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Hawaiian Islands Synoptic Discussion and Guidance

847
FXHW60 PHFO 281359
AFDHFO

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

.SYNOPSIS...
Wet weather will continue over the next couple of days as a deep
low aloft lingers north of the area. A front near Kauai will move
southeast across the area and dissipate near Maui on Thursday.
The threat for flooding rainfall has diminished for Kauai County
but continues for Oahu. Localized heavy showers and thunderstorms
could develop from Oahu to the Big Island through Thursday. The
low will remain north of the state Thursday and Friday. West winds
behind the front will carry bands of showers over the islands.
The low will lift away to the northeast during the weekend,
leading to drier weather.

&&

.DISCUSSION...
Water vapor satellite imagery shows a deep low aloft about 550 miles
north of Kauai near 30N 160W with a sharp trough extending southwest
from the low. A front extending northeast from near Kauai is moving
southeast slowly.

The low aloft has been making the atmosphere unstable and helping
to pull very moist air over the islands. Radars show patchy heavy
showers over water around the islands from Oahu to the Big Island. The
low is forecast to remain north of the area through Friday. The front
near Kauai is forecast to move slowly southeast across the islands
over the next couple of days and dissipate near Maui on Thursday.

The potential for spotty heavy showers will continue state-wide
overnight, but the greatest chances for flooding rainfall are
expected to be along the front as it moves across Oahu. A Flash
Flood Watch remains in effect for Oahu through Thursday afternoon.
Satellite loops show layered clouds over the islands have thinned
quite a bit since last evening, indicating a decrease in moisture
aloft. The Winter Weather Advisory for the high summits of the
Big Island has been cancelled.

On Wednesday night and Thursday, the front will likely move to
Maui County, with widespread rainfall diminishing across Kauai and
Oahu. The GFS and ECMWF show drier air aloft moving over the
island chain, which may limit active showers to along and just
ahead of the front. While the front is expected to focus rainfall
over Maui County by Thursday, upper level support for heavy
rainfall will be decreasing.

On Thursday night and Friday, the front will likely stall and
dissipate near Maui as the parent surface low drops down to
within 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 some showers,
especially along leeward slopes. 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.
Westerly flow will carry bands of showers over the island from
time to time, the atmosphere will be drier and more stable, so
rainfall amounts will be modest.

&&

.AVIATION...
A sharp trough aloft just W of the islands will support areas of
layered mid- and high-level clouds that are expected to gradually
slide toward the E and diminish over the next 24 hours. In the
meantime, light icing is expected in the area from Molokai
eastward (mentioned in the AIRMET ZULU bulletin), and is
supported by a couple of overnight PIREPS. While no AIRMETs are
currently in effect, AIRMET Sierra may be required for mountain
obscuration later today.

Meanwhile, a surface front near Kauai will slide slowly SE, and
act as a focus for +SHRA/+TSRA development today, leading to
increased areas of MVFR VIS/CIG, with localized areas of IFR
VIS/CIG possible. Additionally, the large scale lift provided by
the trough aloft will allow +SHRA/+TSRA to pop up nearly anywhere
across the remainder of the area through the day, especially in
the afternoon over the islands.

&&

.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 the long-period, north-northwest swell that peaked on
Sunday lowers. A short-period, moderate northerly swell associated
with low pressure positioned around 600 nm 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. Looking ahead into the
weekend and early next week, guidance depicts another long-
period, northwest swell arriving Saturday night as a developing
hurricane-force low east of Japan lifts northeastward toward the
Aleutians/Date Line today through Thursday. Guidance shows this
feature dropping southeastward from this position Friday into
Saturday, which could send a north-northwest swell to the islands
Sunday night into Monday if it materializes.

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 a combination of background short-period,
southeast and long-period, south and southwest swells moving
through. An upward trend is expected next week, by or around
Wednesday, due to a late season active pattern evolving down
south, featuring a gale southeast of New Zealand continuing
eastward over the next couple of days (940 mb low with seas
forecasted to hit the 35-40 ft range).

&&

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

&&

$$
DISCUSSION...Donaldson
AVIATION...Birchard
MARINE...Gibbs

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



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