Heavy Rain in Hawaii, September 11 to 14, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
952 AM HST Mon Sep 11 2017

.SYNOPSIS...
Daytime sea breezes will bring scattered afternoon and evening
showers over interior and leeward areas, which may become locally
heavy at times through Wednesday. Land breeze will clear out most
clouds and showers at night. The trade winds are expected back on
Thursday, and continuing into the upcoming weekend.

&&

.DISCUSSION...
Water vapor imagery shows an upper trough with an axis passing
about 75 miles west of Kauai. Morning soundings show a weak and
elevated midlevel inversion in places with precipitable water
(PW) values ranging from 1 inch at Lihue to 1.72 inches at Hilo.

A cold front is currently located 500 miles northwest of the
Kauai, while a east to west oriented ridge axis is located about
125 miles north of Kauai this morning. Both features will move
southward over the next couple of days. The ridge will stall
within 50 miles north of Kauai on Tuesday, while the front will
stall about 150 miles north of Kauai on Wednesday before slowly
dissipating Thursday and Thursday night.

Until the front dissipates on Thursday, the islands will be under
a light easterly wind flow regime which will allow daytime sea
breezes and nighttime land breezes to set up over all of the
islands. The resulting convective weather pattern will feature
cloudy afternoons and evenings with showers, followed by clearing
skies during the overnight and morning hours. The exception to
this forecast will be over the Kona slopes of the Big Island
where persistent clouds and showers have remained over the past
couple of nights, and have kept some clouds and at least a chance
for rain tonight given the persistence and little expected change
to the overall weather pattern over the state through Thursday.

The potential for locally heavy rain, through not widespread, will
remain this afternoon and evening, and again on Tuesday and
Wednesday afternoons and evenings. This is due to the relatively
high PW (particularly across the eastern portion of the state), upper
trough and resulting weak inversion over the state.

By Tuesday, the background winds will shift southeasterly and
bring higher humidity and the potential for vog across the smaller
islands (highest chance for vog would be on Wednesday on Kauai
and Oahu).

High pressure will build in from the northeast and return the trade
winds on Thursday per the GFS solution, while the ECMWF shows them
building in on Wednesday night. The trades will push the remnant
frontal moisture away from the state, and become moderate to fresh
by Friday night. The trades will usher in the typical windward
and mauka clouds and showers into the weekend, with drier and
sunnier weather leeward.

&&

.AVIATION...
Light winds will allow for sea breezes to form today and land
breezes to form tonight. Sea breezes will help focus clouds and
showers along the leeward and interior areas this afternoon. Some
of the clouds and shower build ups this afternoon could introduce
periods of MVFR CIGS/VIS. Land breezes should then help clear out
land areas tonight.

&&

.MARINE...
Trades will weaken as a cold front approaches the region from the
northwest. This will allow land and sea breezes to develop each
afternoon through midweek. Winds across the coastal waters will
remain well below Small Craft Advisory levels during this time.
Trades are expected to gradually strengthen again beginning
Thursday through the end of the week, as the front to our north
dissipates and high pressure rebuilds north of the state. Trades
may reach advisory levels in the windiest areas around the Big
Island by the weekend.

Surf along north and west facing shores will decline today as
a northwest swell continues to fade. Another small northerly
swell should reach the waters Tuesday, then hold through midweek
before slowly trending down. Surf from this source will remain
small along north facing shores.

Surf along south facing shores will remain small through early
this week with a small southeast swell holding through the next
couple of days. A small to moderate southwest (220 deg) swell that
was generated in the Tasman Sea will arrive Tuesday and lead to a
slight increase in surf for a few days along south and west
facing shores.

Surf along east facing shores will remain small through midweek
due to the absence of strong trade winds.

&&

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

&&

DISCUSSION...Foster
AVIATION...Eaton
MARINE...TS