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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 161915

Area Forecast Discussion
National Weather Service Honolulu HI
915 AM HST Thu Aug 16 2018

Surface high pressure will remain far northeast of the area
through early next week, with light to moderate trade winds
prevailing over the islands. Areas of tropical moisture will move
west across the state through the weekend. Showers will focus
mainly over windward slopes, with afternoon clouds and showers, a
few briefly heavy, developing over interior and leeward areas.
Drier and breezy trade wind weather will return Monday, as a new
high strengthens north of the state. Tropical Storm Lane is
forecast to strengthen to a hurricane and move into the central
Pacific during the weekend. It is too soon to know if Lane will
directly affect the Hawaiian Islands.


Mostly dry weather prevails over much of the state on this
Thursday morning, with radar showing most of the scattered shower
activity over the coastal waters, and rain gauges measuring only
isolated very light amounts at some windward locations during the
past six hours. Somewhat wetter conditions prevail over the
windward Big Island, with most rain gauges there recording some
precip during the past six hours. Surface analysis depicts a 1027
mb high centered about 1600 miles northeast of the state. The
distance of this high and a weak cold front moving over the North
Pacific has caused the trade winds to slacken, with just light to
moderate trades prevailing over the islands. Aloft, water vapor
imagery shows a mid/upper level trough over and to the north of
the state. This trough is causing some instability, especially
over the Big Island, where the 12Z Hilo sounding found no
inversion and PW of almost 2 inches. Drier and more stable
conditions prevail at Lihue, where the 12Z sounding found a well-
defined inversion based at 6000 feet and PW of 1.41 inches. MIMIC-
TPW satellite imagery confirms the higher moisture present around
the Big Island, with estimated PW values near 2 inches, while a
drier airmass remains over the western half of the state.

For the rest of today, moisture and instability over the Big
Island may produce some briefly heavy showers and a few
thunderstorms over the slopes and interior with afternoon heating,
as occurred on Wednesday afternoon. Drier weather is expected over
the remainder of the state, with showers mostly remaining over
windward areas, but some clouds and showers developing leeward
during the afternoon. The existing forecast appears to be on
track, and no updates are planned this morning.

Light to moderate trade winds will continue through the weekend,
as the surface high remains far to our northeast and a weak front
lingers far to our north. The combination of enhanced tropical
moisture and lingering instability aloft due to the persistent
mid/upper level trough will keep showers fairly active during this
time. The showers will be focused over mainly windward areas, but
will affect some leeward areas as well, especially during the
afternoons. Thunderstorms will remain possible over the Big
Island during the afternoons and evenings through the weekend.

Forecast uncertainty increases early next week. Tropical Storm
Lane, currently located about 1950 miles ESE of Hilo, is projected
to continue tracking generally westward while intensifying into a
major hurricane during the next several days. At this point, we
expect drier and increasingly breezy trade wind weather on Monday,
due to subsidence ahead of the hurricane and a tightening pressure
gradient between Lane and building high pressure to our north.
Weather conditions thereafter heading into midweek will obviously
depend greatly on the eventual track, intensity and size of Lane.
It is still too soon to know whether Lane will directly impact
the islands.


High pressure to the far northeast will maintain a light to
locally moderate trade wind flow over the islands. Showers riding
in on the trades will bring isolated MVFR conditions mainly to
windward areas in clouds and showers, otherwise expect VFR
conditions to prevail. Slight chance of thunderstorms this
afternoon over the Big Island slopes could also produce briefly
heavy showers with reduced visibilities which could result in IFR

No AIRMETs are currently in effect.


Light to moderate trade winds are expected to continue through the
weekend, remaining below Small Craft Advisory levels. An uptick
in winds is possible beginning early next week. Shower activity
is expected to increase this weekend, particularly over the
windward waters, as remnant tropical moisture moves in from the

No significant swells are expected over the next several days, so
expect surf to remain small on all shores. A small south-southeast
swell is expected to fill in today, peak Friday and hold through
Saturday at heights below the summer average. We will continue to
monitor the development of Tropical Storm Lane, currently located
in the eastern Pacific, which may bring an easterly swell to the
coastal waters as early as Monday of next week.

See the latest Oahu Surf Discussion (SRDHFO) for more details on
surf and swell.





AVIATION...M Ballard

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