Heavy Rain in Hawaii, September 30, 2016

Area Forecast Discussion
National Weather Service Honolulu HI
834 PM HST FRI SEP 30 2016

.SYNOPSIS...
High pressure north of the state will drive fresh trade winds through
the weekend. Showers are expected to increase in coverage during
the weekend, with heavy downpours becoming more likely early next
week, as a moist airmass associated with the remnants of Ulika
moves over the state.

&&

.DISCUSSION...
Typical trade wind weather prevails across the Aloha State on this
Friday evening, with passing showers focused mainly across
windward and mauka areas. Rainfall amounts during the past 12
hours have been mostly light, with a few stations on Kauai
reporting over a quarter inch during that time. Surface analysis
depicts high pressure far north-northeast of the state, with fresh
trade winds over the islands. The 00Z soundings show rather dry
conditions, with PW values near one inch and the trade wind
inversion around 6000 feet. MIMIC-TPW satellite imagery depicts a
dry airmass over the islands, but increased moisture with
estimated PW values up to 1.7 inches approaching the Big Island
from the east. This plume of enhanced moisture extends several
hundred miles further eastward, to the remnants of former Tropical
Storm Ulika about 600 miles east of Hilo. Aloft, water vapor
imagery shows a weak trough extending eastward from the islands to
over the remnants of Ulika, with a ridge centered northwest of the
state.

Overnight, showers are expected to increase across windward
sections, especially over the Big Island and Maui, as the area of
enhanced moisture begins to spread westward over the state. Fresh
trade winds will keep the showers moving along, and conditions
will remain rather stable aloft, so rainfall amounts should not be
particularly heavy. The existing forecast appears to be on track,
and no updates are planned for this evening.

During the weekend, moisture will continue to increase over the
islands from east to west, while the upper trough initially over
and east of the state will evolve into a pair of upper lows, one
centered a few hundred miles northwest of Kauai and the other
centered several hundred miles east of the Big Island. Surface
high pressure will shift slowly southward far to the northeast of
the state, maintaining fresh trade winds over the islands. An
increasingly wet trade wind pattern is the likely result, with
frequent showers over windward/mauka areas and scattered showers
spilling over into many leeward areas. Model consensus indicates
that the atmosphere will become only slightly unstable during the
weekend, so most of the showers should remain light to moderate
although a few heavier downpours will be possible. The frequency
and persistence of the showers could still result in some
considerable rainfall totals during the weekend across windward
areas.

Moisture and instability is forecast to increase further across
the state early next week, as a very moist airmass with PW values
up to 2 inches move overhead, and the closer upper trough drifts
eastward with the trough axis moving over the islands. Localized
heavy downpours appear increasingly likely during this time, and
isolated thunderstorms cannot be ruled out. Low level winds are
forecast to veer slightly toward the east-southeast, as a passing
front over the North Pacific weakens the Northeast Pacific surface
high. This flow pattern could encourage training of showers over
certain favored areas. It is a bit early to add heavy rain or
thunder to the forecast at this point, but the early to middle
portion of next week looks potentially quite active. Will continue
to monitor later model runs as we head through the weekend.

&&

.AVIATION...
Moisture moving westward on the trades will cause occasional MVFR
conditions across windward locations, with a mountain obscuration
AIRMET SIERRA possible for these areas later tonight. Leeward
terminals and terminals on the western islands should remain VFR
through tonight and Saturday. AIRMET TANGO is in effect statewide
for mechanical turbulence below 8000 feet immediately over and
south through west of the mountains.

&&

.MARINE...
The Friday morning ASCAT pass found areas of 25 kt winds in the
Alenuihaha Channel and southwest of the Big Island, and recent
mesonet data suggest that Small Craft Advisory /SCA/ criteria
winds are ongoing in the typically windy channels and bays around
Maui County as well. High pressure far to the north of the state
will keep fresh trade winds in place through the weekend, and the
SCA remains in effect for the typically windy waters around Maui
and the Big Island through Saturday. This advisory will likely
need to be extended through the remainder of the weekend.

Short period easterly swell resulting from the trade winds will
continue through forecast period. A bit of a bump in surf heights
is expected for east facing shores during the weekend as the
remnants of Ulika approach from the east. Surf heights will remain
small through the middle of next week. Models show a strong low
near the Aleutian Islands during the middle of next week that
could bring our first significant northwest swell of the winter
season next weekend.

&&

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

&&

DISCUSSION/MARINE...Jacobson
AVIATION...Powell