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Hawaiian Islands Synoptic Discussion and Guidance

764
FXHW60 PHFO 281958
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
958 AM HST Wed Oct 28 2020

.SYNOPSIS...
Unsettled weather, including spotty heavy showers and
thunderstorms, will continue through tomorrow, mainly along and
ahead of a weak front that pass over much of the state. The front
will move from Kauai this morning, reach Oahu this afternoon,
then stall and gradually dissipate over Maui County tonight and
Thursday. West winds will bring less humid conditions and will
send shallow shower bands over the islands Friday and Saturday,
followed by a brief return of light trade winds early next week.

&&

.DISCUSSION...
Showers remain active along a front located near Kauai, and
although clouds and showers are largely absent in the south to
southwest flow across the eastern half of the island chain, the
atmosphere remains unstable. A deep slow-moving cutoff low sits
roughly 700 miles north of Kauai this morning, and an associated
front runs across waters just east of Kauai. The upper level
trough associated with this complex system is centered just west
of Kauai, and a jet stream aloft east of the trough has weakened,
causing the thick high clouds that were present yesterday to
dissipate overnight.

The front will move slowly eastward today, and while Oahu is
expected to be the focal area for rainfall this afternoon, spotty
heavy showers and thunderstorms will flare up over most islands,
since the atmosphere remains moist and unstable. However, showers
will likely diminish on Kauai later this afternoon as a drier
post-frontal airmass moves in. A Flash Flood Watch remains in
effect on Oahu through Thursday. The flood threat should be
decreasing on Oahu tonight as the dissipating front drifts over
Maui County.

The increasingly shallow and diffuse front will stall and fall
apart near Maui or northern Big Island on Thursday and Friday as
the parent surface low drops down to around 350 miles north of the
islands. Showers will likely remain concentrated along the
weakening front, but moderate to potentially breezy westerly winds
will also deliver a few showers, especially along leeward slopes.
On Friday, these winds could become gusty over some windward
communities due to downsloping off of terrain. In addition, there
will be a noticeable drop in humidity behind the front.

During the weekend, the low will lift off to the northeast, but
the islands will continue to see some showers dropping down from
the north from time to time as the winds switch from westerly to
northerly. The latest runs of the GFS and ECMWF hint at a narrow
upper level trough redeveloping overhead on Sunday, which could
trigger a few heavy showers.

Heading into early next week, the GFS and ECMWF both show a
surface ridge passing north of the state, suggesting a brief
return of light trade winds.

&&

.AVIATION...
NNW wind at Barking Sands indicates the surface front has reached
Kauai. Instability will wane through late morning and early
afternoon as the potential for heavy showers and thunderstorms
rapidly diminishes there. Elsewhere, daytime heating will have the
potential to capitalize on the resident prefrontal moisture axis
from Oahu through Maui today, especially as sea breezes enhance
inland convergence. Locally heavy rain/isolated thunder capable of
brief MVFR/local IFR will be possible as a result. There is only
a low probability that AIRMET Sierra for tempo mountain
obscuration will be required during this time. The front and
attendant band of convection presently east of Kauai will spread
across Oahu late this evening reaching Maui Thursday morning.
AIRMET Sierra may therefore be needed overnight.

&&

.MARINE...
A cold front near Kauai will continue to advance down the island
chain, likely to Oahu later today, then to Maui County tonight
into Thursday as it begins to stall and weaken. Light to moderate
south to southwest winds will continue over Maui County and the
Big Island waters through Thursday. For the Kauai and Oahu
waters, light and variable winds will shift out of the northwest,
potentially increasing into the moderate range, as the front
moves through. Expect this general pattern to continue into the
weekend as a second (much weaker) front approaches and moves into
the area. In addition to the winds, heavy showers and a few
thunderstorms are expected along and ahead of the front into
Thursday.

Surf along north and west facing shores will continue to ease
today as a long-period, north-northwest swell lowers. A short-
period, moderate northerly swell associated with low pressure
north-northwest of the islands is expected arrive tonight into
Thursday. This source will linger into the weekend with the
highest surf expected along northern exposures of Kauai and Oahu.
Another long-period, northwest swell is expected Saturday night,
then shifting from the north-northwest as the source low shifts
northeast toward the Aleutians.

Surf along east facing shores will steadily trend down as the
small, northeast swell eases. Expect this trend to hold into the
weekend.

Surf along south facing shores will remain small through the
weekend with mainly background swells. An upward trend is expected
around next Wednesday, due to a late season active pattern
evolving near New Zealand.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Thursday afternoon for Oahu.

&&

$$

DISCUSSION...Wroe
AVIATION...JVC
MARINE...TS

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