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Area forecast discussion 
National Weather Service Honolulu HI
338 am HST sun Dec 21 2014

Synopsis...
light winds will continue today...allowing for sea breezes to
return this afternoon and generating a few showers over leeward and
interior areas. A cold front will approach the islands from the
northwest...bringing an increase in showers starting later tonight.
There is potential for heavy rain and a few thunderstorms across the
Aloha state from Monday afternoon through Tuesday. Strong northerly
winds will develop behind the front by middle week.

&&

Discussion...
a few showers have been moving across windward and mountain areas
during the early morning hours...with surface winds remaining generally
light. Winds over The Big Island are now veering to a more southeast flow.
An upper level ridge over the Aloha state will help in keeping a
relatively stable airmass aloft with shower activity remaining low
today. At the surface...a broad high far east-northeast of the islands will
continue to migrate east...which will maintain winds on the light side
today and tonight.

Meanwhile...a deepening trough far northwest of the island chain continues
to gradually approach the Aloha state...with an associated cold
front about 250 miles northwest of Kauai also moving towards the islands.
With this overall synoptic setup...expect another round of sea
breezes developing this afternoon with clouds and showers affecting
some leeward and interior areas. Rainfall should remain light.

The trough/low complex will approach the west edge of the island chain
tonight with shower coverage increasing across the area...and with
possibly enough upper level support for some deep convection to
develop. A colder airmass aloft will arrive with the trough and
further destabilize the atmosphere over the islands. Latest model
analyses show 500 mb temperatures dropping to -12c to -14c. However...
compared to previous runs...the most recent trends in the global
model solutions seem less aggressive in terms of overall available
moisture for significant convection to develop with the frontal passage. X-second
analyses depict a rather dry swath of air above 700 mb Monday through
Tuesday. But the trough should provide enough instability to erode low
level inversions and allow for some vertical development of showers
and any thunderstorm cells that may form.

In general...GFS and European model (ecmwf) show fair agreement in terms of the
timing and overall scenario...bringing a frontal passage through the island
chain and a cutoff low developing NE of the Aloha state Monday through
Tuesday. So far it seems the highest impacts weatherwise are expected
over Kauai County and Oahu...where the highest probability of precipitation and chances for
localized heavy rainfall coverage reside. There is also a slight
chance of thunderstorms from Monday afternoon to Tuesday night.

Strong north-northeast winds will arrive in the wake of the frontal passage by middle
week...then gradually relax through the second half of the week.
Wind will finally veer to a more typical NE flow next weekend as
high pressure builds northwest of the Aloha state.

&&

Aviation...
light winds will allow sea breezes to develop over the islands later
this morning...leading to increased cloudiness over interior areas.
A stable atmosphere will limit cloud and rain showers development...with
isolate MVFR visible/ceiling expected. In the mean time...a light but
persistent east/southeast low-level flow over most areas will deliver isolate
MVFR visible/ceiling in rain showers to windward areas of the islands...mainly from
Oahu to The Big Island. A front arriving from the northwest tonight will
bring increasing north to NE winds and MVFR visible/ceiling in rain showers...affecting
Kauai initially...before spreading to Oahu on Monday. The increasing
winds and rain showers will likely require airmets for low level turbulence
to the Lee of the mountains...and mountain obscuration along north
facing slopes.

&&

Marine...
a long period northwest swell generated by a large and powerful low
pressure system will bring increasing surf heights today...reaching
warning levels tonight. A high surf warning is in effect until
Tuesday morning for north and west facing shores of the smaller
islands...and starting later this morning for west facing shores of
The Big Island.

A high surf advisory also remains in effect for east facing shores
of The Big Island through 6 PM HST today.

This increasing swell will also generate Small Craft Advisory
conditions over most coastal waters due to hazardous seas.

&&

Hfo watches/warnings/advisories...
high surf warning until 6 am HST Tuesday for north and west facing
shores of Niihau Kauai Oahu and Molokai and for north facing shores
of Maui.

High surf warning from 10 am HST today to 6 am HST Tuesday for west
facing shores of The Big Island.

High surf advisory until 6 PM HST today for east facing shores of
The Big Island.

Small Craft Advisory until 6 am HST Tuesday for all Hawaiian waters
except Maalaea Bay.

&&

$$

Discussion/marine...reynes
aviation...birchard

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