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Hawaiian Islands Synoptic Discussion and Guidance

179
FXHW60 PHFO 152202 AAA
AFDHFO

Area Forecast Discussion...UPDATED
National Weather Service Honolulu HI
1202 PM HST Fri Dec 15 2017

.UPDATE...
Cancelled the Wind Advisory for Big Island summits and issued a
Winter Weather Advisory for Haleakala summit above 9000 feet for
freezing rain due to expected freezing temperatures and widespread
showers later today into Saturday morning. Freezing rain could
occasionally change over to snow at the summit level in heavier
showers or thunderstorms.


&&

.SYNOPSIS...
Breezy north to northeast winds will continue today, but the
winds will weaken from tonight into the weekend. An upper level
low and increasing moisture will bring unsettled weather to
portions of the state through this weekend. As a result, rain
chances will increase, with locally heavy downpours possible and a
slight chance of thunderstorms. Widespread rainfall is possible
early next week as an upper-level trough pushes toward the
islands from the northwest.

&&

.PREVIOUS DISCUSSION...
A 1035 mb surface high is near 42N138W, or about 1800 miles
northeast of Honolulu. A surface ridge extends southwest from this
high through a point approximately 600 miles north of Kauai.
Elsewhere, a north to south oriented surface trough, which is
moving slowly westward, is evident about 250 miles east of Hilo.
The tight pressure gradient between these features is maintaining
fresh to strong north to northeast winds across the state. A Wind
Advisory remains in effect for the Big Island Summits this
morning. Note that the breezy conditions combined with low dew
points (in the 50s) continue to make sensible temperatures feel
relatively cool.

Aloft, middle and upper-tropospheric lows are located around 240
miles northeast of Hilo. The surface trough described above is a
reflection of these features, which are propagating slowly toward
the west-southwest. The close proximity of the lows appears to
have already destabilized the atmosphere over the eastern end of
the state. The early morning Hilo balloon sounding shows no
significant low-level inversion. Satellite imagery and radar
reflectivity data this morning indicate mainly broken low clouds
with embedded scattered showers moving down from the north and
northeast toward the windward and north facing sides of most
islands east of Oahu.

The upper level lows, as well as the surface trough, will
approach the islands today. As a result, the north to northeast
winds will gradually weaken tonight. The upper lows are expected
to reach a position just northeast of the islands on Saturday, and
then slowly pull away toward the northeast on Sunday. By Saturday,
with the surface trough in the vicinity of the islands, the background
flow will become light and variable across most areas. This will
allow the development of local afternoon sea breezes. By Sunday,
the surface trough is forecast to be located west of the state,
with light southerly winds gradually spreading from east to west
across the area.

The cooler temperatures aloft associated with the middle and
upper-level lows will be close enough to destabilize the atmosphere
today, especially over central and eastern portions of the island
chain. In addition, increasing moisture will move in to the
islands as the surface trough approaches the state. This will
likely usher in an extended period of unsettled weather across the
region, with increasing chances for rainfall, and the possibility
of locally heavy downpours and isolated thunderstorms through the
weekend. In addition, there is a chance of rain mixed with snow
on the Big Island Summits. By Saturday afternoon, daytime heating
and local sea breezes may provide additional support for deep
convection over the islands.

The upper lows will lift away to the north late Sunday, and may
provide a brief respite from the active weather on Monday.
However, an upper- level trough is forecast to dig down toward the
islands from the northwest, with deep tropical moisture pushes up
from the southeast Monday night into Tuesday. This may provide
widespread showers overspreading the islands. In addition, a
couple of cold fronts may sweep across the islands from the
northwest later in the week, which may result in an extended
period of northwest winds.


&&

.AVIATION...
Locally strong north to northeast winds will continue today,
before becoming light and variable Saturday as a surface trough
east of the state moves west. AIRMET TANGO for low level
turbulence is posted across the state. This will likely be able
to be cancelled this evening as winds ease.

An upper level low just northeast of the state will gradually
move west over the next day. This low will help to destabilize
the atmosphere around the state, and will help to enhanced passing
showers showers today. Showers could be locally heavy at times,
carrying MVFR to isol IFR conditions. The forecast also introduces
a slight chance of thunderstorms from Oahu through Big Island
this afternoon. AIRMET SIERRA for mountain obscuration cannot be
ruled out today, and will be issued as needed.


&&

.MARINE...
A surface trough northeast through east of the Big Island will be
moving westward across the coastal waters over the next several
days. The trough will weaken the winds over the eastern waters
initially today then over the western waters later tonight. Winds
should remain light through early next week.

Observations from the nearshore PacIOOS buoys have been showing a
declining northwest swell through the morning. This trend should
continue through the rest of today. The nearshore buoys are also
showing a short period northeast wave component produced by strong
northeasterly winds west of the trough. The northeasterly swell
is large enough to warrant a High Surf Advisory for east facing
shores. As the winds decrease, the northeasterly swell will
decrease and drop below advisory level, possibly by early Saturday
morning.

As the winds and seas gradually subside, conditions are expected
to drop below Small Craft Advisory criteria, possibly by tonight
for all waters through Monday.

Heavy showers and isolated thunderstorms will be possible across
parts of the coastal and offshore waters today through at least
Saturday due to a nearby upper level trough.

See the Collaborative Nearshore Swell and Wind Forecast for
details on expected swell and resulting surf.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Saturday for east facing shores
of Kauai, Oahu, Molokai, Maui and Big Island.

Winter Weather Advisory from 4 PM this afternoon to noon HST
Saturday for Haleakala Summit.

Small Craft Advisory until 6 PM HST this evening for all Hawaiian
waters, except the Big Island Southeast waters.



&&

$$

UPDATE/PREVIOUS DISCUSSION...Foster
AVIATION...Eaton
MARINE...Kodama

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office



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