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Hawaiian Islands Synoptic Discussion and Guidance

864
FXHW60 PHFO 212004
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1004 AM HST Tue Aug 21 2018

.SYNOPSIS...
The weather will remain good today to continue your hurricane
preparations. High pressure to the north and Hurricane Lane to the
south will keep breezy trade winds blowing with only a few
showers. Deep tropical moisture surrounding Lane will start to
spread rainbands and possible thunderstorms over the Big Island by
Wednesday morning, then continuing northward across the state as
Lane approaches. Hurricane Lane could bring damaging winds,
dangerous surf, and intense, flooding rains to various parts of
the state Wednesday through Saturday.

&&

.DISCUSSION...
Please note that tropical cyclone operations are in effect. To
incorporate the latest forecast information from the Central
Pacific Hurricane Center (CPHC), public forecast updates from the
Honolulu Forecast Office will be delayed to immediately follow
the latest forecast package from CPHC.

Good weather to continue hurricane preparations today as trade
winds blow between high pressure about 1100 miles to the NNW, and
Hurricane Lane approximately 375 miles to the SE of Ka Lae on the
Big Island. Satellite imagery shows some minor showery bands of
cumulus extending upwind from Maui County and the Big Island, but
relatively dry and stable clouds around and to the east of the
remaining islands.

The Naalehu radar is already showing the outermost rainbands
associated with Lane about 170 miles to the SE of the Big Island.
This corresponds to the leading edge of a vast area of very deep
tropical moisture surrounding Lane, according to the NESDIS
Blended Total Precipitable Water product. The models remain in
excellent agreement that this vast area of PW well in excess of 2"
will overspread the Big Island by Wed morning, Maui county by Wed
night, Oahu by Thu, and Kauai by late Thu or Thu night. Once this
occurs, it's game on for the possibility of intense, flooding
rainfall in the rainbands and squalls surrounding Lane. There is a
considerable amount of uncertainty in the models as to when the
deep moisture in the wake of Lane departs. The ECMWF shows some
modest drying out over the weekend, while the GFS and to a lesser
extent the NAVGEM maintain high PW over the islands into next
week, which could be troublesome on any areas that get excessive
rainfall.

The combination of a deep moist unstable layer and strong low
level wind shear along and to the east of the track of Lane will
make tornadoes and large waterspouts a distinct possibility in
the rainbands on the right semicircle of the hurricane. A mention
of these have been mentioned in the HLS and TCV products.

As far as wind effects, based on the latest CPHC forecast, the
onset of damaging tropical storm force winds on the Big Island
could be as early as Wednesday, with dangerous hurricane force
winds possible by Wednesday night. On Maui county, damaging
tropical storm force winds could begin as early as Wednesday
night, with dangerous hurricane force winds possible starting on
Thursday. Oahu could see damaging winds by Thursday morning, and
Kauai by Thursday night. Hurricane Watches could be needed for
other areas as Lane draws closer. It's not possible to know who
might get the worst of the storm right now, so the best thing to
do is to get ready. The CPHC forecasts continue to show that all
islands are at risk from direct effects from the core of Hurricane
Lane. Everyone should take this hurricane very seriously and
should prepare accordingly. It's also important to expect that
there will need to be future adjustments to the track and
intensity forecast, particularly with the challenging recurvature
forecast that Lane is presenting. Please consult the latest
advisories from CPHC.

&&

.AVIATION...
With Hurricane Lane approaching from the southeast and a
relatively strong northeast pacific high, expect the pressure
gradient to tighten resulting in increased wind speeds and gusts.
This will be especially true for the Big Island, Maui, Lanai and
Molokai and will affect Oahu and Kauai to a lesser extent tonight
and through Wednesday. There is already an AIRMET in place for
TEMPO moderate turbulence below eight thousand feet on the leeward
side of the mountains with the strongest turbulence on the Big
Island and weakening from the Big Island northwest to Kauai. So
this turbulence will almost certainly be in place at least
through Wednesday into Wednesday night. This would only change if
Hurricane Lane somehow did not follow the current forecast and
remained on a westerly heading, which is extremely unlikely.

The best chance for clouds and showers will be on the Big Island
and Maui through the day Wednesday as Hurricane Lane approaches
from the southeast. The best chance for MVFR conditions and
mountain obscuration will be on these islands, with increasing
chance for MVFR conditions as we progress through the period.
Otherwise expect VFR conditions for the rest of the state.

&&

.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.

Due to increasing confidence of Hurricane Lane turning toward the
Hawaiian Islands Wednesday into Thursday, a Hurricane Watch is in
effect for the coastal waters surrounding Big Island and Maui
County. This may have to be expanded to other coastal waters later
today or tomorrow. There is still some 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. Otherwise,
breezy trade winds are expected to continue through tonight, and
are expected to increase as Lane approaches the Hawaiian Islands.

A High Surf Advisory for E facing shores of Maui and the Big
Island remains in effect. Latest buoy readings from station 51004
located southeast of the Big Island are beginning to show a rise
in swell out of the southeast direction. This southeast swell is
expected to arrive tonight and will likely lead to warning level
surf along the southeast coast of the Big Island. As Hurricane
Lane continues to move 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 smaller islands. Otherwise, a small
NNW swell is expected to peak tonight before gradually
diminishing Wednesday and Thursday.

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

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from Wednesday morning through Friday
afternoon for all Hawaii islands.

Hurricane Watch for Molokai-Lanai-Kahoolawe-Maui-Big Island.

High Surf Advisory until 6 PM HST this evening for Windward
Haleakala-South Big Island-Big Island North and East.

Hurricane Watch for Maui County Windward Waters-Maui County
Leeward Waters-Maalaea Bay-Pailolo Channel-Alenuihaha Channel-
Big Island Windward Waters-Big Island Leeward Waters-Big Island
Southeast Waters.

&&

$$
R Ballard/Chevalier/Kino

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