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Hawaiian Islands Synoptic Discussion and Guidance

991
FXHW60 PHFO 171415
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
415 AM HST Sat Aug 17 2019

.SYNOPSIS...
A surface ridge north of the islands will maintain moderate trade
winds through today. Winds will decrease Sunday and Monday, with
an uptick expected for the middle of next week. Increased shower
activity and muggy conditions are expected starting late Tuesday
through Thursday.

&&

.DISCUSSION...
There is little or no change in forecast philosophy since last
evening. A 1029 mb high near 38N143W, far northeast of the main
Hawaiian Islands, along with an associated ridge extending through
30N150W and 26N170W, continues to drive moderate trade winds
across the area this morning. High clouds along the southern
flank of an upper trough cover skies above the Big Island, while
low clouds brought in by the trades continue to concentrate along
windward and mauka sections of all the islands. Radar shows
scattered to numerous moderate showers embedded within trade flow
also favoring windward and mauka areas. Models show the upper
trough will move east over the next day or so, likely causing the
high clouds to shift east of the state by Sunday.

The high will shift eastward over the next few days, causing the
ridge to our north to elongate and weaken. As a result, trade
winds will decrease today through early next week. By late
Wednesday, another high will build in from the northwest, allowing
trade winds to increase from midweek into next weekend.

Models show a weak trough will move past the islands around
midweek, likely bumping up available moisture for showers. Details
concerning how much moisture will be available and how far up the
island chain the additional showers will develop remain to be
worked out. No strong upper trough is expected during this time,
so at this point we are only forecasting an increase in trade
shower coverage from midweek onward.

&&

.AVIATION...
High pressure northeast of the state will keep moderate to
locally breezy trade winds in place through tonight. Low clouds
and showers will favor windward and mauka areas, bringing
occasional MVFR cigs/vsbys. Showers will be most prevalent during
the pre-dawn and early morning hours, with improving conditions
expected by mid-morning.

AIRMET Sierra is in effect for mountain obscuration across
windward sections of Oahu, Molokai, Maui and the Big Island.
These conditions are expected to improve by mid-morning.

&&

.MARINE...
A surface high pressure system far northeast of the state will
gradually weaken this weekend, which will cause the trade winds
to diminish slightly through early next week. Therefore, the
Small Craft Advisory (SCA), which was in effect over the
typically windier waters adjacent to the islands of Maui County
and the Big Island, has been cancelled this morning. A new
surface high pressure system building far north of the state by
the middle of next week is expected to cause the trade winds to
gradually strengthen starting next Wednesday.

Small south and southwest swells will maintain background
summertime surf along south facing shores this weekend. A large,
long-period south-southwest swell arriving Monday is expected to
peak Tuesday, then gradually lower through the rest of next week.
Surf heights are expected to reach the High Surf Advisory (HSA)
criteria along south facing shores from Monday night through
Tuesday.

The current west swell produced by tropical cyclones in the
Northwest Pacific will continue to slowly decline through Monday.
Surf heights produced by this lingering swell will be largest
along shorelines with a westerly exposure. A small northwest swell
will be possible from next Thursday through Friday. Modest surf
will persist along east facing shores today, then will slowly
subside from Sunday through early next week as the trade winds
weaken.

For additional details on swells and surf, please see the latest
Collaborative Nearshore Swell and Wind Forecast for Oahu (SRDHFO)
product.

&&

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

&&

$$

DISCUSSION...Powell
AVIATION...Jelsema
MARINE...Houston

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