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Hawaiian Islands Synoptic Discussion and Guidance

643
FXHW60 PHFO 181409
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
409 AM HST Fri Aug 18 2017

.SYNOPSIS...
Moderate to locally breezy trade winds will diminish to light to
moderate speeds late in the weekend. The trade winds will never
the less deliver clouds and showers to windward areas well into
next week. The remnants of a former tropical cyclone will move
through the islands through the first half of next week, resulting
in some warm and muggy conditions. Further more, this moisture
will be aided by an upper level low resulting in some spotty
afternoon and evening showers over the interior and lee areas of
the smaller islands. A drier and more stable air mass will return
Wednesday night and Thursday along with a boost in the trades.


&&

.DISCUSSION...
A relatively stable trade wind weather regime will continue into the
early part of the weekend with passing clouds and showers favoring
the windward areas, especially during nights and mornings. the
latest infrared satellite imagery and weather radar shows widely
dispersed rain clouds coming in with the trades, with a tad more
clouds immediately upwind of Maui. Water vapor imagery shows a
dry NW flow aloft over the region associated with a ridge aloft
centered SW of the islands.

An upper level low is noted 540 miles NNE of Oahu at 28n154w. It
will be moving SW toward Kauai, coming to a rest a couple of
hundred miles west of Kauai by Monday night. In doing so, it
weaken and raise the low level inversion from the current 5-6k
feet to 12k feet by Sunday afternoon.

The latest GFS solution shows the strength of the trades holding
at pretty much marginal mainly moderate speed through Saturday
night. A satellite pass over the islands early in the evening,
revealed wind speed of less than 25 kt in the Alenuihaha Channel
and waters south of the Big Island. Still, there is the likelihood
of diurnal pulses reaching 25 kt within these waters later today
due to mixing and differential heating between land and water. The
trades will trend slightly softer over the western half of the
island chain on Sunday to light to moderate speeds. This will be
enough to favor some afternoon clouds and showers over the
interior and lee areas of the smaller islands Sunday afternoon.
The GFS is showing the trades to rebound slightly late Sunday
night and holding through Tuesday although remaining sluggish over
smaller islands. A moderate trade wind flow will stay with the
Big Island into next week.

Next, we will be dealing with the remnants of once T.C. Jova,
which will be pushing through the islands during the first half
of next week. We will likely see a rise in the dew points into
the lower 70's as early as Saturday night. Once again, our comfort
level will be tested. In terms of precipitable water (pw), the pw
will be increasing from the current 1 to 1.3 inches to 1.4 to 1.7
inches by Sunday afternoon, and to 1.7 to 2 inches at its peak
Tuesday.

As we put all these ingredients together, it looks like Sunday
through Tuesday will be an uncomfortable period with high
humidity given the lighter trades and the injection of tropical
moisture. In this hybrid convective and trade regime, daytime
heating and light enough winds will favor spotty afternoon
convective showers across the interior and lee areas of the
smaller islands, while showers riding in with the trades will
likely be enhanced by the upper low. The GFS is suggesting pockets
of dry air within the tropical moisture, which leaves us with some
uncertainty of how widespread these trade showers will be. The
current forecast calls for scattered to likely. Believe this is a
good call, and will stay with it for now. We will be introducing
a slight chance of thunderstorms for the offshore waters north
through west through south of Kauai and Niihau beginning Sunday.
The GFS is hinting of a slight chance of thunderstorms for the 2
Big Island summits both Monday and Tuesday afternoons. It is not
in the forecast now. We will be taking a harder look at this
possibility in the coming days.

Stronger trades will return Tuesday night while the remnants of
Jova gradually exit the region. The last of the tropical moisture
should clear Kauai Wednesday evening. Moderate to locally breezy
trades will persist through the rest of the week with a slight
down turn late in the week.

The area of disturbed weather SE of the Big island is being
monitored closely. In brief, the feature will be severely tested
in the coming days in an unfriendly environment. Please refer to
our Tropical Weather Outlook for more details. At least the GFS
solution has the bulk of the moisture passing south of the
islands Sunday night and Monday.

&&

.AVIATION...
The high pressure ridge north of the islands will weaken slightly
today as the high center drifts eastward. An upper low north of
the islands will also deepen, resulting in a weakening of our
trade winds. Periods of scattered showers are expected becoming
numerous along northeastern slopes of all islands.

AIRMET Tango remains in effect for moderate low level turbulence.
Weakening wind speeds will reduce the threat for turbulence and
allow us to drop this AIRMET later this morning.

&&

.MARINE...
Small craft advisory conditions due to strong winds across the
windier channel waters will continue through this afternoon. Winds
are forecast to dip below advisory criteria tonight through early
next week as the gradient slightly relaxes over the region. A
slight uptick in shower coverage due to an upper low moving over
the region will become a possibility Sunday into next week.

Surf along south facing shores will steadily build through the
morning hours and peak later today as a south-southeast swell
associated with a gale south-southeast of Tahiti late last week
fills in. The latest observations at 51003/51004 across the
offshore waters to the south have begun to reflect this new energy
within the past several hours within the 15-17 second bands. This
energy should translate to an uptick in surf heights through the
day before slowly easing over the weekend. Heights along south
facing shores are expected to remain below the high surf advisory
criteria through the peak of this event.

Surf along east facing shores will trend down through the weekend
as the trades weaken. This trend will likely continue into next
week. Although confidence remains low this far out, there is an
outside chance of a small long-period east (090 deg) swell from a
developing tropical cyclone over the eastern Pacific filling in
through the middle to latter half of next week for eastern
exposures.

Water levels are running around .8 ft above the predicted levels due
to a sea surface height anomaly associated with an oceanic eddy
moving through the islands from east to west. This combined with
the normal King Tide will result in coastal flooding in the
vulnerable low-lying coastal locations each day through the
weekend. Flooding will be expected through the afternoon hours
each day through the weekend around the daily high tide cycles.
The peak of this event is expected Saturday afternoon, when total
waters levels will likely top what was observed in Honolulu at the
end of April earlier this year. See the Special Weather
Statement, SPSHFO, for more details.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST late this afternoon for
Maalaea Bay, the Pailolo and Alenuihaha Channels, and waters south
of the Big Island.

&&

$$

DISCUSSION...Lau
AVIATION...Bohlin
MARINE...Gibbs

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



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