Heavy Rain in Hawaii, September 22 to 27, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
1000 AM HST Sun Sep 22 2019

.SYNOPSIS...
Increasing moisture moving over the islands and lighter winds
will bring increasing showers statewide over the next couple
days. An upper level trough to the west of the islands will
maintain some instability that could cause some of the showers to
be heavy, and could lead to a few thunderstorms. Drier weather is
expected by Tuesday for Maui County and the Big Island, with
lingering showers expected over the western islands through much
of the work week. More typical trade wind weather is expected to
return next weekend.

&&

.DISCUSSION...
Currently at the surface, a weak surface trough of low pressure is
located a couple hundred miles southwest of the Big Island, while
a 1028 mb high is centered around 1150 miles north-northeast of
Honolulu. The gradient remains rather light across the smaller
islands due to blockage from the Big Island in the east-
southeasterly boundary layer flow. Meanwhile aloft, an upper
level low is evident in water vapor imagery around 250 miles west
of Kauai. Visible satellite imagery shows scattered to broken high
clouds moving through the island chain, with some lower clouds
moving into windward locales, resulting in partly to mostly cloudy
conditions across the state. Radar imagery shows a few light
showers moving into windward areas, with most locales experiencing
rain free conditions. Main short term concerns continues to
revolve around the potential for locally heavy rainfall and
thunderstorms over the next couple of days.

Today through Tuesday,
Model solutions are in good agreement showing showing the upper
level low west of Kauai retrograding westward over the next couple
days, dragging the weak surface trough to the south of the islands
steadily westward. Meanwhile, deep layer ridging is forecast to
build well to the northeast of the state through the period. These
features are expected to drag a plume of deep tropical moisture
with precipitable water (PW) values of 2.0 to 2.3 inches from
east to west across the island chain over the next couple days.
The deep tropical moisture is already in place over the eastern
end of the state, and is expected to overspread Maui County and
Oahu later today. The plume will overspread Kauai late tonight or
Monday morning, with some drier air with (PW values below 2.0
inches) moving into the Big Island Monday afternoon. This drier
airmass will continue to spread westward through Maui County and
Oahu Monday night, and shift west of Kauai on Tuesday.

The threat for heavy rainfall and flash flooding does not appear
particularly high given the more unstable air associated with the
upper level low not aligning with the deeper tropical moisture
through the period. That said, given the high PW airmass in place,
some locally heavy downpours can't be ruled out, this
afternoon/evening and again Monday afternoon and evening. Will
keep a mention of thunder in the forecast this afternoon through
Monday, but we may be able to scale this back some as the airmass
moving over the state appears more stable than it did 24 hours
ago.

Showers are expected to favor interior and mauka areas this
afternoon, then transition over to windward areas tonight through
Tuesday as the trades begin to return from east to west across the
island chain. Given the high PW airmass in place however, a few
showers will remain possible in leeward areas through the period,
with the highest chances for rain during the afternoon and early
evening hours.

Tuesday night through Saturday,
There remains some differences in the long range periods, but the
models have come into better agreement compared to 24 hours ago.
Both the GFS and ECMWF linger the band of deep tropical moisture
over or near the western end of the state Tuesday night through
Friday, with the GFS painting a bit wetter scenario for Oahu and
Kauai than the ECMWF which is a bit further west. Overall, winds
are expected to remain rather light through the period, and this
should result in a more windward shower focus during the overnight
and early morning hours, and an interior/mauka shower focus during
the afternoon and early evening hours. More robust trade winds are
expected to return next weekend, and this should bring a return of
more typical trade wind weather to the island chain.

&&

.AVIATION...
A VFR morning thus far, but conditions are favorable for some
spotty heavy showers and even an isolated thunderstorm.
A band of north thru south oriented high clouds lies across
Molokai, and there is more mid and high lying to the south of the
central islands. The mid/upper level low currently just W of
Kauai, is expected to continue moving W the rest of the day,
putting the islands in a deeper southerly wind flow that will lead
to increasing mid and higher clouds across the western half of
the island chain. The wind flow just above the surface is a light
easterly flow, and is weak enough for a seabreeze to develop.
This, combined with ample moisture and daytime heating will lead
to spotty heavy showers/and isolated thunderstorm this afternoon
over land. AIRMET Sierra may be needed. These showers/storms will
be ending area wide after 04z.

&&

.MARINE...
A surface reflection of the upper level trough to the northwest of
the islands is over the state and has allowed winds to veer to the
east-southeast and decrease to light to moderate speeds across
most areas. The channels and windward zones near the Big Island
and Maui County, though, will likely see fresh speeds due to
terrain accelerations later this afternoon. Fresh to locally
strong speeds are expected to return late Monday as the trough
moves west of the state and the persistent high, far north-
northeast of the state, builds back across the area. A brief
return to Small Craft Advisory (SCA) conditions for one or more of
the typically windy zones near the Big Island and Maui is a
possibility starting late Monday.

A front developing northwest of the state early in the week should
cause winds to decrease to light to moderate east-southeast winds
by midweek. Beyond Tuesday, winds are expected to remain in an
east to southeast direction and below SCA levels as the front
remains northwest of the state and the high persists north-
northeast of the islands.

A series of small swells from the southwest and south are
expected through the week. A new moderate period south-southwest
swell is due Monday and will be followed by a larger long-period
swell later in the week. This latter swell may reach advisory
levels along south facing shores late Thursday into Friday. A new
northwest swell filled in overnight and has already peaked at the
Hanalei Buoy. We should see the swell peak today for rest of the
state and lower through the day Monday. A few other small
northwest swells will be possible towards the end of the week.

&&

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

&&

DISCUSSION...Jelsema
AVIATION...H Lau
MARINE...Kino