Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 171409
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
409 AM HST Sun Dec 17 2017

.SYNOPSIS...
A weak pressure pattern will remain in place today through
Monday, with daytime sea breezes and overnight land breezes
expected in most areas. As a result, shower activity is expected
to be most prevalent during the afternoon and early evening hours,
particularly over the eastern and western sections of the state
where moisture from remnant boundaries will reside. An upper
level disturbance and a cold front pushing southeastward across
the island chain is expected to result in an increase in showers
Monday night through Wednesday night, particularly over Maui
County and the Big Island, where periods of locally heavy rain
and thunderstorms will be possible. Cooler and drier conditions
are then expected to overspread the state Thursday through Friday,
with yet another cold front expected to move down the island
chain Friday night and Saturday.

&&

.DISCUSSION...
Currently at the surface, a weak area of low pressure is located
around 150 miles north-northeast of Lihue, with a trough extending
south-southwestward over Kauai. Meanwhile, another weak trough of
low pressure is oriented roughly north to south just to the east
of the Big Island. Aloft, an upper level low is evident in water
vapor imagery around 225 miles north-northeast of Oahu. Infrared
satellite imagery shows a couple bands of enhanced cloud cover
across the island chain early this morning. One band is over Oahu
and the Kauai Channel, with another over and just to the east of
the windward side of the Big Island. Radar imagery shows
scattered showers over Oahu and the Kauai Channel, as well as just
east of the Big Island in association with the two enhanced bands
of moisture. Elsewhere, isolated showers are in place over the
water, with mainly rain free conditions over land. Main short
term concern revolves around rain chances.

Today,
A weak pressure pattern will remain in place across the island
chain, allowing for daytime seabreezes statewide. The band of
moisture over Oahu and the Kauai Channel this morning is expected
to shift westward and over Kauai by late in the day. As a result,
scattered showers are expected to continue over Oahu, with a
transition to interior areas during the afternoon hours. Across
Kauai, expect shower activity to pick up by late morning or early
afternoon, particularly over interior sections of the island. A
drier airmass is expected over Maui County so only a few isolated
showers are expected here during the afternoon hours. Meanwhile
over the Big Island, the proximity to the band of deeper moisture
will keep scattered showers in the forecast, mainly during the
afternoon hours.

Winds at summit level on the Big Island have increased to warning
levels overnight. The winds should begin to ease later this
morning, so will keep the High Wind Warning in tact as is, with an
expiration at noon today. The warning will likely need to be
transitioned over to a Wind Advisory later this morning, with
advisory level winds possibly lingering into the evening hours.

Tonight and Monday,
A weak pressure pattern will remain in place across the island
chain, keeping winds dominated by overnight land breezes and
daytime sea breezes. The band of moisture over Kauai will become
less prominent through the period, with some lingering shower
activity expected, particularly during the afternoon hours on
Monday. Relatively dry weather is expected to affect Oahu, with
perhaps a few afternoon showers developing on Monday. The band of
deeper moisture over the Big Island will begin to shift westward
and over Maui County Monday afternoon. As a result, showers will
continue to affect the Big Island mainly in the evening on Sunday
as well as Monday afternoon. Over Maui County, expect mainly dry
weather Sunday night and Monday morning, with scattered showers
developing Monday afternoon as deep moisture increases.

Monday night through Wednesday night,
A weak pressure pattern will hold over the islands Monday night,
with a cold front then approaching from the northwest on Tuesday,
possibly reaching Kauai by late in the day. The cold front is
then expected to progress southeastward down the island chain
Tuesday night through Wednesday night. Aloft, upper level
troughing will approach the state from the northwest Monday night,
before dampening out a bit over the western islands Tuesday and
Tuesday night. A re-enforcing shortwave trough will dive
southeastward and into the island chain during the middle of the
week.

It appears that shower activity will be most prevalent over the
eastern end of the state, mainly Maui County and the Big Island
through the period, where the deepest moisture is expected to
remain. Here some of the showers could be heavy at times, with
thunderstorms not out of the question due to the colder
temperatures aloft associated with the upper level troughing
moving through. Across the western islands, shower coverage is not
expected to be as great, particularly across Kauai. Across Kauai,
most of the shower activity will likely be associated with the
cold front as it pushes southeastward through the island chain
Tuesday night through Wednesday night. Can`t entirely rule out an
isolated thunderstorm Wednesday or Wednesday night across the
western islands as the re-enforcing shortwave trough dives
through with some very cold air aloft, but deep moisture will not
be as favorable here for thunderstorm activity.

Cooler conditions and locally breezy northerly winds are expected
to overspread the state as the front pushes southeastward down the
island chain Tuesday night through Wednesday night.

The increase in deep moisture over the Big Island along with
temperatures at or below freezing, may result in periods of snow
for the higher elevations above 12 kft. A Winter Storm Watch may
be needed later today or tonight for heavy snow during the Monday
night through Wednesday night time frame.

Thursday through Saturday,
High pressure is expected to build southward and over the islands
Thursday and Thursday night, with another cold front then
approaching from the northwest on Friday. This cold front is then
expected to progress southeastward down the island chain Friday
night and Saturday. Some lingering shower activity may continue
across the eastern end of the state on Thursday, while showers
will be limited mainly to north facing slopes and coasts of the
smaller islands. Drier conditions are expected statewide Thursday
night and Friday, with any showers mainly affecting north facing
slopes and coasts. A band of showers is then expected to accompany
the next front as it moves southeastward down the island chain
Friday night and Saturday.

Cool conditions are expected to continue through the period.
Locally breezy northerly winds on Thursday, will diminish
Thursday night and Friday. Moderate trades are then expected to
overspread the state Friday night and Saturday from northwest to
southeast behind the re-enforcing cold front.

&&

.AVIATION...
The upper level low will slowly drift northward away from the
state over the next 24 hours. Expect continued cool and unsettled
weather today as the low departs, with isolated to scattered
showers moving across the islands from south to north. Deeper
tropical moisture will move into the Big Island later this
morning enhancing shower coverage into the evening hours. Mostly
VFR conditions will prevail with localized MVFR CIG and VIS in
SHRA.

No AIRMETs are currently posted and none forecast.

&&

.MARINE...
Light winds are expected across much of the area into the upcoming
week as a trough of low pressure continues to impact the local
weather. Moderate to fresh northerly winds are forecast to begin
filling in around midweek as another cold front moves down the
island chain, then remain in place across the state through the
second half of the week at moderate to fresh levels.

A new, long-period, west-northwest (300-320 deg) swell associated
with a gale- to storm-force low that developed across the northwest
Pacific this past Wednesday through Friday will fill in through
the day. Surf heights will steadily rise along exposed north and
west facing shores, likely peaking below advisory levels later
this afternoon into tonight, then hold into Monday.

Guidance depicts a gale developing around 1000 nm north of the
islands Tuesday through midweek, then slowly lifting northward
through the second half of the week. A shorter period (11-12 sec)
north-northwest (350-010 deg) swell is expected to fill in through
this time that could result in surf reaching advisory levels along
north facing shores.

A long-period northwest (320-330 deg) swell associated with a large
area of gales forecast to setup over the northwest Pacific between
40N and 50N Wednesday (near the western end of the Aleutians)
through Thursday will fill in Friday. Surf heights should remain
well below advisory levels from this source.

For the extended, guidance remains in decent agreement and shows a
gale developing just east of the date line around 40N Wednesday
through Thursday. A large area of strong- to gale-force winds
associated with this feature is forecast to setup within the 350-360
directional band relative to the islands. Surf associated with this
source will come up, likely exceeding advisory levels over the
weekend along north facing shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until noon HST today for Big Island Summits.

&&

$$

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Gibbs


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