Heavy Rain in Hawaii, July 18 to 19, 2016

Area Forecast Discussion
National Weather Service Honolulu HI
1022 AM HST MON JUL 18 2016

.SYNOPSIS...
The low that was formerly Celia will pass a couple hundred miles
north of the state through tonight. A period of light winds south
of the low will lead to onshore sea breezes and afternoon showers.
The tropical airmass associated with Celia will keep conditions
muggy and help fuel heavier showers, especially during the
afternoon. Breezy and somewhat cooler trade winds will return
Tuesday and Tuesday night, bringing brief windward showers through
Thursday. Moisture from weakening Darby could potentially bring
increased showers for Friday and next weekend.

&&

.DISCUSSION...
Water vapor imagery shows an upper level low far north of the
state, with a trough extending down from it to our northeast. An
upper level high and ridge is located west of the state. At the
surface, the low that is former tropical cyclone Celia is located
about 200 miles northeast of Honolulu, and is moving to the west
around 15 mph. 12z soundings from Lihue and Hilo show inversions
around 12-13kft, with a weak, lower inversion around 6kft at
Lihue. Precipitable waters range from 1.37 at Lihue to 1.63 at
Hilo. Early morning MIMIC total precipitable water imagery shows
the higher moisture near the Big Island extending westward past
Oahu and approaching Kauai. Much higher PW values over 2 inches
are located to the west and north of the center of former Celia.

The track of former Celia north of the state has interrupted the
low-level winds, placing Maui and the Big Island in a weak col
this morning and turning winds more northerly across Kauai/Oahu.
The rainfall pattern bears this out with few clouds to start the
morning over the eastern half of the state and greater
showers/rainfall in the west. Rainfall amounts over the past 12
hours were generally a half inch or less over Kauai and Oahu, and
a quarter inch of less elsewhere (with most of that quarter inch
rainfall occurring last evening).

As former Celia continues westward through tonight, winds across
Kauai/Oahu will back and weaken, and winds near the Big Island
will turn more southeasterly. The light or weakening winds across
the islands will allow for onshore sea breezes this afternoon.
with an increase in showers across interior sections. Between the
available moisture and elevated inversions, some of the showers
could be heavy during the afternoon. Otherwise conditions will
continue to feel muggy with dewpoints in the lower 70s and
relatively light winds.

The progression of low-level winds turning south to southeast,
then backing around to the east Tuesday/Tuesday night, will spread
from east to west across the state as former Celia moves west of
the area. 12z models indicate a trailing convergence band moving
westward across the state tonight into Tuesday, bringing another
burst of showers. A band is apparent on satellite imagery wrapping
around the east side of the low, and is currently located about
150 miles east of Hilo at its closest point. The current forecast
keeps higher rainfall chances tonight, including the potential for
an afternoon convective pattern again on Tuesday.

The deeper moisture associated with the former Celia may not clear
the western islands until early Wednesday, but we could start to
see building trade winds by Tuesday night. These strengthening
trades may become breezy Thursday/Friday as we are located between
the subtropical high far north of the state slowly shifting
westward and a tightening gradient due to the approach of Darby
from the east. Forecast soundings show inversion heights dropping
back to near normal levels, which should give us a couple days of
breezy trade wind weather.

During the weekend we could see an increase in moisture
associated with what is expected to be a significantly weakened
version of current Hurricane Darby. See the latest forecast from
the National Hurricane Center in Miami for details on Darby. It is
still too early to be confident in the details of the forecast,
especially since there remain significant differences between
global model solutions. Our long range forecast grids are more
heavily weighted toward the wetter and windier GFS vice the ECWMF,
which depicts Darby making a sharp northward turn just east of the
Big Island, bringing less wind and showers.

&&

.AVIATION...
A surface low currently 225nm NNE of PHNL will track W at 10 kt.
Showery low clouds in the south semicircle of the low will produce
TEMPO MVFR conditions over Oahu and Kauai for the next 12 hours or
so. AIRMET SIERRA is in effect for MTN OBSC for Kauai and Oahu.
Light winds over Maui County and the Big Island of Hawaii will allow
clouds and showers to develop this afternoon, so MTN OBSC is
possible for those islands later today. A convergence band trailing
southeast from the low will produce more clouds and showers tonight,
and possibly more MTN OBSC.

&&

.MARINE...
Former tropical cyclone Celia is passing north of the state, and
the swell generated by it is backing from east to northeast. With
the low closer and weaker, the period is also expected to drop
today, though it has held steady through the morning so far. A
high surf advisory is currently in effect for east shores through
today. This advisory may need to be extended through tonight for
at least a portion of the area. Seas at the CDIP buoy in Hilo have
started to trend down, but the Pauwela and Mokapu buoys have
continued to gradually rise. Based on surf reports we receive
today and how the buoys respond, we will determine which areas the
high surf advisory will need to be extended.

Although surf may drop below advisory levels along east facing
shores by Tuesday, long-period swells generated by distant
Hurricane Darby are expected to arrive this week, keeping east
shore surf heights elevated. This swell is expected to climb
through the week, and may reach advisory levels again during the
second half of the week. Elsewhere, small to moderate surf along
south facing shores this week will be supplied by relatively
small swells arriving from the southern hemisphere.

While winds are expected to remain below small craft advisory
levels for the next couple of days, we will see hazardous seas of
10 feet for portions of Kauai and Oahu waters through early
Tuesday. Buoy 51000 to the northeast of the state reported seas up
to 14 feet overnight, followed by a rapid drop off through the
morning. The Mokapu CDIP buoy is currently running around 10 feet.
As the Celia remnant low moves NW of Kauai on Tuesday, trade winds
will increase from east to west across the state, potentially
becoming strong Thursday and Friday. The forecast is dependent on
the future track of what is expected to be a significantly
weakened Darby. At a minimum, expect small craft winds/seas in
many areas from late Wednesday into the weekend.

&&

.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-South Big Island-Big Island North and East.

Small Craft Advisory until 6 AM HST Tuesday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Channel-Oahu Windward Waters.

&&

Public/Marine...Bravender
Aviation...Donaldson