Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 211359

Area Forecast Discussion
National Weather Service Honolulu HI
359 AM HST Thu Oct 21 2021

An area of low clouds and light showers is pushing down across
most of the smaller islands early this morning. Easterly trade
winds will gradually strengthen later today and tonight, which
will likely transport additional low clouds and showers into the
windward sides of the islands, especially Kauai and Oahu, from
tonight through Friday. Expect breezy trade winds from Friday
through this weekend. In addition, drier conditions will also
return across most of the smaller islands starting Saturday, but
showers may increase over the windward Big Island early this
weekend. The trade winds are expected to gradually weaken early
next week.


An area of broken low clouds and light showers continues to push
down across portions of the smaller islands early this morning.
This is the remnant moisture from a dissipated frontal boundary.
There are much fewer clouds and showers to the north of this area
of moisture, so it appears that there will be a break in
precipitation over many areas some time after sunrise this

The leading edge of a band of broken low clouds and scattered
showers associated with a weakening frontal boundary is less than
300 miles north of Lihue. Elsewhere, a 1027 mb surface high pressure
system centered near 37N 158W, or about 1100 miles north of Honolulu,
is moving toward the south-southeast at 15 to 20 mph. This feature
is pushing the frontal boundary down toward the state. The close
proximity of the weakening front between the islands and the high
to the north continues to produce light background northeasterly
flow across most of the state early this morning.

The forecast guidance shows the trade winds will gradually become
more easterly and strengthen later today and tonight. These trade
winds will likely transport low clouds and showers over windward
and mauka sections of the islands, especially Kauai and Oahu,
through tonight. A few showers may also reach some leeward sections
of the western islands. By Friday, the trade winds will be locally
breezy. The models still indicate the relatively wet trade wind
weather pattern may continue across windward and mauka areas of
the western islands through Friday as the remnant moisture lingers
and a weak upper-level trough moves into the vicinity of the

The trade winds will likely remain breezy this weekend. At the
same time, mid-level ridging builds in, which will eventually
strengthen the low-level trade wind inversion. The area of low
clouds and showers over the western islands is expected to push
west of Kauai. An area of low-level moisture may be transported
toward the Big Island from the east by the trades Saturday. The
models show this moisture passing south of the rest of the state,
so any low clouds and showers within this area would mainly impact
the windward and southeastern Big Island. The surface high
pressure system to the north may eventually weaken early next
week, which could lead to a weakening trend in trade wind speeds
by next Tuesday.


The overall wind flow remains weak across the state, with land
breezes over the more sheltered leeward areas. The remnants of a
couple of old fronts remain evident in satellite imagery early
this morning, the first of which has reached Kauai and Oahu,
providing a slight increase in shower activity and cloud cover.
The moisture associated with the second batch will become more
diffuse as it nears the islands later today, but should still be
enough to increase clouds and showers over windward areas this
evening and tonight. Trades will restrengthen later today as well,
helping to push a few showers to leeward areas at times.

No AIRMETs are currently in effect, but TEMPO MVFR conditions
with mountain obscuration are possible along windward slopes,
especially for the western half of the state. Trades will increase
further tonight through Friday, which could warrant an AIRMET
Tango for moderate low-level turbulence.


No major changes to the coastal marine forecast this morning, with
things pretty much on track. The remnants of a front will bring
some clouds and showers mainly to the windward waters, but the
organization of this feature is quickly falling apart. Some
evidence of this boundary is expected to remain near the islands
over the next 24 hours or so.

Behind the front, a new high will will build north of the
islands. As the high builds in, expect a gradual increase in
northeast trade winds late today/tonight, shifting to the east at
moderate to locally strong speeds by Friday. As the winds
increase, a Small Craft Advisory (SCA) will be required for the
typical windier waters near Maui and the Big Island. It could be
as early as tonight, but more likely tomorrow as winds reach SCA
levels, with winds expected to remain nearly steady then into next

Nearshore buoy observations are generally on track with the
current forecast. The current north-northwest swell may still
boost surf along exposed shores over the next few hours, but will
then diminish into Friday, as a new north swell is expected to
build. This next swell is being generated by a powerful low moving
into the Gulf of Alaska. While this swell has the potential to
bring surf to near advisory levels, confidence remains not so high
given the relatively small window for the swell generation. Due to
the size and direction of the swell, this has the potential to
produce moderate harbor surges in the Kahului and Hilo harbors. A
low near the Aleutian Islands will generate a small to moderate
northwest swell for Sunday/Monday. A slight increase in long-
period south swell is expected today/tonight. As the trades
increase over the weekend, expect a slight boost in short-period
wind waves along east facing shores.





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