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Hawaiian Islands Synoptic Discussion and Guidance

383
FXHW60 PHFO 240137
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
337 PM HST MON MAY 23 2016

.SYNOPSIS...
Trade winds will gradually decrease over the next couple of days,
with afternoon sea breezes becoming widespread over leeward areas
by Wednesday. Moderate showers will continue to favor windward
slopes into Tuesday, and chances for heavier showers will increase
on Wednesday, especially over interior sections, as the
atmosphere becomes unstable. A more stable, hybrid trade wind and
weak afternoon convective pattern will prevail on Thursday and
Friday, followed by light and variable winds and a land and sea
breeze pattern over most islands this weekend.

&&

.DISCUSSION...
A weakening and somewhat stable trade wind flow will focus clouds
and showers over windward slopes into Tuesday. Surface high
pressure centered roughly 1100 miles northeast of the state is
being eroded and pushed eastward by an approaching front. As a
result, winds at most local observing sites are down compared to
yesterday and will continue a slow decline during the next couple
of days. Low level ridging is keeping the atmosphere somewhat
stable, even though weak upper level troughing overhead is
inhibiting a well-established inversion from forming. Expect
moderate showers to continue to be focused over windward slopes,
with pockets of increased moisture riding through.

On late Tuesday night and Wednesday, unstable conditions will
develop as a somewhat strong mid to upper level trough swings over
and north of the islands. As this feature approaches Tuesday
night, trade winds will become easterly and may shift out of the
east-southeast, possibly setting up conditions for some heavy
showers along windward slopes of the Koolau Range on Oahu. As
trades weaken further on Wednesday and instability is maximized,
afternoon sea breezes will likely trigger spotty convection over
leeward and interior areas of the islands. According to the GFS
and ECMWF, the deepest moisture will be held off to the northeast,
but we have added a chance for heavy showers in the official
forecast. If models trend toward the current GFS solution, we will
have to consider introducing a slight chance of thunderstorms as
well.

A hybrid trade wind / convective pattern will continue on Thursday
and Friday, though conditions are expected to be more stable as
the upper level trough departs. The weak trade winds will produce
modest showers over windward slopes. Meanwhile, afternoon sea
breezes will generate clouds and a few afternoon showers over
leeward and interior sections, followed by clearing at night as
land breezes develop.

Heading into the weekend, trade winds are expected to be cutoff
over most islands. While southeast winds may hang on over the Big
Island, most islands should experience a humid land and sea
breeze regime, featuring afternoon clouds and spotty interior
showers with partial clearing at night. Details remain unclear at
this time, but volcanic emissions may be drawn up over the smaller
islands.

Uncertainty increases for early next week. Odds are high that
trade winds will remain suppressed and that a land and sea breeze
regime will continue. The GFS and ECMWF are hinting at another
upper level trough dropping toward the state and pushing a
weakening front near the islands. This could enhance convection
again on the Memorial Day holiday.

&&

.AVIATION...
Trade wind flow will focus patchy clouds and showers along the
windward slopes and coasts of all islands. Isolated MVFR ceilings
and visibilities are expected in passing showers.

Weak sea breezes and abundant moisture will combine along the
Kona coast of the big island to encourage afternoon and evening
convective build-ups over inland areas. Partial clearing is
expected across leeward big island after midnight.

VFR conditions are expected to prevail through at least early
Tuesday. No AIRMETS in effect or anticipated at this time.

&&

.MARINE...
Trade winds will be gradually declining during the next couple of
days. We have canceled the Small Craft Advisory (SCA) for the
typically windy waters around the Big Island and Maui, and there
is high confidence that the SCA flag will remain down into early
next week. In fact, we could see variable winds develop near the
smaller islands during the holiday weekend.

Small surf will dominate on most shores this week. The one
exception is a small to moderate southwest swell that will build
on Thursday, peak on Friday, then decline during the weekend. This
swell will originate from the Tasman Sea, and given the large
amount of south Pacific island blocking on swells from this
direction, surf locally tends to be inconsistent. See the recently
issued Collaborative Nearshore Swell and Wind Forecast (SRDHFO)
for more details.

&&

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

&&

$$

Public and Marine...Wroe
Aviation...Bedal

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office



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