Flash Flood and Heavy Rain in Hawaii, February 19 to 23, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
645 AM HST Tue Feb 19 2019

.SYNOPSIS...
Widespread light to moderate rain will affect the islands from Oahu
to the Big Island this morning, with brief heavy showers possible on
Kauai. Thunderstorms will be possible as the island atmosphere is
moist and unstable, but rain chances will be diminishing over Kauai
and Oahu by afternoon. A continued drying trend is expected nearly
statewide tonight into Thursday, with light winds expected. Moisture
may linger near the Big Island into Friday, but a light trade wind
regime will prevail statewide by the weekend, with limited windward
showers favoring nights and mornings.

&&

.DISCUSSION...
Satellite imagery shows a deep moisture plume across most of the the
state this morning, with a few thunderstorms embedded in the deep
southerly flow. Widespread light to moderate rain is affecting the
area from Oahu to the Big Island, while convective showers and
thunderstorms are noted near and W of Kauai. Expect enhanced shower
activity over the eastern half of the state through Wednesday, with
a drying trend from W to E expected later today in response to a
lifting trough aloft that also slides E.

Showers will trend higher through the day over Maui and Hawaii
Counties as low level convergence associated with the surface trough
lines up with an area of upper level divergence. Periods of heavy
rain showers are forecast to develop along with isolated to
scattered thunderstorms through the afternoon hours. On Wednesday,
this trough and moisture plume will change direction and drift back
eastward with decreasing showers over Maui County, and continued
showers over the Big Island.

A Flash Flood Watch remains in effect for Maui and Hawaii Counties
through this afternoon. This watch may be extended into Wednesday if
heavy rainfall is forecast to continue. Temperatures will remain cold
enough to keep snow levels around 12,000 foot elevation. This means
periods of heavy snowfall is likely over the Big Island summits of
Maunakea and Maunaloa through Wednesday morning. Wind speeds at the
summit level are currently reaching advisory criteria this morning
and a Wind Advisory remains in effect for both mountain peaks. A
combination of heavy snow, icy roads, and strong winds will create
treacherous winter driving conditions with periods of near zero
visibility in white out conditions on the road to the summits. A
Winter Storm Warning remains in effect through Wednesday morning.

On Thursday the surface trough will move over the eastern side of
the Big Island and elevated showers will likely develop over the
eastern and southeastern sections of the island. From Friday
onward model solutions continue to diverge between the American
(GFS) and the European (ECMWF) models. These differences appear to
be related to the strength and eastward progression of the high
pressure center moving into the Central Pacific basin. The latest
model run from the ECMWF is showing a stronger high center far to
the north of Kauai with a drier weather solution for the islands.
While the GFS model on the other hand shows a weaker high
northwest of Kauai with a wetter solution with scattered showers
across windward and mountain areas. The precipitation grids for
this weekend were created using a blended approach to minimize
errors in one model solution.

&&

.AVIATION...
A deep upper level trough lies nearly stationary just to the west
Kauai, but it is expected to move E while the parent upper low lifts
northward.

With the upper trough west of Kauai, abundant moisture mainly
above 10k feet, continues to stream north across all but kauai
from the tropical south, with the base of this layer at around 12k
feet, extending to fl350. Conditions are ripe for light icing
between 15k and 25k feet through today into tonight. Also, the air
mass will become more supportive for isolated CB/thunderstorm
activity throughout today, and lasting through tonight.

AIRMET Sierra, for mountain obscuration is in place for the
eastern part of the Big Island, from Laupahoehoe to Mountain View
to South Cape, and this AIRMET is likely to continue through
tonight.

There is a SIGMET in our local area, basically from just east of
the upper trough axis to western Big Island, between fl280 and
fl330. The SIGMET includes the area of sharp moisture gradient as
depicted on water vapor imagery. AIRMET Tango is in effect for
tempo moderate turbulence elsewhere across the rest of the island
chain between fl250 and fl350.

This heavy weather is not expect to advance over to Kauai. But the
Garden Isle will experience some scattered showers throughout
today. No AIRMETs are expected, however.

&&

.MARINE...
A trough of low pressure will move into the waters east of the
Big Island today, then linger in this vicinity through late this
week. Light to locally moderate east to southeast winds will
continue today, before becoming light and variable tonight
through Wednesday night. The one exception, is for the coastal
waters east of the Big Island, where moderate to strong southeast
winds could develop later today and tonight. Weak high pressure is
expected to develop north of the state Thursday through the
weekend and we should see a return of light to locally moderate
north to northeast winds when this occurs.

As the current north swell declines today, a new short-period
east swell will begin to build along east facing shores of the Big
Island and Maui. This east swell will then build westward across
the rest of the smaller islands tonight. The swell is expected to
peak on Wednesday and gradually decline Thursday through rest of
the week. Due to the new incoming east swell, a High Surf Advisory
(HSA) remains in effect for east facing shores of Maui and the
Big Island through Thursday. Surf has dropped below advisory
levels along east facing shores of Kauai, Oahu and Molokai this
morning, as the north swell has diminished. The new east swell is
not expected to build surf to advisory levels in these areas until
tonight, so the HSA was adjusted to begin at 6 PM this evening.
The HSA for north facing shores of Kauai and Niihau has been
cancelled.

A Small Craft Advisory (SCA) is in effect for most waters around
Kauai this morning, with seas expected to drop below advisory
thresholds by noon today. A SCA is also in effect for the windward
waters of Maui County and the Big Island today through Wednesday
night due to elevated seas. This SCA will likely need to be
expanded to include most of the windward waters around Oahu and
Kauai tonight or Wednesday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory from 6 PM this evening to 6 PM HST Thursday for
East facing shores of Kauai Oahu and Molokai.

Flash Flood Watch through this afternoon for Maui and Hawaii
Counties.

High Surf Advisory until 6 PM HST Thursday for East facing shores
of Maui and the Big Island.

Wind Advisory until 6 AM HST Wednesday for Big Island Summits.

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

Small Craft Advisory until 6 AM HST Thursday for Maui County
Windward Waters-Alenuihaha Channel-Big Island Windward Waters-
Big Island Southeast Waters.

Small Craft Advisory until noon HST today for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters.

&&

DISCUSSION...Bohlin/Birchard
AVIATION...Lau
MARINE...Jelsema