Flash Flooding and Damaging Wind in Hawaii, February 15 to 19, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
317 AM HST Thu Feb 15 2018

.SYNOPSIS...
The wet pattern will continue into the weekend as a trough remains
in the area. The best chance for heavy showers will remain over the
eastern end of the state through Friday, but will begin to shift
back toward the west later in the weekend and early next week as
another upper low drops south into the area. Winds will remain light
and variable through early next week.

&&

.DISCUSSION...
Light to moderate showers continue to develop and lift northward
over the islands this morning due to a weakening front near Maui and
an upper low centered a couple of hundred miles northwest of Kauai.
Overnight rainfall accumulations have remained light except for Maui
where showers near the front continue to train over the same areas.
Similar to Oahu through the day Wednesday, peak totals at the gages
on Maui have reached the 2 to 3 inch range (3.19" at Puu Kukui as of
1 AM HST ) over the past 12 hrs. Although rainfall rates have
remained steady at around .25" per hour, heavier showers with
locally higher rainfall rates will continue to develop periodically.
As a result, the flash flood watch remains in place today for Maui
and the Big Island.

The latest hi-res model reflectivities support this and depict the
line of showers continuing to impact Maui through the morning hours
today, then gradually shifting eastward and becoming focused over
the Big Island later today through Friday. Although a storm or two
can't be ruled out over these areas today, the best chance for
thunderstorms will remain north of the islands across the offshore
waters where a better source of deeper moisture and instability
reside. Over the western end of the state, mainly sprinkles with
some light showers will remain possible through the morning hours
before gradually clearing.

Friday through early next week, although the upper low northwest of
Kauai is forecast to lift northward and away from the local area
Friday, guidance remains in good agreement and depicts another upper
low dropping south into the area just west of Kauai come Sunday. The
lingering surface trough over the eastern end of the state will
begin to drift back toward the west. This surface trough combined
with falling upper heights, cold pool aloft (-15 C at 500 mb near
the center of the upper low) destabilizing the atmosphere, and a
sufficient amount of deep moisture will support increasing shower
and even thunderstorm chances late Saturday night into next week.
This general pattern will hold into midweek before the upper low and
surface trough begin to shift westward and away from the state.

&&

.AVIATION...
AIRMET SIERRA for occasional mountain obscuration is in effect for
the smaller islands through the day as bands of showery low
clouds along a front affect the islands. AIRMET TANGO for
occasional moderate turbulence from 150 to FL350 is in effect for
the entire area. Most TAFS contain TEMPO groups for MVFR
visibilities and ceilings within showers.

&&

.MARINE...
An ill-defined front is temporarily stalled near Maui this early
Thursday morning. Some of the models are nudging the front
eastward onto the Big Island later this morning, where it will
stall and weaken to a north to south oriented trough. This trough
then shifts westward across the rest of the island chain during
the course of the three day holiday weekend.

The current NW swell is hanging tough, but is starting to show
signs of lowering at a faster pace in the past hour. A series of
NW swells, from 310 degrees, will be reaching our north and west
facing shorelines well into the middle of next week. These swells
will be small to moderate in size except for one, where it will be
large enough to warrant a high surf advisory. Fore-runners from
this swell are expected to reach our north and west shores Tuesday
night, and max out Wednesday night at 7 feet and 18 seconds. It
should be noted that the first of this series of NW swells, is
expected to reach buoy 51001 now, located 200 nm NW of Kauai. It
is a small swell of 3 feet and a period of 18 seconds. So far, it
has not.

Surf will be building along our E facing shores over the weekend
and into early next week. The source of this swell is from a
broad and strong surface high located far to our NE. Surf heights
are expected to be below advisory level.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through this afternoon for Kahoolawe-Maui-Kona-
South Big Island-Big Island North and East-Kohala-Big Island
Interior.

Wind Advisory until 6 PM HST this evening for Big Island Summits.

&&

AVIATION...Powell
MARINE...Lau
REST OF DISCUSSION...Gibbs