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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 241953

Area Forecast Discussion
National Weather Service Honolulu HI
953 AM HST TUE MAY 24 2016

Trade winds will continue a gradual decline, with showers remaining
focused over windward slopes today. Widespread afternoon sea
breezes will trigger heavier showers on Wednesday, especially
over interior sections, as the atmosphere becomes unstable. A more
stable but humid, 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.


Today will likely be the last true trade wind weather day through
the holiday weekend. A surface ridge driving the trades is located
about 550 miles north of the state and is being eroded by a slow-
moving front roughly 550 miles northwest of Kauai. Winds are down
at the typically windy observing sites compared to this time
yesterday, and a further decline in the trade winds will occur
today. Showers will remain focused over windward slopes today
though clouds and a few leeward showers will develop along leeward
terrain as sea breezes become more widespread this afternoon. The
mid level ridge capping the showers will be weakening, and a few
briefly heavy showers could develop.

The atmosphere will become increasingly unstable tonight as a
rather strong upper level trough approaches from the northwest.
The highest chances for showers will be on windward slopes, and as
easterly trade winds possibly shift out of the east-southeast,
conditions will likely become conducive for heavy shower
development 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 convection over island
terrain and leeward areas. The GFS and, to a lesser degree, the
ECMWF continue to hold the deepest moisture off to the northeast
of the state, but there should be ample moisture to generate
localized heavy showers over interior and mauka areas. The upper
level trough will drop 500 mb temperatures over the islands to -10
to -11C, and if current trends hold, we will be adding a slight
chance of thunderstorms.

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


Clouds and showers will continue to favor windward and mountain
areas under a weakening and veering light to moderate trade wind
regime. AIRMET Sierra for mountain obscuration along windward
oahu has expired, but conditions are expected to redevelop late
tonight in response to an approaching surface through.

Meanwhile across the leeward coast of the Big Island, sea breezes
and convective showers are expected to return by early afternoon.
The air mass over the big island is especially moist today so
embedded MVFR conditions can be expected in locally heavy
showers. An AIRMET for mountain obscuration across big island
leeward coast could become necessary late this afternoon or early

Elsewhere, over the smaller islands, except for isolated MVFR
ceilings in passing showers VFR conditions are expected to prevail
through the remainder of today.


Today will likely be the last true trade wind day through the
holiday weekend. The are no marine advisories in effect, and we do
not anticipate seeing a Small Craft Advisory (SCA) through the
week. Trade winds shift easterly later tonight, and they are
expected to remain out of the east to east-southeast through
Thursday. During that time, winds could be accelerated to near SCA
strength over waters north and east of Maui and the Big Island.
Lighter winds are due heading into the weekend.

Small surf will dominate on most shores this week. The one
exception is a potentially 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
the Collaborative Nearshore Swell and Wind Forecast (SRDHFO) for
more details.





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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
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