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Hawaiian Islands Synoptic Discussion and Guidance

436
FXHW60 PHFO 210151
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
351 PM HST Mon Aug 20 2018

.SYNOPSIS...
High pressure north of the state will provide for a typical trade
wind weather pattern across the area through Tuesday. Clouds and
passing light showers will favor windward and mauka areas with
just a few brief light showers expected over select leeward locations.
For the remainder of the week and on into the weekend, our weather
pattern will be highly dependent on the track and intensity of
Hurricane 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
are received from CPHC.

Tonight through Tuesday, high pressure will continue building in
north of the area with moderate trade winds being the result. A
mid level ridge over the area is providing for a rather stable
airmass. Moisture embedded the trades will provide for some
passing showers, mainly light, over windward and mauka areas. Most
leeward areas will remain rather dry with just a few brief light
showers possible. However, daytime heating will likely allow for
some clouds and showers to build up over the Kona slopes during
the afternoon and evening hours.

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 that Lane
may have on the state. The latest forecast track from the Central
Pacific Hurricane Center still has Lane passing south of the main
Hawaiian Islands. But the latest does have it tracking closer to
the state. If this track holds, the potential impacts on our
islands weather could include 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...
Northeast trade winds will strengthen as surface high pressure
builds north of the area and Hurricane Lane approaches from the
southeast. Moderate turbulence could develop to the lee of the
mountains as the winds strengthen and the atmosphere becomes more
stable. AIRMET Tango may be needed by tomorrow.

The high pressure building to the north will maintain mostly dry
weather over the area through Tuesday and VFR will prevail. Clouds
and showers are likely to increase later in the week as Hurricane
Lane approaches. The weather over the area will be greatly
dependent on the track of hurricane.

&&

.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
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) remains in effect for the windier areas
around Maui and the Big Island for these stronger winds. As
Hurricane Lane moves into an area S of the islands by mid-week,
the pressure gradient will likely increase further, potentially
requiring an expansion of the SCA. More significantly, a Tropical
Storm Watch has been issued for the coastal waters south of the
Big Island. This may need to be upgraded to a Tropical Storm
Warning or Hurricane Watch/Warning as the storm gets closer. 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 continue to build at
buoy 51004 to the SE of the Big Island and at the PacIOOS buoy in
Hilo. A High Surf Advisory for E facing shores of Maui and the
Big Island is 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 and directions leading to rough
and potentially damaging surf. There is still significant
uncertainty and it is difficult to know the level of impacts, but
there is potential for large seas/swells from Lane leading to
warning-level surf along E through S facing shores of the islands.
Otherwise, 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.

Tropical Storm Watch for Big Island Leeward Waters-Big Island
Southeast Waters.

Small Craft Advisory until 6 PM HST Tuesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel.

&&

$$

SYNOPSIS/DISCUSSION...Burke
AVIATION...Donaldson
MARINE...TS

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

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