Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 230206

Area Forecast Discussion
National Weather Service Honolulu HI
406 PM HST Wed Feb 22 2017

Current strong trade winds will gradually decrease in speed
through Friday night. Clouds and showers will favor windward and
mauka areas, with brief showers occasionally spreading leeward of
the smaller islands. An upper level disturbance near the Big
Island will depart east of the area tonight bringing a return to
more stable weather.


Water vapor imagery shows an upper level trough with an axis
passing over the Big Island this afternoon. The instability
provided by the upper trough, along with the lift and moisture
pushed upslope by the sea breezes on the Kona coast, continues to
produce active showers and thunderstorms this afternoon over the
interior of the Big Island. Frequent lightning strikes have been
observed within these thunderstorms this afternoon. The 00Z
sounding at Hilo showed little change over the 18Z launch this
morning, with a 500 mb temperature of -15 degrees celsius and a
700 mb temperature of 3 degrees celsius. The soundings showed a
high lapse rate between 8 to 28 kft with a low level inversion
below 8 kft. We anticipate the thunderstorms to weaken and
dissipate around sunset as land breezes set in and the upper
trough drives eastward.

The smaller islands have been dominated by strong trade winds and
upper level ridging filling in from the west. While winds were
breezy today, most areas stayed below wind advisory criteria for
sustained winds and gusts, through not too far off the mark. The
strong winds are driven by a tight pressure gradient south of
strong high pressure cells centered northeast and northwest of
islands. The trade winds are expected to gradually weaken through
Friday night and maintain moderate breezes into the weekend.
Passing showers moving along with the trades will focus over
windward areas, but winds will occasionally carry showers to
leeward sections of the smaller islands. Moisture associated with
an old front is expected to push in with the trades around Friday,
but mid level ridging is expected to limit total rainfall.

Another upper trough will begin to dig towards the state around
Sunday and may bring another round of active weather through
around Tuesday night or Wednesday. However, there is still
significant uncertainty due to model differences. A surface low
developing far north northwest of the area will likely lead to a
reduction in the local pressure gradient after Sunday, but the GFS
develops another low to the east of the islands that leads to very
light winds. The ECMWF does not develop this second low, and
maintains a light to moderate ESE flow that becomes quite wet due
to the development of a nearby mid-level low. The GFS keeps the
bulk of the moisture east of the islands, and with the second
surface low, but does depict another trough aloft near the
islands. The current forecast is a blend of the two solutions, and
indicates a light to moderate ESE flow, with a broad brush
approach to the PoP forecast.


High pressure north of the state will keep a breezy trade wind
flow in place over the smaller islands through tomorrow. Clouds
and showers will continue to be focused mainly over windward and
mauka areas. Isolated MVFR conditions expected in passing
showers, otherwise, VFR conditions prevailing across all smaller

On the Big Island, light trade winds combined with an unstable
airmass has resulted in scattered light showers along the
Windward slopes and coasts and scattered thunderstorms over the
Interior and the Leeward coast. Light shower activity will
persist overnight on the Windward side, but thunderstorm activity
on the Leeward side will taper off after sunset. Isolated
MVFR/IFR conditions with gusty winds and small hail should be
expected in and near thunderstorms. Mountain obscuration could
become a concern along windward slopes late tonight.

AIRMET Tango remains in effect for low level turbulence over and
downwind of the terrain across all islands. Conditions are
expected to persist into the evening hours.


An ASCAT pass from late this morning showed Small
Craft Advisory (SCA) winds over most coastal waters. Gale
conditions have dropped out of the Alenuihaha and Pailolo Channels
and the Gale Warning has been cancelled. Channels where the wind
speeds are further increased due to nearby islands will still
likely have seas above the 10 ft threshold, otherwise seas are
expected to be just below, as the new incoming west-northwest
swell has been coming in a little lower than expected. This swell
has been following the ECMWF guidance in terms of coming is lower
and slower than the Wave Watch III, but still remains a foot or
two below the ECMWF. Buoys 51101 and 51001 look to be reaching
their peaks, with the buoys reporting near 6 ft, 14 sec as of 3 pm
HST. The front runners started reaching the Hanalei buoy (51208)
around 12 pm HST, and as of 3 pm HST are reporting 2.3 ft, 15 sec.
Within the last hour the Waimea buoy (51201) has started to see
front runners too, reporting 2 ft, 15 sec. The swell should peak
late tonight near the islands and hold through Thursday. Surf will
remain below advisory levels for the exposed north and west facing

Another, slightly larger, long period west-northwest swell is
forecast to start filling in late Thursday night and peak Friday
night, reaching near to advisory levels before subsiding Sunday. A
small pulse from the north is expected Friday and Saturday from a
low to the north. As the trade winds start to trend down over the
next few days, the rough choppy surf along the east facing shores
will also ease up a bit.


Small Craft Advisory until 6 PM HST Thursday for all Hawaiian



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