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Hawaiian Islands Synoptic Discussion and Guidance

496
FXHW60 PHFO 011324
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
324 AM HST SAT OCT 1 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 locally heavy downpours becoming more likely
early next week, as a moist airmass associated with the remnants
of Ulika moves over the state.

&&

.DISCUSSION...
Showers embedded in fresh trade wind flow have increased across
windward and mauka areas during the night, with some of the
showers spilling over into leeward areas of the smaller islands.
Rainfall amounts during the past 12 hours have been mostly light,
although an isolated heavy shower over Honaunau produced over
3 inches of rain during a couple of hours late Friday evening.
Surface analysis depicts high pressure far north-northeast of the
state, with fresh trade winds over the islands. The 12z Hilo
sounding shows PW values increasing to 1.83 inches with the
inversion rising to over 12 thousand feet, while the 12Z Lihue
sounding continues to depict typical trade wind conditions with PW
of 1.15 inches and the inversion near 7000 feet. MIMIC-TPW
satellite imagery depicts an area of increased moisture with
estimated PW values up to 1.8 inches moving into the Big Island
and Maui, with a drier airmass retreating west of Oahu. The plume
of enhanced moisture extends several hundred miles further
eastward, to the remnants of former Tropical Storm Ulika about 550
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 north of the state.

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 will cause the smaller islands to become
blocked by the Big Island, with afternoon seabreezes likely on
Tuesday and Wednesday. These could act as a focus for locally
heavy downpours. The heavy rains will likely be somewhat spotty
and not occurring all the time, but there is sufficient confidence
at this point to add locally heavy rainfall wording to the
forecast for Monday through Wednesday. While later shifts may need
to refine the timing of this event, a flash flood watch could
become necessary for the early to middle part of next week if
current forecast trends continue.

Trade winds should gradually return late next week, with moisture
and instability slowly decreasing as the upper trough moves away
and a drier airmass filters back into the state.

&&

.AVIATION...
Moisture moving westward on the trades will continue to push
occasional MVFR conditions across windward locations. AIRMET
SIERRA, for mountain obscuration above 2500 feet, is now in
effect for north and east sections of Maui and the Big Island.
Leeward terminals and terminals on the western islands should
remain VFR through the rest of tonight and Saturday. Thanks to
locally strong trade winds, AIRMET TANGO remains in effect
statewide for mechanical turbulence below 8000 feet immediately
over and south through west of the mountains.

&&

.MARINE...
Fresh to strong trade winds associated with high pressure north of
the region will continue through the weekend. The highest winds and
seas are forecast across the typically windier locations between
Molokai and the Big Island each day. The small craft advisory will
remain in place for these windier zones and has been extended
through Sunday. Near advisory-level seas will be expected across the
Alenuihaha Channel and the waters south of the Big Island through
this time. Winds and seas will trend down next week as high pressure
north of the region weakens.

Rough surf will persist along east facing shores into Monday in
response to slightly enhanced trades between former tropical cyclone
Ulika and high pressure to the north. Surf along east facing shores
will begin to trend down by mid-week as the winds ease and shift
toward the east-southeast. Elsewhere, small surf is expected through
the upcoming week. Surf along north facing shores will likely build
next weekend due to a gale that is projected to develop over the
northwest Pacific by Wednesday.

&&

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

&&

$$

DISCUSSION...Jacobson
AVIATION...Powell
MARINE...Gibbs

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office