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Hawaiian Islands Synoptic Discussion and Guidance

550
FXHW60 PHFO 231349
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
349 AM HST Mon Oct 23 2017

.SYNOPSIS...
Heavy showers and thunderstorms associated with a strong cold front
and upper trough approaching could lead to flash flooding,
especially from Oahu to the Big Island today through Tuesday. The
front will reach Kauai tonight, then continue down the island chain
and shift east of the Big Island Wednesday night into Thursday.
Cooler and drier air with breezy northerly winds will follow the
front through the second half of the week.

&&

.DISCUSSION...
Animated water vapor imagery showed an amplifying upper pattern
across the northern Pacific that was characterized with a building
ridge east of Japan and a shortwave trough digging southeastward
toward the Hawaiian Islands. The latest surface analysis showed a
cold front associated with the upper trough extending southwest to
just south of Midway from a developing gale-force low centered
around 1000 miles north-northwest of the state. A ridge axis was
analyzed extending west-southwest across the islands from 1031 mb
high pressure centered over the far eastern Pacific. The early
morning radar loop showed patches of heavy showers developing over
the waters south of Oahu and Kauai lifting northward. Some of these
showers are moving ashore along the leeward coasts of Oahu
providing some much needed rainfall this morning.

Rainfall coverage will steadily increase through the day today
across the island chain as deep tropical moisture pools northward
ahead of the approaching cold front and upper trough. The latest
satellite-derived precipitable water (PW) imagery showed a large
area of around 2" values filling in from the south. High-resolution
model reflectivities are initializing well with the current radar
pattern and depict heavy showers continuing to develop and expanding
in coverage along a pre-frontal convergence boundary near Oahu
today, then shifting eastward across the eastern half of the state
tonight through Tuesday as the front begins to move down the island
chain.

Impacts will include: the potential for flash flooding, strong
thunderstorms (an isolated severe storm can't be ruled out), gusty
downslope winds mixing down over windward areas (Oahu and Kauai),
and the potential for a wintry mix and gusty southwest winds over
the summits tonight into Tuesday.

A flash flood watch remains in effect for Oahu, Maui County and the
Big Island through Tuesday. A high wind watch remains in place for
the summits on the Big Island for late today through Tuesday as
southwest winds ahead of the upper trough strengthen and become
hazardous (gusts to 65 mph).

Drier and cooler air with north-northeast flow will fill in across
the islands through the second half of the week as the band of deep
tropical moisture associated with the front exits to the east of the
Big Island. Some lingering moisture can't be ruled out over the Big
Island through this time, if the boundary were to stall. Dewpoints
are forecast to reach the 50s over Oahu and Kauai Tuesday through
midweek in the wake of the front, then steadily climb back into the
60s through the second half of the week. Winds are forecast to
weaken once again over the upcoming weekend as another, slightly
weaker, cold front approaches the state.

&&

.AVIATION...
While VFR conditions and light winds persist across most areas
this morning, low clouds and showers are beginning to increase
across the area. MVFR and possible IFR conditions may occur later
this morning as heavier showers and possible thunderstorms gradually
develop. If this happens, AIRMET SIERRA will be issued for
mountain obscuration. A cold front approaching from the northwest,
along with associated prefrontal convergence bands, may cause
ceilings and visibilities to drop at the terminals today through
Tuesday.

&&

.MARINE...
The background flow will be out of the south and southwest today
as we front approaches from the northwest. The front is expected
to reach the Kauai waters by this evening, and then continue down
the island chain, reaching the Big Island waters Tuesday. Heavy
showers and thunderstorms will be possible along and east of the
front. Some of the thunderstorms may be strong enough to warrant
Special Marine Warnings.

Behind the front, fresh to locally strong northerly winds are
expected, but should become light to moderate northerlies by
Wednesday, and persist through the remainder of the week.

The current northwest swell is peaking along north facing shores
just below advisory levels, and is expected to decline today. A
bigger northwest swell on Tuesday and Wednesday will likely
produce surf well above the advisory threshold for north and west
facing shores. The swell will also boost combined seas above the
10 foot level, so a Small Craft Advisory will likely be needed for
exposed waters. An even larger northwest swell is forecast to
arrive Friday. That swell could produce warning level surf along
north and west facing shores. No significant change to the
forecast with the latest model run.

A low east of New Zealand generated a swell which will produce surf
near the advisory level along south facing shores through Monday.

See the latest Oahu Surf Discussion (SRDHFO) for additional
details on surf and swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Tuesday afternoon for Oahu-Molokai-
Lanai-Kahoolawe-Maui-Big Island.

High Wind Watch from this evening through Tuesday afternoon for
Big Island Summits.

&&

$$

DISCUSSION...Gibbs
AVIATION...Powell
MARINE...M Ballard

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