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Forecast Discussion for Honolulu, HI

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000
FXHW60 PHFO 270203
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
403 PM HST Thu Apr 26 2018

.SYNOPSIS...
Diminishing trade winds will deliver low clouds and showers to many
areas through the evening, especially windward. Winds will weaken
and shift to the southeast overnight as a trough develops near
Kauai, and this will allow for some brief reduction in showers. A
developing front will move down the island chain Friday and
Saturday, and then stall near the Big Island Sunday into next week.
The front will bring a period of clouds and showers to both windward
and leeward portions of the islands, with some briefly heavy
showers. While a cool and mostly dry air mass will prevail over over
the smaller islands after the front passes, the stalled front will
keep wet weather over the Big Island for several days.

&&

.DISCUSSION...
Trade winds will diminish tonight as a N-S oriented surface trough
develops near Kauai, in response to a deep-layer cut-off low that
develops about 900 miles N of the islands. This process is underway
with cyclogenesis appearing to take place near 33N 166W. Weakening
trade winds are advecting an area of showery low clouds over
windward portions of the islands, which also helped to fuel showers
developing on Oahu`s leeward-side sea breeze. As the trades weaken
and veer to the SE overnight, some of this moisture will slide N of
the islands, and a developing land breeze should lead to decreasing
shower coverage overnight.

Since the low becomes cut-off, it will be slow to clear out of the
area, instead lingering N of the islands through early next week.
The low is expected to send a developing front across the island
chain, with the front expected to move across Kauai during the day
Friday, Oahu and Maui county Friday afternoon/evening before
spreading to the Big Island by Saturday. As the slow-moving low
wobbles around N of the islands through the weekend, the associated
frontal band will likely stall near the Big Island, acting as a
focus for cloud and shower development. While the front is expected
to bring a period of clouds and showers that will affect both
windward and leeward portions of the islands, instability will be
limited and a deep moisture tap is not expected, at least initially.
Issuance of a Flash Flood Watch is therefore not planned for the
smaller islands, but it (and a Winter Storm Watch) may be necessary
later if the models continue to indicate the front stalling near the
Big Island over the weekend. Timing and confidence don`t support
their issuance at this time.

A cool NW to N flow will prevail over the smaller islands after the
front passes, with dew points expected to drop into the mid 50`s.
This should bring cool and mostly dry weather Saturday into Sunday,
but a shortwave rotating around the closed low aloft may bring an
increase in low clouds later Sunday and Sunday night. Lack of
moisture should limit the coverage and intensity of showers, with NW-
N winds focusing this moisture along coasts and slopes that face N.
As this shortwave passes over the islands, associated strong winds
will likely be felt over the summits of the Big Island, where
warning-level wind speeds are possible Sunday/Monday. It is too soon
for the issuance of a High Wind Watch.

As the low gradually moves NE during the early to middle part of
next week, high pressure will build to the NW of the islands,
supporting NE trade winds that will deliver a few windward showers
while keeping a cool air mass in place. Moisture associated with the
front will continue to linger near the Big Island for most of next
week, where a prolonged period of cloudy and showery weather appears
increasingly likely.

&&

.AVIATION...
AIRMET Sierra is in effect for the windward side of the mountains on
all islands except Kauai causing areas of mountain obscurations due
to showers and cloud cover moving across the state. With moderate
trade winds still in place at least into the late afternoon hours,
some of these showers may drift to the leeward side of the mountains
but VFR conditions are expected in most leeward areas.

&&

.MARINE...
An ASCAT pass this morning showed winds dropping off as expected,
with just a few 25 kt areas in the Alenuihaha Channel. With models
showing a downward trend continuing, feel confident with winds over
the waters below Small Craft Advisory (SCA) tonight. As such the SCA
has been cancelled. We will likely have to issue another SCA by the
end of the weekend for some combination of winds and seas.

The general forecast philosophy remains the same. A developing low
north of the islands will bring a front down the island chain over
the next couple of days, with a trough forming ahead of the front.
The front is expected to stall near the Big Island Saturday night
and Sunday, with moisture lingering there during the first part of
the week. Initially, right behind the front, northerly winds could
briefly reach SCA criteria in some of the marine zones, particularly
over the waters near the smaller islands. The low will send a mix of
swells and seas toward the islands this weekend into next week, with
associated seas rising above 10 feet in most zones late Sunday and
into Monday, thus the mention above about the possible need for an
SCA by the end of the weekend.

A moderate long-period NNW swell is showing signs of arriving at the
nearshore buoys, and its expected to peak Friday, with peak surf
heights below advisory levels. The low that develops north of the
islands will remain in place until early next week. The amount of
swell/surf that arrives in the islands is somewhat uncertain, and
will depend on the evolution of the low and associated fetch.
Current indications are that a fairly significant, but poorly
organized, swell will produce advisory- level surf along exposed
north and west facing shores, with the peak of the swell around
Sunday.

Several pulses of relatively small southwest to south swells are
expected over the next week or so, with a peak in swell energy
around Sunday. Rough and choppy surf along east facing shores will
diminish through Friday as trades weaken.

See the latest Oahu Surf Discussion (SRDHFO) for additional details
on surf and swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Birchard
AVIATION...Chevalier/Foster
MARINE...M Ballard

Previous Forecast Discussions may be found at
NWS Honolulu, HI (HFO) Office Forecast Discussions.
(Click 'Previous Version' there to view past versions successively.
Some may differ only in time posted.)

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