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Hawaiian Islands Synoptic Discussion and Guidance

552
FXHW60 PHFO 070617
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
817 PM HST Thu Aug 6 2020

.SYNOPSIS...
Breezy trade winds will prevail through the weekend, then trend
down early next week as high pressure to the north-northeast
weakens. A disturbance moving through aloft combined with plenty
of moisture in the area will keep the rainfall chances up for
windward and mountain locations through Friday night, especially
during the overnight and early morning periods. Drier conditions
will be possible over the weekend as the upper disturbance and
moisture shift west of the area.

&&

.DISCUSSION...
Latest surface analysis and satellite imagery showed the
subtropical ridge anchored north of the islands and patchy low-
level clouds approaching and moving through from east to west.
Some of these clouds are generating brief/mostly light showers as
they move through - mainly over windward and mountain areas. Rain
gages around the islands reflected this and showed peak
accumulations ranging from a quarter to a little over half of an
inch in the past 12-Hrs (through 7 pm HST). Locally higher
amounts were measured on the Big Island ranging from 1-1.29
inches. Expect the showery pattern to persist through Friday
night in response to a weak upper trough moving through and
plenty of moisture in the area (above average: 1.6-1.8" PWs).
Some showers may become locally heavy, especially over the
leeward areas of the Big Island Friday afternoon.

Guidance has initialized well with the current pattern and shows
upper heights gradually rising over the weekend as the
aforementioned upper trough shifts west of the area. This
combined with lowering PWs will translate to a drier and more
stable trade wind pattern returning Saturday through Sunday.
Trades will become brisk through this time as high pressure
strengthens to the north, but should trend down through the first
half of next week. Another area of lower heights moving in aloft
early next week could enhance shower coverage once again through
midweek, mostly for windward and mountain locations. The one
limiting factor, however, will be less moisture in the area
compared to the current environment.

&&

.AVIATION...
High pressure far north of the state will bring persistent
moderate trades to the islands tonight. Thus, tempo moderate
turbulence will likely form in the lee of the mountains. An area
of enhanced moisture and weak instability caught up in the flow
will pass over the smaller islands and bring a greater chance of
showers to Oahu and Kauai. However, windward locations on all
islands may experience periods with MVFR conditions as scattered
to numerous showers pass through.

AIRMET TANGO remains in effect over and immediately south thru
west of mountains for tempo moderate turbulence.

AIRMET SIERRA remains in effect for north thru east sections of
the Big Island and Kauai above 2000 feet for temp mountain
obscuration from SHRA and clouds.

&&

.MARINE...
Strong high pressure located far north-northeast of the area will
remain the driving factor through the weekend and into early next
week. Moderate to locally breezy trade winds will continue to
support the Small Craft Advisory (SCA) for the typical windier
areas near Maui County and the Big Island. Winds are expected to
pick up a bit this weekend, which may require the SCA to be
expanded to additional coastal waters, before backing down a bit
early next week. The SCA will most likely continue into early
next week for the typical windier areas.

A small south swell will be on the decline through Friday. Surf
along south facing shores should return to background levels on
Saturday and will remain small through next week. Small 2 to 3
foot short-period southeast swell will also continue over the
next several days, but the Big Island will block some of this
swell from reaching the rest of the islands. Surf along east
facing shores will build through the weekend as trade winds
strengthen. Surf heights are expected to remain just below the
advisory levels along east facing shores.

&&

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

&&

$$

DISCUSSION...Gibbs
AVIATION...Thomas
MARINE...M Ballard

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



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