Heavy Rain in Hawaii, April 7 to 9, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
1004 AM HST Sat Apr 7 2018

.SYNOPSIS...
Wet and unsettled weather will disrupt the trade wind flow this
weekend. A band of deep moisture pooled along a dissipating front
will remain over the state, and an upper level disturbance passing
overhead will trigger bouts of heavy showers and thunderstorms.
Drier conditions and rebuilding trade winds will develop for the
western islands on Monday, while wet and unstable conditions
persist over the eastern end of the island chain. The band of
moisture will gradually break up on Tuesday and Wednesday, and
strengthening trade winds will focus clouds and showers along
windward slopes.

&&

.DISCUSSION...
The tail end of a nearly stationary and weakening front sits over
the Big Island this morning, while a broad and rather deep mid to
upper level trough approaches the state from the west. Meanwhile,
surface high pressure several hundred miles to the north is
maintaining a moderate trade wind flow. The overnight soundings
show the shallow trade wind flow, along with a deep and very
moist atmosphere with precipitable water (PW) values in the 1.8
to 2.1 inches range. The approaching upper level trough is
blanketing all islands with mid to high level clouds and is
triggering heavy showers and thunderstorms along the decaying
front over and around the Big Island. Rainfall has been the
focused across windward slopes this morning, especially on the
Big Island where generally less than an inch has fallen in the
last six hours.

Mostly cloudy conditions and a threat for heavy, flooding
rainfall and thunderstorms is in store today as trade winds
weaken. A Flash Flood Watch went into effect this morning and runs
through Sunday afternoon. While flooding could develop anywhere,
the main concern will be across windward slopes along the decaying
front. With the front drifting northward, this places the
greatest flood risk over Maui County and the Big Island.

The flood threat continues state-wide tonight and Sunday. The mid
to upper level trough will move over the state tonight then
stall, and deep moisture with PW values in the 1.75 to 2 inch
range will linger. This will further destabilize the atmosphere,
and the GFS and ECMWF both show a trough developing within the old
front, which will disrupt the trade wind flow. Again, while
flooding rainfall could occur anywhere, this places the greatest
threat along the forming surface trough, which should be stretched
across the central islands. Thick mid to high level clouds will
continue to blanket most of the state.

Conditions will slowly improve from north to south on Monday. The
upper level will begin to shift eastward, and surface high
pressure will build far northwest of the state. Strengthening
northeasterly trade winds will filter in on Kauai as early as
Sunday night, then build down to Oahu and Molokai on Monday. While
this will bring drier and more stable stable conditions to the
western end of the state, the eastern half of the island chain
will remain under very moist and unstable conditions with PW
values likely holding in excess of 1.75 inches. As a result, a
flood threat could linger.

Moderate to breezy trade winds will prevail on Tuesday and
Wednesday, and the stalled and gradually dissipating band of
moisture will maintain wet windward weather for the eastern half
of the state. Mid level ridging is expected to slowly build in
from the west, but unsettled weather could linger on the Big
Island.

&&

.AVIATION...
With low level moisture remaining in the area after a frontal
passage, mainly windward areas will see periods of MVFR with
lower ceilings and showers as low level flow continues from the
east. Initially, the Big Island will see the most precipitation
since old frontal moisture is closer to that isle. AIRMETs for
mountain obscuration are currently in effect for the windward
sections of all islands because of the moist flow.

An upper air disturbance is expected to move over the state
beginning later today, and this feature will bring the possibility
of more widespread showers and isolated thunderstorms through
Sunday.

&&

.MARINE...
Heavy showers and a few thunderstorms are possible over the
waters through the weekend. Moderate to fresh trades, strongest
over the typically windier areas around Maui County, will hold
through tonight, then weaken through the weekend. Improving
conditions are expected late Sunday into next week with a return
of fresh to strong trades as high pressure builds north of the
region and drier air fills in.

A series of northwest swells is expected over the weekend and
through the upcoming week. The first in the series has already
begun to fill into the local area this morning and is expected to
remain below advisory level today as it peaks along north and
west facing shores. This source will slowly fade through the
second half of the weekend. The second in the series of northwest
swells is expected Monday through Tuesday and should be at least
advisory level, possibly near warning, along north and west facing
shores around the peak Monday night. This one will fade through
midweek. Another long-period reinforcement is possible out of the
northwest Thursday, which could become reinforced by a larger
source out of the same direction Friday into the weekend.

Surf along east facing shores will begin to rise next week as fresh
to strong trades return locally and upstream of the state.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Sunday afternoon for all islands.

&&

Discussion...Wroe
Aviation...Kinel
Marine...TS