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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 200151

Area Forecast Discussion
National Weather Service Honolulu HI
351 PM HST Wed Sep 19 2018

A trough of low pressure will move across the area over the next
couple of days. Winds will be light favoring daytime sea breezes
and nighttime land breezes. An increase is showers is expected
with the troughs passage. Trade winds will return Friday and on
through the weekend. An area of low pressure is expected to
develop northwest of the area early next week with winds becoming
lighter and out of a southeastly direction. Shower activity may
increase over the west end of the state around the middle of next


A weak trough of low pressure lies near the Big Island. The
trough is forecast to move from east to west across the island
chain over the next couple of days. Winds will generally be on the
light side in the troughs vicinity with daytime sea breezes and
nighttime land breezes dominating in many locations. Higher
moisture content is also present along and just east of the trough
axis. The result will be a more humid airmass in place and well
as increased shower chances. Increasing rainfall will begin first
over the Big Island tonight and then gradually spread up the
island chain Thursday and Thursday night. The highest
probabilities for showers will be over interior and mauka areas
but showers are possible over some leeward areas as well.
Daytime heating along with the increase in moisture has caused
for some thunderstorm development over interior sections of the
Big Island. Thus a slight chance for thunderstorms are in the
forecast for tonight over eastern and interior sections of the Big
Island. Lingering moisture over the Big Island on Thursday along
with daytime heating may allow for a few thunderstorms to develop
over the Kona slopes and interior sections during the afternoon

High pressure will build in behind the trough by Friday with
trade winds making a gradual return. A drier airmass will also
filter in from east to west with shower activity tapering off.
Passing showers will tend to favor windward and mauka areas.

Beginning early next week, an area of low pressure is forecast to
develop northwest of the area. This will gradually turn our winds
to a southeasterly direction. There is some differences in the
global models on how close its associated moisture field will be
to islands. It appears that the west end of the state could see an
increase in rainfall chances toward the middle of the week but it
is still to early to forecast on when and where this might occur.


Weak east to southeasterly flow will persist. The air mass over
the Big Island will become increasingly moist through Thursday
morning. Layered clouds moving into Windward Big Island may
necessitate an AIRMET for mountain obscuration late tonight or
early Thursday.

Elsewhere, light sea breezes will give way to weak land breezes
after sunset. Sea-breeze driven cumulus build-ups over interior
portions of the smaller islands and Leeward Big Island will
dissipate by late evening, but will likely redevelop tomorrow
afternoon. VFR conditions will prevail in these areas.

The air mass over the state will remain unstable overnight.
Slight chance of thunderstorms over Interior and Windward Big
Island through the early morning hours.


A surface trough moving across the island chain from the east
will disrupt the trade wind flow through Thursday. Mainly gentle
to moderate east to east-southeast winds will prevail, with
stronger winds holding around the Big Island and Maui. As the
trough lifts away, easterly trade winds will rebuild, possibly
reaching Small Craft Advisory strength across the typically windy
waters around the Big Island and Maui into the weekend. Winds will
likely veer out of the southeast late Sunday and early next week
as an area of low pressure forms several hundred miles northwest
of Kauai.

Surf will remain below advisory levels along all shores this
week. A south swell is peaking near summer average this
afternoon and will slowly subside Thursday and Friday. The
current very small northwest swell will slowly fade tonight. As
trade winds rebuild late Thursday and Friday, rough, short-period
surf will briefly rise above summer average, then drop during the






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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
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