of unsettled weather is possible
Saturday night through Monday, as a potent low aloft passes over the
area, bringing the potential for thunderstorms, and a few locally
heavy showers. A more settled weather pattern is expected thereafter.

&&

.DISCUSSION...
A high to the distant N and a persistent low to the NE of the
islands is maintaining light to locally moderate NE trade winds over
the area this evening, with satellite and radar indicating a few
showers embedded in the scattered low clouds moving in on the
trades. These are primarily affecting windward areas, while
scattered to locally broken high clouds are thickest over the Big
Island. Dew points in the upper 50s to lower 60s F are making it
feel a little cool. Little overall change is expected in the short
term, with just a few windward showers through the night.

The low-level pressure gradient near and NE of the islands will
become quite weak on Saturday as the low to the distant NE tracks W
and (partially) moves between the islands and the trade-wind-
supporting high. In this regime, light and variable winds will
prevail over Maui and the Big Island while light NE winds are
expected over Kauai and Oahu. Either way, winds will likely be
sufficiently light to allow afternoon sea breezes to drive some
cloud formation over leeward and interior portions of the islands.
The island atmosphere will likely remain mostly dry and still mostly
stable, thus shower coverage and intensity should be limited.

A potent shot of mid-level cold air is expected to accompany a
shortwave trough aloft that will sharpen as it rotates around the
parent low aloft to the NE, moving over the islands Saturday night
through Monday. This will significantly destabilize the island
atmosphere as mid-level temperatures plummet to -16C, but PWAT will
remain on the lower side (near 1.2"). Locally derived guidance
depicts thunderstorm chances steadily increasing Saturday night,
peaking Sunday, then slowly diminishing and shifting E on Monday.

Mid-level lapse rates will be sufficient for thunderstorm
development as early as Saturday night, but land breezes may keep
them offshore - mainly to the NE. On Sunday, as temperatures aloft
over the islands reach their minima, thunderstorms will be possible
statewide, shifting to the E end of the state Sunday night and
Monday as the trough shifts E. Periods of wintry weather will be
possible over the Big Island Summits and upper slopes with any
convection that develops, and the freezing level may become low
enough to support a brief period of wintry weather over the summit
of Haleakala.

Models indicate surface low pressure will linger NE of the islands
into the middle of next week, leading to moderate NE winds over
Kauai and Oahu, and lighter NE winds over Maui County and the Big
Island, with mostly dry and stable conditions in place. A weak cold
front may move into the area from the N around Thursday.

&&

.AVIATION...
A moderate northeast trade wind flow tonight will ease down a notch
Saturday. The weakness of this flow and daytime heating will allow
for local daytime sea breeze over the interior and lee areas of the
smaller islands, resulting in some afternoon clouds and possibly a
few showers. Leeward Big Island will face a similar pattern Saturday
afternoon as well.

For the overnight hours, VFR will be the prevailing flying
condition, although there will be some brief MVFR ceiling and vis
from showers coming in with the northeast flow. These showers will
favor the windward and mountain areas. Windward Kauai may encounter
a little more clouds and showers as to the other islands. The Kona
coast of the Big Island may see a late night shower or two in the
coming hours, but be gone by sunrise.

No AIRMETS, and none are expected through Saturday morning.

&&

.MARINE...
Moderate to fresh northeast trade winds are expected to trend down
over the weekend as the ridge weakens to the north. Winds should
become light enough for a land and sea breeze regime to setup for
most areas. The background flow will shift out of a more northerly
direction (stronger Kauai and Oahu waters) due to a persistent
trough/low lurking northeast of the state. In addition to light
winds, a few heavy showers and thunderstorms will become a
possibility Saturday night through Sunday as an upper disturbance
drops south over the islands - best chance windward waters and
eastern half of the state. Northeast trade winds will gradually
return early next week, potentially reaching the fresh to locally
strong category by midweek.

Surf along south facing shores will gradually rise over the weekend
as a long-period, south-southwest swell arrives from recent activity
within Hawaii's swell window near New Zealand. This source should be
fully filled in by Monday, then hold into midweek before lowering.
Heights should peak just below the advisory threshold early next
week (8 ft faces).

Surf along north and west facing shores will remain small through
the weekend, then trend up late Sunday through early next week as a
small, west-northwest swell arrives from a gale that developed near
the Kurils earlier this week. Although confidence remains low, a
moderate, short-period north-northeast swell is depicted late next
week due a strong gradient setting up once again to the north
between strong high pressure near the Date Line and low pressure to
its east.

Surf along east facing shores will lower over the weekend as the
persistent north-northeast swell fades and the local winds diminish.
Surf will remain small early next week, then trend up by the second
half as almost a repeat pattern sets up over the northern Pacific
and drives another moderate, north-northeast swell through the area.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$


DISCUSSION...Birchard
AVIATION...Lau
MARINE...Gibbs

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



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