Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 240647

Area Forecast Discussion
National Weather Service Honolulu HI
847 PM HST Sun Apr 23 2017

A ridge of high pressure to the north of the islands will maintain
a trade wind flow into the middle of the week. Limited shower
activity is expected into Tuesday. From Wednesday and into the
weekend, increasing showers are expected, with the potential for
a prolong period of unsettle weather.


The clouds and showers eased off across Kauai, finally, and had
really nice sunset. The last visual satellite imagery, at 5 pm
HST, showed very scant low cloud coverage across the island`s
windward water...and this clearing extended upwind of the islands
for several hundred miles. Low cloud coverage will be increasing
as the evening progresses, however, and so will the showers. The
forecast was updated to reflect this scenario, and also to lower
the cloud coverage and POPs for Kauai in particular.

Dense cirrus clouds, at around 25k feet, have increased across
the eastern half of the island chain this afternoon and evening.
These clouds are expected to linger through the rest of the night
before retreating south and eastward Monday clearing Maui County
in the morning, and the Big island late in the afternoon.

The cirrus is being carried over the islands from an upper level
trough orientate NE to SW, and located just W of Kauai. A low is
forecast to form along the trough W of Kauai later tonight and
drift S to a position of 250 miles SW of Kauai late Monday
afternoon. Then the low turns into a deep trough by Wednesday.
During this period, the main surface feature will be the surface
high of 1026 mb located 1200 miles NE of the main Hawaiian
Islands. The associated ridge extends westward from the high
passing some 450 miles N of Kauai. Through Tuesday...the ridge
will be hold its ground while a front tries to advance eastward.
Trades, therefore, will continue across the Hawaiian Islands.

Thereafter, the forecast may start to turn sour. An upper level
trough with a pool of unstable cold air is forecast to drop down
onto the main Hawaiian Islands Thursday night. The air mass will
start destabilizing across the area as early as Wednesday night
leading to widespread showers, a few locally heavy including a
slight chance of thunderstorms. Various part of the island chain
will be affect by this weather system. The surface ridge breaks
down and the front advances toward the islands, reaching Kauai
early Friday morning as per ECMWF solution. The GFS has a slightly
different picture with the formation of a N to S oriented low
level trough over Maui County while the front passing just N of
Kauai. But any way you look at it, the trade flow will be
disrupted and there will be some significant weather around as
early as Thursday night.

The weather systems lingers into the upcoming weekend and into
next week Monday. A surface low may even form along the surface
trough near the islands thereby enhancing some of the showers.
Both the ECMWF and GFS are in on this. More information will be
proved in the coming days should the models continue on this very
wet outlook.


Expect VFR conditions to dominate tonight and Monday. A diffuse
band of moisture that affected the western end of the state today
is dissipating, allowing a drier trade wind flow to fill in from
the east. AIRMET Sierra remains in effect over windward terrain of
Kauai, but it will likely be dropped before sun down. An upper
level trough centered about 250 nm west of Kauai will generate a
few high clouds, but as a low level ridge builds in tonight, the
entire state will be stable, with isolated -SHRA confined to
windward slopes.

A stable easterly trade wind flow prevails from the Big Island to
Oahu and will spread to Kauai later today. The afternoon Hilo
sounding showed an inversion near 6000 ft, and since east winds at
850 mb (roughly 5,000 ft) will increase tonight, some low level
turbulence will occur to the west of terrain on the Big Island and
Maui County. However, we do not anticipate needing AIRMET Tango
tonight, and conditions will be border line on Monday.


The Small Craft Advisory (SCA) has been extended through Monday
night for coastal water zones surrounding the Big Island and some
areas around Maui County due to strong ESE winds interacting with
the island terrain. Wind speeds will maintain through at least
Monday night with possible extension needed into Tuesday.

A number of mainly small swells will affect the state this week,
but High Surf Advisories are not expected for at least the first
half of the week. Wind waves, along with a little bit of a trade
wind fetch upstream, will bring small, but choppy waters across
the east facing shores through most of the week. Small north and
northwest swells will fade in and out over the next week as well.
Small, long period south swells will also fade in and out this
week, but will need to be monitored for possible advisory level
surf along south facing shores toward the end of this week.

A stationary upper trough with an axis approximately 250 nm west
of Kauai will keep the chance for scattered thunderstorms in the
northwest offshore waters through tonight. These thunderstorms are
not expected to reach the western coastal waters.

Small Craft Advisory until 6 AM HST Tuesday for the windward
waters of Maui and Hawaii Counties, waters south of the Big Island
and the Pailolo and Alenuihaha Channels.



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