Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 250117

Area Forecast Discussion
National Weather Service Honolulu HI
317 PM HST TUE MAY 24 2016

Trade winds will continue a gradual decline, with showers, some
locally heavy, remaining focused over windward slopes tonight.
Widespread afternoon sea breezes will trigger isolated heavy
downpours on Wednesday, mainly over interior and mauka sections,
as the atmosphere becomes increasingly unstable. A more stable
but humid, hybrid trade wind and weak afternoon convective pattern
will prevail on Thursday, followed by light and variable winds
and a land and sea breeze pattern over most islands heading into
the weekend.


Trade winds have continued a downward trend today as the surface
ridge far north of the state has been eroded by a deepening trough
to the northwest of Kauai. Windward rainfall has been modest,
with the exception of Kauai, where a pocket of moisture moving
along the trade wind flow has been generating some enhanced
rainfall along northern and interior slopes. The afternoon Lihue
sounding showed this moisture and the easterly trade wind flow
confined to the lowest 13000 ft of the atmosphere. This moisture
is expected to linger into the evening, then move off to the west.
Elsewhere, the decreasing trade winds gave way to leeward sea
breezes near high terrain, and with the inversion elevated and
weakening, isolated showers, some locally heavy, have been flaring
on mainly leeward Big Island and Maui slopes. This activity should
diminish toward sunset.

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,
especially across the eastern end of the state, and as easterly
trade winds shift out of the east-southeast to southeast in the
lowest 10000 ft, conditions will likely become conducive for heavy
shower development along windward slopes of the Koolau Range on
Oahu overnight.

As trades weaken further on Wednesday and instability is maximized,
afternoon sea breezes will 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 given the shower activity we have seen today, a slight
chance of thunderstorms has been added for some interior areas.

A hybrid trade wind / convective pattern will prevail on
Thursday, 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 holiday 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 during the weekend.


Clouds and showers will continue to favor windward and mountain
areas under a veering light to moderate trade wind regime.
Meanwhile along the leeward coasts, light sea breezes are driving
the development of convective showers over the southwest slopes of
the big island and maui. Windward showers are expected to increase
or persist throughout the night, but leeward slopes and coasts
should experience partial clearing overnight.

The air mass over the islands is especially moist today so
isolated MVFR conditions are expected in locally heavy showers.
The air mass over the state is also becoming increasingly unstable
in response to the upper level trough gradually approaching from
the northwest. Shower activity tonight is expected to be greatest
just to the north of the smaller islands and over windward slopes.
An AIRMET for mountain obscuration may become necessary on some
islands overnight.


Today will 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 will shift easterly 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 friday through the holiday 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





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