Flash Flood Watch 7, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
319 AM HST Sun Oct 22 2017

.SYNOPSIS...
Afternoon clouds and showers are expected to become focused over
interior and leeward areas today as sea breezes develop and push
inland. Some heavier showers over the eastern half of the Big Island
will be possible later today and tonight as moisture begins to
increase and fill in from the east. Monday through Wednesday, heavy
showers and thunderstorms associated with a strong cold front that
is forecast to move down the island chain could lead to flash
flooding from Oahu to the Big Island. Winds are forecast to become
gusty across the Big Island Summits late Monday through Tuesday as a
strong upper trough digs south over the region. Cool and dry air
will fill in behind the front through the second half of the week.

&&

.DISCUSSION...
After a long stretch of breezy to windy conditions across the
islands, trade winds have dropped off considerably through the
overnight hours. Observations are depicting light offshore winds
(land breezes) this morning at many stations across the islands. The
low-level wind flow will gradually shift toward the south through
the day as a cold front begins to approach from the northwest. The
latest surface analysis showed this frontal boundary north of Midway
extending westward from a developing storm-force low around 1000
miles north-northwest of Kauai. A weak surface trough was analyzed
north-northeast of Maui County, marking the leading edge of a broad
area of higher moisture. The latest satellite-derived precipitable
water (PW) imagery reflected this and showed a large area of 1.75 to
2" PWs approaching the Big Island from the east.

The latest short-term (today-Tuesday) guidance remains in good
agreement and indicates increasing deep layer moisture filling in
across the islands as a cold front and upper trough approach from
the northwest. Model PWs are forecast to rise into 1.75 to 2" range,
with the highest moisture expected to become focused over the
eastern half of the state in response to a pre-frontal convergence
boundary setting up Monday. The low-level flow is forecast to shift
toward the south today, then toward the south-southwest and
strengthen Monday and Monday night (850 mb SW winds 25-35 mph) ahead
of the front. The front is forecast to reach Kauai Monday night,
then progress eastward over the rest of the island chain Tuesday
through Tuesday night. A combination of falling upper heights
associated with the upper trough, increasing instability and
isentropic lift, shear (effective 0-6 km bulk shear up to 40 kt),
and plenty of deep tropical moisture will support heavy rain and
thunderstorms developing.

Impacts associated with this front and upper trough will include:
the potential for flash flooding, strong thunderstorms (an isolated
severe storm can't be ruled out), gusty downslope winds mixing down
over windward areas (Oahu and Kauai), and the potential for a wintry
mix over the summits Monday night into Tuesday. A flash flood watch
has been issued this morning that includes Oahu, Maui County and the
Big Island for the Monday through Tuesday period. A high wind watch
has been issued for the summits on the Big Island for late Monday
through Tuesday as southwest winds ahead of the upper trough
strengthen and become hazardous (gusts to 65 to 70 mph).

Drier and cooler air with north-northeast flow across the islands
is anticipated to fill in from west to east Tuesday through
midweek in the wake of the front. Dewpoints are forecast to reach
the 50s over Oahu and Kauai Tuesday through Wednesday. Guidance
supports this drier air spreading across the eastern half of the
state through the second half of the week as the slowly nudges
eastward.

&&

.AVIATION...
Light and variable winds today will lead to afternoon sea breezes
in many locations, which will result in afternoon clouds and
showers. Shower activity will continue tonight and into Monday as
a front approaches from the northwest. VFR conditions are expected
to prevail today, with some brief periods of MVFR in the afternoon
showers. Shower activity is expected to increase and possibly be
heavy late tonight and Monday, increasing the possibility of MVFR
or even IFR conditions. No AIRMETs at this time, but cannot rule
one out late today or tonight.

Overnight soundings show the trade wind inversion at both Lihue
and Hilo beginning to break down. Cloud tops this morning are
likely to be between 6500 and 8000 ft, but with afternoon shower
development and a more unstable airmass moving overhead, cloud
tops could reach over 12000 ft tonight, with the highest tops most
likely over the southern half of the island chain.

&&

.MARINE...
A ridge north of the area will shift south ahead of a front
approaching from the northwest. Winds will weaken and shift out
of the southeast today as the ridge moves over the area. Winds
may shift out of the south Monday and could become locally fresh
again as the front approaches. The front is expected to reach the
Kauai waters Monday evening and push down to the Big Island waters
by Tuesday evening. Heavy showers and thunderstorms are possible
ahead of and along the front. Fresh to locally strong north winds
will spread over the islands behind the front. The winds will
slowly veer to northeast by the end of next week.

A northwest swell expected later today and Monday will likely
produce surf near the advisory threshold along north facing shores
overnight. This swell will diminish Monday night, but a bigger
northwest swell Tuesday and Wednesday will likely produce surf
well above the advisory threshold for north and west facing
shores. The swell will also boost combined seas above the 10 foot
level, so a Small Craft Advisory will likely be needed. An even
larger northwest swell is forecast to arrive Friday. That swell
could produce warning level surf along north and west facing
shores.

A low east of New Zealand generated a swell which will produce surf
near the advisory level along south facing shores later today and
Monday. See the Collaborative Surf Discussion for details.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from Monday morning through Tuesday afternoon
for Oahu-Molokai-Lanai-Kahoolawe-Maui-Big Island.

High Wind Watch from Monday evening through Tuesday afternoon
for Big Island Summits.

&&

DISCUSSION...GIBBS
AVIATION...BALLARD
MARINE...DONALDSON