Area Forecast Discussion
Issued by NWS Honolulu, HI

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180
FXHW60 PHFO 041328
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
328 AM HST Sun Dec 4 2016

.SYNOPSIS...
An unsettled weather pattern will continue across the state today,
as an upper level trough and deep tropical moisture move over the
area. Active weather may linger over central and eastern parts of
the state tonight into Monday, but somewhat drier conditions
should return to western sections. Another round of wet weather
with locally heavy rains will be possible around midweek. Winds
will be rather light for the most part favoring daytime sea
breezes and nighttime land breezes.

&&

.DISCUSSION...
Abundant high cloudiness is streaming over the state early this
morning. Radar shows shower activity increasing during the past
hour west of Kauai and Niihau, but little activity across the
remainder of the state. Surface analysis depicts a surface trough
just west of the state, with another weak trough or low located a
couple hundred miles northeast of Maui. Weak low-level wind flow
prevails over the islands due to these troughs. Aloft, water vapor
imagery shows a deep/sharp longwave trough axis a couple hundred
miles west of Kauai and moving slowly eastward. The 12Z Lihue
sounding shows a fairly moist and unstable profile, with no
inversion and PW value of 1.40 inches, while the 12Z Hilo sounding
has temporarily dried out and stabilized with PW down to 1.12
inches. MIMIC- TPW imagery shows a moist plume extending northward
from the deep tropics over the state, to the east of the upper
trough. There is a dry pocket evident over and east of the Big
Island, which likely accounts for the drier 12Z Hilo sounding.

Today and tonight, the upper level and surface troughs are
forecast to move slowly eastward across the state. Models show
another surge of deep moisture and synoptic-scale lift moving
northward over the islands beginning later this morning and
continuing into this evening. A mid-level dry slot edges eastward
into Kauai by late this evening, then over Oahu by early Monday
morning, but deep moisture remains over Maui and the Big Island
during that time. While it appears that the onset of increased
shower and isolated thunderstorm activity may be delayed by a few
hours this morning from previous thinking for Oahu through the
Big Island, there will still be the potential for locally heavy
rains across the entire state by later this morning through the
rest of the day, and the Flash Flood Watch remains posted for the
entire state through today. The watch may need to be extended for
eastern and possibly central parts of the state through tonight,
but will let the day shift make that decision.

A Winter Storm Warning remains in effect for the Big Island
summits for today and tonight. More significant snowfall is
likely for the summits during this time. Strong winds will also
cause significant blowing and drifting of snow, along with very
low wind chill readings.

A short break in the unsettled weather is expected from later
Monday through Tuesday as weak shortwave ridging builds over the
state and a somewhat less moist airmass passes through. However,
with light winds continuing, scattered showers will remain
possible along localized convergence zones such as afternoon
seabreezes. It appears that rain chances during this time will be
highest over the Big Island, and lowest over Kauai.

By Wednesday, global models continue to show another deep
mid/upper level trough developing west of the state, with a weak
surface trough over or just west of the islands. This will
maintain deep southerly flow over the islands, although surface
winds will remain rather light. Additional surges of deep moisture
with the potential for more heavy rains over the islands will be
possible during the second half of the week, although pinpointing
details of timing and location is not possible at these extended
time ranges. Overall, there is no indication of a return to quiet
trade wind weather anytime through the next several days.

&&

.AVIATION...
A sharp trough aloft about 200 nm W of Kauai will continue to
support an unsettled weather regime over the islands while
tracking slowly E today. Overcast mid- and high-level clouds
associated with the trough are spreading over all islands from the
SW while isolated thunderstorms are noted to the distant NW and NE
of Kauai. Despite this, the expectation is for somewhat quiet
weather to prevail over most islands this morning, as light large-
scale winds have allowed island-scale land breezes to promote
clearing over land. The exception may be Kauai and adjacent
waters, as dynamic forcing from the trough aloft will drive the
formation of +SHRA/+TSRA that may move ashore, with associated IFR
conditions possible. Elsewhere, just a few showers are possible
over the coastal waters this morning, with ISOL MVFR conditions.

AIRMET SIERRA for mountain obscuration was dropped for Kauai
earlier in the night, but will likely need to be re-issued later
this morning, with additional areas likely to be included as the
weather potentially becomes more active later today and tonight.
AIRMET ZULU for icing within the overhead layered cloud field has
been expanded to include the entire state. This moisture field
should gradually move E, potentially moving E of Kauai and Oahu
later today and tonight. AIRMET TANGO is posted for the entire
area due to moderate turbulence in the upper-levels, associated
with the trough aloft.

Sea breeze convergence this afternoon combined with the passage
of the trough aloft is expected to lead to the formation of
+SHRA/+TSRA over the islands, while the large-scale lift supplied
by the trough may lead to the development of an area of RA
spreading toward Maui and the Big Island from the SW late in the
day.

&&

.MARINE...
While the Saturday evening ASCAT pass found light winds across the
Hawaiian coastal waters, the nearshore buoys off Hanalei and Hilo
continue to report seas near 10 feet, while the buoys off Pauwela
and Mokapu Point are reporting somewhat lower seas. It appears
that a combination of north and east swell is affecting the
waters, and have extended the Small Craft Advisory for many zones
through today. Seas should drop below 10 feet across the area by
tonight.

With seas at the Hilo buoy lingering near 10 feet early this
morning, we have extended the High Surf Advisory for east facing
shores of the Big Island through today. The advisory has been
dropped for east facing shores of the smaller islands.

Winds will be light and variable through much of the week. Surf
is expected to remain below High Surf Advisory levels from tonight
through the rest of the forecast period.

Unsettled weather across the islands is expected to continue today
and tonight due to an upper level trough passing over the area.
Heavy rain and isolated thunderstorms will be possible.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through 6 PM HST this evening for all Hawaii
islands.

High Surf Advisory until 6 PM HST this evening for South Big
Island-Big Island North and East.

Winter Storm Warning until 6 AM HST Monday for Big Island
Summits.

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

&&

$$

DISCUSSION/MARINE...Jacobson
AVIATION...Birchard



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