Flash Flood and Heavy Rain in Hawaii, September 25 to 29, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
336 AM HST Tue Sep 25 2018

.SYNOPSIS...
Low pressure northwest of Kauai will lift northeast during the
next couple of days, dragging a trough slowly eastward toward the
islands. High moisture levels with southerly winds ahead of the
trough will bring frequent showers to Kauai today and tonight,
with heavy rainfall possible. Muggy conditions are expected to
continue across the state through much of the week as the trough
slowly weakens. A more typical trade wind pattern should return by
Thursday night or Friday, with trades becoming breezy during the
weekend.

&&

.DISCUSSION...
Mainly stratiform rain with hourly rates of a quarter inch or
less has fallen on Kauai for the last several hours, but a large
area of heavier rain is now moving into the island from the south.
A few lightning strikes have been detected from time to time
across the offshore waters south through west of Kauai. Generally
dry conditions prevail across the remainder of the state. Surface
analysis depicts an elongated low pressure trough several hundred
miles to the northwest and west of Kauai. Southeast to south winds
ahead of this trough are bringing a muggy airmass northward over
the state, with dewpoints well into the 70s. Aloft, water vapor
imagery shows a cutoff low about 500 miles northwest of Kauai,
while a ridge lies just to the east of the state. The 12Z Lihue
sounding showed a moist airmass in place, with PW of 2.13 inches,
and only a very weak inversion near 6000 feet. The 12Z Hilo
showed significant changes during the past 12 hours, with the
inversion disappearing and PW jumping to 2.08 inches. MIMIC-TPW
imagery depicts greater than 2 inch PW values extending northward
from the deep tropics over Kauai, and another area of high PWs
near 2 inches moving into the Big Island from the southeast.
Somewhat lower PWs of 1.5 to 1.8 inches extend from Oahu eastward
through Maui.

Today, high moisture with PW values of 2 inches or more will
continue over Kauai, edging eastward into the Kauai Channel, as
the surface and upper troughs remain stalled to our west. Models
also show some mid-level ascent over Kauai during this time due to
the proximity of the upper trough. Would expect frequent showers,
some heavy, along with a slight chance of thunderstorms, and the
Flash Flood Watch will remain posted for Kauai and Niihau. The
remainder of the state will see a continuation of southeast/south
background flow and a sea breeze pattern, with scattered showers
possible over the islands, especially in the afternoon.

Tonight, the high moisture edges slowly eastward into the Kauai
Channel and toward Oahu, while the large-scale ascent gradually
lifts out to the north of the state along with the upper trough.
Some heavy downpours will remain possible over Kauai during this
time, and will have to keep an eye on the eastward progression of
this activity as it may reach Oahu.

Model guidance shows above normal moisture lingering over much of
the state from Wednesday through the remainder of the work week.
Heavy rains will be less likely, as the upper trough flattens out
to our north resulting in the loss of forcing over the islands.
Surface troughing also to our north will maintain light southerly
flow through at least Thursday, therefore expect warm and muggy
weather to continue. Trade winds should resume by Thursday night
or Friday as weak high pressure builds to our north, and a
tropical low pressure area possibly develops to our south. Longer
range guidance shows the tropical low developing further during
the upcoming weekend as it tracks off to the southwest and west of
the islands, with breezy trades and a drier airmass returning to
the state.

&&

.AVIATION...
A nearly stationary surface trough to the northwest of Kauai will
keep the low level surface flow weak and out of the southeast.
Weak flow will allow light sea breezes to predominate most areas
in the afternoon and early evening hours and light land breezes in
the late night and early morning hours.

The surface through will also continue to draw moisture and
instability out of the south. Deep layered clouds and persistent
shower activity over Kauai continues to necessitate AIRMET Sierra
for mountain obscuration. Conditions are expected to persist
through at least tomorrow evening. Isolated thunderstorms and
heavy downpours possible over Kauai and its adjacent waters
through Tuesday night.

Elsewhere, higher clouds moving in from the west will spread
across the remaining smaller islands by morning and the Big Island
in the afternoon. Localized MVFR conditions are possible in
scattered showers, otherwise VFR conditions will prevail through
Tuesday evening.

&&

.MARINE...
Light to moderate southerly winds will continue across the
Hawaiian waters through midweek as low pressure to the northwest
slowly lifts north-northeastward. Plenty of moisture and
instability will support moderate to heavy showers at times,
especially over the western waters. Winds and seas will be higher
in and around any heavy shower or storm that develops. The
pressure gradient will begin to tighten over the weekend as high
pressure builds to the north and a potential tropical cyclone
develops and passes to the south. Seas will quickly respond and
build, potentially reaching advisory levels Saturday through
Sunday over exposed waters.

Surf along south facing shores will remain small through midweek
with mainly background south-southeast energy expected. A small to
moderate south-southwest swell is expected to fill in Thursday
night through Friday, then hold into the upcoming weekend before
easing. Observations at the PacIOOS American Samoa buoy are
already reflecting this energy filling in within the 16-18 sec
band. A slightly larger reinforcing south-southwest source will be
possible early next week. Overnight satellite data reflected a
large area of southwest gales south of New Zealand with seas
reaching as high as 30 to 33 ft. Guidance depicts this source
peaking at the Samoa buoy Friday night into the weekend, which
would result in an arrival locally through the day Tuesday of next
week.

Surf along north facing shores is expected to reach moderate
levels over the upcoming weekend. Guidance continues to show a
large area of strong north-northwest winds setting up over the
central Pacific later tonight through midweek as low pressure
slowly lifts northeastward. WAVEWATCH III and ECMWF-Wave both
indicate similar solutions with peak seas reaching the 14 to 18
ft range Wednesday through Wednesday night within this development
region. Proximity to the islands (1100 nm) will result in shorter
periods upon arrival (10-12 sec), which will support heights
remaining below advisory levels.

See the latest Oahu Surf Discussion (SRDHFO) for more details on
surf and swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through late tonight for Niihau-Kauai.

&&

DISCUSSION...Jacobson
AVIATION...Bedal
MARINE...Gibbs