Heavy Rain in Hawaii, September 20, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
350 PM HST Thu Sep 20 2018

.SYNOPSIS...
A weakening surface trough in the will be exiting Oahu early this
evening,and pass through Kauai County late tonight. High pressure
following the trough will usher in a trade wind flow with speed
ranging from moderate to locally strong across Maui and Big Island
counties to light to moderate across Kauai. An area of low pressure
is expected to develop northwest of the islands over the weekend,
bringing in a period of southeast winds to the area that will
linger through the first half of next week. Expect an uptick in
showers as well, especially around Kauai by the middle of next
week.

&&

.DISCUSSION...
A surface trough is currently located over Oahu, moving west at
about 10 mph. The trough has been losing its cloud cover since
daybreak and what is left is a line of clouds approaching Oahu's
windward shoreline. Otherwise, as of 230 pm HST, showers have been
pretty spotty thus far. There are scattered showers over interior
Kauai, an isolated shower over Wahiawa on Oahu and upcountry Maui
south of Pukalani. Daytime heating with ample moisture and a
unstable air mass over the Big Island, has resulted in a
thunderstorm over the higher elevations just northwest of Mauna
Loa. We will likely encounter a few more showers before sunset
commencing at 630 pm HST.

The story though, has been the humid air mass following
immediately behind the trough. Dew points have risen to as high
as 76 degrees fahrenheit. And, with the abundant sunshine, the
air temperatures were in the upper 80's to lower 90's. Warmest
spot was Kahului Airport with 94 degrees F. But, with a dew point
of 76, the heat index hit 104 degrees F. Honolulu Airport had a
88 degrees F this hour. With a dew point of 76, the heat index was
98 F. Areas where there are trades are helping cool things out.
Lowering of the dew points are expected overnight for all but
Kauai.

With the trades returning, it mean the trade showers will be
coming back. This will be true for Kauai, but the  trades will be
weak enough for the continuation of hybrid mix of a sea breeze and
trades. So the interior/lee areas can expect some afternoon build
ups and a few showers each afternoon into the weekend. Otherwise,
the models are indicating a normal trade shower pattern into the
weekend affecting the windward areas of Maui and the Big Island,
and somewhat a drier trade pattern for the remaining islands.

From Sunday on, expect the winds shifting to the southeast across
Oahu and Kauai, and the remaining islands by Monday evening. This
comes about as surface low forms northwest of Kauai over the
weekend. Although the low moves north, it will leave behind a
nearly stationary trough just to the west of Kauai through at
least the middle of next week. We are expecting an uptick in
showers, especially around Kauai, towards Wednesday.

&&

.AVIATION...
A few TCU, with an isolated CB have formed over the upper Kona
slopes of the Big Island. These are expected to diminish rapidly
by sunset.

VFR generally persisting through tonight. Some clouds and showers
will develop across windward sides by Friday morning as a trade
wind flow is expected by Friday.
&&

.MARINE...
Trade winds have returned to waters east of Oahu and is expected
to spread westward towards Kauai tonight. Trade winds will begin
to strengthen on Friday and is expected to stay breezy through
Saturday. A Small Craft Advisory (SCA) is posted for the Pailolo
and Alenuihaha channels and the waters south of the Big Island for
tonight and is expected to expand to Maalaea Bay on Friday as
winds increase. Starting Sunday, winds will likely weaken and
shift around to the southeast through early next week as an area
of low pressure forms several hundred miles northwest of Kauai.

Surf will remain below advisory levels along all shores through
early next week. The current south swell will continue to subside
on Friday. As trade winds strengthen Friday, rough, short-period
surf will briefly rise to around the summer average along east
facing shores, then decline during the weekend. Elsewhere, no
significant swells are expected.
&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Saturday for Alenuihaha
Channel-Big Island Leeward Waters-Big Island Southeast Waters.

Small Craft Advisory from 6 AM Friday to 6 PM HST Saturday for
Maalaea Bay.

Small Craft Advisory from 6 PM this evening to 6 PM HST Saturday
for Pailolo Channel.

&&

Lau/Brenchley/Kino