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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 161336

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

A high will remain far northeast of the area through early next
week. Tropical moisture will move west across the area though the
weekend. Drier air will move over the islands on Monday. A high far
northwest of the area will move east to a position north of the
islands by the middle of next week. Tropical Storm Lane will likely
strengthen to a hurricane and move into the central Pacific over
the weekend. It is too soon to know if Lane will directly affect
the Hawaiian Islands.


The surface analysis at 200 AM shows a 1027mb surface high around
1700 miles northeast of the islands near 43N 140W. The high is
forecast to move northeast slowly through early next week. This
distant high will maintain gentle to moderate easterly trade winds
over the islands through early next week.

An area of moist air associated with a trough in the lower
atmosphere has been moving west across the islands. A trough
higher in the atmosphere northwest of the islands is making the
atmosphere somewhat unstable. The combination of moisture and
instability will keep showers active for the next few days. Since
trade winds will continue, showers will be focused over mainly
windward areas but clouds will build high enough to spill over to
some leeward areas. Thunderstorms will be possible over the Big
Island during the afternoons and evenings. A new area of tropical
moisture will spread over the islands during the weekend and
enhance showers even more.

Tropical Storm Lane, now far southeast of the islands, is forecast
to strengthen to a major hurricane and move into the central
Pacific over the weekend. It is too soon to know if Lane will
directly impact the islands. An indirect impact from Lane is
possible early next week as drier air from subsidence ahead of
the hurricane spreads over the islands on Monday.


Light to locally moderate trade winds are expected through tonight,
supported by a surface high to the distant northeast. An area of
low-level moisture embedded in the trade wind flow lies over
windward waters from Oahu to the Big Island. With a nearby upper-
level trough providing instability, showers are more active and
heavier than usual, but most have remained offshore over the past
several hours. The most persistent and enhanced area of showers
has been over near shore windward Big Island waters, where cloud
tops up to 30000 feet have been observed for most of the night.
With most of the showers currently offshore, no AIRMETs are in

The light trade winds will keep clouds and showers focused over
windward areas this morning, with isolated MVFR conditions expected
over land. Afternoon sea breezes will drive the formation of some
leeward clouds and showers on all islands, increasing the potential
for MVFR conditions over leeward areas. On the Big Island there
is potential for locally heavy and/or thunderstorms, mainly over
the interior portions and upper slopes, which could lead to brief
periods of IFR conditions, and may require an AIRMET for mountain


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

No significant swells are expected over the next several days, so
expect surf to remain small on all shores. A small south 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 cyclone Lane, which is currently
located in the eastern Pacific. Lane may bring an easterly swell
to the coastal waters as early as Monday of next week.

See the latest Collaborative Surf Discussion for more details on
surf and swell.





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