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Hawaiian Islands Synoptic Discussion and Guidance

228
FXHW60 PHFO 261418
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
418 AM HST Wed Sep 26 2018

.SYNOPSIS...
A low pressure system north-northwest of Kauai will continue to
draw up tropical moisture from the south through Thursday
producing light south to southeasterly winds across the state.
Expect hot and humid weather conditions across all islands, with
shower activity primarily focused over Kauai and Oahu through
Thursday along with scattered showers possible elsewhere. The low
will lift slowly towards the north over the next 36 hours with a
high pressure ridge building back into the region allowing trade
winds to return 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 Hawaiian Islands
continues to produce broad southerly to southeasterly flow across
the islands. 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 drifted
over Kauai and Niihau helping to trigger heavy showers over both
islands today. The Flash Flood Watch for Kauai and Niihau has
been extended through the afternoon hours. Thunderstorms remain
possible along this trough axis today, and over the Big Island
today and Thursday afternoon. The potential for heavy rain showers
will gradually diminish over the next 24 hours as this low center
continues to drift northward decreasing the dynamic lift of
moisture 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 continue to show 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...
The trough of low pressure northwest of Kauai will remain nearly
stationary. Light south to southeasterly flow ahead of the trough
will continue to draw moisture and instability towards the
islands.

AIRMET Sierra for mountain obscuration remains in effect for
Kauai and is expected to remain necessary through this afternoon.
Isolated thunderstorms and locally heavy downpours through this
evening across Kauai and adjacent waters. Conditions could spread
to Oahu towards evening.

Elsewhere, expect sea-breeze driven cumulus development over
island interiors in the late afternoon and evening hours;
localized MVFR ceilings and visibilities in light showers; and
isolated IFR ceilings this morning in patchy stratus. Slight
chance of thunderstorms over Windward and Interior Big Island late
this afternoon and early evening.

&&

.MARINE...
Light to moderate southerly winds will continue across most
Hawaiian waters through Thursday due to a weak surface trough
located just west of Kauai. The pressure gradient will tighten
from Thursday night and Friday into this weekend as surface 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 latest forecast
continues to show Small Craft Advisory (SCA) winds developing over
the typically windier waters adjacent to the Big Island and the
islands of Maui County starting Friday night. 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 SCA criteria of 10 feet over additional waters starting
Saturday.

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

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 through this afternoon 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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