Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 231526 CCA
AFDHFO

Area Forecast Discussion...updated
National Weather Service Honolulu HI
525 AM HST Tue May 23 2017

.update...
The zone for north and east Big Island has been updated,
with lower POPs and more morning sun.

&&

.SYNOPSIS...
High pressure to the north of the islands will continue to
provide the islands with trade wind weather for the rest of the
week. Moderate to strong trades will weaken during the second half
of the week, becoming light and variable by the start of the
holiday weekend. A surface trough is expected to settle over Kauai
by Wednesday and linger there into the Memorial Day weekend.

&&

.DISCUSSION...
We are looking a breezy trade wind pattern for the next 24 hours
before a weakening trend settles in for the second half of the
week. Showers could also be on the uptick for especially the
western part of the island chain within the next 48 hours.

The breezy trades, generated by a strong ridge of high pressure
located north of the islands, will be undercut by a surface trough
during the second half of the week. This trough which is now a
stalled front and shearline, extends from 1000 to 1200 miles NE
of Oahu to 85 miles N of Kauai. The models agree on the trough
drifting and settling across Kauai Wednesday night. The showery
weather may come sooner, as early as Wednesday afternoon, from
pockets of showers running ahead of the trough. Oahu will be at
the fringe of the activity so there will be periods of passing
showers on Thursday. The GFS solution which is the aggressor of
the two, holds on to a showery pattern for Kauai lasting into the
weekend.

Light to moderate trades should be in place by Friday. The trades
will be weak enough for the development of local sea breezes on
the smaller islands. This hybrid convective trade wind pattern
will favor the central islands stretching into the weekend.

At the upper levels, it was first thought the upper trough will
play a bigger role in the weather this weekend but the models have
eased off from that assessment. The islands will be at the fringe
of a broad upper low with 500 mb temps of around minus 7 C on
Thursday. The air mass cools to between minus 8 and minus 10 C by
Friday afternoon with the cooler temps over Kauai. This means the
air mass over the islands destabilizes to where it will support
locally heavy showers for particularly Kauai.

From this point on, the EC and GFS differs on the upper air
pattern. While the GFS pinches off a low southwest of Kauai, the
EC pushes the trough through the island chain and is followed by
warming aloft. At the surface, both models have the trough moving
just west of Kauai by Saturday night while retaining a showery
pattern for the Garden Isle. Marginal low level moisture will
likely limit the afternoon convective showers to scattered at most
across the central island over the weekend.

The background trade wind flow shifts to the south and southeast
on Sunday as this surface trough deepens north and west of Kauai.
As for Memorial day, it remains inconclusive at this point of the
wind field. But it looks like light variable winds with a chance
of afternoon showers.

Currently, a patch of trade wind clouds and showers are moving
through the central islands. It should reach Kauai around day
break. Expect these clouds and showers to diminish and become
isolated to scattered by noon. With the breezy trades, some of
these showers will easily spill over into the leeward sections
of the smaller islands. More shower bearing clouds are noted on
satellite upwind of the islands. We expect these showers to reach
the islands around sunset this evening.

A thin layer of broken to overcast cirrus clouds over the Big
Island will clear out later this afternoon or tonight.

&&

.AVIATION...
High pressure well north of the state will keep a breezy trade
wind flow in place through tonight. Clouds and showers will favor
windward and mauka areas, but will spread leeward across the
smaller islands from time to time due to the strength of the
trades. VFR conditions are expected at most of the TAF sites, but
MVFR cigs/vsbys will impact windward areas, including PHLI, PHMK
and PHTO from time to time.

AIRMET Sierra remains in effect for mountain obscuration across
windward sections of Oahu Molokai Maui and the Big Island, with
these conditions expected to spread to Kauai after 15Z. Conditions
should then temporarily improve from east to west later this
morning, with another round of enhanced clouds and showers likely
spreading from east to west and requiring a mountain obscuration
AIRMET for some windward sections of the state this afternoon
into this evening.

AIRMET Tango remains in effect for low level turbulence over and
downwind of the terrain of all islands due to the breezy trade
winds.


&&

.MARINE...
Breezy trade winds will continue across large areas of the
coastal waters through tonight, as high pressure passes far north
of the islands. Trade winds will begin to diminish on Wednesday,
then decrease further toward the end of the week. The Small Craft
Advisory is posted for much of the waters through tonight, and has
been extended for the typically windy zones around Maui County
and the Big Island through Wednesday night.

Northwest swell appears to be peaking now at the nearshore buoys,
and should gradually decline today. The resulting surf will remain
below advisory levels on north and west facing shores. This swell
is forecast to decline further while veering around to the NNW
tonight and Wednesday. A series of small and short-period NNE
swells are expected later this week into the weekend.

The current small south swell will linger through Wednesday.
A larger south swell is expected to arrive Friday and continue
through most of the Memorial Day weekend. This swell may produce
advisory level surf of 8 feet or greater along south facing
shores. In addition, the combination of swell energy, high
astronomical tides associated with the new moon, and ongoing
higher than predicted water levels due to oceanographic factors,
will likely result in minor coastal flooding due to wave run-up
along south facing shores later this week and during the weekend,
especially near the high tide times.

East facing shores will see short-period swell and choppy surf
during the next couple of days due to the breezy trade winds.


&&

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

Small Craft Advisory until 6 AM HST Wednesday for Kauai
Northwest Waters-Kauai Leeward Waters-Kauai Channel-Oahu
Windward Waters-Oahu Leeward Waters-Kaiwi Channel.


&&

$$
DISCUSSION...Lau
AVIATION...Jelsema
MARINE...Jacobson


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