Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 251944
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
944 AM HST Sun Jun 25 2017

.SYNOPSIS...
Trade winds in the light to moderate range will allow leeward sea
breeze development this afternoon, resulting in an increase in
leeward shower activity later today. A mid-level trough will
shift west of the state tonight, giving high pressure to our north
a tighter grip on trade flow and generating breezy and locally
windy conditions Tuesday through Thursday. Drier and more stable
conditions are expected Monday through Thursday, but a more
showery trade wind pattern will return late in the work week as
an upper level low approaches the islands from the east.

&&

.DISCUSSION...
A 1033 mb high far north of the main Hawaiian Islands is driving
light to moderate trade winds across local waters this morning.
Satellite loop shows a plume of broken to overcast low clouds
extending west from the Big Island across waters south of the
smaller islands, while patchy broken low clouds embedded within
trade flow lie just east of the islands. Radar shows very little
shower activity over the islands, with scattered showers noted
across waters south of the islands within the Big Island plume.
Overnight soundings show a neutral to slightly unstable airmass,
with less than 1.2 inches of PW.

Trade winds are light enough to allow development of afternoon
sea breezes across leeward areas today. We expect trades will
continue to push showers across mainly windward areas through the
forecast period, but this will be augmented by sea breeze-
generated leeward showers this afternoon. A mid-level trough will
exit to the west of the state over the next few days,
strengthening the hold the high to our north has on our trade
flow. We expect trades to increase beginning tonight and Monday,
then reach breezy to locally windy levels Tuesday through
Thursday. In the short run, there may be more windward shower
activity tonight due to the convergence associated with the
returning trades. However, by Monday and continuing through
Thursday, models show a drier and more stable airmass will move
into the area.

Models show an upper low will track eastward toward the islands
Thursday night and Friday, then lift northward and away from the
islands for the first half of next weekend. High pressure will
persist north of the islands through the period, with breezy trade
winds expected to continue through at least Thursday night.
Trades are then expected to weaken Friday through Saturday as the
upper low approaches from the east. We expect deep layer moisture
will increase beginning Thursday night and remain over the islands
through next Saturday. As a result, we should see wetter trades
by the end of the work week, with showers continuing to favor
windward and mauka areas.

&&

.AVIATION...
Light to moderate east to east-southeast background flow around
the state has allowed land breezes to clear out most of the land
areas early this morning. The light flow will also allow for a
hybrid trade wind/sea breeze pattern to set up today. Leeward and
windward areas should see cloud build up this afternoon. Some
showers this afternoon may introduce brief periods of MVFR
CIGS/VIS. There are no AIRMETS in effect.

&&

.MARINE...
Gentle to moderate trade winds will gradually increase later
tonight and Monday as the surface ridge strengthens well north of
the state. Fresh to locally strong trade winds will then prevail
through much of the upcoming week, with a Small Craft Advisory
(SCA) expected for the typically windy areas around the Big Island
and Maui County.

A small increase in short-period trade wind swell is due this
week, though a more noticeable increase will come from a NE swell
due to build later today through Monday. This swell was produced
by a fetch of gales off the Oregon and California coast and will
have periods of around 12 to 13 seconds. This creates some
difficulty in sorting out the arrival of the swell at the NOAA
buoys NE of the islands, since the the ongoing SSE swell is
characterized by similar wave periods. We will monitor the NOAA
and PacIOOS buoys through the next couple of days, as there is a
low probability that this swell could potentially produce east
surf near the advisory level on Monday and Tuesday. Some of this
swell energy will also wrap into exposed north facing shores.

Pulses of swell from the SSE and at times from the SSW will
produce south shore surf near to just below summer average during
the upcoming week.

Extreme tides that have been observed over the past several days
will trend back toward normal into the upcoming week. As a result,
flooding impacts along the coast will diminish.

&&

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

&&

$$

DISCUSSION...Powell
AVIATION...Eaton
MARINE...Wroe


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