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Hawaiian Islands Synoptic Discussion and Guidance

337
FXHW60 PHFO 170639
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
839 PM HST Wed Aug 16 2017

.SYNOPSIS...
Moderate to locally breezy trade winds will continue through the
remainder of the work week, then ease to light and moderate
levels over the upcoming 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 1029 mb high centered around 1650
miles northeast of Honolulu is driving moderate to locally breezy
trade winds across the island chain this evening. Meanwhile,
invests 90C and 91C remain around 700-800 miles southeast of Hilo.
Infrared satellite imagery shows scattered to broken cloud cover
drifting into windward areas with the trades, with some cloud
buildups lingering along leeward sections of the Big Island.
Leeward areas of the smaller islands are generally partly cloudy.
Radar imagery shows scattered showers moving into windward areas,
with a few of the showers drifting into leeward areas from time
to time. Main short term concern over the next couple of days
revolves around rain chances.

Tonight through Friday night,
High pressure northeast of the State will keep a moderate to
locally breezy trade wind flow in place through the remainder of
the work week, with a slight easing of the trades expected as we
head into the weekend. Precipitable water (PW) values are forecast
to remain below normal through the period, generally in the 1.0
to 1.2 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 as an upper level low north of
the state pivots 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 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 the 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 continues to forecast a medium chance
for the two invest areas to merge and develop into a single
tropical cyclone over the next few 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 evening for low level
turbulence over and south through west of all mountains.
Wind speeds may decrease slightly on Friday 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 is currently in effect through Thursday afternoon.
The high is expected to drift to the east during the next few days,
and a slight downward trend in trade wind speeds is expected from
Friday through this weekend.

A small, long period south-southeast swell is expected to fill in
on Thursday, and then peak Friday, before gradually subsiding
this weekend. Another small, reinforcing long-period southwest
swell is forecast to reach the islands this weekend.

Rough moderate surf will persist along east facing shores due to
the breezy trade winds. Surf may 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 early next week.

Another period of above normal high tides is expected through
this weekend. These tides are forecast to be a little lower than
in July, and no significant swells are expected during this time.
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 Thursday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$

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

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

Current Conditions
Temp		2.4 C
RH		14 %
Wind		NE 9 mph
Road		Open (4x4)