Heavy Rain in Hawaii, July 7 to 9, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
331 AM HST Sat Jul 7 2018

.SYNOPSIS...
The current trade wind pattern, with clouds and showers focusing
across windward and mauka areas, will continue through the
forecast period. Enhanced showers and increased shower coverage
will persist through today as a weak surface trough moves across
the islands. A band of moisture will affect windward portions of
Kauai and Oahu on Sunday. Trade winds will decrease slightly on
Monday and Tuesday as enhanced shower activity clears from east to
west.

&&

.DISCUSSION...
A 1037 mb high near 40N165W, far north northwest of the main
Hawaiian islands, continues to drive breezy trade winds across
local waters this morning. Satellite loop shows an area of
enhanced moisture moving across waters lee of the Big Island and
Maui while the islands farther to the west have clearer skies.
However, another area of enhanced moisture lies across windward
waters and is poised to move across island chain within the next
few hours. Radar shows scattered to numerous showers across
windward portions of the islands east of Oahu, scattered showers
across Oahu and isolated showers across Kauai. A small area of
enhanced showers persists across the active volcanic rift zone in
Puna district.

Breezy and wet conditions are expected for much of the state
today. Trade winds will persist as high pressure holds to the
north. The area of enhanced moisture just upwind of the islands
will spread westward through the day. Showers should be active
along windward slopes, with periods of passing showers expected
across leeward sections from Maui to Kauai. The combination of
this moisture and a weak mid level trough will erode the trade
wind inversion, leading to spotty heavy showers and perhaps a
thunderstorm on the Big Island, mainly within the volcanic plume
in Puna and over the interior.

Breezy trade winds will decrease a bit Sunday and Monday. As the
area of enhanced moisture clears the state from east to west, a
separate band of low level moisture, currently sitting less than
200 miles north of Kauai, will move over the northern end of the
state. This points towards wet windward conditions for Kauai and
Oahu, while the rest of the state experiences a decrease in
rainfall.

A drier and more stable trade wind flow will settle over the
entire state by the middle of next week. High pressure will remain
north of the state, driving moderate to locally breezy trade
winds. Ridging aloft will bring stable conditions. No areas of
enhanced moisture are seen at this time.

&&

.AVIATION...
A strong high pressure system far to the north of the islands will
continue to produce breezy to locally windy trade winds through
the weekend. A low pressure trough moving through the state today
will continue to produce clouds and widespread showers spreading
westward across the island chain. A wet trade wind weather pattern
will continue through Sunday with the highest shower activity
forecast along windward and mountain slopes with periods of showers
drifting leeward.

AIRMET Sierra in effect for mountain obscurations above 2000 feet
over mountains and north through east sections of all islands. These
low clouds will likely last through Sunday morning.

AIRMET Tango in effect for tempo moderate low level turbulence
over and south through west of mountains with conditions lasting
through the weekend.

&&

.MARINE...
The Friday evening ASCAT pass showed swaths of 25 kt winds in the
channels between all of the islands from the Big Island to Kauai.
These strong trade winds combined with both east and south swells
will maintain rough boating conditions through the weekend. A
Small Craft Advisory remains in effect for all Hawaiian waters
through 6 AM Sunday morning, and this advisory may need to be
extended by later shifts for some waters through Sunday afternoon.
Winds and seas should drop below advisory levels across most or
all of the waters by Monday. Advisory-level winds may return to
the typically windy areas around Maui County and the Big Island on
Tuesday or Wednesday.

With Barber's Point still reporting 4 feet at 13 seconds as of
2 AM, we have extended the High Surf Advisory (HSA) for south
facing shores through today. Offshore buoy reports suggest that
the swell and surf should begin to decrease during the afternoon.

Along east facing shores, we expect a mixture of long period swell
from former Hurricane Fabio and short period trade wind swell to
generate rough surf today and tonight, and the High Surf Advisory
remains in effect through 6 AM Sunday morning.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST this evening for Niihau-Kauai
Leeward-Oahu South Shore-Waianae Coast-Molokai Leeward-Lanai
Makai-Kahoolawe-Maui Leeward West-Maui Central Valley-Leeward
Haleakala-Kona.

High Surf Advisory until 6 AM HST Sunday 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 Sunday for all Hawaiian
waters-

&&

DISCUSSION...Powell
AVIATION...Bohlin
MARINE...Jacobson