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Hawaiian Islands Synoptic Discussion and Guidance

437
FXHW60 PHFO 280148
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
348 PM HST Tue Oct 27 2020

.SYNOPSIS...
Unsettled wet weather will prevail over the next couple of days
as deep low pressure to the north pushes a front across portions
of the island chain. While heavy showers and thunderstorms could
develop anywhere, the greatest threat for heavy flooding rainfall
will be over the western half of the state through Wednesday. The
low will remain north of the state Thursday and Friday, likely
sending bands of showers across the state. The low will lift away
to the northeast during the weekend, leading to decreased shower
activity.

&&

.DISCUSSION...
Mostly cloudy skies with a few heavy showers and thunderstorms are
noted this afternoon. A slow-moving diffuse front sits just
northwest of Kauai, while a sharp upper level trough remains
centered just west of the islands. A jet stream aloft running
along the eastern flank of the upper trough continues to produce
instability and broad lift that is generating thick high clouds
over most of the state. At the lower levels, southeasterly winds
around the Big Island quickly veer southerly near Maui and become
variable across the western half of the state. This flow has been
producing showers across the eastern slopes of the Big Island
today. Otherwise, heavy showers have been limited to Maui and
Molokai, where surface convergence as greatest due to the
approaching front.

The potential for spotty heavy showers will continue state-wide
overnight, especially near Maui County this evening, but the
greatest chances for flooding rainfall are expected to be along
the front, which will push down to near or over Kauai. The GFS
model shows continued mid level moisture being pulled over the
eastern end of the state tonight, and a Winter Weather Advisory
has been issued for the high summits of the Big Island, where a
mix of rain and snow is expected above 13,000 ft.

On Wednesday, the GFS and ECMWF depict the front moving slowly
across Kauai and Oahu, while a persistent jet stream aloft
maintains unstable conditions. The highest chances for heavy
rainfall will be along the front, but the remainder of the state
will continue to have a threat for spotty heavy showers and
thunderstorms.

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. The Flash Flood Watch for Kauai and Oahu
currently runs through Thursday afternoon, though the flood threat
could be diminishing on these islands. While the front is expected
to focus rainfall over Maui County by Thursday, upper level
support for heavy rainfall will be decreasing. Thus, we will
leave the watch as is due to uncertainty.

On Thursday night and Friday, the front will likely weaken and
stall over Maui County 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, but
the islands will remain under southwest to northwest winds with
bands of showers likely dropping down from the north from time to
time. Significant rainfall is unlikely.

&&

.AVIATION...
Prevailing VFR continues to be the rule this afternoon. Light
stratiform radar returns have been occasionally interspersed with
a low coverage of deeper convection, mainly over higher terrain.
In the near term, occasional MVFR ceilings will remain possible
over the Big Island as deeper moisture grazes the island but not
yet sufficient in coverage or duration to warrant issuance of
AIRMET Sierra.

Mid-level southwesterlies will consolidate in advance of the front
tonight through early wednesday morning bringing deeper moisture
and stronger ascent for Kauai through Molokai. Convection
bringing MVFR/localized IFR and isolated thunder will become
increasingly common during this time.

No AIRMETs currently in effect. However, AIRMET Sierra will likely
be needed for tempo mountain obscuration late tonight or early
Wednesday morning.

&&

.MARINE...
A ridge near Maui will bring southeast flow across the area with
the strongest winds near Maui and the Big Island. A front
currently northwest of Kauai will move into the coastal waters
Wednesday and push east through Thursday. West-northwest winds
will follow behind the front and southerly winds ahead of the
front. An upper level disturbance linked with the front will keep
heavy rain and thunderstorms in the area through Thursday.

The current northwest swell will diminish through Wednesday. A
short period northwest swell will bump surf up along exposed north
and west facing shores Thursday and hold through Friday. A larger
long-period northwest swell is expected to arrive late Saturday
and may produce advisory level surf along north and west facing
shores early next week.

A small northeast swell is on the downtrend tonight. A series of
small, long period south swells are anticipated through the week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Thursday afternoon for Oahu, Kauai, and
Niihau.

Winter Weather Advisory until 6 AM HST Wednesday for Big Island
Summits.

&&

$$

DISCUSSION...Wroe
AVIATION...JVC
MARINE...Almanza/Burke

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