Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 260158

Area Forecast Discussion
National Weather Service Honolulu HI
358 PM HST Mon Sep 25 2017

A slow increase in the trades is expected over the next few days as
a surface ridge to our north strengthens. The trades will peak on
Friday at locally breezy levels. Some increase in trade wind
showers is expected Tuesday night through Wednesday night as a
band of moisture pushes across the islands. Our winds will shift
to southeast by the weekend as the ridge pushes south over the


The afternoon soundings came in rather dry with precipitable water
of only 1.03 inch at Lihue and 1.10 inch at Hilo, along with
relatively weak inversions near 6500 feet. Some sea breeze
cloudiness developed over leeward sections, but rain has been
quite sparse in most areas. One exception to that is over the
leeward and Kau slopes of the Big Island where a few showers have
popped this afternoon.

The current gentle trade wind scenario is due to a weak
subtropical ridge about 300 miles NW of Kauai, and a weakening
surface trough nearly stationary E of the Big Island near 154W.
Our trades will make a slow, gradual, and short-lived comeback
the next few days as the subtropical ridge strengthens to our
north, reaching locally breezy levels by Thu.

At the same time, the airmass will remain relatively dry with
plenty of fine weather until a band of remnant moisture once
associated with the surface trough moves in from the E. Models
show this moisture getting to the windward side of the Big Island
late Tue night, spreading across the chain Wed, and starting to
exit Wed night. This will bring us a few more windward and mauka
showers, but will not be a major rain event by any means with the
air remaining quite stable.

Changes are in the cards for the end of the week. By Friday models
are showing a fall-like low pressure center reaching 40N 170W, or
roughly 1400 mi NW of Kauai. This low will not affect us directly
but it will weaken the subtropical ridge and push it rather
quickly S over the islands over the weekend. Trades will shift
rather abruptly to SE flow and weaken over the islands this
weekend. There`s some minor disagreement in the models about how
much moisture will be available, but it seems likely there will be
some increase in afternoon and evening clouds and showers, as
well as considerable vog as well.

Earlier runs of the ECMWF held out hope for trade winds returning
for Sunday and Monday, but the 12Z run looks a lot more like the
recent GFS, keeping SE flow going through the end of the forecast


Localized sea breezes developed across sheltered leeward areas
this afternoon, leading to afternoon cloud buildups and general
VFR ceilings. As the sea breezes weaken and leeward clouds
dissipate with sunset, windward clouds will fill back in under a
gentle trade wind pattern. Brief MVFR conditions are possible in
showers over windward and mauka areas; otherwise VFR conditions
will prevail.

No AIRMETs are currently in effect or anticipated.


Light to moderate trades are expected to persist through the first
half of the week as a weak surface trough approaches the islands
from the east. An upper level trough above the surface trough is
providing sufficient instability to support thunderstorms to the
east of the offshore waters. As the surface trough moves west,
some thunderstorms may develop over the eastern offshore waters on
Tuesday before the upper level trough departs to the northeast. A
return of moderate to locally fresh trades is forecast through
the second half of the week as the high pressure ridge north of
the state rebuilds. Some of the typically windier channel waters
may even reach Small Craft Advisory levels Thursday through

Surf is expected to remain below advisory levels through the
forecast period. Surf along south facing shores will remain small
through the week with mainly a mix of background southeast and
southwest swells expected. The next long-period pulse out of the
southwest should fill in Tuesday night through midweek, leading to
a slight increase in surf.

Small, short period, northwest swell energy should be enough to
keep the surf from going flat along north facing shores through
the week. A reinforcing north-northwest swell should reach the
local waters late Wednesday through Thursday.

Surf along east facing shores will remain small through the first
half of the week as the trades locally and upstream of the state
remain light. Small and choppy surf will likely return as the
trades fill back in through the second half of the week.




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