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Hawaiian Islands Synoptic Discussion and Guidance

841
FXHW60 PHFO 191341
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
341 AM HST Mon Aug 19 2019

.SYNOPSIS...
A ridge of high pressure north of the state will keep trade winds
blowing through the 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 tonight through Tuesday night, particularly 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 east to west oriented ridge of high
pressure is located around 325 miles north of Kauai. The resulting
gradient is producing light to moderate trade winds across the
island chain early this morning. Infrared satellite imagery shows
clear to partly cloudy skies in place in most areas, with some
more extensive cloud cover lingering over leeward sections of the
Big Island. Radar imagery shows a few light showers moving into
windward areas, with mainly rain free conditions in leeward
sections. 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 today, with light showers
mainly affecting windward and mauka areas. With the trough sliding
by south of the state, we should see an increase in trade wind
showers tonight 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 state 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...
A nearly east to west oriented surface ridge located about 300 nm
north of Lihue is forecast to weaken and shift southward later
today. This will result in light to moderate northeast trade
winds across the Hawaiian coastal waters. As a new surface high
pressure system builds north of state from tonight 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 Wednesday night. A weak surface front
approaching the area from the north later this week is expected
to cause a slight disruption in the pressure gradient over the
region. Therefore, the trade winds are forecast to weaken
slightly from Thursday through Friday.

Surf along south facing shores will remain small early today,
with mainly background swell energy from the southwest and
southeast. An upward trend in surf is anticipated beginning later
today 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 tonight. We will be monitoring observations in
case a High Surf Warning might be required during the peak of
this event on Tuesday or Tuesday night. 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 today. Surf along
north and west facing shores of the smaller islands may increase
from late Wednesday through Friday as small northwest swells
spread down the island chain. Small, choppy surf may 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



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