High Wind and Heavy Rain in Hawaii, April 27 to 30, 2017

Area Forecast Discussion...UPDATED
National Weather Service Honolulu HI
1013 AM HST Thu Apr 27 2017

.SYNOPSIS...
Trade winds will ease and shift to the southeast today and tonight
as a cold front approaches the state. On Friday, the front and
associated upper trough will bring increasing showers that may
become heavy at times. The front will move through Friday and
stall over Maui County or near the northern Big Island over the
weekend. Wet and unstable conditions are expected over much of the
state again this weekend and into early next week.

&&

.DISCUSSION...
Trade winds will begin to weaken and veer southeasterly today as
a cold front moves approaches from the northwest and the subtropical
high retreats away from Hawaii. Aloft, a narrow mid level ridge
is located over the state, between an upper level low several
hundred miles southeast of the Big Island and a large mid/upper
level trough moving east from the international dateline. The 12Z
soundings reflect a dry and stable atmosphere remaining in place,
with PW values ranging from 0.92 to 1.11 inches, and strong
inversions based near 7000 feet.

The weakening southeast background flow will allow humid
conditions and seabreezes to develop in over much of the state
this afternoon, while trade winds will hang on a little bit longer
across windward parts of Maui and the Big Island. A deep
mid/upper level trough driving the surface front will also advance
southeastward today, displacing the mid level ridge initially
over the islands. This will cause the inversion to begin to weaken
and lift, and this decreased stability combined with marginal
moisture availability and a low level trigger provided by the
expected seabreeze regime could allow for some spotty heavier
showers to develop during the afternoon, especially over leeward
and interior areas.

The atmosphere will become more unstable as the upper trough
brings colder temperatures over the state by early Friday morning.
Models depict a cold pool for -12 celsius 500 mb temperatures by
morning. The instability has the potential to tap into the prefrontal
moisture over the state. Thus, have added a slight chance of
thunderstorms and possible locally heavy rain for most of the
state on Friday and Friday evening. The front itself will reach
Kauai Friday afternoon and, Oahu on Friday night, then stall over
eastern Maui County or the northern Big Island by Saturday
afternoon.

A much drier airmass and breezy north winds is expected to funnel
into the western end of the state early Saturday morning. This may
may provide a brief reprieve from the active weather on Saturday
for Kauai and maybe Oahu. With the front expected to stall near
Maui County and the Big Island, higher moisture levels and light
winds will likely keep the potential for heavy rain for the
eastern end of the state on Saturday.

Meanwhile, a very cold upper level low will be cut off and drop
southward toward the islands, bringing increasing instability over
beginning around Sunday morning. This will lead to the potential
for heavy rainfall and thunderstorms over portions of the state
continuing into early next week.

The latest GFS and ECMWF continue to show varying solutions on
how quickly this upper low will move southward over the islands,
with the EC bringing the core of the low over Oahu Sunday
afternoon while the GFS is a full day slower, bringing the core of
the low near Oahu Monday afternoon. Timing and position
differences with the mid level low have persisted for the last
several model runs, which causes us to have rather low confidence
in these details at this point. The models do agree on a lower
level convergence band forming along the old frontal boundary
located someone near Maui County or the Big Island Saturday night
through Monday, with this feature becoming the focus for heavy
rain. In addition, thunderstorms will be likely near the core of
the upper low due to very cold temperatures aloft, but it is not
yet possible to pin down the exact timing or location for the
thunderstorm threat. For now, we have introduced a slight chance
of thunderstorms for the entire state for Sunday and Sunday night,
and for all islands except Kauai on Monday.

Deepening moisture and cold temperatures aloft, combined with
strengthening winds associated with the mid level trough, could
produce wintry conditions on the Big Island summits starting
during the weekend and continuing through Monday.

Overall, there appears to be a substantial threat for heavy rains
and thunderstorms during the weekend and into early next week,
with the greatest threat seeming to focus over the eastern half of
the state at this point, but it is likely going to take another
few model cycles before we can have more confidence in the timing
and location details. The forecast will continue to be refined
during the next couple of days.

Gentle trade winds and much more stable weather should return by
the middle of next week, as the upper trough weakens and moves
east of the area.

&&

.AVIATION...
An approaching cold front has weakened the trade winds and is
gradually shifting winds from the southeast. Widespread leeward
sea breezes are expected this afternoon with convective cloud
build ups and ISOL MVFR over mountain and interior sections. Skies
will partially clear tonight with downslope flow expected. The front
will reach Kauai Friday with TEMPO MVFR expected in showers. There
will be a chance for scattered prefrontal showers over the rest of
the state.

AIRMET Sierra is in effect for mountain obscuration across
windward sections of Kauai. Conditions are expected to improve
late this morning.

&&

.MARINE...
A ridge of high pressure located 400 nm north of Kauai, is
forecast to move east during the next several hours due to a
front advancing eastward. Winds will become light and variable
today as a north to south oriented trough forms over the central
islands ahead of the advancing front.

The front is forecast to reach the NW offshore waters early this
evening, Kauai Friday morning, and by Oahu Friday afternoon,
before stalling over Maui County through the weekend. Breezy NE
winds behind the front will warrant a Small Craft Advisory for
the coastal waters from around and west of Oahu. The front is forecast
to inch slowly eastward to the Big Island late in the weekend with
the NE winds weakening below advisory level. The arrival of the front
will be enhanced by a strong upper level trough resulting in an
unsettled weather period this weekend, including a chance of thunderstorms
over the surrounding waters through Monday of next week.

Swell-wise, the currently NW swell will continue to subside today.
A moderate NW swell with an 8 second period arrives Friday night
and peaks Saturday. A similar size swell from the north follows
for Sunday night and Monday. A small NW swell with a 14 second
period is on tap for Tuesday night and Wednesday.

The south facing shores will be impacted by a couple of south
swells during the forecast period, the first of which is due to
arrive early Friday morning. Surf from this swell could be large
enough to warrant an high surf advisory for the south facing
shores. A gradual lowering will follow early Saturday morning.
Another southerly swell is expected to arrive Monday afternoon,
but the majority of this energy appears to stay east of Oahu,
focusing instead on Maui County and the Big Island.

Smaller surf can be expected along the east facing shores over
the weekend as the trades take a breather.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

DISCUSSION...Foster
AVIATION...Morrison
MARINE...TS