Flash Flood and Heavy Rain in Hawaii, November 11 to 12, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
1029 AM HST Sat Nov 11 2017

.SYNOPSIS...
An area of moisture moving across the islands will interact with
an upper level disturbance pushing over the state this weekend to
bring wet weather to many locales. The weather will start to improve
from east to west on Sunday as the moisture area moves away from
the islands, with more settled trade wind weather expected for
most of next week. A strong surface high developing north of the
islands next week will strengthen the trades back to breezy levels
for most area by Wednesday.

&&

.DISCUSSION...
A pocket of low level moisture (with CIMSS MIMIC precipitable
water values around 1.75" or so) is pushing across the islands
this morning. Meanwhile, a weak but digging mid- level shortwave
is approaching the islands from the NW, bringing cooler air aloft.
The combination of these two features is destabilizing the
airmass and enhancing showers from Oahu to the Big Island at mid-
morning, with even a few heavy thundershowers near Hilo where
orographic lift is enhancing the rainfall even more. So far, Kauai
has been fairly quiet, with a nice Veterans Day morning. The
surface pressure gradient over the islands is fairly slack,
keeping mostly gentle trades in the background flow around the
islands.

The pocket of low level moisture is expected to continue to move
rather slowly westward across the state, bringing increasing
showers to Kauai by afternoon. As the upper trough continues to
dig over the islands, expecting a few more showers to become
locally heavy, along with the odd thundershower or two as well.
Not expecting to see a lot of anchoring or training of convection
with background trades blowing, but will need to keep an eye out,
especially on windward facing slopes.

Enhanced showers should continue into the evening across the
state, and with the airmass becoming more unstable, we could see a
few more locally heavy showers or thundershowers. The forecast is
a bit tricky as the deeper moisture will be pushing west of the
islands overnight, as the same time as the coolest mid-level
temperatures are arriving. The high resolution convection-
allowing guidance responds by indicating a decreasing trend in the
showers, especially later in the night. This expectation is
currently reflected in our forecast, but will be monitoring trends
today to see if any adjustments need to be made to PoPs, shower
intensities, or thunder areas for the nighttime period.

Although there should be fewer trade-wind showers on Sunday and
Sunday night, rather chilly 500 mb temperatures from -10C to -12C
should provide some oomph to whatever showers do ride in. Primary
focus should return to more of a windward and mauka focus, but the
heavier showers could make their way over to some adjacent leeward
areas over the smaller islands. The unstable air and lingering
moisture will probably lead to some locally heavy afternoon
showers or thundershowers over the lee slopes of the Big Island on
Sunday, and will likely be adding these to the forecast grids
soon.

Monday into Tuesday should be relatively quiet days with drier,
more stable air as the trough axis weakens and shifts to the E of
the islands. More typical trade wind weather is expected.

The remnants of an old front will likely drop south over the
islands Tuesday night, along with another shot of modestly cooler
air aloft, and this should bring another increase in showers
Tuesday night into Wednesday. The trades will increase back to
breezy levels as this boundary pushes through, as strong high
pressure far to the N drops S. A weak cold front could bring
another band of showers through late in the week, followed by
cooler and drier air. Although the models are in fairly good
agreement with this feature, there are some timing differences and
this is pretty far out in time.

&&

.AVIATION...
A moist and somewhat unstable trade wind flow will persist over
the state into tonight. An area of enhanced low level moisture
associated with a weak surface trough stretches from the Big
Island to Oahu and is poised to reach Kauai late this morning. An
approaching mid to upper level trough has wiped out the inversion
according to recent aircraft data and has triggered a few
thunderstorms around the Big Island this morning. AIRMET Sierra
for mountain obscuration will remain in effect today and likely
tonight for widespread MVFR conditions over mountains and along
windward slopes. This AIRMET currently covers from the Big Island
to Oahu but will be expanded to cover Kauai before noon. VFR
conditions will mostly prevail over leeward sections of the
smaller islands today, though brief periods of MVFR conditions
will occur as showers periodically push over the terrain. In
addition, an isolated thunderstorm cannot be ruled anywhere
today.

&&

.MARINE...
The combination of enhanced moisture and an upper level trough
moving over the state will lead to occasionally heavy showers and
isolated thunderstorms today. Isolated heavy showers could linger
through the weekend.

The High Surf Advisory has been extended through this afternoon
for east facing shores on all islands, except for Lanai, due to
a north-northeast swell (010-020). The period has dropped a bit
from yesterday, but the swell heights continue to run between 7
and 8 feet at buoy 51000, and 5 and 6 feet at buoys 51202 and
51206. Swell and surf should gradually lower into tonight.

A west-northwest (290-300) swell is forecast to fill in Monday
night and Tuesday, before peaking Tuesday night. The forecast has
the swell peaking near 5 feet, 14-15 sec. Another north-northeast
(360-020) swell is forecast to peak midweek, near 8 to 9 feet,
10-11 sec. This swell will likely bring advisory, to potentially
warning level, surf to the east facing shores again. Advisory
level surf is also possibility for north facing shores as well.

A series of small, long period south to southwest swells will move
through the islands through the up coming week, producing small
bumps in surf along south facing shores.

Winds and seas should stay below the Small Craft Advisory (SCA)
threshold at least into midweek.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST this evening for Kauai
Windward-Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-
Windward Haleakala-Big Island North and East.

&&

R Ballard/Wroe/EATON