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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 200627

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

A weak surface trough will move slowly westward across the area
through Thursday. Winds will remain light, which will allow local
nighttime land breezes and afternoon sea breezes to develop. An
increase in showers is expected as the trough passes. Trade winds
will return by Friday, and will persist through this weekend. An
area of low pressure is expected to develop northwest of the
islands early next week, which will again cause the winds to
become lighter, and shift to a southeasterly direction. Clouds and
showers may increase over the western end of the state by the
middle of next week.


The southern end of weak north to south oriented surface trough
is evident just north of the Hawaiian Islands appears to be
drifting slowly westward. The close proximity of this feature to
the state is causing the winds to remain light. This is allowing
local nighttime land breezes to develop early this evening over
most of the islands as daytime heating is ending.

Increased moisture is evident east and southeast of the trough
axis. Showers continue to move into the windward Big Island and
eastern Maui early this evening. The presence of this humid air
mass may cause isolated thunderstorms to develop tonight over the
coastal and upslope sections of the windward and interior Big
Island. The land breezes are expected to keep the remainder of the
state west of Maui mostly dry through early Thursday morning.

The forecast guidance indicates the light wind regime will persist
Thursday. As a result, we anticipate local afternoon sea breeze
circulations will develop over most islands. In addition, the
higher moisture values may spread westward, which could aid in the
development of showers over interior and mauka areas, as well as
over some leeward sections. Daytime heating combined with lingering
moisture over the eastern end of the state may allow a few
thunderstorms to develop over the Kona slopes and interior
sections of the Big Island Thursday afternoon. By Thursday
evening, we expect the local nighttime land breezes will begin to
clear out most of the showers from interior and leeward areas of
the state. Isolated showers may continue over some windward facing
slopes Thursday night.

As the surface trough shifts away from the region, we expect
surface high pressure will build north and northeast of the state.
This will tighten the pressure gradient across the region, which
will cause the trade winds to return. In addition, a drier air
mass will also filter in from east to west across the state. This
will result in brief trade showers over some windward and mauka
areas, but most leeward sections of the smaller islands will be
relatively dry this weekend.

Beginning early next week, an area of low pressure is forecast to
develop northwest of the islands. This will cause the pressure
gradient to relax over the region, which will result in light
winds once again. In addition, the winds will shift out of the
east-south or southeast. The forecast models continue to show
some variations in the location of the area of low pressure, as
well the areal coverage of its associated moisture field.
Depending on the proximity of these features to the state, there
could be an increase in rainfall chances over the western end of
the island chain starting around the middle of next week. However,
it is still too early to forecast with any confidence that this
will occur. Regardless of the chances of rain, expect very warm
and muggy conditions to develop starting early next week. Note
that with the lack of any significant volcanic emissions on the
Big Island, we are not including any volcanic haze in our forecast
for the smaller islands next week.


Light to moderate ESE low-level flow near the Big Island and Maui
will bring clouds and showers to E and SE facing slopes and
adjacent waters overnight and Thursday as moisture increases.
Periods of MVFR VIS/CIG are expected, with a slight chance of an
overnight thunderstorm. Coverage may increase to the point that
AIRMET SIERRA for mountain obscuration becomes warranted.

A light wind regime and a drier air mass from Molokai to Kauai
will allow overnight land breezes to clear skies over land, with
little in the way of showers over adjacent waters. VFR conditions
will prevail through Thursday morning.

As moisture spreads NW over the smaller islands on Thursday, and
afternoon sea breezes lead to convergence over the islands,
clouds and showers are expected over interior and upslope areas
of all islands. Some of the showers could be briefly heavy,
especially over interior Big Island.


A surface trough will move across the island chain from the east
to west tonight and Thursday. In its wake, easterly trades will
strengthen beginning Thursday afternoon across the eastern islands
and Thursday night across the western end of the state. Small
Craft Advisory (SCA) level winds will be possible across some of
the typically windy waters around Maui and the Big Island Thursday
afternoon into the weekend. Winds will likely shift around to the
southeast late Sunday through 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 through
early next week. The current south swell will remain near the
summer average tonight, then slowly subside Thursday through
Friday. As trade winds rebuild late Thursday and Friday, rough,
short-period surf will briefly rise above the summer average
along east facing shores, then drop during the weekend. Elsewhere,
no significant swells are expected.






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

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