Heavy Rain in Hawaii, February 7, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
1008 AM HST Wed Feb 7 2018

.SYNOPSIS...
An unstable airmass over the state this afternoon and early
evening will bring the threat of locally heavy showers and
isolated thunderstorms. Drier and more stable weather should
prevail tonight through the weekend as trade winds gradually
return.

&&

.DISCUSSION...
Satellite, surface analysis, and radar reflectivity motions
indicate that a surface trough is located near Molokai, and an
upper trough axis over the Big Island this morning. MIMIC-TPW
imagery depicts the axis of highest moisture now east of the
state, with estimated precipitable water (PW) values between 1
and 1.2 inches over the islands. The 12Z soundings reflect this,
showing rather low PWs near one inch at both Lihue and Hilo, but
also remaining quite unstable with steep mid level lapse rates and
notably cold 500 mb temperatures near -14C.

Skies are mostly sunny this morning with isolated showers popping
up over the coastal waters around Kauai and Oahu. The pleasant
conditions will continue through the rest of the morning. This
afternoon, we anticipate sea breezes to bring cloud build ups,
scattered showers, with a slight chance for heavy rain and
thunderstorms over the islands. This type of weather pattern also
typically brings a higher chance of funnel clouds or waterspouts,
as was observed yesterday. The lift and atmospheric instability
will be in place, however, the low PWs may be the limiting factor
for the heavy rain/thunderstorm chances. The Flash Flood Watch
remains in effect statewide through this afternoon.

Model guidance shows the surface and upper troughs finally easing
to the east of the state tonight and Thursday, with a ridge aloft
building into the area from the west. A weak cold front will also
reach Kauai Thursday morning, then dissipate as it moves down the
island chain through Friday. These developments will bring a
drier and much more stable airmass over the state, with surface
high pressure behind the front bringing increasing northeast trade
winds to the islands during this time. Benign and rather dry
trade wind weather should then continue through the weekend, with
passing mostly light showers over windward areas and mainly dry
conditions leeward.

Forecast uncertainty increases significantly early next week. The
latest GFS, GFS Ensemble Mean, and ECMWF all continue to develop
a deep trough aloft somewhere over the Central North Pacific
during that time, but with major differences in the exact
timing/amplitude/location of the trough. We will need to wait for
later model cycles to sort out these differences, but at this time
there does appear to be the potential for another round of wet
and possibly stormy weather across the islands during the early to
middle part of next week.

&&

.AVIATION...
A strong trough at low and mid levels of the atmosphere is
keeping the airmass over the state quite unstable. Limited
moisture and light winds will be enough to fire another round of
spotty, hit-or miss afternoon convection today, some of which
could briefly be rather vigorous owing to the very cold mid-level
temperatures aloft. MVFR conditions expected in CB, and briefly
MT OBSC/IFR possible in the heaviest activity. Conditions will
improve quickly this evening as the showers fade away and a more
stable northerly flow begins to replace the recent light wind
pattern.

&&

.MARINE...
The current small short period northwest swell will continue to
lower today through Thursday. A new large northwest swell is
expected to arrive by Thursday night. This swell will peak on
Friday with surf heights exceeding advisory levels and could get
to near low end warning levels for a brief time on Friday if the
swell is bigger than Wavewatch guidance. This swell will gradually
lower Friday night through Saturday night. A moderate size
northwest swell is expected to fill in Saturday night and Sunday
with surf heights possibly reaching low end advisory levels. A
series of larger reinforcing northwest swells are expected early
next week. Surf heights will exceed advisory levels and could
reach low end warning levels during the peak of the swells.

Winds and seas will remain below Small Craft Advisory levels
through Thursday. Strengthening trade winds along with a large
northwest swell will likely require a Small Craft Advisory for the
majority of the Hawaiian coastal waters beginning Thursday night
and continuing on into early Saturday. Winds and seas will likely
be lower later Saturday and on into Sunday.

An unstable airmass in the island vicinity may allow for some
thunderstorms to develop over the Hawaiian coastal waters today
and tonight. Locally gusty winds and possible waterspouts may
occur within these thunderstorms.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch until 6 PM HST this evening for all Hawaiian
Islands.

&&

Discussion...Foster/Brenchley
Aviation...R Ballard
Marine...Burke