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Hawaiian Islands Synoptic Discussion and Guidance |
FXHW60 PHFO 192006
Area Forecast Discussion
National Weather Service Honolulu HI
1006 AM HST Sun Aug 19 2018
A rather humid airmass is in place due to a weak trough of low
pressure near Kauai. Showers and a slight chance of thunderstorms
are in the forecast for portions of the state today. A drier
airmass along with strengthening trade winds are in the forecast
for 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 shower activity
will be the likely result based on the latest forecast for Lane.
Areas of high pressure are currently centered far north-northeast
and far northwest of the state with a weak westward moving trough
of low pressure centered near Kauai. Instability associated with
the trough will allow for the possibility for a few thunderstorms
to form over and around Kauai today and possibly over the leeward
slopes on the Big Island this afternoon. Otherwise scattered
showers can be expected across portions of the area today
especially over the west end of the state. The highest chances for
rainfall will be over windward and mauka areas as well as some
interior areas during the afternoon hours.
For Monday and Tuesday, the two highs will gradually merge and
take up a position far to our north. This will cause for an uptick
in trade winds speeds across the area. The trough of low pressure
will continue to move west away from the area and gradually
dissipate west of the state. Thus a drier airmass is forecast to
move into the area from the east though some passing light showers
will still be possible over some windward and mauka areas.
During the Wednesday through Friday time frame, our weather
pattern will be highly dependent on the track and intensity of
Hurricane Lane. It remains too early to forecast the exact track
of Lane and whether it will directly impact portions the state.
The latest forecast track from the Central Pacific Hurricane
Center has Lane passing by south of the main Hawaiian Islands. On
this track, the main impacts on the islands weather would be for
an increase in trade wind speeds across the area due to a
tightened pressure gradient between high pressure north of the
state and Lane to our south. Moisture around the periphery of Lane
would likely cause an increase in shower activity over east and
southeast sections of the Big Island, and then eventually
spreading up the island chain through the remainder of the week.
Everyone is encouraged to monitor the latest advisories for Lane
from the Central Pacific Hurricane Center.
A high far northeast of the area will maintain moderate east
winds through tonight. A weak trough near Kauai will move west at
10 kt. Moisture associated with this trough may produce heavy
showers and isolated thunderstorms through today near Kauai and
over the Big Island slopes. AIRMET Sierra is in effect for MTN
OBSC on Kauai. AIRMET Sierra for MTN OBSC may be needed this
afternoon for Lanai and the Big Island.
The atmosphere will become drier and more stable this evening with
VFR conditions prevailing area-wide.
A weak trough of low pressure will continue to move through the
western coastal waters today producing some locally heavy
showers and isolated thunderstorms over the western half of the
state. Light to moderate trade winds are expected to continue
today then strengthen to fresh to strong speeds tonight. A Small
Craft Advisory (SCA) will likely be issued with the afternoon
package today for the typically windy waters around Maui county
and the Big Island. The latest forecast for Hurricane Lane has it
passing by to the south of the Big Island Tuesday night through
Wednesday. As a result, trade winds will likely increase further
and the SCA may have to be expanded to additional zones during the
middle of the week. It is still too early to determine the exact
impacts that Hurricane Lane may have on the coastal waters, so
mariners are urged to keep up to date with the latest forecasts.
Long period energy from Hurricane Lane is expected to begin
filling in along east and southeast facing shores of the Big
Island and Maui tonight and Monday. As Hurricane Lane moves
closer to the islands around midweek, swell is expected to
increase and will likely result in large surf for east and south
facing shores of the Big Island and possibly Maui county. We will
continue to monitor the track and intensity of Hurricane Lane, as
any changes in the track or intensity will significantly affect
surf heights. A small, north-northwest swell is expected to
arrive Monday and continue through Tuesday. A larger north swell
is expected to arrive late Tuesday, peak Tuesday night and early
Wednesday, then slowly fade through Thursday.
See the latest Oahu Surf Discussion (SRDHFO) for more details on
surf and swell.
Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office