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Hawaiian Islands Synoptic Discussion and Guidance

492
FXHW60 PHFO 050138
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
338 PM 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...
Made some additional tweaks to the PoPs and associated fields over
the next 24 hours, but no changes of significance.

High pressure off the northeast, with a ridge to the north will
maintain a trade wind flow over the islands into the weekend. Observed
winds over the typically windier spots of Maui County and the Big
Island have had some gusts reach near 30 mph today. Winds are
expected to remain nearly steady as we head into the weekend.
There is the possibility that winds could get stronger this
weekend.

The satellite derived precipitable water (PW) continues to show an
increase in moisture now about 150 miles upstream of the Big
Island and a little further upstream of the smaller islands. The
satellite imagery doesn't show a lot of showery clouds with this
moisture, but the models are picking up on it as well. The
forecast continues to reflect an uptick in showers later tonight
and tomorrow as that moisture reaches and interacts with the
islands.

Dew points are still expected to creep up into the lower 70s,
raising humidity and giving the air a warmer feel, even with the
trades blowing over the next several days.

A mid-level trough, loosely associated with that increase in PW,
will also move over the islands from the east, helping to 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, which could lead to an increase in trade winds.

&&

.AVIATION...
An elongated ridge of high pressure north of the islands will
continue to generate moderate trades through this afternoon and
into the evening. Thus expect tempo moderate turbulence along the
lee of the mountains.

There will be isolated showers over windward locations during the
next several hours 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...
A high north of the area will remain nearly stationary and slowly
strengthen through the weekend. A Small Craft Advisory (SCA) continues
for the waters around Maui County and the Big Island through
Friday due to strong easterly trade winds. The SCA may need to be
expanded over the weekend.

A small, and fairly short-period, southeast swell will persist
into next week but the Big Island will block some of this swell
from reaching the rest of the islands. Long-period swells from
deep in the southern hemisphere will remain small. Small surf
along east facing shores will build through the weekend as trade
winds strengthen.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 9 AM HST Saturday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.


&&

$$


DISCUSSION...M Ballard
AVIATION...Thomas
MARINE...Donaldson

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



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