Area Forecast Discussion
Issued by NWS Honolulu, HI

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635
FXHW60 PHFO 191924
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
924 AM 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
will support a few showers over windward areas, mainly over the
eastern end of the state through the morning hours. Otherwise,
mostly dry and pleasant conditions will prevail across the
islands. Satellite-derived precipitable water (PW) imagery and
upper air soundings at Lihue and Hilo reflect the dry conditions
with values measured around and just under an inch beneath a
strong subsidence inversion (around 7,000 ft).

Guidance remains in good agreement through much of the week and
depicts mostly dry and stable conditions persisting. A few windward
showers, however, will remain possible tonight into Monday due to a
combination of the aforementioned pulse aloft moving through and
slightly higher dewpoints filling in as the trade winds become
more easterly (low to mid 60s). Upper heights will then rise
through the day Monday as this pulse of energy shifts east of the
area. Model PWs respond and drop to around .75 inches (below
average for this time of year).

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. 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...
A stable deck of high-based stratocumulus is moving across much
of the island chain this morning. Showers will be hard to come by
today thanks to a strong inversion and mid level ridging, so
expect VFR conditions to continue. Trades remain breezy as a high
passes north of the state through Monday.

Because the trades are still elevated and visible satellite
imagery shows some wave cloud features over and downwind of the
mountains, will continue AIRMET Tango for tempo moderate low
level turbulence below 7,000 feet.

&&

.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 tonight. As the high moves steadily E the
next couple of days, 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 weak front
toward the islands from the N, and support moderate trade winds
heading into next weekend, with the front currently expected to
stall and dissipate near Kauai and Oahu.

The strong trade winds are producing choppy surf along E facing
shores, but surf is below advisory levels, and will continue to
gradually diminish as winds ease. Long-period forerunners
associated with a new NNW swell are now arriving in the islands,
and this swell is expected to near advisory levels tonight before
gradually diminishing late Monday. Observations at buoys
51001/51101 NW of Kauai will be closely monitored to determine the
need for a High Surf Advisory, which could be issued this
afternoon. A large NW swell building Wednesday 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...
Small Craft Advisory until 6 AM HST Monday for all Hawaiian
waters-

&&

$$

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



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