Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Honolulu HI
330 PM HST Thursday Apr 17 2014

Synopsis...
trade wind weather will continue across the main Hawaiian islands
through the forecast period as high pressure dominates to our north.
Clouds and showers will favor windward and Mauka areas. Clouds and
showers will increase each afternoon and evening across leeward
portions of The Big Island. Total rainfall will be light.

&&

Discussion...
there is little to no change in forecast philosophy from this
morning. A 1024 mb high near 30n148w...about 870 miles northeast of
the main Hawaiian islands...has an associated ridge extending east
through 30n140w and west through 25n180w. This ridge has moved very
little over the past 12 hours...and the pressure gradient between
this ridge and equatorial low pressure is sufficiently steep to
support moderate to locally strong easterly trade winds across local
waters this afternoon. Small craft advisories remain in effect for
the traditionally windier channels and waters around Maui and The
Big Island. These have been extended through Saturday given latest
model guidance. Scant showers embedded within the trades continue to
see enhancement from a persistent upper trough across the islands. A
ragged band of high clouds extending southwest through northeast
across The Big Island and adjacent waters Marks the eastern flank of
this upper trough. Airmass stability varies from slightly unstable
near Kauai with cape over 750 j/kg and a negative lifted index
value...to stable over The Big Island with cape less than 50 j/kg
and a positive lifted index value. The trade inversion from
yesterday is higher and weaker this afternoon...indicating that the
upper trough continues to have a destabilizing effect.

Models show stronger high pressure will build along the ridge
northwest of the main islands over the weekend...then push eastward
early next week. This may give a slight boost to trade winds across
local waters from Sunday Onward as the gradient steepens. The upper
trough will persist into the middle of next week before
filling...keeping any trade showers active especially at night.

In general...expect trade wind weather to continue through the rest
of the forecast period with rainfall focusing over windward and
mountain areas. The main exception will be the leeward side of The
Big Island...where afternoon and early evening clouds and showers
are expected each day as onshore flow develops.

&&

Aviation...
radar VAD wind profiles and 00z radiosonde observations show winds generally 15-20
knots. Only acars flights from Kahului have winds of 25-30 knots.
Conditions are borderline for Lee turbulence...and we have not
received any low-level reports since last night. However...models
show winds picking up again through the night. With some strong
winds at the surface and the stronger low-level winds near Maui...we
will keep the airmet going for moderate low-level turbulence.

An airmet is also in effect for moderate upper level turbulence.
Strong wind shear and deformation accompany the upper trough that
extends across the state. GFS guidance highlights the greatest
potential for clear-air turbulence in the area to the northeast of
the state. However...recent water vapor imagery shows this area
moving westward. This potential for high-level turbulence may
continue through the night...continuing its westward movement.

Clouds and showers remain focused primarily across windward
sections. Conditions will be predominately VFR...with occasional
MVFR ceilings and visibilities in showers. Clouds east of Maui have
thinned out this afternoon...but are expected to redevelop and
enhance showers across windward sections overnight into Friday
morning. We may eventually need an airmet for mountain obscuration
if the low ceilings become more widespread and persistent.

&&

Marine...
a Small Craft Advisory remains in effect through Friday afternoon
for typically windier areas around Maui and The Big Island.

A small swell from the west northwest will peak below advisory
levels later today. No other significant swells are expected.

&&

Hfo watches/warnings/advisories...
Small Craft Advisory until 6 PM HST Saturday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island leeward waters-Big
Island southeast waters.

&&

$$

Discussion/marine...Powell
aviation...bravender