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Hawaiian Islands Synoptic Discussion and Guidance

426
FXHW60 PHFO 161351
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
351 AM 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 heavy showers in the forecast through
tonight. A return of drier air and more stable conditions is
anticipated from east to west Tuesday through the second half of
the week. 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 few hundred
miles north of the islands with plenty of middle- to upper-level
moisture pooling northward across the region to its east. This upper
feature combined with a surface trough north of Kauai and plenty of
deep tropical moisture continues to fuel spotty heavy showers this
morning. Recent satellite-derived precipitable water (PW) imagery
reflected this with pockets of higher moisture near or around two
inches, which is well above average. The latest rainfall summary
showed peak accumulations over Kauai in the past six hours
(through 2 am HST) with totals up to 4.54 inches at Mount
Waialeale and 1.65 inches at the N. Wailua Ditch gage.

The short-term guidance (through Tuesday) remains in good agreement
and depicts a similar pattern continuing, which will keep the rain
chances in the forecast through tonight. The better instability
and deep tropical moisture are forecast to hold over the western
end of the state. Model PWs will hold around two inches over these
areas. Light to moderate east-southeast winds will continue,
which will give way to localized sea breezes, potentially
triggering afternoon and early evening showers over leeward areas
today. Conditions will gradually begin to improve over the eastern
end of the state by Tuesday, as drier air and more stable
conditions 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 statewide. The low-level flow (up to 10,000 ft)
is forecast to shift back to a more typical easterly direction
and strengthen. Model PWs should drop back to and around normal
through this period. Clouds and showers will focus over windward
and mountain areas, especially through the overnight and early
morning periods as pockets of higher moisture move through from
east to west.

Despite some differences between the various model solutions, a
return of light winds and increasing moisture (two inch PWs) will be
possible next 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 low pressure system north of Kauai will continue to move
northward away from the islands. Elevated amounts of tropical
moisture will keep low clouds and showers in the forecast with
periods of MVFR/IFR conditions expected. Drying trends are
forecast to spread from east to west across the islands through
Tuesday with the increasing trade winds.

AIRMET Sierra remains in effect for Mountain Obscuration over
north through east sections of Oahu and Kauai.

AIRMET Zulu remains in effect for light icing 160-FL280.

&&

.MARINE...
High pressure far north of the state is expected to strengthen
slightly and slowly move east through the week. This will lead to
moderate to locally strong easterly trades through most of this
week. Winds may veer to the southeast across the far western
Hawaiian waters near the middle of the week as a low pressure
system tracks south to north, west of the state. The Small Craft
Advisory (SCA) remains in effect for waters around Maui and the
Big Island through this afternoon. We will continue to monitor the
winds today to see if the SCA needs to be extended. Nonetheless,
borderline SCA level winds are expected for Monday night into
Tuesday.

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. The size of the southwest swell will
depend on the development of the low pressure system currently
near the dateline. The latest model guidance is showing 3 feet 10
seconds out of the 230-250 degree direction and 2 feet 17 seconds
out of 190 degrees for Friday into Friday night. 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...
Small Craft Advisory until 6 PM HST this evening for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Windward Waters-
Big Island Leeward Waters-Big Island Southeast Waters.

&&

$$

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

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



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