Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 231132 CCA
AFDHFO

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
132 AM HST Tue Jan 23 2018

.UPDATE...
The Pacific Tsunami Warning Center has cancelled the Tsunami
Watch for for the state of Hawaii as of 110 AM HST.

&&

.SYNOPSIS...
A ridge of high pressure north of the state will keep trade winds
blowing through Thursday, with clouds and showers focused
primarily over windward and mauka areas. The trades will diminish
late in the work week as a front approaches from the northwest,
and this should allow for a land and sea breeze pattern with
convective shower development over the interior of the islands
during the afternoon hours on Friday. Gentle trade winds are then
expected to return over the upcoming weekend, with showers
transitioning back into the more typical windward and mauka areas.

&&

.DISCUSSION...
Currently at the surface, a west-northwest to east-southeast
oriented ridge of high pressure located around 800 miles north of
Kauai, is driving moderate trade winds across the island chain
this evening. Meanwhile, a cold front is located around 1450 miles
west of Kauai. Aloft, water vapor imagery shows a southwest to
northeast oriented upper level trough over the western islands.
Infrared satellite imagery shows extensive cloud cover over
windward sections of the Big Island, with a broken band of low
clouds associated with an old front extending over 1000 miles to
the east of the state. Across the smaller islands, partly to
mostly cloudy skies are in place over windward areas, with clear
to partly cloudy conditions in leeward locales. Radar imagery
shows widespread showers moving into windward sections of the Big
Island, with isolated to scattered showers moving into windward
sections of the smaller islands. Main short term concern overnight
revolves around rain chances.

Tonight,
The ridge of high pressure north of the islands will maintain a
moderate trade wind flow across the island chain. The low level
boundary layer flow will remain easterly to east-southeasterly,
and with upper level troughing over the western islands, the plume
of deeper moisture currently over the Big Island, should lift
northward into the smaller islands overnight. As a result, we
should see an increase in windward showers over the smaller
islands as the night progresses, with wet conditions continuing
over windward Big Island. A few showers may also spill leeward on
the smaller islands, with the best chances for this occurring
after midnight. Additionally, enough instability is in place
across the Big Island overnight, that an isolated thunderstorm
can`t be ruled out over windward areas and the adjacent coastal
waters.

We will also need closely monitor the Big Island summits for
potential wintry weather overnight, as relative humidity values
remain between 95 and 100 percent with temperatures below
freezing. Model solutions do indicate that offshore low level
flow should keep precipitation east of the summits however, so we
are not anticipating a need for a Winter Weather Advisory at this
time.

Tuesday through Thursday night,
A ridge of high pressure will remain in place to the north and
northeast of the state through the period, while a cold front
slowly approaches the islands from the northwest. Moderate trade
winds are expected to continue through Wednesday, with the trades
then trending lighter from west to east across the state Wednesday
night through Thursday night. Meanwhile aloft, the upper level
trough will shift into the central and eastern islands Tuesday
through Wednesday, while a weak ridge aloft builds over the
western end of the state. The upper level troughing will then lift
back across the entire island chain Wednesday night through
Thursday night.

As for sensible weather details, the band of deeper moisture will
remain over the entire island chain on Tuesday, then shift
southeastward and over the eastern islands Tuesday night through
Wednesday. Showery trade wind weather is expected over windward
areas of all islands on Tuesday, with shower coverage diminishing
across the smaller islands Tuesday night and Wednesday, while
windward Big Island remains wet. Shower coverage should increase
over windward sections of the smaller islands Wednesday night and
Thursday, while wet conditions continue over windward Big Island.
Meanwhile, a few showers may occasionally drift into leeward
areas through the period, while leeward Big Island sees isolated
to scattered afternoon shower development each day.

Additionally, an isolated thunderstorm will be possible over the
Big Island on Tuesday as the atmosphere will remain unstable
here.

Friday through Sunday,
Model solutions begin to show some subtle but important
differences over the weekend, so forecast confidence is a bit
lower than normal during this time frame. Both the GFS and ECMWF
show a front stalling out a couple hundred miles northwest of
Kauai on Friday, so confidence in a land and sea breeze dominant
pattern is fairly high for Friday and Friday night. Solutions
then diverge over the weekend, with the GFS keeping troughing in
place over the islands, while the ECMWF shows ridging north of the
islands being dominant with little in the way of troughing over
the state. The GFS solution would suggest light winds with land
and sea breezes dominant through the weekend, while the ECMWF
solution would suggest light to moderate trade winds. Given the
differences in the solutions, will take a blended forecast
approach and show a gentle trade wind regime prevailing over the
weekend.

As for sensible weather details, expect a convective pattern
Friday and Friday night, with a more typical trade wind pattern
with windward/mauka focused shower activity over the weekend.

&&

.AVIATION...
A large band of MVFR clouds and showers continues to stream into
the eastern slopes of the Big Island this evening. Moisture with
this band will drift northward spreading MVFR clouds and showers
into windward Maui later this evening. An upper level disturbance
aloft may produce enough lift for isolated thunderstorms near the
Big Island and Maui tonight. Elsewhere mostly VFR conditions will
prevail.

AIRMET Sierra is in effect for mountain obscuration across
windward sections of the Big Island. These conditions are
expected to persist through the night. This AIRMET may need to be
expanded to include windward sections of Maui later this evening.

AIRMET Tango remains in effect for moderate low level turbulence
over and downwind of the terrain of all islands. The AIRMET is
expected to remain in place through tonight.

AIRMET ZULU is in effect for TEMPO light to moderate icing in
clouds around the Big Island from 12000 to 20000 feet. This AIRMET
may be canceled later tonight.

&&

.MARINE...
Guidance remains in good agreement and depicts fresh to strong trade
winds continuing through midweek as the ridge of high pressure
strengthens. The ASCAT passes earlier today and last night reflected
this with up to 25 kt shown across the Alenuihaha Channel and south
of the Big Island. A combination of winds and seas will support
small craft advisory conditions over most waters through Wednesday.

ECMWF and GFS both support a downward trend for the local winds
through the second half of the week as a storm-force low passes
north of Midway tonight into Tuesday, then tracks slowly
northeastward to an area well north of Hawaii Wednesday through
Friday. Light and variable (land/sea breeze conditions) winds can`t
be ruled out for the northwest islands as early as Thursday, then
for the entire state by Friday, which could linger into the weekend.
Largest model differences are shown Thursday, where the ECMWF
depicts moderate to fresh trades holding and the GFS transitions
quickly to more of a land/sea breeze setup. Differences are also
noted over the upcoming weekend as the ECMWF keeps more of a light
trade pattern in place (GFS holds with variable wind pattern). A
blend of these solutions will be included in the package.

The high surf advisory in place for east facing shores continues
through 6 PM HST Tuesday. Observations and reports today reflected
this with up to 8 ft faces. A combination of the fresh to strong
onshore breezes associated with the trades locally and a background
moderate easterly swell (10 sec) from a persistent large area of
fresh to strong breezes upstream of the state will likely support
advisory-level surf through Wednesday. For the second half of the
week, guidance supports the solid easterly swell holding, despite
the local winds trending down. The easterly swell will gradually
trend down Friday through the weekend.

For north and west facing shores, surf associated with the long-
period, west-northwest (300-310 deg) swell that began filling in
Sunday night is around or just past its peak this evening. The
current advisory for north and west facing shores remains in place
through 6 PM HST Tuesday, but may need to be trimmed back to noon
depending on how quickly the swell begins to fade. Will continue to
monitor observations overnight and make the final decision in the
late night package. This source will steadily fade through
Wednesday.

For southern shores, surf will remain up through Tuesday due to an
out-of-season, long-period, south (170-180 deg) swell that has
filled in from recent activity of the southern Pacific. A new, long-
period, background south-southwest (210-220 deg) Tasman swell, is
forecast to fill in by Wednesday, then hold into Thursday, which
should be enough to keep southern shores from going flat.

A small west-northwest (300-310 deg) will be possible Thursday
through Friday from the previously discussed gale- to storm-force
low currently north-northwest of Midway near the date line. Although
the bulk of the winds associated with this feature will focus energy
away from the islands, the ECMWF and GFS support a window of time
late Tuesday into Wednesday where a decent area of strong- to gale-
force west-northwest winds are focused toward the islands within the
300-310 deg directional band. Provided its proximity to the
state, shorter periods (13 sec) will accompany this small swell
with small surf expected.

A small northerly (000-010 deg) swell associated with a complex and
broad gale-force low parked over the Gulf of Alaska can`t be ruled
out Thursday through the weekend. A recent ASCAT pass showed a
decent sized area of strong- to gale-force northerly winds focused
toward the islands earlier today (strongest northerlies near
Aleutians). WAVEWATCH III is depicting 2 ft at 13 sec beginning
Thursday, then becoming reinforced Friday into the weekend within
the 12-14 sec band. Guidance typically handles the timing of this
source well, but under predicts the peak slightly.

A small, long-period, west-northwest (300-310 deg) swell will also
become a possibility late Friday through the weekend. The ECMWF and
GFS both support a decent sized area of persistent westerly gales
setting up off the coast of Japan Tuesday night through Wednesday
night. Small surf will result (largest on Kauai considering
direction and shadowing impacts) over the weekend for north and
west facing shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Tsunami Watch cancelled for all Hawaiian islands.

High Surf Advisory until 6 PM HST this evening for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Olomana-Molokai-Maui Windward West-Maui Central Valley-
Windward Haleakala-South Big Island-Big Island North and East.

Small Craft Advisory until 6 PM HST this evening for Kauai
Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-
Kauai Channel-Oahu Windward Waters-Kaiwi Channel-Maui County
Windward Waters-Maalaea Bay-Pailolo Channel-Alenuihaha Channel-
Big Island Windward Waters-Big Island Leeward Waters-Big Island
Southeast Waters.

&&

$$

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



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