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Hawaiian Islands Synoptic Discussion and Guidance

589
FXHW60 PHFO 041946
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
946 AM HST Wed Aug 4 2021

.SYNOPSIS...
High pressure to the north will keep breezy trade winds over the
islands into the weekend. Low clouds will deliver a few showers,
mainly to windward slopes during night and morning hours, while most
leeward areas remain dry.

&&

.DISCUSSION...
Surface high pressure far NNE of the islands will drive breezy
trade winds into the upcoming weekend as it shifts slowly W. The
central pressure of the high is expected to be around 1030 mb for
the next several days, thus locally breezy trade winds are not
expected to change all that much. A low-level trough (remnants of
what eventually becomes of T.S. Hilda and T.D. 9-E remnants) may
approach and pass near or N of the islands next week, potentially
leading to some reduction in trade wind speeds late this weekend
and early next week, but confidence in this occurring is low.

Model guidance does not explicitly highlight any significant
moisture source to bring enhanced rainfall to the islands, but does
indicate that the center of a mid-level ridge will move N and away
from the islands over the next couple of days. The cloud-capping
subsidence inversion currently based between 8000-10000 feet will
remain sufficiently high to allow low clouds arriving on the trades
to deliver a few showers, while island terrain also helps to drive
some shower formation. Most of this light to moderate rainfall will
remain close to the windward mountains, and will be most likely
during nights and mornings. Any light rain that occasionally falls
over windward coastal areas, or that spreads to leeward communities,
will do little to override the drying effects of the breezy trades
and strong solar insolation.

&&

.AVIATION...
VFR conditions prevail at most TAF sites with the exception of
PHTO where occasional MVFR conditions are occurring in low clouds
and shra. Periods of MVFR conditions can also be expected over
windward slopes as low clouds interact with island terrain. Expect
these conditions to continue through the day today and on through
the evening hours. AIRMET Tango is currently post for low level
turbulence over and downwind of mountain terrain.


&&

.MARINE...
Fresh to strong trade winds continue as strong high pressure
remains anchored about 1000 nm north-northeast of the state.
Little change in the strength of the high is expected over the
next several days, but the pressure gradient will weaken over the
islands, causing the trades to ease slightly. A Small Craft
Advisory (SCA) is in effect for all Hawaiian waters today and may
need to be extended into tonight. The SCA will then scale back to
the typically windy waters around the Big Island and Maui through
Friday, though SCA conditions will likely last through Saturday.
Weaker trade winds are expected Sunday and early next week.

Surf will remain highest along east facing shores for the next few
days. Near shore PacIOOS buoys continue to show trade wind swell
around 7 ft at 7 to 9 seconds, which is producing rough east
shore surf slightly higher than seasonal average. As trades drop a
notch, expect east shore surf to fall to seasonal average
tomorrow, then gradually decrease during the weekend and early
next week. The current small bump in south swell is hanging around
today. This will fade tomorrow, followed by another small pulse
this weekend. South shore surf may reach or possibly exceed
seasonal average early next week. Expect nearly flat conditions
along most north and west facing shores through much of the week.


&&

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

Small Craft Advisory until 6 PM HST this evening for all remaining
Hawaiian coastal waters.


&&

$$


DISCUSSION...Foster
AVIATION...Burke
MARINE...Wroe

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



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