Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 210651
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
851 PM HST Wed Sep 20 2017

.SYNOPSIS...
A high pressure system located far NNE of the islands will
continue to generate moderate to locally strong trades through
Thursday. Lighter trades will come about starting Friday as the
high drifts further away from the area, and its trailing ridge
shifts closer to the islands from the north. Still, the trade wind
regime will prevail with clouds and showers favoring the windward
and mauka areas. The showers will be most frequent during the
nights and mornings.

&&

.DISCUSSION...
A 1032 mb high far NNE of the main Hawaiian Islands continues to
produce moderate to locally strong trade winds across the area
this evening. Satellite imagery shows a narrow band of showers
nearing the windward shores of the smaller islands, especially
Oahu to Maui. Expect some scattered windward showers for these
three islands within the next hour. Showers from this same band
will affect windward Kauai in an hour or two. Cloud and shower
distribution further upwind is scattered at best now with a couple
of large breaks. We expect a slight increase to these trade
showers as the night progresses but we do not expect anything out
of the ordinary. There is no organized cloud masses that will
significantly boost the overnight showers like last night. In
brief, it is more or less a typical trade wind pattern this
evening. The windward and mountain areas will absorb most of these
showers, leaving the lee side dry.

Weather radar is picking up a couple of moderate to heavy showers
over Kailua-Kona Town now. There has been scattered showers
along the entire stretch of the Kona coast prior to these showers
We would not rule out a few pop up showers along the coast lasting
into the early morning hours of Thursday.

The Big Island is cloudier than usual, thanks to a patch of mid
level clouds between 15k and 20k feet. Water vapor imagery shows
a shortwave trough extending from the upper low, located 700
miles NE of the Big Island, through Oahu. The trough is moving
southeastward and should kick out the mid level clouds later
tonight or Thursday morning.

Satellite derived moisture content or precipitable water (PW) was
between 1.3 and 1.6 inches this afternoon as per soundings from
Lihue and Hilo, with the highest over on the Big island. The
outlook is for no significant change for the next few days.

The latest GFS solution keeps the upper low where it is now
through Friday evening. This suggest little change to the current
marine layer, that is it slopes from 7k feet over Kauai to 10k
feet over Maui to 16k feet over the Big Island. The upper low
weakens on Saturday. And this will allow for mid level ridging to
build across the Big island, which in turn lowers the marine layer
to 13k feet over the weekend.

As noted above, we are looking at slightly weaker trades beginning
Friday. This is due to the surface high moving further away from
the area and its associated ridge being a bit closer to the
islands. It looks like the strength of the trades will ward off
widespread daytime onshore sea breezes. And this mean, limited
afternoon clouds to perhaps the leeward shores like the Waianae
Coast on Oahu.

In summary, trades will continue through the weekend although at
tad weaker starting Friday. Embedded trade showers will continue
to favor the windward and mountain areas, especially nights and
mornings. Plenty of sun all around but especially along the
southern shores.


.AVIATION...
High pressure north-northeast of the State will continue to drive
a moderate to breezy trade wind flow across the islands through
Thursday. Clouds and showers will favor windward and mauka areas,
with a stray shower reaching leeward locales from time to time.
Predominantly VFR conditions are expected at the terminals through
06Z Friday, but some brief MVFR cigs/vsbys will be possible in
windward areas as trade showers move through.

AIRMET Tango remains in effect for low level turbulence over and
downwind of the terrain of all islands. The AIRMET will likely
continue through the day on Thursday.

AIRMET Sierra may be needed for mountain obscuration across portions
of the island chain later tonight.

&&

.MARINE...
Fresh to locally strong trades associated with 1033 mb high pressure
north-northeast of the state across the eastern Pacific will hold
through Friday before trending down over the upcoming weekend and
early next week. Strongest winds are expected across the Pailolo
and Alenuihaha Channels, Maalaea Bay and the waters south of the
Big Island through this time. Trades should reach the light to
moderate range by the end of the weekend as high pressure shifts
eastward and weakens in response to a frontal boundary passing far
north of the state. This pattern should hold through the first
half of the upcoming week (potential for land/sea breezes for some
leeward areas setting up Monday through Tuesday).

A combination of moderate to strong breezes locally and upstream of
the islands south of high pressure will continue to generate rough
surf along east facing shores into Friday. Surf along east facing
shores will trend down over the upcoming weekend as the trades
weaken locally and upstream. Guidance suggests a weak or broad
surface trough developing east of the islands Friday due to a
broad upper low centered northeast of the state, which will weaken
the easterlies upstream of the state and lower the trade wind
swell.

Surf along south facing shores will remain small through early next
week with mainly background southeast and southwest swell energy
expected. The next potential long-period pulse out of the southwest
from the Tasman Sea will be possible by Sunday, which should lead to
a slight increase in surf. For the extended, strong- to gale-force
southwest winds associated with a low that has been impacting the
Tasman Sea over the past 24 hrs may lead to another small southwest
(210-220 deg) swell Tuesday through midweek.

A small northwest (310 deg) swell associated with broad and complex
low pressure that has been centered over the central Pacific near
the Date Line over the past couple of days is expected to fill in
locally Thursday and hold into the weekend before easing. The latest
surface analysis showed 989 mb low centered near the Aleutians
around the Date Line with a decent pocket of strong- to near gale-
force winds focused within the 320 to 330 deg band relative to
the islands. This may end up reinforcing what is left of this
weekend`s 310 deg energy by Monday, before completely fading
Tuesday through Wednesday. As a result, a slight increase in surf
is expected along north and west facing shores beginning Thursday,
which should hold into early next week before dropping off.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Thursday for Maalaea Bay,
Pailolo and Alenuihaha Channels, and waters south of the Big
Island.

&&

$$

Lau/Jelsema/Gibbs


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