Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 240211

Area Forecast Discussion
National Weather Service Honolulu HI
411 PM HST Mon Oct 23 2017

Heavy showers and isolated thunderstorms associated with a strong
cold front and upper trough approaching could lead to flash
flooding, especially from Oahu to the Big Island 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 northerly winds will follow
the front through the second half of the week.


Water vapor loop across the Central Pacific shows an amplifying
upper pattern across the northern Pacific with a shortwave trough
digging southeastward toward the Hawaiian Islands. A cold front
associated with the upper trough is located about 130 miles
northwest of Kauai this afternoon advancing towards the southeast
at around 25 mph. Meanwhile, an area of pre-frontal showers and
thunderstorms are lifting up from the southwest over Kauai and
Oahu, and moving onto Maui County this afternoon. Satellite
derived precipitable water (PW) imagery showed a large area of
around 2 inches filling in over the state from the south.
Soundings from this afternoon show unstable conditions across the
western side of the state with deep vertical moisture and a CAPE
value of 1334 J/kg at Lihue. While persistent rain has impacted
Kauai and Oahu much of the day today, the thunderstorms have been
triggering 40 to 60 miles south of both islands through much of
the day.

The attention will turn towards Maui County this evening and to
the Big Island later tonight. A Flash Flood Watch remains in
effect through Tuesday for Oahu, Maui County and the Big Island
due to the threat of flooding from heavy rain. There will also be
the potential for isolated thunderstorms with gusty winds a times
tonight. As the upper trough swings by the state tonight and
Tuesday, strong winds aloft will impact the Big Island summits. A
High Wind Warning has been issued for the expected winds at the
summits, but no other areas around the state should be impacted.
There is also a chance for light snowfall on the Big Island
summits tonight through Tuesday.

The cold front will reach Kauai later tonight and will provide a
brief uptick in showers that will spread southeast down the island
chain on Tuesday. Drier and cooler air will fill in west to
behind the front Tuesday into Wednesday. Some lingering moisture
can`t be ruled out over the Big Island into Wednesday if the
boundary stalls. 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


Waves of MVFR showers with embedded IFR visibilities are
affecting Kauai and Oahu, located within a larger prefrontal
convergence band. This band will shift eastward this evening,
beginning to affect Maui County. Isolated thunderstorms are
embedded within the convergence band as well and will begin to
affect Oahu late this afternoon. Elsewhere, thick mid-level
clouds extending across Maui County have helped to inhibit
convection. Stronger heating over the Big Island has led to a
couple thunderstorms across the interior. The cold front will
approach Kauai this evening, bringing another period of potential
thunderstorms and localized IFR conditions, followed by more
prevailing VFR conditions behind it.

Most of the prefrontal rain has been steady stratiform, and we
have received a few light icing reports. Have updated the AIRMET
bulletin to include a mention of the icing in order to provide
some timing information (i.e., diminishing near Kauai after 04z
but continuing into the evening elsewhere). We have also received
several moderate turbulence reports. Some may be related to
localized convective updrafts in the embedded showers, but many of
the mid-level ones appear to be associated with the larger-scale
debris clouds. Have also issued an AIRMET for moderate turbulence
in the 080/200 layer.


The background flow will be out of the south and southwest ahead
of a front crossing the island chain through Tuesday. The front
is expected to reach the Kauai waters by this evening, then
continue down the island chain, reaching the Big Island waters
Tuesday, with winds shifting around to the north behind the front.
Heavy showers and thunderstorms are likely along and east of the
front and some 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 by
Wednesday, and persist through the remainder of the week.

The current northwest swell is peaking along north facing shores
right around advisory levels. A High Surf Advisory is currently in
effect for north facing shores. Even though this swell and
associated surf will decline slightly tonight, a bigger northwest
swell on Tuesday and Wednesday will produce surf above the
advisory threshold again for north facing shores. Therefore, the
current High Surf advisory will run right through Wednesday
afternoon. The Tuesday/Wednesday swell will also boost combined
seas above the 10-foot level, beginning as early as tonight
around Kauai, so a Small Craft Advisory (SCA) has been issued for
exposed waters. The SCA will go into effect tonight for Kauai
waters and tomorrow morning for the remaining northern marine
zones, and run through Wednesday for all affected areas. An even
larger northwest swell is forecast to arrive Friday and could
produce warning level surf along north and west facing shores.

A low east of New Zealand generated a swell which produced surf
near advisory level along south facing shores yesterday and today.
this swell will decrease over the next few days.

The Oahu Surf Discussion (SRDHFO) has been updated today with
additional details on surf and swell.


Flash Flood Watch through Tuesday afternoon for Oahu, Molokai,
Lanai, Kahoolawe, Maui, Big Island.

High Surf Advisory until 6 PM HST Wednesday for north facing
shores of Niihau, Kauai, Oahu, Molokai and Maui.

High Wind Warning until 6 PM HST Tuesday for Big Island Summits.

Small Craft Advisory until 6 PM HST Wednesday for Kauai
Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters.

Small Craft Advisory from 6 AM Tuesday to 6 PM HST Wednesday for
Kauai Channel-Oahu Windward Waters-Kaiwi Channel-Maui County
Windward Waters-Big Island Windward Waters.



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