Flash Flood Watch 4, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
824 PM HST Mon Apr 13 2020

.SYNOPSIS...
An upper level disturbance will continue to bring unsettled
weather conditions with periods of showers and isolated
thunderstorms across the island chain through Tuesday. High
pressure will build in north of the state on Wednesday with a more
easterly trade wind flavor to the weather pattern with drier
trends on rainfall coverage lasting through the upcoming weekend.

&&

.DISCUSSION...
A low pressure system far to the north of the Hawaiian Islands
will continue to weaken the high pressure ridge north of the state
keeping light and variable winds in the forecast through Tuesday
night. An upper level trough shown in satellite water vapor
imagery, drifting into the region from the northwest, will keep
an active shower pattern in the forecast with isolated
thunderstorms and locally heavy rain possible through Tuesday.
Radar imagery this evening shows several thunderstorms developing
30 miles north of Molokai, and some deeper showers forming 30
miles southwest of Oahu.

On Tuesday, the upper level trough axis moves into the western
half of the state. Upper level forcing from this trough will
interact with surface heating over each island to produce periods
of heavy showers and some thunderstorms. Localized flooding is
possible through Tuesday night due to slow moving and terrain
anchored storms. The upper trough stalls over the islands on
Tuesday afternoon forming a weak cut off low center at the 500 MB
level.

Wednesday through Friday, The unstable energy associated with
this stalled cut off low will drift south of the state from
Wednesday into Thursday as high pressure builds back into the
region. This pressure pattern change will allow light to moderate
easterly trade winds to return from Wednesday onward. Unsettled
weather will transition to a drier pattern on Wednesday as a high
pressure ridge aloft will stabilize conditions across the state.
Trade wind inversion heights are predicted to range from 5000 to
6000 feet from Wednesday night onward, keeping shower activity
more limited during this time period. Mid range forecast guidance
is currently in good agreement with this trade wind solution.

Saturday and Sunday, longer range guidance shows the weak and
unstable cut off low south of the state will kick out to the
northeast moving away from the state as a 1024 MB surface high
moves from west to east across the region. This keeps light to
moderate trade winds in the forecast through the upcoming weekend
in a generally stable and fair weather pattern.

&&

.AVIATION...
A moist and unstable south to southeast boundary layer flow will
continue across the state during the next 24 hours. Showers will
favor southeast and south facing slopes and coasts at night and
early Tuesday, and the island interiors Tuesday afternoon and
early evening. Some MVFR cigs and MVFR/IFR vsbys will accompany
the showers as they move through. Additionally, thunderstorms will
be possible across all islands Tuesday afternoon.

AIRMET Sierra remains in effect for eastern sections of Maui and
the Big Island. These conditions could hold in place through much
of the night. Mountain obscuration could also affect east through
south facing sections of Kauai later tonight.

AIRMET Tango may be needed for upper level turbulence over the
western end of the state late Tuesday.

&&

.MARINE...
Guidance depicts a trough lingering in the area into Tuesday,
while a front passes to the north. Local winds will respond and
shift out of the east-southeast direction tonight through Tuesday.
Although this will give way to a land and sea breeze pattern over
the western end of the state near the coasts, pockets of fresh
southeast winds will become possible over the typical areas around
Maui County and the Big Island by Tuesday (Hamakua coast/near
South Point/north of Maui). The trough lingering in the area
combined with an upper disturbance moving across the region will
keep the threat for heavy showers and thunderstorms across the
island waters. A return of a more typical trade wind pattern is
anticipated through the second half of the week as high pressure
builds to the north.

Surf along north and west facing shores will trend up Wednesday
through the upcoming weekend due to an active pattern evolving over
the northwest Pacific featuring a series of winter-caliber systems.
The first in the series quickly reached storm-force category as it
pushed off the coast of Japan late Friday, then tracked
northeastward to its current position near the Aleutians/Date
Line. A broad fetch of gale- to storm-force westerly winds
focused at the islands within the 295-320 degree directional band
has been the result over the past 48-hrs. Satellite data
reflected this and showed seas up to around 25-30 ft in this area.
Forerunners from this source should start filling in locally
by Wednesday morning, with a peak expected Wednesday night. The
ECMWF-Wave solution does reflect a slightly later arrival time
through the day Wednesday vs the GFS-WAVEWATCH III solution, which
leads to lower confidence in the details regarding arrival time.
Surf will exceed advisory levels around the peak for exposed north
and west facing shores and may near the warning levels if this
source fills in larger than anticipated Wednesday night.

As this source begins to fade late Thursday through Friday, a
similar long-period, northwest swell will begin to build down the
island chain Friday night into the weekend from a developing storm
currently near Japan that will take a similar track
today/Tuesday. This source will peak by Saturday night, then
slowly ease Sunday into next Monday.

Surf along south facing shores will remain small through the week
with mainly small, background southerly energy moving through. For
the extended, things have become active within our Southern
Hemisphere swell window. Latest surface analysis from Australia
showed a 970 mb low centered southeast of New Zealand with a
tight gradient over the Tasman Sea/New Zealand driving wind
speeds up to storm-force levels. Wave model guidance remains in
decent agreement shows this swell arriving at Pago Pago Friday
night (peaking around 6 ft 16 sec at Pago Pago), which should
begin arriving here in Hawaii by next Monday (20th). Heights will
likely exceed the south shore advisory level as it fills in and
peaks early next week.

Surf along east facing shores will be small as the trades remain
disrupted locally and upstream. An upward trend is anticipated
through the second half of the week as the trades return.

&&

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

&&

DISCUSSION...Bohlin
AVIATION...Jelsema
MARINE...Gibbs

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