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Hawaiian Islands Synoptic Discussion and Guidance

686
FXHW60 PHFO 071325
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
325 AM HST Fri Aug 7 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 tonight, 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 three quarters of an inch
in the past 6-Hrs (through 2 am HST). Expect the showery pattern
to persist through tonight 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 be locally heavy, especially over
the leeward areas of the Big Island this 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 depicted in the area compared to
the current environment.

&&

.AVIATION...
High pressure far north of the state will bring persistent
moderate trades to the islands through this morning. 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 MVFR conditions as scattered
to numerous showers pass through. Isolated activity expected
elsewhere.

AIRMET TANGO remains in effect over and immediately south thru
west of mountains below 9000 feet for tempo moderate turbulence.

AIRMET SIERRA remains in effect for north thru east sections of
Kauai , Oahu, Maui, and Lanai above 2500 feet for tempo mountain
obscuration from SHRA and clouds. The Big Island also has an AIRMET
Sierra due to tempo mountain obscuration.


&&

.MARINE...
Strong high pressure located far north-northeast of the area will
remain the driving feature for our weather 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 into early next week. 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 current SCA is in effect through Saturday
afternoon, but its possible that additional area could be added
Saturday or Saturday night.

A small south swell will continue to decline today, with south
facing shore surf returning to background levels by tomorrow.
Small short period southeast swell will continue, with the Big
Island blocking some of that energy from reaching the smaller
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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