Heavy Rain in Hawaii, June 14 to 15, 2017

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
1044 AM HST Wed Jun 14 2017

.SYNOPSIS...
Ridging far north of the islands will maintain moderate trade
wind through Thursday. The trades will ease thereafter and into
the weekend as the ridge is moved south closer to the islands.
An unstable airmass brought on by an upper level disturbance
over the west end of the island chain will remain through
Thursday, providing a chance of thunderstorms and the potential
for heavy rainfall for this State. A drier more stable airmass
will then build across the islands Friday through early next
week, with showers favoring windward and mauka areas.

&&

.DISCUSSION...
The upper level disturbance is currently over the south part of
the Kauai Channel. It is forecast to linger there through tonight
before making a move toward the west-southwest late tonight and
Thursday morning. Any significant improvement or lesser threat
of heavy rains and thunderstorms severe weather, won't come until
late Thursday or Thursday night.

So far the weather has been behaving, pretty much some enhance
scattered trade showers. There were a thunderstorm over Kauai
overnight, and one over Oahu at sunrise. A batch of thunderstorms
remains over the open waters some 40 to 120 miles south of Kauai.

cor
The forecast package was updated, to trim back the sky cover over
Kauai and Oahu, where we anticipate the bulk of the weather to occur.
We are still tending to this scenario this afternoon with the
addition of daytime heating and more of the sunshine. Low level
moisture is marginal which could be the reason of frequency of
the showers. This will change later this afternoon and evening as
we are eyeing a large patch of moisture  just upwind of Maui.
It is slated to reach Maui at just or a little after 12 noon, and
Oahu around sunset. Satellite is detecting isolated thunderstorms
with this batch of moisture about 100 miles northeast of Maui.
A trailing piece of moisture will also reach the Hamakua coast of
the Big Island early this afternoon. So, some worsening weather
appears on the way. The rest of the rain forecast is on track.

Currently at the surface, a 1025 mb surface high is centered 1450
miles northeast of Oahu. The associate surface ridge extends from
the high to 645 miles north of Kauai. This ridge will be moving
south toward the islands, and will come to a stop about 250 to
300 miles north of Kauai Friday evening. This action will cause
the trades to weaken from moderate to light to moderate over the
islands by Friday. And this will result in local daytime sea
breezes for the islands. By Friday, the air mass over the islands
will be more stable with the departure of the upper low toward
the west. With this said, expect only spotty afternoon showers
over the lee and interior areas of the smaller island.

The ridge remains 280 miles north of Kauai until Sunday, where it
starts to shift north. As it shifts north, the stronger trades
will slide in. Expect the trades to reach locally strong by Monday,
with a more typical trade wind pattern.

&&

.AVIATION...
An upper level low over Kauai is destabilize the air mass over
the state. There is chance of thunderstorms statewide through 24
hours. Thunderstorm activity will carries MVFR to IFR conditions,
with gusty winds, lightning, and possible icing.

Moderate trade wind flow will continue to focus clouds and
showers along mountain and windward areas. Tempo MVFR and isol
IFR conditions are expected along north through east slopes and
coasts with AIRMET SIERRA for mountain obscuration in effect for
some areas.

&&

.MARINE...
The latest short-term model guidance remains in decent agreement and
shows a weakness within the ridge axis north of the islands
developing through the second half of the week as a series of cold
fronts pass from west-to-east across the central Pacific far north
of the local area. This weakness combined with a southward shift of
the ridge axis toward the local waters will translate to slightly
weaker trade winds across the waters, especially over the smaller
islands Thursday night through Friday night.

Sunday into Monday, trades are forecast to build back across the
waters as the ridge axis shifts northward. This will likely result
in Small Craft Advisory (SCA) conditions returning across the
windier channel waters.

Choppy, short-period, surf will persist along east facing shores
through the period, with a slight downward trend expected Thursday
through Saturday as the winds slightly weaken. Surf will likely
become rough Sunday into next week as the trades strengthen
locally and upstream across the eastern Pacific south of high
pressure.

Surf along south facing shores is expected to pick up over the
upcoming weekend and through much of next week as back-to-back south
swells fill in from distant sources across the southern Pacific. The
first south swell should begin to fill in through the day Friday,
peak over the weekend, then slowly ease into the upcoming week. Surf
should remain just below advisory level through the peak.

The second south swell will be slightly larger and may reach
advisory levels once it fills in late Monday night through midweek.
The source associated with this swell is currently evolving just
south and southeast of New Zealand. The latest surface analysis and
guidance reflects this and shows a complex gale to storm-force
system centered just southeast of New Zealand with a favorable
batch of southwest winds focused within the 190-200 directional
band relative to the local area. An early morning altimetry pass
through the center of this fetch region showed peak seas of 39 ft,
which lines up well with the latest ECMWF-Wave guidance (slightly
higher than the GFS-WAVEWATCH III solution). Guidance indicates
this source peaking slightly higher than previous model cycles
over Pago Pago Friday night at 9 ft 16 seconds, which will likely
correspond to advisory-level surf along south facing shores here
locally Tuesday through midweek.

An out of season, short period, northwest swell associated with low
pressure system that is forecast to setup across the northwest
Pacific and track southeastward toward and across the Date Line
through the second half of this week will fill in Sunday and hold
through Monday before trending down.

&&

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

&&

DISCUSSION...Lau
AVIATION...Morrison
MARINE...Foster