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Area forecast discussion...updated 
National Weather Service Honolulu HI
708 am HST Tuesday Sep 1 2015

Update...
sent an updated coastal hazards message to heighten wording about
impacts from high surf. Seas at the cdip buoy off Pauwela Maui
peaked overnight and have lowered to 10 feet...though the mokapu
buoy on Oahu and Hanalei on Kauai are now holding steady near 10
feet. The morning tide is near its peak right now and will drop a
foot and a half through the morning...which may help alleviate some
of the coastal impacts from the swell by midday.

&&

Synopsis...
very humid conditions with spotty heavy showers...mainly each
afternoon...will continue through most of the week. The extended
forecast will depend heavily on the track of tropical cyclone
Jimena...but easterly winds may return briefly.

&&

Discussion...
showers embedded in the very moist air mass to the southwest of
tropical cyclone Ignacio moved ashore and intensified along north
slopes of the main Hawaiian islands. This activity was strongest
over Kauai...where the Hanalei river still ran high after persistent
accumulation through the last 24 hours. Lihue shattered the daily
rainfall record for Monday. The threat of flooding remained real...
but also somewhat remote unless the pace of shower activity picks
up...which may occur this afternoon as sea breezes support
independent shower formation over higher terrain inland.

Ignacio will cut off the trade winds entirely as it passes northeast
of the islands through tonight. In principle...southwest winds will
prevail once the storm moves north of a given island...but the
current forecast track meant that these winds will be very weak. In
practice...diurnal circulations will remain most important. Many
people may find conditions uncomfortably stagnant and humid despite
any wind that can develop in this environment.

From Wednesday into the weekend...prevailing winds will become more
southerly...but still light enough so that diurnal effects will
remain more important. Similarly...advection from the south will do
little to dry the air mass. The heat index will approach the
threshold for an excessive heat advisory in some areas...but for now
it looked like this product will not be needed.

A middle-level trough trailing south from Ignacio will linger over the
state through this time period. A substantial fraction of solutions
predicted a surface convergence zone to develop under or near this
feature...focusing convection in the very moist air mass. With
little prevailing wind through much of the atmosphere...resulting
showers or thunderstorms may move very slowly. Considering the
unrelentingly high precipitable water totals...resulting showers may
be very heavy as well. However...after more careful analysis...the
precise mechanics of such a scenario remained obscure. Models also
could not agree on the width of the resulting linear convective
feature. Some kept it entirely offshore in the channels between the
islands...while others moved it over land. The potential for heavy
rain certainly will be present...but distilling a deterministic
forecast from these ingredients proved difficult.

Drier easterly flow may develop by Sunday...but the extended
forecast will depend heavily on the track of tropical cyclone
Jimena. The current forecast track predicted this storm to meander
far east to northeast of the state.

&&

Aviation...
see tc significant meteorological information series Tango for tropical cyclone Ignacio.

The passing of Ignacio a few hundred miles north of the state will
greatly influence the wind. West of the main storm
circulation...expect moderate north to NE winds. Winds south of the
storm will back more westerly...and eventually turn more southerly
and weaken as the center of Ignacio moves west.

Shower activity will continue to favor northeast facing slopes and
coasts of the smaller islands. On The Big Island....mostly clear
skies this morning over leeward and interior areas will be replaced
by afternoon and early evening showers over leeward and interior
areas. Isolated MVFR conditions are to be expected in showers
otherwise VFR conditions should prevail throughout the day and into
coming night.

The airmass over the state remains moist and unstable. Isolated
thunderstorms with heavy downpours are possible over island
interiors this afternoon.

&&

Marine...
large swells from Hurricane Ignacio will affect the state during the
next couple of days...with reinforcing swells from distant Hurricane
Jimena coming in during the rest of the week. The largest swells
will remain aimed at The Big Island and east Maui...but a high surf
warning remained in effect for east-facing shores of most islands.
Even with swell energy from Ignacio fading on Wednesday...easterly
swell from Hurricane Jimena will keep East Shore surf elevated...
probably at warning levels...through the weekend.

A Small Craft Advisory /sca/ also remained in effect through tonight
for waters exposed to easterly swell from Hurricane Ignacio. The Small Craft Advisory
may need to be extended further if seas remain elevated. Winds are
expected to remain below the Small Craft Advisory threshold this week.

Long-period swells from the south-southwest may produce
advisory-level surf along south-facing shores this weekend.

Please refer to the latest collaborative nearshore swell and wind
forecast /srdhfo/ for more details on the swells.

&&

Hfo watches/warnings/advisories...
high surf warning until 6 am HST Wednesday for Kauai windward-
Oahu koolau-Olomana-Molokai-Maui windward west-Maui Central
Valley-windward Haleakala-south Big Island-Big Island north and
east.

Small Craft Advisory until 6 am HST Wednesday for Kauai
northwest waters-Kauai windward waters-Kauai Channel-Oahu
windward waters-Kaiwi Channel-Maui County windward waters-
Pailolo Channel-Alenuihaha Channel-Big Island windward waters-
Big Island southeast waters.

&&

$$

Public and marine...ryshko
aviation...Bedal

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