Heavy Rain in Hawaii, August 17 to 19, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
341 AM HST Fri Aug 17 2018

.SYNOPSIS...
A high will remain far northeast of the area through early next
week. Tropical moisture will increase over the area though the
weekend, then drier air will move over the islands on Monday. A
high far northwest of the area will move east to a position north
of the islands by the middle of next week. Hurricane Lane will
likely move into the central Pacific over the weekend. It is too
soon to know if Lane will directly affect the Hawaiian Islands.

&&

.DISCUSSION...
The surface analysis at 200 AM shows a 1032mb surface high around 1900
miles northeast of the islands near 44N 137W. The high is forecast to
move northeast slowly through early next week. This distant high will
maintain gentle to moderate easterly trade winds over the islands.

An area of moist air associated with a trough in the lower atmosphere
lingers over the islands. A trough higher in the atmosphere northwest
of the islands is keeping the atmosphere somewhat unstable. The combination
of moisture and instability will keep showers active for the next few
days. Since trade winds will continue, showers will be focused over
mainly windward areas but some clouds will build high enough to spill
over to leeward areas. Thunderstorms will be possible over the Big Island
slopes during the afternoons and evenings.

A cloud vortex associated with the remnant of a tropical disturbance
centered about 600 miles east southeast of the Big Island has moved
west around 15 mph overnight. The leading edge of the cloud area
is about 325 miles east of the Big Island. This cloud area corresponds
well with an area of high precipitable water depicted by the GFS
model. This moist air will move over the islands during the weekend
and make the atmosphere even more humid. Heavy rain will be possible
on all islands.

The atmosphere is expected to become drier and more stable on Monday.
This trend could be amplified by subsidence ahead of Hurricane Lane.
Lane, still far southeast of the islands, is forecast to strengthen
to a major hurricane and move into the central Pacific over the weekend.
It is too soon to know if Lane will directly impact the islands.

&&

.MARINE...
Light to moderate trade winds are expected to continue through
this weekend, but remain below Small Craft Advisory levels.
Periods of heavier showers are expected this weekend, particularly
over the windward waters, as tropical moisture moves in from the
east. Hurricane Lane, currently about 1350 nm east southeast of
Hilo may bring tropical storm conditions to waters south of the
Big Island next week.

No significant swells are expected through the weekend, so expect
surf to remain small on all shores. A small north northwest swell
is expected to arrive Tuesday. Hurricane Lane may bring an easterly
swell to the coastal waters as early as Sunday night.

See the latest Collaborative Surf Discussion for more details on
surf and swell.

&&

.AVIATION...
High pressure far northeast of the islands will maintain a light
to locally moderate trade wind flow through tonight. Clouds and
showers riding in on the trades are being enhanced by an upper-
level trough that is making the atmosphere somewhat unstable. As
a result, isolated cloud tops to FL350 are expected.

While VFR conditions will prevail overall, most windward areas
will see periods of MVFR conditions, and coverage currently
warrants an AIRMET for mountain obscuration for tempo MVFR on
windward Big Island. While most showers are expected over windward
slopes through tonight, daytime heating is once again expected to
lead to the formation of clouds and showers over leeward areas.
Isolated +SHRA/+TSRA over the Big Island this afternoon could
potentially lead to isolated IFR conditions.

&&

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

&&

DISCUSSION...Donaldson
AVIATION...Birchard
MARINE...Kino