Heavy Rain in Hawaii, July 4, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
955 AM HST Thu Jul 4 2019

.SYNOPSIS...
A stable and somewhat weak east to southeast flow will prevail
for the next couple of days, producing modest rainfall over
windward slopes and spotty afternoon showers over leeward areas.
Trade winds will build late Saturday and Sunday as high pressure
northeast of the state strengthens. An increase in rainfall and
winds is looking increasingly likely on Monday and Tuesday as the
remnants of Hurricane Barbara move through.

&&

.DISCUSSION...
Stable conditions and somewhat weak easterly winds remain in
place. The subtropical ridge sits less than 200 miles north of
Kauai this morning, driving gentle easterly trade winds over most
islands, with a shift out of the east-southeast noted near Kauai.
An unseasonably deep trough passing north of the state has eroded
the subtropical ridge and displaced it well south of its normal
July location, but a mid-level ridge is hanging on over the
islands, maintaining stable conditions with an inversion between
5,500 to 8,500 ft. The higher inversion was captured within an
area of slightly enhanced low-level moisture currently affecting
the Big Island and Maui. As far west as Oahu, expect a small
uptick in windward showers compared to yesterday as the pocket of
moisture advances westward, along with more extensive leeward
cloud cover and a few showers this afternoon as sea breezes kick
in. Dew points will also creep up to the lower 70s, and with winds
somewhat light, humidity will likely feel worse than yesterday,
even though daytime temps will be near normal.

Little change is expected into Saturday. Winds will briefly shift
southeasterly on Friday as the subtropical ridge is displaced a
bit closer to the islands, but overall, the stable weather
picture will only subtly change. Modest rainfall will persist
along windward slopes, while afternoon sea breezes produce clouds
and spotty afternoon showers over leeward areas. The pocket of
moisture currently affecting the eastern end of the state will
drift to Kauai and Oahu on Friday as another area, now sitting
about 150 miles east of Hilo, moves in. No areas of organized
moisture are noted for Saturday.

Trades will make a comeback late Saturday and Sunday. The trough
north of the islands will fill in and lift northward, allowing
high pressure to the northeast of the state to strengthen. As a
result, trades will build to moderate strength, while a ridge
aloft maintains stable conditions. Expect typical shower activity
to be focused across windward areas.

Although the details remain elusive, an increase in rainfall and
winds is looking increasingly likely on Monday and Tuesday. The
official National Hurricane Center forecast for Hurricane Barbara,
currently located about 1,600 miles east of Hawaii, calls for the
system to weaken to a remnant low far east of the state on
Saturday. The GFS and ECMWF are in fairly good agreement in
showing the remnant circulation of Barbara passing near or over
the islands on Monday and Tuesday. While the details remain
somewhat uncertain, expect an increase in shower activity and
winds during this time.

&&

.AVIATION...
A weakness in the surface ridge north of the islands will maintain
a light background trade wind flow across the area through Friday,
but also bring on sea breezes over the lee and interior areas of
especially the smaller islands.

Expect a predominantly VFR situation area wide for the rest of
today. The light trade wind flow will lead to some spotty cloudiness
with a shra or two over the interior and lee areas of all islands.
The weak trade wind flow, itself, will continue to carry in a few
trade wind clouds and showers to the windward areas of all islands.
Do expect some brief periods of MVFR ceiling/vis with these
clouds/showers. Expect the max tops of these clouds at 12k feet.
AIRMET Sierra for mountain obscuration over the slopes above Hilo,
Hawaii, was just lowered. We do not anticipate any more AIRMET
Sierra or any other AIRMETs for anywhere across Hawaii-nei for the
rest of today and into the evening hours.

&&

.MARINE...
A weak surface trough continues to linger northwest of Kauai,
while a weak surface ridge is located just north of Oahu. This
will keep relatively light east to east-southeast winds across
most of the coastal waters through early Friday. The surface
trough is forecast to move westward away from the region, with
high pressure building far north and northeast of the state this
weekend, allowing easterly trades to strengthen slightly.

The latest advisory for Hurricane Barbara from NHC continues to
indicate weakening as it approaches the central Pacific, possibly
down to a post-tropical remnant low just east of the islands early
next week. Winds will strengthen and back to the northeast as this
system approaches. SCA conditions will likely develop Monday across
most of the coastal waters, especially the channels and waters
adjacent to Maui County and the Big Island. In addition, expect a
significant increase in shower coverage as it nears the islands.

New swells from the South Pacific are expected to keep surf near
the HSA criteria along south facing shores this weekend and
moderate surf into early next week.

Forerunners from the large swell produced by Hurricane Barbara are
expected to arrive along east facing shores of the Big Island and
Maui late tonight or early Friday morning, reaching advisory level
Friday or Friday evening. This swell is also expected to cause
advisory level surf for east facing shores of most other Hawaiian
islands this weekend into early next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

DISCUSSION...Wroe
AVIATION...Lau
MARINE...TS