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Hawaiian Islands Synoptic Discussion and Guidance

246
FXHW60 PHFO 200130
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
330 PM HST Sun Jan 19 2020

.SYNOPSIS...
Dry and pleasant conditions in place are expected to continue
through the week. Breezy trade winds will hold through Monday,
then shift out of the southeast and diminish Tuesday through
Friday. A mostly dry land and sea breeze pattern will result, with
the best chance for a few clouds and showers setting up over
interior areas through the afternoon periods. Moderate to breezy
trades may return next weekend as high pressure builds to the
north behind a passing front.

&&

.DISCUSSION...
Water vapor imagery showed a weak pulse aloft dropping southeastward
into the area and a broad ridge extending northward to the west.
Despite the dry conditions in place with dewpoints hovering in the
upper 50s to low 60s, this upper pulse of energy moving through
combined with a pocket of moisture moving up the island chain
will support showers over windward areas, mainly overnight through
the early morning hours Monday. Elsewhere, mostly dry and
pleasant conditions with breezy trade winds will prevail across
the islands through Monday.

A progressive pattern setting up over the northern Pacific is
expected through the week as the jet becomes highly zonal. Two back-
to-back fronts passing to the north Tuesday and again Thursday into
Friday will be the result. The subtropical ridge will shift south
into the area beginning Tuesday and remain in place through Friday.
Trades will respond and shift to the southeast and diminish,
allowing a land and sea breeze regime to setup. Rain chances will
remain limited due to the dry air and strong subsidence expected
through this time as upper heights rise. The best chance for a
few clouds and light showers will shift over interior areas
through the afternoon hours each day Tuesday through Thursday.

The tail-end of the second front passing to the north may move over
the western end of the state Friday through Saturday before stalling
and diminishing. If this evolves, rain chances may trend up,
especially for windward and north facing slopes of Kauai and Oahu.
Trades should return next weekend as high pressure builds north
of the area behind the passing front.

&&

.AVIATION...
Cloud cover has diminished this afternoon across the western half
of the island chain as drier air moves in with the trade flow
associated with a high off to the NW of the state. The majority
of the clouds and a few showers are situated over Maui and the Big
Island. Expect that windward areas of all islands will likely see
an uptick in shower activity during the overnight hours before
things clear out again for Monday. Brief periods of MVFR
conditions are possible with these showers, but expect VFR to
prevail overall.

Trade winds remain elevated and are forecast to continue to be
strong enough to warrant the continuation of AIRMET Tango for
tempo low level turbulence below 7,000 feet through at least the
first half of tonight.

&&

.MARINE...
A surface high passing far N of the islands is supporting strong
NE trade winds, and a Small Craft Advisory (SCA) remains posted
for all waters through Monday. As the high moves steadily E the
next couple of days and a front approaches from the NW, winds
will veer and gradually weaken as the associated ridge moves
closer to the islands. Winds will become E on Monday, light SE on
Tuesday, and light and variable Wednesday and Thursday as the
ridge moves over the islands. A new high passing N of the islands
around Friday will push a second front toward the islands from
the N, and support moderate trade winds heading into next weekend,
with the front currently expected to stall near Kauai and Oahu.

A new long-period NNW swell will build overnight, and surf is
expected to peak at advisory levels, thus a High Surf Advisory
has been issued for exposed N and W shores. This swell will
diminish late Monday and Tuesday, but will be followed on
Wednesday by a significantly larger NW swell that will likely
peak near High Surf Warning heights, with an even larger swell
potentially arriving next Saturday. Seas will be sufficiently
large to warrant an extension of the existing SCA for some marine
zones on Monday, but diminishing winds and seas will allow its
cancellation by Tuesday at the latest. Seas associated with the
two large NW swells will warrant a SCA for most areas.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Monday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Olomana-Molokai-Maui Windward West-Maui Central Valley-
Windward Haleakala.

Small Craft Advisory until 6 PM HST Monday for all Hawaiian
waters-

&&

$$

DISCUSSION...Gibbs
AVIATION...Dye
MARINE...Birchard

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



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