Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Honolulu HI
330 PM HST Wednesday Apr 16 2014

Synopsis...
trade wind weather will persist through the forecast period as high
pressure dominates to our north. Winds will increase over the
weekend into early next week as this high pressure strengthens.
Clouds and showers will continue to favor windward and Mauka areas.
Leeward areas of The Big Island will see increased clouds and
showers each afternoon and early evening.

&&

Discussion...
there is little to no change in forecast philosophy from this
morning. The axis of a surface ridge north of the main Hawaiian
islands extends through 34n140w and 26n180w...spanning the central
North Pacific. This ridge will play a key role in our weather
through the next week by keeping trade winds blowing across local
waters. Water vapor loop shows a northeast to southwest oriented
upper trough across our area...with an axis extending through
29n140w and 15n165w. Latest soundings show evidence of both weather
features...with strong trade inversions marking subsidence
associated with a low-level ridge and negative lifted index values
marking the mild instability associated with the upper trough.

The pressure gradient between the ridge to our north and equatorial
low pressure is sufficiently steep to support moderate to fresh
northeast trade winds across local waters this afternoon. Winds
remain slightly stronger in the traditionally windy channels and
waters...and small craft advisories will remain in effect there
through Friday afternoon. However...small craft advisories elsewhere
are cancelled since wind speeds overall are beginning to decrease.
Wind waves have decreased in height as well...so the high surf
advisory for east-facing shores is also cancelled. Models show the
ridge is expected to weaken slightly over the next few days as a
developing low tracks northeastward within the storm track to its
north. A stronger high is expected to develop along the ridge west
of 160w by Sunday...then continue to strengthen as it tracks
eastward early next week. This will cause trade winds to strengthen
again.

Clouds and showers embedded within the trades will favor windward
and Mauka areas through the forecast period. However...return flow
and daytime heating will also cause increased clouds and showers
across leeward Big Island areas each afternoon and early evening.
Continued low level ridging will tend to lower the inversion height
over time...so we expect overall shower activity will diminish.
However...models show a modest plume of more humid air will move
over the eastern islands from Thursday through Friday...briefly
increasing trade shower coverage across those areas.

In short...we expect trade wind weather through the forecast
period...with minor variations in trade wind speeds and trade shower
coverage.

&&

Aviation...
moderate trade winds from surface to about 15k with subsidence
inversions between 8k and 10k aloft expected to persist through 24
hours and beyond. Will be keeping airmet Tango for mechanical turb
in effect indefinitely.

Clouds and showers will favor the typical windward and mountain
areas with a few of these showers drifting across leeward portions
of the smaller islands. MVFR ceilings are isolated...but are expected to
be on the increase this evening along the NE slopes and coast of The
Big Island. Airmet Sierra for for Montana obscure may be necessary for NE
Big Island starting late this evening.

Sea breeze driven clouds and isolated showers along the Kona coast
and adjacent waters this afternoon will dissipate after sunset.

&&

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 Thursday below
advisory level. No other significant swells are expected.

&&

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

&&

$$

Marine/public...Powell
aviation...Bedal