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Hawaiian Islands Synoptic Discussion and Guidance

717
FXHW60 PHFO 301355
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
355 AM HST THU JUN 30 2016

.SYNOPSIS...
High pressure north of the state will keep solid trade winds in
place through the middle of next week. The trades will ramp up
Tonight through the Holiday weekend, with windy conditions
expected across the Aloha state. Typical trade wind showers are
expected Today in advance of a much drier airmass approaching from
the east. This dry and stable airmass will overspread the state
Tonight, keeping trade showers to a minimum Tonight through
Saturday. The airmass will moisten up quickly Saturday night as
some deep tropical moisture gets drawn northward into the islands,
bringing a wet trade wind pattern to the state through the
remainder of the Holiday weekend and into the middle of next week.

&&

.DISCUSSION...
Currently at the surface, a 1030 mb high is centered around 1550
miles northeast of Honolulu. Aloft, an upper level low is centered
around 350 miles west-northwest of Kauai. Across the islands,
infrared satellite imagery shows partly to mostly cloudy skies in
place, with radar imagery showing isolated to scattered showers
moving into windward areas, with the majority of the activity
focused across Maui and the Big Island. Main short term concerns
revolve around rain chances Today and increasing winds late in the
week and over the Holiday weekend.

Today,
High pressure well northeast of the state, will shift westward to
a location around 1200 miles north-northeast of Honolulu by late
this afternoon. This will keep moderate to locally breezy trade
winds in place across the island chain. Aloft, the upper level low
west of Kauai will retrograde further west and away from the state
as ridging aloft builds in from the east. There is a distinct
drier airmass, with origins from the desert southwest in the
Mainland, now evident in satellite imagery around 120 miles to
the east of Hilo. MIMIC total precipitable water (PW) values
quickly drop to between 1.0 and 1.2 inches within this drier
airmass. Ahead of this drier airmass, PW values are generally
between 1.4 and 1.6 inches, and these values jump to between 1.6
and 1.8 inches immediately in advance of the drier airmass. As a
result, we should see an increase in shower activity over
windward/mauka sections of the eastern islands this morning, with
the activity then translating westward through the remainder of
the day. Shower activity is expected to diminish quickly behind
this push of enhanced moisture as the dry/stable airmass takes
hold.

Tonight through Saturday,
Strong high pressure will remain centered around 1100 to 1200
miles north of Honolulu, resulting in breezy to windy conditions
across the island chain. What will also be enhancing the winds
over the area is the very stable and dry airmass overspreading the
state, with inversion heights dropping below 5 kft. This will
likely lead to gusty downslope winds in the leeward areas. As a
result, a wind advisory may eventually be needed for portions of
the state both Friday and Saturday. As for sensible weather
details, shower activity will be greatly limited due to the dry
airmass and very low inversion heights, with only isolated to
widely scattered showers possible in windward areas, primarily
during the overnight and early morning hours. Despite the dry
airmass, upper level cirrus will continue to stream through the
area associated with the jet stream and upper level low to the
west of the state. This will lead to some increased cloud cover
across the area through the period.

Saturday night through next Wednesday,
Strong high pressure will remain nearly stationary to the north of
the state, keeping a breezy to locally windy trade wind pattern in
place. Both the 00z GFS and the 00z ECMWF show some deep tropical
moisture streaming back into the state beginning Saturday night,
and remaining over the islands through the middle of next week. As
a result, we should see a wet trade wind pattern return across
the area. Clouds and showers will focus mainly over windward and
mauka locales, but the strong trades will deliver occasional
showers to leeward areas as well.

&&

.AVIATION...
Clouds and scattered showers will remain focused over windward
areas through Today, with isolated MVFR conditions possible. VFR
conditions will prevail elsewhere across the state. A drier
airmass will move into the state from the east this afternoon and
Tonight, with showers decreasing even over the windward areas.

Trade winds will increase Today and Tonight, and AIRMET TANGO for
low level turbulence over and south through west of the mountains
will remain in effect.

&&

.MARINE...
A Small Craft Advisory, or SCA, is in effect for most Hawaiian
waters already, or starting Tonight. The SCA will be continuing
for all of these waters through Saturday afternoon.

A Gale Watch has also been issued for the Pailolo and Alenuihaha
Channels and the waters south of the Big Island starting Tonight
and continuing through Saturday as the trades are forecast to
strengthen to gale force over these waters later Tonight or
Friday.

Surf is expected to remain below the High Surf Advisory criteria
along all shorelines during the next couple of days. As the trade
winds strengthen, expect elevated rough surf along windward
shores, and a High Surf Advisory for the east facing shores will
likely be necessary starting Saturday, and continuing into early
next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Gale Watch from this evening through Saturday afternoon for
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

Small Craft Advisory from 6 AM this morning to 6 PM HST this
evening for Big Island Leeward Waters-Big Island Southeast
Waters.

Small Craft Advisory from 6 AM this morning to 6 PM HST Saturday
for Kauai Northwest Waters-Maui County Windward Waters-Big
Island Windward Waters.

Small Craft Advisory until 6 PM HST this evening for Pailolo
Channel-Alenuihaha Channel.

Small Craft Advisory from 6 PM this evening to 6 PM HST Saturday
for Kauai Leeward Waters-Kauai Channel-Oahu Windward Waters-Oahu
Leeward Waters-Kaiwi Channel-Maui County Leeward Waters.

Small Craft Advisory until 6 PM HST Saturday for Maalaea Bay.

&&

$$

DISCUSSION...Jelsema
AVIATION...Jacobson
MARINE...Houston

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



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