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Hawaiian Islands Synoptic Discussion and Guidance

866
FXHW60 PHFO 040655
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
855 PM HST Sat Dec 3 2016

.SYNOPSIS...
An rather unsettled weather pattern can be expected to continue
through the next several days as a low pressure trough and deep
tropical moisture linger near the state. Locally heavy rainfall
can be expected at times though not all locations will experience
these conditions. There will be breaks in the weather at times
with some sunshine possible as well. Winds will be rather light
for the most part favoring daytime sea breezes and nighttime land
breezes.

&&

.DISCUSSION...
Locally heavy shower activity flared up during the afternoon
across parts of the state, but has died down early this evening.
Radar shows only isolated showers over Kahoolawe and East Maui,
with larger areas of showers and isolated thunderstorms over
the offshore waters well north of Maui and southwest of Kauai.
The heaviest rains during the past six hours occurred on Kauai,
where some local amounts approached 3 inches, and flooding closed
Kuhio Highway at Hanalei River Bridge for a few hours during the
late afternoon and early evening.

Surface analysis depicts a surface trough just west of the state,
with another weak trough or low located just north of the central
islands. 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, with another much weaker shortwave trough
exiting northeast of the Big Island. The 00Z Lihue sounding shows
a moist and unstable profile, with no inversion and PW value of
1.47 inches, while the 00Z Hilo sounding was also moist with PW of
1.63 inches. MIMIC-TPW imagery shows a moist plume extending
northward from the deep tropics over the state, to the east of
the upper trough.

Overnight, radar trends and incoming model guidance suggest that
the focus of activity will be over Kauai, with the convection
currently lurking southwest of that island likely to spread over
Kauai late tonight. Elsewhere, a bit of a mid-level dry slot and
lack of forcing/trigger along with developing land breezes may
suppress shower activity over the other islands through the night,
although with an unstable environment in place an isolated heavy
shower or even a thunderstorm cannot be ruled out. Will likely
update the forecast to increase rain chances over Kauai and
decrease rain chances a bit elsewhere through the rest of the
night.

For Sunday and Sunday night, the upper level and surface troughs
are forecast to move slowly eastward across the state. Models show
another massive surge of deep moisture and synoptic-scale lift
moving northward over the islands during this time. A mid-level
dry slot edges eastward into Kauai by Sunday evening, then over
Oahu by Monday morning, but deep moisture remains over Maui and
the Big Island during that time. Heavy showers and isolated
thunderstorms will remain likely during this time period, and the
Flash Flood Watch remains posted for the entire state through
Sunday afternoon. The watch may need to be extended for eastern
and possibly central parts of the state through Sunday night, but
will likely let the Sunday day shift make that decision.

A Winter Storm Warning remains in effect for the Big Island
summits for Sunday and Sunday night, and this warning appears to
be well-justified by the new model guidance. More significant
snowfall is likely for the summits during this time.

A brief 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 250 nm W of Kauai will continue to
support an unsettled weather regime over the islands while
tracking slowly E, currently promoting the development of ISOL
+TSRA over waters about 100 nm SW of Kauai, and spreading a mid-
and high-layered cloud field toward Kauai. Despite this, the
expectation is for somewhat quiet weather to prevail in most areas
overnight. This is due to light winds on the large-scale allowing
island-scale land breezes to promote clearing over land. The
exception may be Kauai, as dynamic forcing from the trough aloft
will drive the formation of +SHRA/+TSRA over adjacent waters that
may move ashore, with associated IFR conditions possible.
Elsewhere, just a few showers are expected over the coastal
waters, with ISOL MVFR conditions.

Considering this scenario, it should come as no surprise that
Kauai is covered by AIRMET SIERRA for mountain obscuration and
also AIRMET ZULU for icing within the layered cloud field. ZULU
may need to be expanded eastward overnight into Sunday as the
trough aloft and associated large-scale ascent shift eastward.
AIRMET TANGO is posted for the entire area due to moderate
turbulence in the upper-levels, associated with the trough aloft.

Sea breeze convergence Sunday afternoon and the passage of the
trough aloft will likely 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...
East facing shores remain under a High Surf Advisory through
tonight due a moderate short-period east swell. Buoys exposed to
this swell continue to show seas ranging from 7 feet at Mokapu
Point to near 10 feet at Hilo, but this swell is expected to
gradually decrease overnight as the upstream fetch weakens.

A Small Craft Advisory (SCA) for most waters will remain in effect
through tonight due to a combination of the east swell and a
moderate northwest swell. The SCA and High Surf Advisory are
likely to be dropped early Sunday morning. Winds will be light and
variable through much of the week. Surf is expected to remain
below High Surf Advisory levels from Sunday through the rest of
the forecast period.

Unsettled weather across the islands is expected to continue
through Sunday night 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 Sunday afternoon for all Hawaii
islands.

High Surf Advisory until 6 AM HST Sunday for east facing shores of
Kauai Oahu Molokai Maui and the Big Island.

Winter Storm Warning from 6 AM Sunday to 6 AM HST Monday for Big
Island Summits.

Small Craft Advisory until 6 AM HST Sunday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Channel-Oahu Windward Waters-
Kaiwi Channel-Maui County Windward Waters-Alenuihaha Channel-Big
Island Windward Waters.

&&

$$

DISCUSSION/MARINE...Jacobson
AVIATION..Birchard

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



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