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Hawaiian Islands Synoptic Discussion and Guidance

423
FXHW60 PHFO 260649
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
849 PM HST Tue Sep 25 2018

.SYNOPSIS...
A low pressure system north of Kauai this evening will continue to
draw up tropical moisture from the south through Thursday producing
light south to southeasterly winds across the islands. Expect hot
and humid weather conditions across all islands, with shower
activity primarily focused over Kauai and Oahu over the next two
days with scattered showers possible elsewhere. The low will lift
slowly towards the north over the next 48 hours with a high
pressure ridge building back into the region with trade winds
returning from Friday onward. A wet trade wind weather pattern
remains in the forecast on Friday and Saturday with clouds and
showers favoring windward and mountain areas.

&&

.DISCUSSION...
A low pressure system north-northwest of the islands continues to
weaken the subtropical surface ridge and produce a broad southerly
to southeasterly flow across the island chain. This southerly kona
wind flow continues to bring up hot and humid air for the deeper
tropics. Precipitable Water (PW) levels over the islands will
remain elevated in the 1.5 to 2.0 inch range. At these higher PW
levels shower activity can more easily develop. Meanwhile a weak
low level trough has stalled near Kauai and Niihau helping to
trigger deeper convection and thunderstorms over both islands this
evening. A Flash Flood Watch remains in effect trough early
Wednesday morning over Kauai and Niihau. Thunderstorms remain
possible along this trough axis tonight, and over the Big Island
interior on both Wednesday and Thursday afternoons. The potential
for heavy rain showers will gradually diminish over the next 48
hours as this low center continues to drift northward decreasing
the dynamic lift along the low level trough.

Weather models continue to hint at a tropical disturbance
developing southeast of the Hawaiian Islands over the next few
days. We will closely monitor this area of disturbed weather for
tropical cyclone development. Model solutions continue to show
this low passing south of the islands on Friday and Saturday
likely enhancing trade wind speeds due to stronger surface
pressure gradients. The strength of these winds are highly
dependent on the depth of the low center. Either way deeper
tropical moisture is likely and wet trades will remain in the
forecast on Friday and Saturday with clouds and showers favoring
windward and mountain areas.

Longer range models are showing drier air moving in behind the
tropical disturbance starting from Sunday into Monday with the
potential for southerly winds returning by next week Tuesday.
Stay tuned for changes to the forecast as this dynamic weather
pattern evolves over time.

&&

.AVIATION...
Light south to southeasterly flow will continue to draw moisture
and instability towards the islands. AIRMET Sierra for mountain
obscuration remains in effect for Kauai. Low ceilings over Kauai are
expected to persist through Wednesday afternoon and could spread
to Oahu as early as Wednesday morning.

Elsewhere, expect sea-breeze driven cumulus development over
island interiors in the late afternoon and evening hours.
Localized MVFR ceilings and visibilities are to be expected in
isolated to scattered light showers, otherwise VFR conditions
will prevail.

&&

.MARINE...
Light to moderate southerly winds will continue across most
Hawaiian waters through Thursday night due to a weak surface
trough located just west of Kauai. Abundant moisture and unstable
atmospheric conditions will support moderate to locally heavy
showers and a slight chance of thunderstorms at times, especially
over the waters adjacent to Kauai and Niihau. The pressure
gradient will begin to tighten over the weekend as high pressure
builds to the north. In addition, an area of low pressure, which
may be a developing tropical cyclone, is also expected to pass to
the south of the area. The increasing trade winds, plus the
potential of a fetch developing with a tropical cyclone, will
likely cause seas to build. As a result, seas may reach the Small
Craft Advisory (SCA) criteria of 10 feet starting Saturday.

Small south-southeast swell energy will maintain background surf
along south facing shores through Thursday. A small to moderate
south-southwest swell is expected to fill in Thursday night
through Friday, then hold into this weekend before easing. A
slightly larger reinforcing south-southwest swell source will be
possible early next week.

A new north-northwest swell arriving Thursday night is expected to
cause surf to reach moderate levels along most north and west
facing shores of the smaller islands from Friday through this
weekend. The close proximity of the swell source to the islands
will result in shorter periods upon arrival (10-12 sec), which
will support surf heights remaining below the High Surf Advisory
(HSA) criteria.

As noted above, if a tropical cyclone were to develop southeast
of the islands later this week, it could eventually increase surf
heights along some shorelines by later this weekend or early next
week. It is still too early to know if there may be a need for a
HSA along some shorelines. Regardless of whether or not a new
swell will arrive, expect the strengthening trade winds to cause
rough surf to build along most east facing shorelines from Friday
into early next week.

See the latest Collaborative Nearshore Swell and Wind Forecast
for Oahu (SRDHFO) for more details on swells and surf.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch until 6 AM HST Wednesday for Niihau-Kauai.

&&

$$

DISCUSSION...Bohlin
AVIATION...Bedal
MARINE...Houston

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



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