Flash Flooding and Heavy Rain in Hawaii, November 27 to 30, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
843 PM HST Mon Nov 27 2017

.SYNOPSIS...
Gusty trade winds will continue to deliver abundant showers this
week, keeping windward areas quite wet as deep moisture near Maui
and the Big Island spreads over the rest of the state. Wet weather
may continue into next weekend.

&&

.DISCUSSION...
Water vapor imagery shows a broad nearly stationary trough aloft
extending across a large portion of the tropical central north
Pacific, extending from N of the islands to well SW of the islands,
with an embedded low centered about 1300 miles to the WSW. Diffluent
flow E and SE of the trough axis is resulting in overcast mid and
high level clouds over a large area, including the Hawaiian Islands.
At the surface, a pair of highs are in the process of consolidating
about 1000 miles to the NE, supporting gusty E trade winds. A moist
and nearly saturated air mass (PWAT > 2") in place over the Big
Island and Maui is supporting widespread windward rainfall, where a
Flash Flood Watch (FFA) is in effect.

Rainfall rates have been averaging a steady .50"/hour at many of the
gages on windward Big Island/Maui, resulting in 12 hour rainfall
totals up 6", and satellite and radar indicate no end in sight. Thus
far no significant flooding issues have been reported, and Big
Island gages at Honolii Stream and Wailuku River show a leveling-off
in water levels this evening, after rising several feet during the
day. Trade wind showers are also fairly active elsewhere in the
state, although the air mass is not nearly as saturated, with the
00Z PHLI sounding moisture as deep as 10 kft. Big Island summit
temperatures are near or above freezing, but the potential for heavy
showers to lower temperatures warrants a continuation of the Winter
Storm Warning (WSW), with some snow already on the ground from
deeper convection this afternoon.

UW-CIMSS MIMIC2 total precipitable water imagery shows deep moisture
of 2 inches of higher extending well E and SE of the islands.
Forecast models indicate relatively strong trade winds to continue
through most of the week, supported by a pair of highs to the NE and
NW of the islands by midweek. Deep moisture will spread up the
island chain, and although the atmosphere will be only moderately
unstable as the trough aloft gradually weakens, windward areas will
likely receive a steady barrage of gusty showers through at least
Thursday. Whether or not a continuation and/or an expansion of the
FFA will be warranted remains a question due to the fact that
showers will be moving along with the trades, but may be
sufficiently incessant to result in significant stream rises and/or
flash-flooding. Warming summit temperatures may allow the WSW to be
dropped Tuesday, but confidence is low. No changes are expected
overnight.

Later in the work week, the latest 00Z GFS guidance has shifted from
earlier runs, and other available model guidance, and forecast
confidence is quite reduced. A new mid-level shortwave is expected
to digging into the longwave trough to the W, and the models
continue to waver in their handling of its evolution. Most models
generally have the idea that surface cyclogenesis will occur
somewhere southwest of the state starting on Thursday. However, 00Z
GFS and ECWMF have both backed way off on the depth of the low and
its proximity to the islands, instead developing a trough to the SW
that allows a strong trade wind (or ESE) flow to persist over the
islands. Despite the run-to-run gyrations the models have exhibited,
a wet trade wind weather pattern continues to be depicted through
the week, and we are feeling fairly confident about that. The
weekend forecast continues to remain rather uncertain.

High clouds streaming over the islands may thin late Tuesday into
Wednesday, only to thicken again later in the week as the new
shortwave digs in to the W.

&&

.AVIATION...
High pressure north of the state will keep a moderate to breezy
trade wind flow in place through Tuesday. Clouds and showers will
favor windward and mauka areas, but will also affect leeward areas
from time to time. The deep tropical moisture and the most unsettled
weather will remain over Maui County and the Big Island through
Tuesday. Current radar returns show a slight decrease in the showers
across windward Maui, while locally heavy showers continue to pulse
mainly along the coast from Upolu point to Cape Kumukahi to South
Cape. AIRMET Sierra will remain in place through the rest of
tonight, and likely into Tuesday morning for windward mountain
slopes of Molokai, Maui, and the Big Island.

Otherwise, it is predominantly VFR across Oahu, Kauai, leeward
sections of Maui, Molokai, and the Kona section of the Big Island.
There have been brief periods of MVFR conditions over on the
windward and mountain areas of Oahu and Kauai, but not extensive or
long enough to bring on AIRMET Sierra. Conditions, however, will be
monitored closely. Tops of the lower clouds and showers are between
15k and 20k feet across Maui and the Big Island, and around 12k feet
over the remaining islands.

Besides the lower clouds, a large swath of layered clouds, mainly
cirrus, will continue its prescience over the main Hawaiian through
Tuesday. The base of the cirrus is at 20k feet. The cirrus is part
of an active jet stream with its core located just to the north of
Kauai at its nearest point to the islands. Please refer to SIGMET
Whiskey series which cover this jet stream and its potential for
severe turbulence between fl280 and fl420. Forecast models suggest
the turbulence to gradually diminish on Tuesday.

AIRMET Tango for ocnl moderate turbulence between 8k and 25k feet
over the Big Island, and between 15k and 35k feet over the smaller
islands, will be reassessed. AIRMET Tango for low level turb 8k feet
or less will continue through 1600z, and likely beyond.

&&

.MARINE...
A Small Craft Advisory (SCA) remains in effect for all Hawaiian
coastal waters due to a combination of winds and seas. Fresh to
locally strong trade winds, mainly through the channels, will
continue through midweek. A combination of a declining north swell,
building north-northwest swells, and wind waves from the trade
winds, will help keep combined seas above SCA levels through most of
the coastal waters for the week.

A High Surf Advisory is posted across east facing shores through
Tuesday afternoon for the increased trade wind swell from the strong
winds. However, surf along east facing shores may hover around the
advisory level from Wednesday through the rest of the week.

The current north swell will continue to subside for the next few
days. Surf along north facing shores will rise again Tuesday
afternoon as a new north-northwest swell is forecast to build. This
swell is expected to peak early Wednesday morning, with surf along
north facing shores nearing advisory levels again Tuesday evening
into Wednesday. A larger north-northwest swell is then forecast to
build Wednesday and Wednesday night before peaking Thursday. This
swell could potentially bring warning level surf to north facing
shores and advisory level surf to west facing shores from Wednesday
night into Thursday night.

A high amount of uncertainty remains around the weather forecast for
the upcoming weekend. Please stayed tuned to later discussions for
mention of wind and local wave forecasts.

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

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until 6 PM HST Tuesday for Big Island
Summits.

Flash Flood Watch through Tuesday afternoon for Maui and the Big
Island.

High Surf Advisory until 6 PM HST Tuesday for East facing shores of
Kauai Oahu Molokai Maui and the Big Island.

Small Craft Advisory until 6 PM HST Tuesday for all Hawaiian
waters.

&&

Discussion...Birchard
Aviation...Lau
Marine...M Ballard