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Hawaiian Islands Synoptic Discussion and Guidance

581
FXHW60 PHFO 271945
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
945 AM HST Mon Jan 27 2020

.SYNOPSIS...
During the next couple of days, moderate easterly trade winds
will weaken and shift out of the southeast from Kauai to Molokai,
bringing a return of stable daytime sea breezes and overnight land
breezes. Trade winds will be disrupted state-wide Wednesday night
and Thursday as a surface ridge is pushed over the islands by an
advancing front. The front will potentially bring wet and
unsettled weather to portions of the state Friday into the
weekend.

&&

.DISCUSSION...

A stable, light to moderate easterly trade wind flow is on a slow
decline. The trades are being driven by a surface subtropical
ridge sitting roughly 325 miles north of Kauai, and a strong mid
level ridge over the islands is maintaining very stable conditions
with an inversion based between 4,000 and 5,000 ft. Precipitable
water in the trade wind flow is running below January normal at
mainly less than 1 inch. As a result, small pockets of moisture
have produced meager rainfall along windward slopes this morning,
while leeward areas have been dry. Easterly trade will hold at
moderate strength around the Big Island and Maui today, but as the
subtropical ridge drifts closer to the state, expect winds to
weaken and possibly shift out of the east-southeast across the
western half of the island chain. While still low, the highest
chances for modest showers will remain along windward slopes, and
leeward sea breezes will become more widespread during the
afternoon, leading to an increase in leeward cloud cover with
mainly dry conditions holding.

Stable and somewhat dry conditions will persist for the next few
days as trades eventually become disrupted. The subtropical ridge
will continue to creep toward the state through Wednesday,
weakening the background flow and allowing land and sea breezes to
dominate most areas from Kauai to Maui while easterly trade hang
on across the Big Island. By Wednesday night, the subtropical
ridge will be pushed over the islands by an approaching front,
disrupting the the trade wind flow state-wide. The mid level ridge
will remain in place overhead and maintain stable conditions. As
a result, expect light windward showers to favor Big Island and
east Maui, and the rest of the state will be under a stable land
and sea breeze pattern featuring afternoon clouds and a few light
showers with clearing at night.

Wet and unsettled weather will potentially develop by Thursday
night or Friday across portions of the state. A front will drop
down from the northwest and stall near Kauai or Oahu, and surface
high pressure behind the front will usher in building trade
winds. An upper level trough will dig southward over the state
during this time, but the GFS and ECMWF have yet to come into
agreement on the strength of the upper level trough and position
of the stalled front. Uncertainty remains high, but the greatest
potential for wet and unsettled conditions will be over windward
areas, especially over the western end of the state near the
stalled front.

Uncertainty on shower coverage lingers heading into the weekend,
while it appears that moderate trade winds will prevail. The
latest runs of the GFS and ECMWF wash out the front, perhaps
leaving some shallow moisture behind on Saturday. Both models
suggest mid level ridging will build overhead as trade winds
hold, favoring a typical pattern of windward rainfall.

&&

.AVIATION...
A ridge axis north of the islands will allow for light to
occasionally moderate easterly background flow. Diurnal sea
breezes will dominate the afternoon, especially along the
leeward coasts, while land breezes will develop tonight. Isolated
showers may affect areas along the windward sides, but not
expecting anything widespread. Otherwise, VFR conditions with
mainly clear skies elsewhere.

&&

.MARINE...
A ridge of high pressure north of the area will maintain moderate
to locally strong trade winds across most coastal waters through
today. Starting tonight, the ridge will move south and winds will
veer to the southeast as a front passes north of the state.
Winds are expected to be light and variable across Kauai and Oahu
and moderate to locally fresh out of the east to southeast
direction over Maui County and the Big Island through most of the
week. A shift in weather pattern is possible Thursday into Friday
as a stronger front approaches from the northwest.

The current northwest swell continues to gradually decrease and
will remain at advisory levels across exposed north and west
facing shores of smaller islands through today. The high surf
advisory could possibly be cancelled later today in response to
declining swell.

Latest observations show swell heights and winds have dropped
below the Small Craft Advisory (SCA) thresholds for Kauai and
Oahu. However, the SCA remains in effect for most areas around
Maui County and the Big Island through today. The SCA may be
dropped later today as winds and swell continue to decline.

Surf will remain elevated along north and west facing shores as a
progressive weather pattern across the northern Pacific keeps a
series of moderate to large northwest swells moving through the
islands. The next northwest swell is expected to arrive Tuesday
night and peak at solid advisory levels sometime Wednesday. A
larger, potentially warning level, northwest to north-northwest
swell is expected towards the end of the week for exposed north
and west facing shores by Friday night.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST this evening for north and west
facing shores of Niihau, Kauai, Oahu, and Molokai and for north
facing shores of Maui.

Small Craft Advisory until 6 PM HST this evening for Maui County
Windward Waters-Pailolo Channel-Alenuihaha Channel-Big Island
Windward Waters-Big Island Leeward Waters-Big Island Southeast
Waters.

&&

$$

DISCUSSION...Wroe
AVIATION...Thomas
MARINE...TS/Almanza

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



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