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Hawaiian Islands Synoptic Discussion and Guidance

898
FXHW60 PHFO 201340
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
340 AM HST Fri Jul 20 2018

.SYNOPSIS...
A ridge of high pressure far north of the islands will maintain
trade winds through the weekend and into next week. The locally
breezy trades will ease off to moderate speed by tonight, and
continue through the weekend. A relatively dry and stable air mass
will limit the trade showers to until late Sunday when some
tropical moisture roll in from the east.

&&

.DISCUSSION...
The pressure gradient will continue to relax over the islands
today as a weak low pressure area some 550 miles SW of Oahu
continues to move west, further away from the islands. Expect the
current locally windy trades to settle to moderate speed by this
evening.

The air mass over the islands is stable, with a strong inversion
between 5k at Lihue to 6k feet at Hilo this morning. So, any
showers will be light in intensity. Windward Big Island had a few
showers, amounting to a few hundredths of an inch. The Kona coast
also observed a few showers in the evening. Otherwise, lots of
zeroes in the rainfall report.

A layer of cirrus clouds continue to shroud the skies from Oahu
eastward. Models suggest the cirrus to linger around the islands
into the weekend. An upper low centered 600 miles NNW of Kauai,
is responsible in drawing the cirrus to over the islands from the
tropics. Virga was detected by radar early in the evening falling
from the layered high clouds, but has since dissipated. The upper
low is expected to move SW in the coming days, keeping the islands
under a SW wind flow into Saturday. Its influence on the islands
will be limited to filtering some of the sunshine, and causing a
slight chance of a thunderstorm to the offshore waters today.

By late Sunday afternoon, the leading edge to a slug of tropical
moisture should be at the Big Island door steps. This moisture
will be working gradually westward Sunday night and Monday,
finally clearing Kauai by Tuesday afternoon of next week. What
the islands will be getting will be a boost of enhanced trade
showers.

There could be a surge in the trade winds on Tuesday, right
behind the tropical moisture. This boost in the trades is from
the tightening of the pressure gradient brought on by a passing
disturbance south of the islands. The disturbance and moisture
area are two different separate sources that in due course almost
come together. The slug of moisture can be seen on satellite,
some 1170 miles east of Cape Kumukahi, marked by a towering
cumulus cloud. The disturbance is from the thunderstorm activity
about a thousand miles SE of the Big Island.

Additional pockets of moisture follow these features that may keep
the islands under a wet trade pattern the rest of next week.

&&

.AVIATION...
Moderate to strong high pressure northeast of Hawaii will keep
the breezy/windy conditions in placed though today. AIRMET Tango
continues for mechanical turbulence south through west of the
mountains of all islands as a result. Tango could be taken down
by 210400 UTC.

Expect some clouds and showers over the typical windward and
mountain areas with brief MVFR conditions. Otherwise, expect VFR
conditions to prevail. Additional cirrus and some mid level clouds
are moving over the Big Island from thunderstorms south-
southwest of the Big Island.

&&

.MARINE...
Moderate to locally breezy trade winds continue. A disturbance
located about 500 miles southwest of Hawaii will continue to track
west and away from the islands tonight. The Small Craft Advisory
(SCA) has been extended for the typically windier waters around
the Big Island and Maui County through Friday. The passing of
this disturbance has helped to tighten the local pressure gradient
over the state, but this will begin to weaken as the disturbance
tracks farther from the state. Winds are then expected to decline
a notch by Friday night, possibly below SCA thresholds. Another
weak disturbance is then forecast to pass south the of state
early next week, once again tightening the pressure gradient over
the state, leading to an increase in winds.

A series of reinforcing south-southwest swells will keep surf
along the south facing shores slightly above the summer average
through Friday. A larger and longer period south-southwest swell
is expected to begin filling in Saturday and peak on Sunday. This
swell may cause surf to approach the High Surf Advisory (8 ft)
threshold along south facing shores from Saturday into Monday.
Surf along east facing shores will continue its downward trend
Friday into Saturday as the winds ease.

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

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST this evening for Maalaea Bay,
the Pailolo and Alenuihaha channels, and waters south of the Big Island.


&&

$$


Lau/Chevalier/Eaton

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



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