Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 200654
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
830 PM HST Sun Aug 19 2018

.SYNOPSIS...
A weak trough of low pressure just west of Kauai will continue to
advance west, away from the islands, with a drier airmass and
strengthening trade winds returning to the state Monday and Tuesday.
For the remainder of the week, the forecast will be highly dependent
on the track and intensity of Hurricane Lane. An increase in trade
winds and showers will be the likely result based on the latest
forecast for Lane.

&&

.DISCUSSION...
Still some lingering instability over the western end of the state
this evening results in active weather for Kauai and vicinity as
well as western Oahu coastal waters, with isolated thunderstorms
and locally heavy showers. The feature that is responsible for the
weather is a trough that is currently just west of Kauai. This
feature will continue to track west, exiting the state. Hence,
weather conditions for the western end will slowly improve overnight
into early Monday as a somewhat drier but definitely more stable
airmass spreads from the east over the area. Surface high pressure
far northeast of the state providing the trade winds is moving
south-southwest. Meanwhile another high pressure far northwest of
the state is advancing east as well. This leads to a tightening of
pressure gradient across the area, resulting in an increase in trade
winds for the state as well.

The aforementioned two highs will gradually merge and take up a
position far to our north in the next couple of days. This will lead
to a further strengthening in trade wind speeds across state. A
somewhat drier and more stable airmass is also anticipated in the
island vicinity. Therefore, expect breezy trade wind weather for the
islands in the next couple of days, 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. If this track holds, the main impacts on the islands
weather would be for an increase in trade wind speeds due to a
tightened pressure gradient between high pressure north of the state
and Lane to our south. Also moisture around the periphery of 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 low level trough near Kauai continues to drift westward with
enough elevated moisture and instability for continued showers and a
few thunderstorms over the Kauai region through around 10Z. Stronger
trade winds will bring drier and more stable air filtering in from
the east on Monday morning with improving weather trends across The
Aloha State.

AIRMET Sierra is in effect for MTN OBSC on Kauai and Lanai. These
AIRMETs will likely drop early Monday morning.

As winds strengthen and the atmosphere becomes more stable,
turbulence in the lee of the mountains may become moderate and
AIRMET Tango may be needed.

&&

.MARINE...
A weak surface trough just W of Kauai will move W away from the area
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 later in the
week, the pressure gradient will likely increase after the middle of
the week, likely leading to an expansion of the SCA. However, there
is significant uncertainty regarding the position and intensity of
Lane later in the week. It is too early to determine what impacts
Lane may eventually bring, and mariners need to continue to monitor
subsequent forecasts.

Long period swells from Hurricane Lane are expected to begin
arriving along E and SE facing shores of the Big Island and Maui on
Monday, and a High Surf Advisory is posted for E facing shores of
those islands beginning Monday morning. As Hurricane Lane moves
closer to the islands around midweek and potentially lingers S of
the islands through the end of the week, swell is expected to
increase. Difficult to know what level of impacts this may have, but
there is potential for large seas/swells from Lane to lead to
warning-level surf along E through S facing shores of all 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 from 6 AM Monday to 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


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