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Hawaiian Islands Synoptic Discussion and Guidance

345
FXHW60 PHFO 042002
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1002 AM HST Tue Aug 4 2020

.SYNOPSIS...
Moderate to breezy trade winds will continue through the week, and
may get a little stronger by the weekend. The trade winds will
deliver passing low clouds and showers that will primarily focus
over windward areas, with a few showers occasionally spreading
leeward on the smaller islands. Increased moisture and a passing
disturbance aloft are expected to bring an uptick in showers from
late tonight into Friday.

&&

.DISCUSSION...
No significant changes to the forecast with the morning update.
Some very minor tweaks to the winds, with winds around the Big
Island and Maui County today expected to be just below Wind
Advisory levels.

High pressure to the northeast has a ridge to the north of the
islands, and is driving the moderate to locally breezy trade winds
across the region. Satellite derived precipitable water (PW) shows
the decrease in PW that was observed in the overnight soundings.
The satellite is picking up on an increase in PW about 200 miles
upstream of the islands. Infrared and visible satellite shows
little change in the clouds in that area, but it could indicate
more moisture available as that area interacts with the islands
later tonight or tomorrow. The current forecast reflects the
uptick in showers that is possible with that increased moisture.

The trade winds are expected to persist through the week with a
slight trend down this weekend. Dew points will likely creep up
into the lower 70s, raising humidity and giving the air a warmer
feel, even with the trades blowing.

A mid-level trough, loosely associated with that increase in PW,
will also move over the islands from the east, destabilizing the
lower atmosphere. The combination is expected to lead to periods
of enhanced showers, mainly over windward areas, with the trades
driving a few rain drops leeward on the smaller islands. The Big
Island's leeward slopes will see afternoon and evening clouds
developing in response to sea breeze convergence that will drop a
few showers.

By the weekend, a return to a more typical trade wind pattern is
expected, with mainly brief windward showers as moisture gradually
diminishes. High pressure north-northeast of the islands will
also strengthen, driving increased trade winds.

&&

.AVIATION...
An elongated ridge of high pressure north of the islands will
allow for moderate trades to persist through the remainder of the
day. This flow may lead to tempo moderate turbulence along the lee
of the mountains. Expect mainly isolated showers over windward
locations through this afternoon, with coverage increasing to
scattered tonight. Brief MVFR ceilings and visibility remain
possible with this activity.

AIRMET TANGO remains in effect below 8000 feet for areas over and
immediately south through west of mountains for tempo moderate
turbulence.


&&

.MARINE...
Ridging positioned to the north of the islands through the weekend
will support locally fresh to strong trade winds. A Small Craft
Advisory remains in effect for the waters around Maui County and
the Big Island over the next several days.

Small, long period southerly swell will arrive through the week
with surf heights remaining below advisory levels. A short period
southeast swell will persist through the week and maintain
slightly elevated surf along exposed shorelines. Easterly trades
will continue to produce elevated surf along east facing shores.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Wednesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.


&&

$$


DISCUSSION...M Ballard
AVIATION...Thomas
MARINE...Blood

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



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