Flash Flood Warning 5, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
332 PM HST Tue Feb 28 2017

.SYNOPSIS...
Moist and unstable conditions will maintain a threat for heavy
flooding rainfall and thunderstorms through Wednesday. A deep
trough currently several hundred miles west of Kauai will near
the state late tonight then move over the islands on Wednesday and
Wednesday night. The trough will lift northeast on Thursday,
leaving the islands under a humid southwesterly flow capable of
generating spotty heavy rainfall. A cold front is expected to move
over the northern half of the island chain during the weekend.

&&

.DISCUSSION...
A deep trough currently located about 450 miles west of Kauai is
producing cloudy, moist, and unstable conditions across the state
and is interacting with a surface high centered far to the
northeast to result in moderate east to east-southeasterly trade
winds. The approaching trough has eliminated the inversion and
moistened the depth of the atmosphere to cause precipitable water
(PW) values to climb to 1.5 to 1.6 inches. An area of difluence
aloft on the eastern flank of the trough has been triggering heavy
showers and thunderstorms over Kauai this afternoon, but this area
of active weather has been skirting south of Oahu and Maui County.
Elsewhere, moderate showers have been confined to windward slopes.
On the high Big Island summits, snow started overnight, and a
Winter Storm Warning remains in effect.

The greatest thunderstorm and flash flood threat will be tonight
and Wednesday. As the deep trough advances toward the islands
tonight, the GFS models shows mid level lift increasing, leading
to a further increase in PW and boosting odds for heavy shower and
thunderstorm development. Easterly low level winds will continue
to focus rainfall along windward slopes, though heavy downpours
and thunderstorms could develop anywhere as mid level moisture is
expected to surge up from the south. As the negatively tilted
upper trough axis nears the islands on Wednesday, the GFS and
ECMWF are showing a possible surface low forming near Kauai or
Oahu. In addition to the heavy shower and flood risk, increased
vertical wind shear and instability could lead to the potential
for strong thunderstorms, with the highest odds over the smaller
islands. The window for heaviest snowfall on the high summits of
the Big Island should be at this time as well.

The trough will swing over the state on Wednesday night then
slowly lift off to the northeast on Thursday. The Flash Flood
Watch for the entire state and the Winter Storm Warning for the
high summits of the Big Island currently run through Wednesday
afternoon. If unstable conditions linger as suggested by recent
model runs, we will likely have to extend these watch and warning
products through at least Wednesday night. On Thursday, a humid
south to southwesterly flow will develop as a surface ridge builds
over the islands from the east. The mid to upper level trough may
lag over the islands to trigger a few heavy showers, mainly across
the eastern half of the state.

The humid south to southwesterly flow will persist on Friday as
the surface ridge remains in place. Weak mid level ridging will
build overhead, leading to somewhat dry and stable conditions.

A weak cold front will bring a round of modest rainfall across
portions of the island chain during the weekend.

&&

.AVIATION...
A trough aloft several hundred miles W of Kauai will move E and
deepen. Thick layered clouds with embedded CB associated with the
trough will blanket the area around the main Hawaiian Islands
through Wednesday morning. AIRMET SIERRA for MTN OBSC, AIRMET
TANGO for turbulence aloft and AIRMET ZULU for moderate icing
aloft are in effect and will likely remain in effect through
Wednesday morning. SIGMET SIERRA is in effect for embedded TS
southwest through northwest of the area. The SIGMET is likely to
be extended and may cover the area around the islands at times.

There will be widespread MVFR conditions and isolated IFR conditions
due to low ceilings and low visibility in heavy rain over the next
24 hours.

&&

.MARINE...
Small to moderate NW and NE swells will slowly drop through the
rest of the week with surf remaining below advisory levels. Moderate
to strong winds out of the ESE direction will continue SCA winds
through tonight for the Pailolo and Alenuihaha Channels and waters
south of the Big Island. A surface low forming near near Oahu or
Kauai late tonight and Wednesday is expected to shift winds out
of the southeast over eastern waters with SCA winds expected over
windward Big Island and Maui waters, waters near South Point, and
in the Alenuihaha and Pailolo Channels. Winds then shift out of
the south Wednesday with possible SCA winds over the windward and
leeward Big Island waters.

For the extended, models depict a gale- to storm-force low
developing a few hundred miles WNW of Midway Atoll Wed through Thu,
then tracking toward the NNE across the Date Line Thursday.
Along the way, the low will send a large WNW swell toward the
islands that may result in advisory-level surf over the weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Wednesday afternoon for all Hawaii
islands.

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

Small Craft Advisory until 6 AM HST Wednesday for Pailolo
Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

&&

Public...Wroe
Aviation...Donaldson
Marine...Morrison