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Hawaiian Islands Synoptic Discussion and Guidance

917
FXHW60 PHFO 201355
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
400 AM HST Mon Aug 20 2018

.SYNOPSIS...
A drier airmass will persist across the state through Tuesday, with
trade winds strengthening. For the remainder of the week, the
forecast will be highly dependent on the track and intensity of
Hurricane Lane. An increase in winds and showers will be the likely
result based on the latest forecast for Lane.

&&

.DISCUSSION...
Please note that tropical cyclone operations are in effect. So as
to incorporate the latest forecast information from the Central
Pacific Hurricane Center (CPHC), public forecast updates from the
Honolulu Forecast Office will come shortly after forecast updates
from CPHC, namely after 5 and 11 am/pm, instead of the normal 4
am/pm.

Drier airmass has spread across the state since last night, though
early morning Lihue sounding only indicated a slight drying, while
Hilo sounding has a more typical trade wind weather type profile.
Latest radar data revealed not much showers, though satellite
imagery, even with some high clouds obscuring, indicated there are
embedded low clouds just upstream east of the islands. A surface
high pressure far northeast of the state providing the trade winds
continues to drop south-southwestward. Meanwhile another high
pressure far northwest of the state is advancing east as well. This
will allow a tightening of pressure gradient across the area,
resulting in an increase in trade winds for the state. Regardless,
the more typical trade wind weather is back across area, with
passing low clouds and showers affecting mainly windward and
mountain areas. High clouds will stream over the area at times today
and will keep skies a little more opaque at times.

The aforementioned two highs will gradually merge and take up a
position far to our north by Tuesday. This will lead to a further
strengthening in trade wind speeds across area. The somewhat drier
and more stable airmass will persist in the as well. Therefore,
expect breezy trade wind weather for the islands Tuesday, with
passing low clouds and showers affecting mainly windward and
mountain areas. Lee areas of the smaller islands may also see some
clouds and showers as winds may be strong enough to carry them over.

As for Wednesday through the weekend time frame, our weather will be
highly dependent on the track and intensity of Hurricane Lane. It is
still too early to determine the weather impacts from Lane will have
on the state. The last forecast track from the Central Pacific
Hurricane Center still has Lane passing south of the main Hawaiian
Islands, but has it tracked closer to the island chain. If this
track holds, the potential impacts on the islands weather would be
an increase in wind speeds due to a tightened pressure gradient
between high pressure north of the state and Lane traversing to our
south. Also moisture associated with Lane would likely bring an
increase in shower activity over the Big Island Wednesday, mainly
east and southeast and then eventually spreading up the island chain
through the remainder of the week.

Everyone is encouraged to monitor the latest advisories on Hurricane
Lane from the Central Pacific Hurricane Center.

&&

.AVIATION...
A high pressure ridge north of the islands will produce stronger
trade winds today with with improving weather trends as drier more
stable air moves in from the east.

No AIRMETs in effect. As winds strengthen and the atmosphere
becomes more stable this afternoon, turbulence in the lee of the
mountains may become moderate and AIRMET Tango may be needed.

&&

.MARINE...
Please note that tropical cyclone operations are in effect. So as to
incorporate the latest forecast information from the Central Pacific
Hurricane Center (CPHC), marine forecast updates from the Honolulu
Forecast Office will come shortly after forecast updates from CPHC,
namely shortly after 5 and 11 am/pm, instead of the normal 4 and 10
am/pm.

A weak surface trough W of Kauai will move W away from the area
today and tonight as high pressure builds to the N. This will result
in a gradually increasing trade wind flow through Tuesday. A Small
Craft Advisory (SCA) has been posted for the windier areas around
Maui and the Big Island in anticipation of the increasing trade wind
flow. As Hurricane Lane moves into an area S of the islands by
mid-week, the pressure gradient will likely increase, likely leading
to increasing trade winds, and potentially an expansion of the SCA.
More significantly, Tropical Storm and/or Hurricane Watches may be
required for portions of the southern coastal waters later today.
There is significant uncertainty regarding the position and
intensity of Lane later in the week, so it is too early to determine
what impacts Lane may eventually bring, and mariners need to
continue to closely monitor forecasts.

Long period ESE swells from Hurricane Lane are building at buoy
51004 to the SE of the Big Island this morning, while the PacIOOS
buoy in Hilo is just beginning to detect long-period energy from the
E. A High Surf Advisory for E facing shores of Maui and the Big
Island is now in effect. As Hurricane Lane moves into an area S of
the islands later this week, S swell will likely increase, with a
wide variety of wave periods leading to rough and potentially
damaging surf. Still significant uncertainty and difficult to know
the level of impacts, but there is potential for large seas/swells
from Lane to lead to warning-level surf along E through S facing
shores of the islands. A small NNW swell is expected to arrive
Monday and peak on Tuesday before gradually diminishing Wednesday
and Thursday.

See the latest Oahu Surf Discussion (SRDHFO) for more details on
surf and swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Tuesday for Windward Haleakala-
South Big Island-Big Island North and East.

Small Craft Advisory until 6 PM HST Tuesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$

SYNOPSIS/DISCUSSION...Hui
AVIATION...Bohlin
MARINE...Birchard

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



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