Heavy Rain in Hawaii, July 14 to 15, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
355 PM HST Sat Jul 14 2018

.SYNOPSIS...
High pressure far north of Hawaii will drive moderate to fresh
trade winds over the state through next week. Showers carried by
the trades will affect the islands at times, with locally heavy
downpours over windward sections possible into Sunday. A slight
chance of thunderstorms is forecast for the Big Island and leeward
Haleakala slopes in Sunday afternoon. Drier and more settled
weather is expected to return across the islands Monday and
Tuesday, followed by the potential for an increase in rainfall
over the eastern part of the island chain around Tuesday night.

&&

.DISCUSSION...
A surface high far north of the Hawaii will continue to bring
moderate to fresh trade winds across the state through next week.
A nearly stationary low aloft centered about 600 miles north
northwest of the Kauai and will remain there through Sunday night,
allowing for somewhat unstable weather to persist across the
state this weekend.

00Z sounding at Lihue shows decent lapse rates, 500 mb temperates
of -8 degrees Celsius and 807 j/kg MLCAPE. The Hilo sounding
shows a weak and elevated inversion near 11 kft, a bit warmer
aloft as expected and similar MLCAPE values. Both soundings show
very moist precipitable water values 1.8 to 1.9 inches. The
instability, combined with the abundant moisture, may trigger
locally heavy downpours through Sunday statewide (Sunday night for
the western half of the state). Most of the incoming showers will
be directed over windward and mauka sections due to the breezy
trade winds.

In addition, a slight chance for thunderstorms is forecast for
the following areas through Sunday: Puna District on the Big
Island due to the low-level heat/instability generated from the
lava flow. The Kona slopes of the Big Island and leeward Haleakala
as onshore flow interacts with the increased moisture/instability
Sunday afternoon. We do not currently anticipate thunderstorms
elsewhere around the state, since they will be capped by more
stable (but wet) trade winds.

The showers will begin to focus over the western portion of the
state by late Sunday, and clear all areas Monday morning, with
drier and more stable trade wind weather taking through Tuesday.

Forecast models show a tropical disturbance moving westward
passing just south of the state on Wednesday, with an area of
moisture passing near the Big Island, with some associated showers
pushing onshore by the trades. This implies that another increase
in rainfall beginning around Tuesday night mainly for windward
and southeast slopes of the Big Island. GFS and ECMWF do not show
direct impacts to Hawaii at this time, but we will continue to
monitor.

&&

.AVIATION...
Moderate trade wind flow to continue, with clouds and showers
focused over windward and mountain areas. Expect MVFR conditions
in clouds and showers. AIRMET Sierra continues for mountain
obscuration over Kauai and Oahu for clouds and showers over the
windward sides of those islands. Expect these conditions to
continue into tonight. Additional areas of mountain obscuration
may prompt additional AIRMETs.

An unstable airmass over the islands has sparked at least one
thunderstorm over the Big Island this afternoon. Expect the
possibility for additional thunderstorms through the afternoon,
and again Sunday afternoon.

&&

.MARINE...
Strong high pressure far north of the area will continue to
produce fresh to locally strong trade winds across the coastal
waters into early next week. The Small Craft Advisory for the typically
windy waters around Maui County and the Big Island has been
extended through Monday, but could need a further extension in
time. There is also a slight chance of thunderstorms over the
coastal waters north of Kauai and Oahu tonight through Sunday due
to an upper-level trough NW of the state. Building seas are
possible next week, as models continue to indicate a low pressure
system passing south of the state around Wednesday.

Easterly trade winds will continue to produce rough, choppy surf
along east facing shores through early next week. A new southwest
swell from the Tasman Sea is expected to fill in tonight, peak
Sunday night and slowly drop on Monday. A series of storms south
and southeast of New Zealand over the past several days should
provide surf near the summer average for the second half of next
week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Monday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

DISCUSSION...Foster
AVIATION...M Ballard
MARINE...TS