Area Forecast Discussion
Issued by NWS Honolulu, HI

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232
FXHW60 PHFO 291400
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
400 AM HST Wed Mar 29 2017

.SYNOPSIS...
A trade wind regime will persist through the upcoming weekend and
into the early part of next week as a ridge of high pressure
remains north of the islands. Showers embedded in the trade wind
flow will favor the windward and mountain. The frequency of these
showers will higher during the nights and mornings. The trades
will turn windy over the weekend and lasting through early next
week.


&&

.DISCUSSION...
A loose area of clouds and showers embedded in a mainly moderate
trade wind flow have been affecting mainly windward Maui and Big
Island this early Wednesday morning. Otherwise, it has been a
rather dry evening for the remaining islands. But as more shower
bearing clouds roll in from the east, the showers will eventually
reach Kauai by this evening. The clouds and showers are part of a
much larger area of scattered to broken low clouds that stretched
beyond a thousand miles east of the Big Island. It will take
until Friday morning for this batch of moisture to finally blow
through.

In addition to the trade showers, a tinge of weakness of the
slightly south of east wind flow over the western half of the
island chain will favor some afternoon clouds and a few showers
for the lee and interior areas of the smaller islands. We
anticipate a repeat of this pattern for both Thursday and Friday
afternoons.

On Friday night, the surface high feeding the islands with the
gentle trades will be reinforced by another surface high. This
will take place some 1200 miles NNE of the islands resulting in a
strong 1034 mb high. This translate in a boost in the trades to
moderate to strong speeds. At this time, there is a chance of a
wind advisory for the typically windy areas of places like the
Waimea Saddle on the Big island and or Lanai over the weekend.
This boost in the trades will be accompanied by an uptick in the
frequency of trade showers.

The weather pattern takes a change north of the islands Sunday
into Monday where the high is shoved off far to the east by a
broad surface low. The extension of the surface ridge from the
high will be suppressed southward closer to the islands which
will then lead to a weaker trade wind flow by Wednesday. And
with the weaker trades, there will be fewer trade showers.


&&

.AVIATION...
With a broad surface high far northeast of the state, trade winds
will be light to moderate. Low clouds, and any accompanying
showers, moving along on the trades will mainly affect windward
and mauka sections on the individual isles, but conditions overall
will remain VFR.

There are no AIRMETs currently in effect.


&&

.MARINE...
The latest surface analysis showed a 974 mb storm-force low
centered around 1500 nm northwest of Kauai near the Date Line
(around 40N) and a ridge axis extending westward from a 1030 mb
high centered off the coast of California to just north of the
islands. A very strong pressure gradient was depicted south of
this low northwest of the state, that was supported by an ASCAT
pass earlier where a large area of west-northwest storm-force
winds (focused within the 290 to 310 directional band relative to
the islands) were shown out to around 500 nm south-southwest from
the low to near 30N (leading edge of the fetch was around 1200 nm
northwest of the islands). According to the latest altimeter
passes, seas associated with this system continue to range from 30
to 36 ft over the heart of the fetch region focused toward the
islands, which has been persistent for the past several passes
over the past 24 hrs. Despite some small differences between
solutions, the latest model guidance remains in decent agreement
with these latest satellite trends and shows this system
continuing on an east-northeast track from the Date Line early
this morning around 40N to the Gulf of Alaska through the second
half of the week.

A large west-northwest swell (290-310 deg) associated with this
system discussed above will result across the region, that should
reach the islands Thursday night, peak through the day Friday and
Friday night, then slowly ease over the weekend. Warning-level
surf and advisory-level seas will be expected Thursday night into
the weekend before trending down over exposed waters and beaches.
For timing, the ECMWF-Wave, GFS-WAVEWATCH III and ensemble mean
solutions all remain in close agreement and depict seas ramping up
quickly around midnight Thursday night to Small Craft Advisory
levels, peaking within the 10 to 14 ft range Friday, then slowly
easing through the weekend. A few of the ensemble solutions are
coming in slightly higher (by a couple of feet) at the northwest
buoys and converge at a peak near 15 ft Thursday night. If the
swell ends up coming in larger than predicted or later, warning-
level surf could continue into Saturday before dropping to
advisory levels along exposed north and west facing shores. Surf
should dip below advisory levels Sunday.

Local winds and seas will likely remain below advisory levels
through Friday as the ridge of high pressure north of the state
slightly weakens in response to a cold front that is forecast to
approach the region later in the week, then stall and weaken north
and northwest of the area into the upcoming weekend. Advisory-
level trade winds will return across portions of the waters over
the weekend as high pressure builds north of the state behind this
front.

Prior to the arrival of the large swell expected Thursday night
through the weekend, surf along north and west facing shores will
hold at levels below the advisory criteria through Thursday due
to a lingering northwest swell from earlier this week.

Surf along east facing shores will remain small and choppy through
the week due to moderate to fresh onshore winds persisting. Surf
will begin to build over the weekend as the winds strengthen locally
and upstream of the islands and may near or reach advisory levels
along east facing shores by the end of the weekend and into early
next week.

A slight increase in surf along south facing shores will be possible
Thursday through Saturday due to a long-period swell (190 deg)
associated with recent activity across the southern Pacific.
In addition to this southerly swell, wrap from the previously
discussed large west-northwest swell will add to the mix by Friday
at select spots along exposed southern facing beaches.

&&

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

&&

$$

Discussion...Lau
Aviation...Kinel
Marine...Gibbs



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