Flash Flooding and Debris Flows in Hawaii, February 22 to 26, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
401 AM HST Thu Feb 22 2018

.SYNOPSIS...
Another round of unsettled weather is upon the islands once more,
with the wet weather spreading to the smaller islands from the
south and east today. Periods of heavy rains and a slight chance
of a thunderstorm will persist into the weekend. Possibly some
improvement on Sunday.

&&

.DISCUSSION...
A Flash Flood Watch will include all of the major Hawaiian
Islands, beginning shortly at 6 am HST. Also, a Winter Weather
Advisory is in effect for the Big Island summits. These weather
bulletins are good until late Friday afternoon, with the
possibility of an extension.

It has been a relatively dry night overall. The windward side of
Oahu is currently undergoing another flare up in showers after
being quiet all night. There has been a couple of surges of brief
showers across windward Maui and windward Big Island as well but
nothing sustaining. There is abundant of showers located over the
adjacent waters of eastern Big Island. Most of these showers are
embedded in the low level east to southeast flow. Some of these
showers are having a tough go in reaching the shoreline, possibly
due to an off shore land breeze.

There is a surge of thick, dense layered clouds with patchy
stratiform rain, toward the north and the smaller islands. This
weather should be here by sunrise, should it survives. A piece of
this weather is already affecting part of leeward Big Island, with
some light rain.

Water vapor imagery shows the upper low in the process of setting
up shop near 26n170w, or 750 miles WNW of Kauai. Associated temps
of minus 12 to minus 14 degree C at 500 MB, will cover mainly
Kauai and Oahu. A few spotty, isolated thunderstorms are possible
under this air mass. Bulk shear values are marginal at best, 25 to
30, certainly no where near to that of a couple of days ago with
values in the 60's. Thunderstorms are most likely over Kauai and
Oahu this afternoon as per GFS solutiion.

The low will linger there to about Saturday night, before lifting
off toward the NW on Sunday. In doing so, a stiff moderate to
strong trade wind flow fills in across the islands that will last
well into the new week. The upper trough re-deepens on Tuesday, a
tad further west along 175w longitude, meaning Kauai will be just
out of reach of any inclement weather.

The current low level flow is easterly, light to locally moderate
in speed. Both the ECMWF and GFS solutions are point to a SE flow
across the smaller islands today, and an easterly flow around the
Big Island. This pattern is expected to hold into Saturday. Under
this flow, the smaller islands will be blocked, leading to
lighter easterly or variable winds. The SE winds blowing across
the exposed windward waters of Maui and the Big Island, will
strengthen to locally strong by this evening, leading to the
issuance of a Small Craft Advisory which will come later this
morning. The waters immediately south of the Big Islands will
undergo this process today as well.

&&

.AVIATION...
A low aloft about 650nm NW of Kauai is forecast to move SE
slowly. SW flow aloft is carrying middle and high clouds over the
main Hawaiian islands and the approaching low is making the
atmosphere increasingly unstable. Light icing is possible in the
middle and high clouds. This possibility has been mentioned in the
AIRMET bulletin, but conditions are expected to remain below the
AIRMET threshold.

Near the surface, a trough to the west and a ridge to the east are
producing E to SE flow. The onshore flow has produced MVFR ceilings
over the E sides of most islands. AIRMET SIERRA will remain in effect
for MTN OBSC

There have been isolated thunderstorms in the coastal waters
overnight. As the low aloft moves closer today, thunderstorms are
likely to increase in coverage and develop onshore, producing
localized IFR ceilings and visibility.

&&

.MARINE...
The northwest swell will hold today, but surf heights should
remain below the advisory level. After this swell fades Friday
into the weekend, long term guidance does not favor another WNW/NW
swell through at least next week.

Persistent high pressure northeast of the state will continue to
produce an area of strong winds directed towards the area. ESE/SE
winds are expected to increase around the state today, and remain
elevated through at least the middle next week. The strongest
winds will be around the Big Island and windward Maui County
waters. The swell generated from these winds will produce elevated
surf along east-facing shores. A High Surf Advisory will likely
be needed for these areas by Thursday night or Friday and could
potentially reach warning levels by the middle of next week. The
combination of the elevated seas and increased winds will produce
Small Craft Advisory conditions starting tomorrow evening, mainly
for the channels and exposed windward waters.

See the latest Oahu Surf Discussion (SRDHFO) for additional
details on surf and swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from 6 AM HST this morning through Friday
afternoon for Niihau-Kauai-Oahu-Molokai-Lanai-Kahoolawe-Maui.

Flash Flood Watch through Friday afternoon for Kona-South Big
Island-Big Island North and East-Kohala-Big Island Interior.

Winter Weather Advisory until 6 PM HST Friday for Big Island
Summits.

&&

DISCUSSION...Lau
AVIATION...Donaldson
MARINE...TS