Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Honolulu HI
400 PM HST Friday Aug 28 2015
hot and humid conditions will continue as weak trade winds gradually
rebuild. Although showers will favor windward and Mauka areas...
some clouds and showers are still possible over leeward areas
Saturday afternoon. Trade winds will strengthen during the
weekend...though humid conditions will continue. Hurricane Ignacio
will approach the state late in the weekend and will affect island
weather next week.
the near-term forecast features a brief respite to the active
weather...then attention turns to the approaching Ignacio.
Water vapor imagery shows an upper level high just south of the
state...with an elongated trough farther to the north. Hurricane
Ignacio is located far to the east-southeast and tropical storm kilo
is located far to the west-southwest. At the surface...a weak trough
is located northeast of the state...with a large high far to the
north. 00z soundings show precipitable waters ranging from 1.41 at
Hilo to 1.84 at Lihue...with both showing drying throughout the
column as well as weak stabilization. Early afternoon mimic total
precipitable water imagery is still on the high side compared to
radiosonde observations and GPS-met sensors on Kauai/Oahu/Big Island...but does show
qualitatively drier air east of The Big Island.
Forecast soundings show subtle middle-level drying and a weak inversion
spreading from east to west across the state tonight/Saturday. The
surface trough to our northeast has all but dissipated...allowing
the high to our north to exert more influence across the state.
Surface pressures at airports across the state are up to a millibar
higher than 24 hours ago...and trade winds will continue to
gradually strengthen through Saturday. While we will still have
localized sea breezes across sheltered leeward areas...there will be
enough of a gradient to keep most showers focused across windward
and Mauka areas. Rainfall coverage will be slightly lower than
normal with subtle synoptic scale subsidence under a weak middle-level
ridge. However any inversion will be weak...and with lingering
moisture we could still see an isolated heavy shower.
Although afternoon high temperatures will be around seasonable
normals...dew points in the low to middle 70s will maintain humid
conditions with heat index values rising to the middle to upper 90s.
Even with a slight increase in trades...wind speeds will remain on
the lighter side of normal...making the humid conditions feel even
Northeasterly trades will strengthen on Sunday...in part due to a
tightening gradient ahead of Hurricane Ignacio. The middle-level ridge
will also give way to a weak trough from the northeast Saturday
night/Sunday...and forecast soundings show the weak inversion
eroding. Between the increasing moisture/instability and
strengthening trades...we could see a flare up of windward showers
Forecast uncertainty increases significantly on Monday and
Tuesday...hinging on the eventual track of Hurricane Ignacio. See
the latest discussion issued by the Central Pacific Hurricane Center
for specifics related to the track and intensity...issued under
AWIPS header tcdcp3 and WMO header wtpa43 phfo. The Cone of
uncertainty...which is the potential track given historic track
error...includes the entire state. If Ignacio shifts to the
right...we could see light winds...more humid conditions...and
localized heavy rain. On a more leftward track closer to the
islands...we could see stronger/damaging winds and more widespread
heavy rain/flooding. Everyone should continue to monitor the
progress of Ignacio during the next several days.
localized seabreezes allowed for interior and leeward areas on most
islands to cloud up this afternoon. Generally towering cumulus tops were reaching
heights between 150 and fl200...with some isolated tops between
fl250-300 mainly on The Big Island and Kauai. These clouds and
showers will begin to clear out with sunset...leaving leeward areas
with mostly clear and VFR conditions overnight.
The trade winds will continue their gradual return and clouds and
showers will favor windward and Mauka sides...some showers may
introduce brief periods of MVFR tonight. Otherwise...VFR is expected
to prevail at most terminals.
all mariners should monitor the progress of Hurricane Ignacio during
the next several days.
Winds and seas should remain below Small Craft Advisory levels into
Saturday...with elevated seas becoming a concern by Sunday for
portions of the state.
In the short term...a west to west-northwest swell will be the main
concern. Buoy 51001 and the Hanalei buoy showed an increase in long
period energy late last night/this morning. While the impacts of
this swell will be modest on most islands...it will reach advisory
levels for west facing shores of The Big Island...where the
advisory threshold is lower than the other islands. Based on timing
from the buoys...this swell should peak near The Big Island tonight
and gradually diminish through the day on Saturday.
Uncertainty is very high regarding the potential impacts from
Hurricane Ignacio early next week...but we are confident that an
east-southeast swell will arrive on Saturday for The Big Island.
This swell will initially reach advisory levels...and may approach
warning levels for east facing shores late Sunday. Based on the
arrival timing during the day on Saturday...we have issued a high
surf advisory for The Big Island. This will likely be expanded to
include east Maui for Saturday night. Timing for islands farther
west will be delayed due to shadowing/blocking from The Big Island
and Maui. Warning level surf is also likely early next week...though
specifics are highly dependent upon the strength and track of
high surf advisory until 6 PM HST Saturday for west facing shores of
The Big Island.
High surf advisory from 6 am Saturday to 6 PM HST Sunday for east
facing shores of The Big Island.