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Hawaiian Islands Synoptic Discussion and Guidance

465
FXHW60 PHFO 221945
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
945 AM HST Sun Oct 22 2017

.SYNOPSIS...
Afternoon clouds and showers are expected to become focused over
interior and leeward areas today as sea breezes develop and push
inland. Some heavier showers over the eastern half of the Big Island
are expected later today and tonight as moisture increases and
fills from the east. Monday through Wednesday, southerly winds
ahead of a front will bring deep tropical moisture with the
potential for heavy rain and flooding. Cool and dry air will fill
in behind the front through the second half of the week.

&&

.DISCUSSION...
A front approaching from the northwest has pushed a surface ridge
over the islands today allowing for light and variable winds with
widespread afternoon sea breezes and interior clouds and showers
expected. Near and over the Big Island, southeasterly winds will
bring increasing humidity and moisture from the tropics allowing
for heavier showers this afternoon and continuing tonight, mainly
over the Kau, Puna, and Hilo districts.

As the front closes in on 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 likely produce 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
night.

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.

&&

.AVIATION...
Weak flow at the surface and aloft will permit afternoon sea
breezes to prevail over most areas today. Convective development
over island interiors is expected throughout the afternoon and
evening hours. Airmass instability is expected to increase
throughout the day today as the inversion aloft begins to break
down. Cloud tops this morning are mostly below 8000 ft but could
exceed 12000 ft by tonight. Shower activity will likely become
heavy, especially over the southern half of the island chain where
the greatest air mass instability is anticipated.

Isolated MVFR conditions possible in showers today, otherwise VFR
conditions are expected prevail through at least the afternoon
hours. More frequent MVFR or even IFR conditions could develop
this evening as shower activity intensifies. An AIRMET for
mountain obscuration could become necessary, especially across
windward Big Island.

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

&&

.MARINE...
Winds have weakened and shifted to southeasterlies as a ridge
moves near the state in response to a cold front approaching from
the northwest. The winds should continue to veer through tonight
and become southerlies or southwesterlies by Monday. The front is
expected to reach Kauai waters Monday evening and will move down
the island chain, reaching Big Island waters by Tuesday evening.
Heavy showers and thunderstorms are possible 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 week.

The long period forerunners of a new northwest swell have reached
Oahu. Surf is expected to build today and get close to the
advisory threshold along north facing shores. This swell will peak
tonight and diminish Monday night. A bigger northwest swell
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.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
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.


&&

$$

Morrison...Discussion
Kodama...Marine
Bedal...Aviation

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