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Hawaiian Islands Synoptic Discussion and Guidance

251
FXHW60 PHFO 190626
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
826 PM HST Sun Aug 18 2019

.SYNOPSIS...
A ridge of high pressure north of the state will keep trade winds
blowing through next weekend. The trades will strengthen a bit
during the middle of the work week, and again next weekend,
otherwise light to moderate trades will prevail. Showers will
favor windward and mauka areas, particularly during the overnight
and early morning hours. A bit of an increase in shower activity
is expected late Monday night through Tuesday night, mainly across
windward sections of Maui County and the Big Island. A bit more
showery weather is also expected Thursday night through Friday
night, as a front approaches from the north. Otherwise fairly dry
trade wind weather will prevail.

&&

.DISCUSSION...
Currently at the surface, a 1020 mb high is centered around 1500
miles northeast of Honolulu, with the ridge axis extending
west-southwest to a location a few hundred miles north of Kauai.
The resulting gradient is producing light to moderate trade winds
across the island chain this evening. Infrared satellite imagery
shows a few light showers diminishing in leeward areas and picking
up over the windward terrain. Main short term concerns revolve
around rain chances and trade wind trends over the next few days.

A ridge of high pressure will remain north of the state through
the forecast period, keeping trade winds blowing across the island
chain. Light to moderate trades will continue through Tuesday
night, although we should see a slight increase in trade wind
speeds Monday night through Tuesday night as a weak trough of low
pressure slides by to the south of the islands. Generally dry,
stable trade wind weather will prevail through late Monday night,
with light showers mainly affecting windward and mauka areas. With
the trough sliding by south of the state, we should see a bit of
an increase in trade wind showers late Monday night through
Tuesday night, particularly across windward sections of Maui
County and the Big Island.

Drier more stable conditions along with moderate to locally
breezy trade winds are expected to return Wednesday through
Thursday, with windward and mauka areas remaining the main shower
targets. A digging upper level trough north of the state in the
vicinity of 30N, will drive a frontal boundary southward toward
the islands Thursday night through Friday night, easing the trade
winds back into the moderate range. Although the front isn't
expected to make it southward into the state, elevated inversion
heights and an increase in boundary layer moisture should bring
more trade wind showers to the islands for the tail end of the
work week as well.

High pressure will then build northeast of the islands next
weekend, with deep layer ridging stabilizing the airmass over the
islands as the upper level trough retrogrades westward. As a
result, we should see a return of fairly dry trade wind weather
and moderate to locally breezy conditions.

&&

.AVIATION...
Light to moderate trades winds will continue through the period.
Clouds and showers will favor windward and mountain locations,
mainly through the overnight and early morning hours. Mostly dry
conditions will prevail over leeward areas, with the exception
being over the Big Island where clouds and showers form (best
coverage through the afternoon and evening hours). Brief periods
with MVFR CIGS and VSBYS will be possible in and around showers.

No AIRMETs are currently in effect.

&&

.MARINE...
An east-northeast to west-southwest oriented surface ridge
located about 300 nm north of Lihue is forecast to weaken and
shift southward through Monday. This will result in light to
moderate northeast trade winds across the Hawaiian coastal waters
on Monday. As a new surface high pressure system builds north of
state from Monday night through mid-week, east-northeast trade
winds will gradually strengthen. Based on the latest forecast,
trade winds are expected to reach the Small Craft Advisory (SCA)
threshold starting Tuesday night across the typically windier
waters adjacent to the islands of Maui County and the Big Island.
These SCA conditions may persist over these waters through
Friday.

Surf along south facing shores will remain small through early
Monday, with mainly background swell energy from the southwest
and southeast. An upward trend in surf is anticipated beginning
late Monday as a new large, long-period south swell begins to
spread across the Hawaiian waters. Surface analysis charts showed
a compact 956 mb storm-force low southeast of New Zealand early
last week, which produced a captured fetch aimed toward Hawaii's
swell window. Data from scatterometer passes confirmed an area of
storm-force winds aimed northward during this time period. In
addition, altimeter passes within this area showed seas ranging
from 35 to 42 feet. This swell has already moved through the
nearshore PacIOOS buoy at Pago Pago, which showed seas and energy
levels that were slightly higher than our recent mid-July 2019
warning-level event along south facing shores. The possible
limiting factor for this swell event in Hawaii was that the storm
was slightly more compact compared with other high surf events.
Expect a High Surf Advisory (HSA) to be issued for south facing
shores starting Monday night or Tuesday. We will be monitoring
observations in case a High Surf Warning might be required during
the peak of this event. The surf may not drop below the HSA
criteria along south facing shores until Thursday based on the
latest guidance.

The small west swell associated with recent tropical cyclones in
the Northwest Pacific will steadily trend down through Monday.
Surf along north and west facing shores of the smaller islands may
trend up from late Wednesday through Friday as a small northwest
swell spreads down the island chain. Elsewhere, choppy surf is
expected to trend up slightly along east facing shores later this
week as the trades strengthen.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Jelsema
AVIATION...Gibbs
MARINE...Houston

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