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Hawaiian Islands Synoptic Discussion and Guidance

647
FXHW60 PHFO 170614
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
814 PM HST Mon Sep 16 2019

.SYNOPSIS...
Moist and unstable conditions associated with an upper disturbance
and surface trough of low pressure in the vicinity of the islands
will keep the potential for locally heavy showers in the forecast
overnight into Tuesday. A return of drier air and more stable
conditions is anticipated from east to west Tuesday night through
Thursday. Increasing moisture and rain chances will become a
possibility once again over the upcoming weekend.

&&

.DISCUSSION...
Water vapor imagery showed a broad upper low centered a several
hundred miles north-northwest of the islands with plenty of middle-
to upper-level moisture pooling northeastward across the region to
its southeast. Recent satellite-derived precipitable water (PW)
imagery reflected this with pockets of higher moisture near or
around two inches over the western end of the state, which is well
above average. This upper feature combined with a tropical
disturbance several hundred miles south-southwest of Kauai and
plenty of deep tropical moisture continues to fuel spotty heavy
showers this evening. Latest radar loop showed a plume of moderate
to heavy showers developing downwind of Oahu over the Kauai Channel
that was expanding in coverage toward Kauai.

The short-term guidance (through Tuesday) remains in good agreement
and depicts a similar pattern continuing, which will keep the
potential for localized heavy showers in the forecast. The better
instability and deep tropical moisture are forecast to hold over the
western end of the state as the aforementioned tropical disturbance
south-southwest of Kauai slowly lifts northwestward. Model PWs will
hold around two inches over these areas (highest Kauai). Light to
moderate low-level east-southeast flow will prevail, which could
translate to localized sea breezes Tuesday afternoon. Conditions
will gradually improve over the eastern end of the state, as more
stable and drier air move in from the east.

Guidance remains in decent agreement Tuesday night through the
second half of the week and reflects a return of drier and more
stable conditions. Consensus shows the bulk of the deep tropical
moisture shifting west of the islands, closer to the tropical
disturbance lifting northwestward. This will translate to decreasing
rainfall coverage. East-southeast low-level flow will likely hold
through midweek, depending on how this tropical feature evolves,
before shifting back to a more typical easterly direction around
Thursday.

Despite some differences between the various model solutions, a
return of light winds and increasing moisture (two inch PWs) will be
possible Friday night into the upcoming weekend as a trough
associated with a tropical disturbance to the south moves through.
Added instability associated with an upper low setting up over the
state combined with this moisture will support increasing rainfall
chances.

&&

.AVIATION...
A high pressure center far north of the Hawaiian Islands will keep
light to moderate trade winds in the forecast for the next 24
hours. Sea breezes will develop along the sheltered western slopes
of each island with some variability in wind directions along
most southwest sides of each island.

Deeper moisture and instability will linger over Kauai on Tuesday
with enhancements to both clouds and showers, with brief periods
of MVFR conditions possible. Elsewhere expect drier conditions
with more typical windward and mountain showers favoring the
overnight and early morning time periods.

AIRMET Zulu remains in effect for light icing 160-FL280. No other
AIRMETs in effect.

&&

.MARINE...
Moderate to locally fresh east-southeast winds will hold through
midweek as a tropical disturbance to the south-southwest lifts
northwestward. A shift back out of a more typical east to
northeast direction is expected Thursday through Friday. This
transition will be short-lived, however, due to an approaching
trough from the east-southeast. Guidance shows a return of light
to moderate east-southeast flow late Saturday through the second
half of the weekend, which would support localized land and sea
breeze conditions near the coast.

A series of small swells from the southwest, south, and southeast
is expected this week. The largest of these swells looks to be
towards the end of the week as a combination of a long-period
south-southwest swell and potential short-period southwest swell
moves in across the area. Surf heights are expected to be below
advisory levels with these swells.

Small surf is expected along north, east and west facing shores
through the new week. Expect a small east swell from Tropical
Cyclone Kiko as early as Wednesday along exposed east facing
shores and likely continuing through the weekend. Exposed west
shores may see some of the aforementioned southwest swell on
Friday. A small northwest swell will be possible late in the
weekend into early next week.

&&

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

&&

$$

DISCUSSION...Gibbs
AVIATION...Bohlin
MARINE...Kino

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