Heavy Rain in Hawaii, August 2, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
341 AM HST Wed Aug 2 2017

.SYNOPSIS...
Light to moderate trade winds will persist through Thursday, then
become locally breezy Friday through the upcoming weekend. Clouds
and showers will favor windward and mauka areas. Leeward sections of
the Big island will see increased clouds and showers each afternoon
and early evening. Drier and breezier trades are expected for the
weekend and early next week.

&&

.DISCUSSION...
Recently animated water vapor loop showed plenty of middle- to upper-
level dry air in place across the islands between an upper ridge
positioned south of the state and an upper trough extending
southward across the eastern Pacific. The early morning Hilo and
Lihue soundings reflected this mid-level dry air, however, showed
plenty of low-level moisture below 10 ft. The latest satellite-
derived precipitable water (PW) imagery reflected this and
depicted a surge of moisture moving from east to west over and
just south-southwest of the Big Island (PWs up to around 1.6")
and another batch of higher moisture pushing west of Kauai. These
areas of enhanced moisture combined with light to moderate trade
winds have led to some beneficial rainfall overnight across the
windward sections of the northwest islands and the windward and
southeast sections of the Big Island (peak rainfall totals over
the past 24 hrs are up to around 1.5" over Kauai and the Big
Island).

The latest short-term model guidance has initialized well with the
current pattern and depicts these areas of enhanced moisture
discussed continuing westward within the trades through Thursday.
Although the bulk of the low-level moisture currently located over
and southwest of the Big Island will miss the smaller islands to the
south as it continues westward, the northern extent of it could clip
the smaller islands and support better windward and mauka rainfall
chances tonight into Thursday morning. Additionally, trades are
expected to relax into the light to moderate range due to a weakness
within the subtropical ridge that has developed north of the state.
Hi-res model guidance supports sea breezes developing over some
leeward areas, which may translate to better afternoon cloud and
shower coverage.

For the upcoming weekend and early next week, the extended guidance
remains in decent agreement and supports moderate to locally breezy
trades returning across the state as the subtropical ridge
strengthens north of the islands. Stronger trades combined with
drier air (PWs around or just over an inch) will limit rainfall
chances.

&&

.AVIATION...
A light to moderate easterly wind flow will persist across the
area today. The widely dispersed showers riding in with this wind
flow will favor the windward and mountain areas. Expect VFR
conditions to rule the air space today, but will be interrupted
time to time with MVFR conditions associated with these showers.
Additional isolated to scattered showers may fill in this
afternoon across the interior and lee areas of most islands as a
result of the light wind, daytime heating, and ample low level
moisture. Tops of the showers have ranged between 10 to 15k with
the higher tops being more common in the vicinity of the Big
Island. A mid level trough over Maui and Big Island counties will
likely maintain these tops through today.

No AIRMETs currently in effect, and none is expected today.

&&

.MARINE...
Moderate trade winds will persist through most of the work week.
Winds over the coastal waters are forecast to remain below the
Small Craft Advisory (SCA) threshold of 25 knots at least into
Friday. The typically windier waters around the Big Island and
Maui County could experience SCA winds by the weekend.

Surf around the state will also remain fairly quiet. The typical
rough and choppy trade wind swell affecting east-facing shores
should stay small and tame through the work week. The current
small, 10-12 sec period east swell from distant east Pacific
tropical activity will continue to move through the island chain
through Thursday, giving a small bump in surf along the east
facing shores. Background southeast and south swells will
continue, keeping surf along the south shores in the 2-4 foot
range through Friday. A long period south-southwest swell could
bump the surf up just a notch over the weekend. The west facing
shores may also see a slight ripple from very distant west Pacific
typhoon swells through the week. All surf will stay below advisory
levels.

&&

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

&&

DISCUSSION...Gibbs
AVIATION...Lau
MARINE...Kinel