Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 171333
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
333 AM HST Thu Aug 17 2017

.SYNOPSIS...
Moderate to locally breezy trade winds will continue through the
remainder of the work week, then ease to light and moderate
levels this weekend. The trade winds will deliver periods of
clouds and showers to windward areas, with showers most active
during nights and mornings. While leeward areas of the smaller
islands will remain mostly dry, the leeward side of the Big Island
will see clouds and a few showers during the afternoon and
evening hours each day. An increase in trade wind showers is
possible late in the weekend through early next week.

&&

.DISCUSSION...
Currently at the surface, a 1027 mb high centered around 1625
miles northeast of Honolulu is driving moderate trade winds
across the island chain early this morning. Meanwhile, invests
90C and 91C remain around 700-750 miles southeast of Hilo.
Infrared satellite imagery shows scattered to broken cloud cover
drifting into windward areas with the trades, with partly cloudy
conditions prevailing in most leeward locales. Radar imagery
shows scattered showers moving into windward areas, with a few of
the showers drifting leeward from time to time. Main short term
concern over the next couple of days revolves around rain chances.

Today through Friday night,
High pressure northeast of the State will keep a moderate to
locally breezy trade wind flow in place through Friday, with a
slight easing of the trades expected Friday night. Precipitable
water (PW) values are forecast to remain below normal through the
period, generally in the 1.0 to 1.3 inch range. This in
combination with weak mid and upper level ridging should keep a
fairly dry and stable trade wind weather pattern in place through
the period. There could be a slight increase in trade wind showers
Friday night however, as mid/upper level ridging breaks down in
response to an upper level low north of the state pivoting
southwestward and closer to the island chain. Overall, clouds and
showers will favor windward and mauka areas, with a few showers
reaching leeward locales from time to time. Showers will be most
prevalent during the overnight and early morning hours.

Saturday through next Wednesday,
High pressure will hold in place to the north and northeast of the
State through the period, keeping the trade winds blowing across
the island chain. An upper level low to the north of the islands
will induce weak surface troughing over and to the north of the
State, and this will weaken the trade winds into the light to
moderate range over the weekend. Early next week the upper level
low and weak surface trough are expected to retrograde west of the
islands. At the same time, both the GFS and ECMWF show an area of
low pressure (the consolidated low associated with invests 90C
and 91C) passing by to the south of the State. This should
strengthen the trade winds slightly early next week, with the
trades once again reaching moderate to locally breezy levels by
the middle of next week.

As for sensible weather details, the models are in good agreement
showing fairly dry trade wind weather prevailing through Saturday.
The GFS then begins to bring in some deep tropical moisture in
association with the invest areas Saturday night. Meanwhile, the
ECMWF suggests the deeper moisture will hold off until Sunday
night. Given the dependence on the track and potential development
of the resultant low associated with the invest areas currently
to the southeast of the islands, forecast confidence is low with
respect to timing more showery weather back into the island chain.
What does appear more clear, is that we will see an increasingly
moist and unstable airmass moving into the area as the weekend
progresses, with this airmass then holding over the islands
through early next week. In fact, the GFS and ECMWF show well
above normal PW values in excess of 2 inches by early next week.
The combination of plenty of deep moisture and an unstable airmass
should result in a fairly wet trade wind pattern late in the
weekend through early next week, with some locally heavy rainfall
possible. In addition, humid weather will accompany this surge in
deeper moisture making it feel much more uncomfortable than
normal, with dewpoints surging into the lower and middle 70s.
Again, the forecast late in the weekend and early next week hinges
on the track and degree of development of the low passing by to
the south of the State. By the middle of next week, both the GFS
and ECMWF show a more stable and drier airmass working its way
back into the area, and this should lead to a return of more
typical trade wind weather across the island chain.

The Central Pacific Hurricane Center (CPHC) continues to closely
monitor the two invest areas to the southeast of the islands for
potential development. CPHC is now forecasting a high chance for
a single tropical cyclone to emerge out of the two invest areas
over the next 2 to 3 days, before conditions become unlikely for
development. See the Tropical Weather Outlooks issued by CPHC for
additional details.

&&

.AVIATION...
High pressure northeast of the islands will keep moderate to
locally breezy trade winds in the forecast through Friday. Expect
mostly VFR conditions with brief periods of MVFR ceilings and
scattered to numerous showers over northeastern slopes of all
islands. Isolated to scattered shower activity is forecast
elsewhere.

AIRMET Tango remains in effect this morning for low level
turbulence over and south through west of all mountains. Wind
speeds may decrease slightly by this evening enough to drop this
AIRMET. No additional AIRMET`s are expected.

&&

.MARINE...
Surface high pressure far northeast of the islands will maintain
moderate to locally strong trades winds. This will continue to
result in Small Craft Advisory (SCA) conditions for the typically
windy waters adjacent to the islands of Maui County and the Big
Island. The SCA for these areas has been extended through Friday
afternoon. The high is expected to continue drifting eastward
during the next few days, and a slight downward trend in trade
wind speeds is expected from Friday night through this weekend.

A small, long period south-southeast swell is expected to peak
late Friday, before gradually subsiding this weekend. A small,
reinforcing long-period southwest swell is forecast to reach the
islands Saturday, and continue into early next week.

Surf is expected to trend down along east facing shores this
weekend as the trades weaken slightly. Surf will likely remain
below the High Surf Advisory criteria along all shorelines
through the middle of next week.

Finally, another period of above normal high tides is expected
through this weekend. The water levels associated with these
tides are forecast to be slightly lower than those observed in
July. In addition, no large swells are expected during the next
several days. There could still be some localized coastal
flooding from these tides. See the Special Weather Statement,
SPSHFO, for more details.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Friday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Houston



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