Flash Flooding and Heavy Rain in Hawaii, September 13 to 16, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
342 PM HST Fri Sep 13 2019

.SYNOPSIS...
Trade winds will be on the lighter side over the next few days as
a trough of low pressure north of the area disrupts the wind flow
across the state. An area of enhanced moisture will move from
east to west across the area tonight and on into early next week
causing an increase in shower activity. A drier and more typical
trade wind weather pattern is expected Tuesday through Friday.

&&

.DISCUSSION...
Moisture embedded in light low level trade wind flow along with
daytime heating, has allowed for some shower development this
afternoon especially over interior and mauka areas. Also an area
of enhanced moisture depicted in derived satellite imagery is
located just southeast of the Big Island. This is likely the
reason for the shower development over the windward Mauna Loa
slopes as well as in the volcano area. Southwesterly flow aloft is
drawing up some high clouds over the portions of the state mainly
affecting Maui and the Big Island.

At the surface, a rather complex weather pattern exists in the
Central Pacific. Weak areas of high pressure are located far north
northeast and north northwest of the area, while a weak trough
lies a few hundred miles to our northeast. Also a weak area of low
pressure lies a few hundred miles south of the state.

Aloft, an upper level low is centered several miles north of the
state. This upper low is forecast to drift west southwest over the
next few days. This will cause the airmass to become a bit less
stable across the area. This combined with the higher precipitable
water values seen just southeast of the Big Island will allow for
an increase in shower activity across the area. Thus an increase
in showers is expected tonight over the Big Island then gradually
increase from east to west over the next few days as the moisture
spreads westward. The instability caused by the upper low, may
allow for some deep convection to form especially over the leeward
Big Island slopes as well as over the leeward Haleakala slopes
both Saturday and Sunday afternoon. We have added in a slight
chance of thunderstorms for those areas. The highest probabilities
for showers will be over windward and mauka areas but some
leeward and interior locations will likely experience greater
chances for showers as well.

Trade winds will be on the lighter side for the next few days as
the trough north of the area disrupts the pressure gradient. The
high is forecast to strengthen beginning early next week and the
effects of the trough becomes less. Thus we expect trade winds to
pick back up the moderate levels by Tuesday with these stronger
winds expected to linger through Friday.

Shower activity will also become less frequent as the higher
moisture content moves off to the west and the upper low weakens
and moves off to the northwest of the area. An upper ridge is
forecast to build in from the east and will have a stabilizing
effect on our airmass.

&&

.AVIATION...
A few heavier showers have been impacting leeward areas of Maui
and Oahu this afternoon. The Big Island interior shower activity
has also picked up over the last few hours. High clouds are still
streaming over the Big Island from the southwest, but have moved
off of the other islands.

Trades will continue through the day and early evening before
becoming more east- southeasterly overnight and into the weekend.
This will allow an area of moisture southeast of the Big Island to
move over the island chain, bringing an increase in clouds and
showers.

&&

.MARINE...
A weak surface trough moving between the islands and high
pressure to the north will support light to moderate trade winds
across the area into Sunday. Winds and seas should stay below the
Small Craft Advisory threshold. As the trough passes early next
week, winds should increase, potentially returning Small Craft
Advisory winds to the typically windier waters around the Big
Island and Maui County.

Small long period south swells will continue to produce small surf
along south facing shores. A slight bump in south shore surf
remains possible beginning Tuesday. However, surf is expected to
stay be below advisory levels.

A small, long-period northwest swell will fill in Saturday, then
linger through the day Sunday.

Surf along east facing shores will lower over the weekend due to
the weak trades expected locally and upstream of the state.

&&

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

&&

DISCUSSION...Burke
AVIATION...Dye
MARINE...EATON