High Surf and Heavy Rain in Hawaii, August 6 to 7, 2016

Area Forecast Discussion
National Weather Service Honolulu HI
352 PM HST SAT AUG 6 2016

.SYNOPSIS...
Locally breezy and moist northeasterly winds ahead of a surface
low associated with former Tropical Cyclone Howard will send a
steady supply of showers to mainly the smaller islands through
tonight. Moisture and instability will increase as the
circulation moves across the smaller islands on Sunday, bringing
the potential for heavy flooding rainfall and thunderstorms into
Monday. A drier trade wind weather pattern is expected to return
Tuesday and Wednesday, followed by a possible increase in rainfall
late next week.

&&

.DISCUSSION...
Enhanced low level moisture along the outer fringes of a surface
low associated with former Tropical Cyclone Howard has been
keeping windward areas of the smaller islands rather wet today.
The center of the low is less than 200 miles northeast of Hilo and
is moving westward around 15 mph. Surface high pressure is holding
about 1700 miles north of the state, but due to the approaching
low, locally breezy trade winds are gradually shifting out of the
northeast. Enhanced low level moisture with precipitable water
(PW) values around 1.5 inches along the western periphery of the
low has been sending a steady flow of showers across mainly
windward slopes, where totals of a quarter to a half inch have
been common today. A rather strong mid level ridge over the
islands has provided a strong cap on the moisture, with afternoon
soundings and aircraft data showing an inversion around 11000 ft.

The rather wet conditions will persist over the smaller islands
tonight as the weakening surface low eventually moves to a
position just north of Maui County. Locally breezy northeast
winds will continue over Oahu and Kauai, with winds decreasing
over the Big Island this evening and Maui County late. Where the
northeast winds hold, clouds and showers will remain focused over
windward slopes, with showers periodically passing across leeward
areas. The mid level ridge is expected to maintain an inversion
that should inhibit heavy shower development over the islands.

The greatest chance for flash flooding will occur on Sunday and
Sunday night, when the low opens up into a trough as it moves
across Oahu and Kauai. The mid level ridge overhead will erode,
and an upper level trough will sag southward over the state,
leading to increased instability. Meanwhile, most of the state
will be under a moist and convergent southeasterly low level flow
near and east of the trough with PW values around 2 inches. This
time period poses the greatest risk for heavy flooding rainfall
and thunderstorms on already soaked grounds. The GFS and ECMWF
continue to show the deepest moisture and highest instability over
the northern half of the state and waters to the north, but
flooding cannot be ruled out elsewhere, particularly as
convection flares up in the afternoon. Thus, a Flash Flood Watch
is in effect for all islands during this time period. The Watch is
in effect through Monday, when the flood threat will be
diminishing from east to west as trade winds rebuild and moisture
gradually decreases.

A drier trade wind flow is expected on Tuesday into Wednesday.
Moisture within the trade wind flow will likely be increasing
again later Wednesday or Thursday as what remains from Tropical
Storm Ivette passes over the region.

&&

.AVIATION...
Showers have remained active through the day across much of the
area, with frequent MVFR conditions across windward sections and
brief MVFR leeward as well. Deeper moisture and heavier showers are
located upstream with the remnants of TC Howard, which is expected
to cross the northern islands tonight/Sunday. With the approach of
the remnant tropical system, widespread MVFR conditions and mountain
obscurations will continue through the night, likely encompassing
the entire area instead of just windward sections. An AIRMET for IFR
conditions is not out of question. On Sunday the lowest flight
categories will be concentrated over the western half of the state,
but conditions will remain showery with lingering MVFR elsewhere
across the state as well.

The appearance of wave clouds downwind of the terrain has diminished
this afternoon, and 00z soundings and radar VWPs show weaker low-
level winds than earlier. The RAOBs and ACARS soundings still show a
defined low-level inversion, which is expected to erode this
evening. So far all the pilot reports we have received have been for
none or light turbulence, so with diminishing potential we can go
ahead and drop the AIRMET.

&&

.MARINE...
As the surface low associated with former Tropical Cyclone Howard
reaches just north of Maui County overnight, winds will decrease
over the eastern end of the state and remain in the fresh to
strong range near Oahu and Kauai. The low is currently passing
about 85 nm south of buoy 51000, where winds of nearly 25 kt and
seas to 11 ft have been reported. Based on these data, we have
dropped the Small Craft Advisory (SCA) around the the Big Island
and Maui and have posted an SCA for winds and seas across waters
north of Oahu and around Kauai. The SCA runs through the day on
Sunday but could be dropped earlier. Seas will rapidly drop off
from east to west on Sunday as moderate to fresh east-southeast
winds build in from the east. A fresh trade wind flow will develop
on Monday.

Short period easterly swell from Howard increased today, with
windward CDIP buoys reporting combined seas around 8 ft at 11
seconds. A High Surf Advisory is in effect for east facing shores
through tonight but will likely need to be extended through
Sunday for Oahu and Kauai as seas diminish from east to west.
Another easterly swell, from Tropical Storm Ivette, is expected as
early as Monday night through the middle of the work week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from Sunday morning through Monday afternoon
for all islands.

High Surf Advisory until 6 AM HST Sunday for east facing shores of
the Big Island, Maui, Molokai, Oahu, and Kauai.

Small Craft Advisory until 6 PM HST Sunday for Kauai Northwest
Waters, Kauai Windward Waters, Kauai Channel, and Oahu Windward
Waters.

&&

Public and Marine...Wroe
Aviation...Bravender