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Hawaiian Islands Synoptic Discussion and Guidance

625
FXHW60 PHFO 031356
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
400 AM HST Sat Dec 3 2016

.SYNOPSIS...
Active weather is expected across the island chain through Sunday as
another upper level disturbance passes the area while a deep moist
southerly flow continues across the state. Locally heavy rainfall
and thunderstorms will be possible at times. The weather may calm
down briefly Monday and Monday night, though more troublesome
weather may return through the rest of next week.

&&

.DISCUSSION...
Weather conditions did improve over the Big Island since late
Friday night, though there are still lines of showers moving in
from the southeast at times. Elsewhere some heavier showers have
developed over the coastal waters, especially over Kauai channel
while showers also developed and persisted along the Koolaus on Oahu
earlier this morning under the southeast winds. The axis of the
upper level trough is just about over the islands early this
morning...and will continue to shift north. Early morning soundings
revealed an unstable airmass in the island vicinity, and not
surprisingly, Hilo sounding showed rather deep moisture to well over
fifteen thousand feet while Lihue sounding is much drier. As the
moisture is deep enough to reach the Big Island summits, and with
the cold temperature aloft associated with the passing upper level
trough, wintry weather is expected across the Big Island summits
today. Hence, a Winter Storm Warning remains in effect for the
summits of Mauna Kea and Mauna Loa through this evening.

A rather active weather pattern is in store for the state through
the rest of the weekend into next week as a long-wave trough settles
near the states. In turn, a surface trough will persist near the
area, with low level southerly winds in the vicinity. Winds will be
somewhat light across the islands under the mainly southeasterly
winds, allowing night-time land breezes and day-time sea breezes to
become more widespread. The synoptic southerly winds will bring deep
moist tropical airmass to the area. Furthermore, upper level
features that ride along the upper level westerly flow will amplify
somewhat during their passage across the island vicinity, bringing
rather cold temperatures aloft, leading to the airmass in the area
becoming more unstable at times, allowing deep convection to
develop. Therefore, the threat of heavy rain as well as slight
chance of thunderstorms are expected through the rest of the weekend
for the islands. It will be hard to pin-point exactly where the
threat will be the highest, though with the low level wind flow
still has a little easterly component, windward and mountain areas
may see a higher share. A Flash Flood Watch remains in effect for
all islands until Sunday afternoon and may need to be extended.

The next upper level trough is expected to reach the islands
later today into Sunday. Periods of heavy showers and isolated
thunderstorms are possible across the area during its passage. The
focus of the convection will begin with the western portion of the
state today as the feature approaches from the west, then slowly
shift eastward to the Big Island Sunday and Sunday night. A Winter
Storm Watch remains posted for the potential for additional heavy
snow on the Big Island summits Sunday and Sunday night. Snow levels
should be a little bit higher as forecast model indicated 500 mb
temperatures associated with this next trough is slightly warmer.

A slight break on the active weather is possible on Monday as the
upper level feature exits the state east. Another upper level
trough is forecast to approach the islands from the west Tuesday.
Although there are some differences between model solutions upon how
the surface response from this next system will develop across the
islands, the solutions all agree upon a weak broad surface trough
persisting in the island vicinity, with moisture from the tropics
reaching the area at times. Therefore, expect possibly more
disturbed weather to affect the islands next week, with elevated
rain chances.

&&

.AVIATION...
An upper level disturbance lifting northward across the eastern
end of the state will keep conditions unsettled over windward
sections of the Big Island early this morning, with occasional
MVFR cigs and MVFR/IFR vsbys in SHRA. Conditions should improve by
mid to late morning as the upper level disturbance lifts
northward and away from the area, but brief MVFR cigs/vsbys in
SHRA will remain possible through the day.

Over the other islands, conditions are expected to be less active
through the morning hours. With deep tropical moisture along with
an unstable airmass in place however, brief MVFR cigs/vsbys will
be possible in SHRA. Shower coverage may increase this afternoon,
particularly across Kauai, as an upper level trough west of the
state edges closer to the island chain.

AIRMET Sierra remains in effect for mountain obscuration across
the Big Island and Oahu. Conditions are expected to improve here
later this morning. This afternoon and tonight, mountain
obscuration may once again become a problem across portions of the
island chain, particularly across Kauai.

AIRMET Tango remains in effect for moderate upper level turbulence
across the entire state between FL240 and FL400 due to an upper
level disturbance and upper trough in the vicinity of the islands.
These conditions are expected to remain in place through the day.

AIRMET Zulu remains in effect for light to moderate icing from
Oahu eastward through the Big Island between 120 and FL200. These
conditions are expected to improve later this morning.

&&

.MARINE...
East facing shores remain under a high surf advisory through
today. Buoys exposed to the east swell continue to show seas near
10 feet, but seas and swell will gradually decrease tonight and
Sunday as the upstream fetch weakens. A below average NW swell
will decrease through the weekend. The Small Craft Advisory for
most waters will remain through today with seas remaining over 10
feet and some terrain enhanced winds accelerating southeast winds
up to 25 kt.

Mariners will have to contend with lower visibilities and locally
higher winds and waves near thunderstorms the next couple of days.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Sunday afternoon for all Hawaiian
islands.

High Surf Advisory until 6 PM HST this evening for east facing
shores of Kauai, Oahu, Molokai, Maui and Big Island.

Winter Storm Warning until 6 PM HST this evening for Big Island
Summits.

Winter Storm Watch from Sunday morning through late Sunday night
for Big Island Summits.

Small Craft Advisory until 6 PM HST this evening for Kauai
Northwest Waters-Kauai Windward Waters-Kauai Channel-Oahu
Windward Waters-Kaiwi Channel-Maui County Windward Waters-
Alenuihaha Channel-Big Island Windward Waters-Big Island Leeward
Waters-Big Island Southeast Waters.

&&

$$

AVIATION...JELSEMA
MARINE...MORRISON
REST...HUI

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