Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 230220

Area Forecast Discussion
National Weather Service Honolulu HI
420 PM HST Sun Oct 22 2017

Tonight through Tuesday, southerly winds ahead of a front will
bring deep tropical moisture with the potential for heavy rain and
flooding, mainly from Oahu to the Big Island. The front will reach
Kauai Monday with cool and dry air following as it moves down the
island chain. The front is expected to pass east of the Big
Island Wednesday night with dry northeast flow forecast for the
second half of the week.



A front approaching from the northwest has pushed a surface ridge
over the islands allowing for light and variable winds tonight.
Near and over the Big Island, southeasterly winds, increasing
humidity, and afternoon convection has set off some persistent
showers and isolated thunderstorms along the lower slopes from
Hilo through the Puna and Kau districts. Heavier showers will
continue over the eastern half of the Big Island tonight and start
to fill into Oahu and Maui county late tonight and early Monday

As the front approaches Kauai tonight and Monday, a moisture
convergence band is expected to form over Oahu, Maui County, and
the Big Island. The near surface convergence, deep tropical
moisture, and island terrain will produce areas of heavy rain,
isolated thunderstorms, and the potential of flash flooding over
Oahu, Maui County, and the Big Island Monday through Tuesday. A
flash flood watch is in effect starting Monday morning and
continues through Tuesday afternoon. Sufficient deep layer shear,
as well as turning of the winds with height in the lower levels,
suggest that some of those isolated thunderstorms could be strong
or severe along and ahead of the front. Strong winds across the
Big Island summits are expected Monday night and a high wind watch
is in effect. Some wintry weather is also possible over the Big
Island summits with temperatures approaching freezing monday

Kona winds (southerly) will increase Monday with some potential
for gusty winds accelerating down the terrain into windward areas
of Kauai and Oahu. The front will push over Kauai Monday night
bringing moderate rain with the heaviest falling over the north
and west facing slopes of the Garden Isle. The front will push the
aforementioned convergence band slowly eastward, likely pushing
east of Oahu Tuesday, and then east of the Big Island sometime
Wednesday. North winds behind the front will bring cool and dry
conditions to Kauai Tuesday, and Oahu Tuesday night; while the
convergence band keeps things moist and humid over Maui County and
the Big Island.

High pressure nosing in from the northwest is expected to push the
convergence band east of the Big Island Wednesday with dry north
to northeast flow expected for the second half of the week.


Convective development over island interiors will intensify this
evening as the inversion aloft begins to break down. Shower
activity will likely become heavy at times, especially over the
southern half of the island chain where the greatest air mass
instability is anticipated.

Isolated MVFR conditions possible in showers, otherwise VFR
conditions should prevail through the evening hours. More
frequent MVFR or even IFR conditions could develop late tonight,
especially for eastern slopes and coasts of the Big Island. An
AIRMET for mountain obscuration may become necessary.

A cold front approaching out of the northwest will further
destabilize the airmass and encourage shower activity statewide
on Monday. Reduced ceilings and visibilities will likely spread to
the remaining islands, possibly as early as Monday morning.


The morning ASCAT pass showed light to moderate southeasterlies
over most of the coastal waters and moderate trades south of the
Big Island. The winds should continue to veer through tonight
and become southerlies or southwesterlies by Monday in response to
a front moving toward the island chain from the northwest. The
front is expected to reach Kauai waters by Monday evening and
will move down the island chain, reaching Big Island waters on
Tuesday. Heavy showers and thunderstorms are possible mainly to
the east of and along the front. Some of the thunderstorms may be
strong enough to warrant Special Marine Warnings, especially over
Maui County and Big Island waters. A brief period of fresh to
locally strong northerly winds is forecast to occur after the
front passes but should become light to moderate northerlies by
Wednesday. Winds are forecast to become light to moderate
northeasterlies Thursday and persist through the remainder of the

Buoy reports and observations show that the expected northwest
swell is filling in across the area. Surf is expected to peak
tonight close to the advisory threshold along north facing
shores then diminish Monday. 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. An even larger northwest swell is
forecast to arrive Friday. That swell could produce warning level
surf along north and west facing shores.

A low east of New Zealand generated a swell which will produce surf
near the advisory level along south facing shores later today and
Monday. See the Collaborative Surf Discussion for details.


Flash Flood Watch from Monday morning through Tuesday afternoon
for Oahu-Molokai-Lanai-Kahoolawe-Maui-Big Island.

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



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