Flash Flooding and Debris Flows in Hawaii, April 11 to 16, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
1016 AM HST Wed Apr 11 2018

.SYNOPSIS...
High pressure passing by to the north of the islands will keep
fresh trade wind flow in place through tonight. The trades will
ease Thursday and Thursday night as a front passes by to the north
of the state, with windy trades then returning over the weekend
into early next week. Clouds and showers will favor windward and
mauka areas, with a few showers spilling over into leeward areas
from time to time due to the strength of the trades.

&&

.DISCUSSION...
A 1029 mb high centered around 850 miles north northwest of Kauai
is driving fresh trade winds across the island chain late this
morning. Aloft, upper level ridging is situated to the north of
the islands, with upper level troughing to the south of the state.
The airmass has stabilized quite a bit over the past 24 hours,
with a strong trade wind inversion now around 6.5 kft over Kauai
and a much weaker inversion around 8 kft over the Big Island as
evident in the 12Z soundings from PHLI and PHTO respectively. The
airmass is moistening up over the smaller islands, but a gradient
in deep moisture still exists. The latest UW-CIMSS MIMIC Total
Precipitable Water (PW) imagery along with the 12Z soundings show
PW values between 1.2 and 1.5 inches which is slightly above
normal. Visible satellite shows mostly cloudy skies over windward
areas, with partly cloudy to mostly sunny across most leeward
areas. Radar imagery shows scattered to numerous showers moving
into windward areas (with more showers across the eastern half of
the state), with only very little showers or sprinkles/mist making
it into leeward areas.

High pressure to the north of the state will track steadily
eastward along 33/34N, keeping a fresh trades over the state
through Wednesday. Trades will continue to funnel slightly above
average moisture moisture this week and should keep scattered to
numerous showers rolling into windward areas with isolated to
scattered showers spilling over into leeward areas. Rainfall
maximums will occur during the nighttime and early morning hours.

A cold front will pass by well to the north of the islands during
the end of the work week, and this should result in a slight
easing of the trade winds into the moderate range Thursday and
Thursday night. A digging shortwave trough to the NW of the
islands on Friday will cut off to a weak upper low this weekend a
few hundred miles to the west of the islands. Colder air aloft
associated with this feature will drop 500 mb temperatures to -10
to -12C Friday and Saturday, which will help erode the trade wind
inversion. Models show PW still in the 1.2 to 1.5 inches during
this period. The resulting destabilization should help to enhance
trade wind showers during this period.

The upper low will start to move away to the west late in the
weekend, with modest stablization of the airmass over the islands.
Overall moisture levels look to remain elevated, so there will
still be quite a few showers riding in on the trades. The models
agree that high pressure will remain in control to our north,
keeping trades blowing into next week.

&&

.AVIATION...
Moisture embedded in the trades is providing for a good amount of
cloudiness over windward and mauka areas. AIRMET Sierra is
currently posted over N through E facing slopes of all islands due
to mountain obscuration. AIRMET Tango is posted for low level
turbulence over and immediately S through W of mountain ranges
due to the fresh trade winds blowing across the area. Expect these
conditions to continue through the day and on into the evening
hours.

VFR conditions prevail at the majority of the TAF sites but
occasional MVFR conditions are occurring over windward TAF sites
in low clouds and shra. Expect these conditions to continue
through the day and on into the evening hours.

&&

.MARINE...
Fresh to strong trade winds will hold into tonight as high
pressure passes north of the state. Trades will briefly drop off
on Thursday into early Friday as a weakness develops in the ridge.
A Small Craft Advisory (SCA) is in effect for most waters for
today, and the SCA will likely be trimmed back to the typically
windy waters around the Big Island and Maui County for tonight and
Thursday as trades ease. Strong trades will redevelop on Friday
then persist into early next week as robust high pressure passes
north of the islands. An SCA will likely be in effect for most,
if not all, waters during the weekend.

A series of northwest swells are due this week. The current,
moderate northwest swell is on a steady decline today. Another
moderate northwest swell will build tonight and peak just below
advisory levels for north and west facing shores on Thursday. A
larger northwest swell will arrive on Friday, then peak well above
advisory levels Friday night and Saturday. Surf should trend down
below advisory levels along north and west facing shores on Sunday.

Rough east shore surf will remain below the advisory level during
the next several days. As trades strengthen during the weekend,
east shore surf will likely reach advisory levels during the
weekend and into early next week.

Small background south and southwest swells will continue to
generate small surf along south facing shores through the week.
See the latest Oahu Surf Discussion (SRDHFO) for additional
details on surf and swell.

&&

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

Small Craft Advisory until 6 PM HST this evening for Kauai
Northwest Waters-Kauai Windward Waters-Kauai Channel-Oahu
Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui County
Windward Waters.

&&

DISCUSSION...Foster/R Ballard
AVIATION...Burke
MARINE...Wroe

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE HONOLULU HI
1103 AM HST WED APR 11 2018

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1148 PM     DEBRIS FLOW      1 NE WAIPIO VALLEY      20.14N 155.56W
04/10/2018                   PHZ122             HI   COUNTY OFFICIAL

            COUNTY OFFICIALS REPORT OF A SMALL LAND SLIDE ON THE
            UPPER PORTION OF THE ACCESS RAOD.OVER 6 INCHES OF
            RAINFALL HAS OCCURRED OVER THE LAST 24 HOURS. THE SLIDE
            HAS BEEN CLEARED AND ONLY LOCAL TRAFFIC ACCESS IS
            ALLOWED.

&&

JNS