High Surf in Hawaii, September 26 to 28, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
330 AM HST Thu Sep 26 2019

.SYNOPSIS...
The interaction between a surface trough near Kauai and a couple
of upper level disturbances moving across the area has resulted in
a unsettled environment mainly over the western part of the main
Hawaiian Islands. Unsettled conditions with periods of heavy rains
and thunderstorms are possible through today for Kauai, Niihau,
and Oahu. Improving conditions are instore for Friday and the
weekend with light tradesbuilding to moderate strength over the
weekend.

&&

.DISCUSSION...
Besides Niihau and Kauai, the Flash Flood Watch now includes Oahu,
which is good til 4 pm HST this afternoon. Heavy Showers developes
south of Kauai then moved north. But an apparent land breeze is
keeping the heavy showers from advancing further inland. A Flood
Advisory is in effect for the southern part of the island until 5
am HST with further extension of the advisory if needed. The
surface trough with light westerly winds behind it appears on the
move. It is expected to cross through Kauai this afternoon then
stall in the Kauai Channel this evening then dissipate.

The decison to include Oahu in the Watch is because of the
prescence of low clouds lingering around the island. As the
surface trough and upper level disturbance moves eastward, they
can easily trigger some heavy showers and thunderstorms after
daybreak. So far, Oahu is dry. There may have been some light
showers but not much. Like Kauai, a land breeze is appears to be
keeping some scattered heavy showers at bay a few miles off
Waikiki Beach. Isolated heavy showers are also in the Kaiwi
Channel, brushing west Molokai at times. With this said, the
slight threat for a thunderstorm is included in the forecast for
Molokai today. It is likely Oahu will continue wet this evening
followed by diminishing showers after 10 pm.

Maui and Lanai appears to be the dividing line between showery
and dry. Although Maui is mainly dry under fair skies now.
Afternoon heating though may lead to a burst of showers on the
Valley Isle. Maui, actually is not under a southerly wind flow,
but an easterly one. The Big Island is also under a light trade
wind flow with a few trade wind showers. With this saide, the lee
side of the island will undergo its usual diurnal cycle of morning
sun, then afternoon and evening clouds with scattered showers.

All this weather is east of a front located 390 miles northwest
of Kauai. The front is forecast to stall some 280 miles northwest
of Kauai Thursday, and gradually fall apart there through Friday.

Light trades will be returning on Friday area-wide, but it will
be light enough for the development of daytime sea breezes. Some
lingering instability though, may lead to some scattered showers
over the interior and lee areas of especially Kauai and Oahu.
The trades will continue to strengthen into the through the
upcoming weekend, reaching moderate strength by Sunday, and
locally strong about Tuesday of next week. And with the return of
the trades, our rainfall pattern will transition to windward and
mauka showers, especially nights and mornings.

&&

.AVIATION...
This morning, a weak trough draped near Kauai will generate
southerly flow and bring tropical moisture and instability
to Kauai and Oahu. Scattered to numerous SHRA/SHRA+ are expected
to develop along this trough and move slowly eastward today. A
slight chance for thunderstorms exists, which would have the
potential to lower cloud cover/visibility and produce MVFR or
brief IFR conditions. Otherwise, expect light trade flow across
the remaining islands with only isolated -SHRA.

AIRMET Sierra will remain in effect for Oahu and Kauai for all
areas above 2000 feet due to tempo mountain obscuration.

&&

.MARINE...
Light south to southeast winds will continue for the next few
days as a front approaches the islands from the northwest. The
front is expected to stall over the far northwestern offshore
waters today and dissipate through Friday. Light to moderate
trades will return this weekend as high pressure builds in and
replaces the dissipated front. An upper trough west of Kauai will
continue to support isolated thunderstorms over the western and
southern waters of Kauai, including the Kauai Channel, through
today.

Swells from the south-southwest and south will fill in today,
producing advisory level surf for south facing shores through
Friday before slowly declining into the weekend. A High Surf
Advisory (HSA) is now in effect for these shores. East facing
shores will continue to see moderate surf through Friday before
slowly diminishing. A surface low well north-northwest of Kauai is
generating a moderate swell that is forecast to reach Kauai's
north and west facing shores this morning and spread down the
island chain through tonight. This swell should peak Friday below
advisory level, then slowly decline this weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through this afternoon for Oahu, Kauai, and
Niihau.

High Surf Advisory in effect to 6 PM HST Friday for south facing
shores of all islands.

&&

DISCUSSION...H Lau
AVIATION...Thomas
MARINE...Powell