Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 231359
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
359 AM HST Tue Jan 23 2018

.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 to east oriented ridge of high
pressure located around 800 miles north of Kauai, is driving
moderate trade winds across the island chain early this morning.
Meanwhile, a cold front is located around 1200 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 and Maui County, with a broken band of
low clouds associated with an old front extending over 1000 miles
to the east-southeast of the state. Across the remaining islands,
mostly cloudy skies are in place over windward areas, partly
cloudy conditions in leeward locales. Radar imagery shows
widespread showers moving into windward sections of the Big
Island, with scattered to numerous showers moving into windward
sections of the smaller islands. Main short term concern for today
revolves around rain chances.

Today 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
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 today, then shift
southeastward and over the eastern islands tonight through
Wednesday. Showery trade wind weather is expected over windward
areas of all islands today and tonight, with shower coverage
diminishing across the smaller islands on Wednesday, while
windward Big Island remains wet. Shower coverage should increase
over windward sections of the smaller islands beginning late
Wednesday night and continuing through Thursday, while wet
conditions persist over windward Big Island. Meanwhile, a few
showers may occasionally drift into leeward areas through the
period, while isolated to scattered afternoon shower development
is expected over leeward sections of the Big Island each day.

Additionally, an isolated thunderstorm will be possible over the
Big Island today as the atmosphere will remain unstable here. Some
snow showers and freezing rain will be possible at the highest
elevations of the Big Island today as well, but confidence in
this occurring is too low to warrant the issuance of a Winter
Weather Advisory at this time.

Friday through Monday,
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 into early next week, 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 into early next week.

As for sensible weather details, expect a convective pattern
Friday and Friday night, with a bit of a hybrid convective/trade
wind pattern shower pattern over the weekend into early next week.

&&

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

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

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 this afternoon.

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 this morning.

&&

.MARINE...
Guidance remains in good agreement and depicts fresh to strong trade
winds continuing through midweek as the ridge of high pressure
strengthens. A combination of winds and seas will support small
craft advisory conditions over most waters through Wednesday. The
advisory has been extended for these waters.

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 today, 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 will hold through at least the first half of the day
before slowly trending down into midweek. The current advisory
for north and west facing shores remains in place through 6 PM HST
today.

For southern shores, surf will remain up today due to an out-of-
season, long-period, south (170-180 deg) swell that has filled in
from recent activity from 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 today 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.

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 tonight 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...
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 Wednesday 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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