High Surf in Hawaii, July 18 to 19, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
323 AM HST Wed Jul 18 2018

.SYNOPSIS...
Breezy trade winds will prevail today and tonight, with rather wet
conditions over windward areas, especially on Maui and the Big
Island, and some showers carrying over into leeward sections of
the smaller islands. Drier trade wind weather will return on
Thursday, with the winds easing a bit by Friday. Moderate trade
winds are then expected to continue through the weekend, with
showers focused mostly on windward areas. The leeward slopes of
the Big Island will continue to see the typical summertime clouds
and widely scattered showers each afternoon.

&&

.DISCUSSION...
Breezy trade wind weather continues early on this Wednesday
morning across the state. Shower activity has increased as
expected since last evening across eastern parts of the state,
especially over windward sections of Maui and the Big Island,
where several rain gauges have measured over a half inch of
rainfall during the past six hours. Surface analysis depicts an
elongated east-west area of high pressure located about 1200 miles
north of the islands, with a weak tropical disturbance (Invest
91C) located about 400 miles SSE of the Big Island. The tight
pressure gradient between these features is producing the breezy
trade wind pattern over the islands. Aloft, water vapor imagery
depicts a narrow but strong mid level ridge sitting over the state
and extending off to the northeast, with a large upper level
trough to our northwest and north. The ridge overhead is
maintaining stable conditions. The 12Z Hilo sounding was likely
influenced by nearby showers at release time, but did reflect the
greater moisture which has moved into eastern parts of the state
during the night, with an elevated inversion near 10K feet and PW
of 1.83 inches. The 12Z Lihue sounding remained drier with PW of
1.52 inches, and found a lower inversion near 5500 feet. MIMIC-TPW
satellite imagery depicts the higher estimated PW values
extending south and east from the Big Island, as the northern end
of the westward-moving moisture axis associated with Invest 91C
approaches. The leading edge of a drier airmass with estimated PW
values below 1.4 inches is located about 300 miles east of the Big
Island and is moving steadily westward.

Today, breezy trade winds will continue. Enhanced moisture along
the northern fringe of the passing tropical disturbance to our
south will continue to enhance windward shower activity especially
over Maui and the Big Island. Although the mid level ridge will
maintain stable conditions across the southern half of the island
chain, we cannot rule out localized heavy showers flaring over and
downwind of the active lava flow in the southeast Puna District.
Otherwise, the upper level trough to the northwest will drift
closer to Kauai. This feature has been producing isolated
thunderstorms about 275 miles northwest of Kauai, and as the
trough shifts closer, a few thunderstorms could pop up in the
offshore waters somewhat closer to the state. The trough will also
keep areas of high clouds streaming overhead.

The aforementioned drier airmass will spread east to west across
the state tonight into Thursday morning. This should result in
decreasing shower activity. Trade winds are forecast to weaken
somewhat by Thursday night into Friday, as the weak tropical
disturbance passes far southwest of the state allowing the
pressure gradient to relax a bit. Stable and fairly dry weather
with moderate trade wind flow is then expected to prevail through
the weekend. Passing mainly light showers will focus over
windward areas. Heading into early next week, the last couple of
GFS runs have come into much better agreement with the ECMWF in
keeping deeper moisture associated with the next westward moving
tropical disturbance suppressed mainly to the south of the state.
Therefore have lowered rain chances somewhat for Sunday and
Monday. Another wave of tropical moisture may approach the state
from the southeast by the middle of next week.

&&

.AVIATION...
Breezy trade winds will continue, thanks to persistent high
pressure far north of the main Hawaiian Islands. VFR conditions
will prevail through today, but low clouds and scattered showers
embedded within trade flow will bring periods of MVFR conditions
along windward slopes. Windward slopes of the Big Island are
expected to see more persistent MVFR conditions through this
morning as deeper moisture moves through from the east. Conditions
are expected to improve throughout the afternoon as this moisture
moves off to the west.

AIRMET Tango remains in effect for tempo moderate turbulence over
mountains and south through west sections of each island
including coastal waters. Breezy trade winds will keep these
conditions through today.

&&

.MARINE...
High pressure to the north of the islands will maintain fresh to
strong trade winds. The strongest winds are expected today as a
trough passing south of the state tightens the local pressure
gradient. As a result, the Small Craft Advisory (SCA) has been
expanded from the typically windy waters around the Big Island and
Maui County to cover waters around Oahu. Combined seas around the
Big Island will also rise to near 10 ft. Winds will decline a
notch Thursday and Friday, and the SCA will likely be scaled back
to the typically windy waters through the weekend.

The fresh to strong trade winds will generate rough seas during
the next couple of days. Local PacIOOS buoys have recently been
registering seas of 7 to 9 ft at 8 to 10 seconds. Thus, a High
Surf Advisory has been issued for east facing shores through
tonight. As winds decline Thursday and Friday, east shore surf
will drop back to seasonable levels.

A series of reinforcing south and southwest swells will maintain
small to moderate surf along south facing shores across the state
through Friday. Slightly larger and longer period south-southwest
swells spreading across the state this weekend may cause surf to
approach the HSA threshold along south facing shores from Saturday
into Monday.

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

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Thursday for east facing shores
of Kauai, Oahu, Molokai, Maui and the Big Island.

Small Craft Advisory until 6 PM HST Thursday for Kaiwi Channel-
Maui County Windward Waters-Maalaea Bay-Pailolo Channel-
Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

Small Craft Advisory until 6 AM HST Thursday for Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Maui County Leeward
Waters.

&&

DISCUSSION...Jacobson
AVIATION...Powell
MARINE...Kinel