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Hawaiian Islands Synoptic Discussion and Guidance

933
FXHW60 PHFO 290643
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
843 PM HST SUN AUG 28 2016

.SYNOPSIS...
Moderate trade winds will continue for the next few days as high
pressure persists north of the state. Showery conditions this
evening will diminish overnight, with much drier conditions on
Monday. By the middle of the week, we may begin to feel impacts
from Tropical Storm Madeline, with Hurricane Lester possibly
affecting the islands over the upcoming Labor Day weekend.

&&

.DISCUSSION...
Currently at the surface, a 1027 mb high is centered around 1500
miles to the north of Honolulu, while a weak 1010 mb low is
located around 500 miles south of the state. These two features
are driving the moderate to locally breezy trade winds across the
island chain this evening. Meanwhile, Tropical Storm Madeline is
located around 775 miles east-southeast of the Big Island, with
Hurricane Lester located around 1825 miles east of the state.
Closer to home, a band of enhance clouds and showers is seen in
infrared satellite and radar imagery moving through Maui county
and the Big Island, with some drier air working into Oahu and
Kauai. Main short term concern for tonight revolves around rain
chances.

Rest of tonight,
High pressure to the north of the state and low pressure to the
south, will keep a moderate to locally breezy trade wind flow in
place across the islands. MIMIC total precipitable water (PW)
imagery shows a band of 1.4 to 1.7 inch values extending around 80
to 100 miles upstream of Maui county and the Big Island, with PW
values quickly dropping to between 1.0 to 1.2 inches behind this
band. Expect the showery trades to continue across Maui county and
the Big Island through the evening, with the coverage of the
showers decreasing after midnight as some drier air begins to work
in from the east-northeast. Across Oahu and Kauai where the drier
air has already moved in, expect mainly scattered windward and
mauka showers through the remainder of the night.

Monday and Monday night,
High pressure north of the state will keep a moderate to locally
breezy trade wind flow in place through the period. Both the 00z
GFS and 00z ECMWF are in good agreement showing a dry airmass
remaining over the state, with PW values in the 1.0 to 1.2 inch
range. This in combination with inversion heights at or below 7
kft, should keep shower activity limited, with rainfall amounts
light and confined to primarily windward and mauka areas.

Tuesday and Tuesday night,
Moderate trade winds will continue across the Aloha state as the
pressure gradient remains tight between high pressure well north
of the area and the approach of Tropical Storm Madeline from the
east. Model solutions are in good agreement showing deep layer
moisture associated with Madeline increasing, and this appears on
track given the plume of higher PWs seen in MIMIC total PW
imagery extending to the north of the cyclone. As a result, we
should see shower activity increase from east to west across the
state on Tuesday with a showery trade pattern continuing Tuesday
night.

Wednesday through next Sunday,
The extended portion of the forecast will be highly dependent on
the track and intensity of Tropical Cyclones Madeline and Lester
as they move over or very near the Hawaiian Islands. The latest
track from the Central Pacific Hurricane Center (CPHC) shows
Madeline moving over or near the Big Island Wednesday and
Wednesday night, and Tropical Storm watches may be needed for the
Big Island as early as Monday. On the heals of Madeline, the
latest forecast from the National Hurricane Center (NHC) in Miami,
shows Lester approaching the eastern Hawaiian Islands by late in
the week. Model solutions have shown a bit more variability with
Lester over the weekend, so there is more uncertainty on the
eventual track and potential impacts to the state of Hawaii.
Residents and visitors are urged to closely monitor the latest
forecasts from the CPHC and NHC regarding these tropical cyclones.
Overall, it looks like a pretty unsettled weather pattern across
the state with the tropical cyclones moving through.

&&

.AVIATION...
Trade wind flow will persist through twenty four hours and beyond.
Bands of showers embedded within the trade wind flow will impact
mainly windward and mountain areas. AIRMET Sierra remains in
effect for mountain obscuration along windward slopes of the Big
Island and for north through east facing slopes of Maui and
Molokai. MVFR conditions are expected to become isolated by
midnight and AIRMET Sierra will likely be allowed to expire.
Otherwise VFR conditions are expected to prevail all areas late
tonight through early Tuesday.

&&

.MARINE...
Winds have picked up this evening across the windier waters around
Maui and the Big Island, and as a result a Small Craft Advisory
(SCA) has been issued for Maalaea Bay and the Alenuihaha and
Pailolo channels through Monday night. There may be a slight
downward trend in winds on Tuesday, but they should pick up once
again by mid week as Tropical Storm Madeline approaches from the
east. Depending on the eventual track of Madeline, a Tropical
Storm watch may be needed for portions of the marine area as early
as Monday.

No significant swells are expected through Monday, though surf
along east facing shores will be slightly elevated due to the
stronger trades. Winds and seas, as well as surf along east
facing shores, will increase from east to west late Tuesday and
Tuesday night with the approach of Madeline. The east swell may
peak Wednesday, and heights will depend on the eventual track and
intensity of the system. However, warning level surf along east
facing shores will be possible. We may also start to see a swell
from Hurricane Lester as early as Wednesday or Thursday, with
heights building into the weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Tuesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel.

&&

$$

DISCUSSION/MARINE...Jelsema
AVIATION...Bedal

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



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