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Hawaiian Islands Synoptic Discussion and Guidance

309
FXHW60 PHFO 080200
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
400 PM HST Tue Jul 7 2020

.SYNOPSIS...
Breezy trade winds will persist into early next week. Showers
embedded within the trade wind flow will be focused over the
windward sides of the islands, primarily during the nights and
early morning hours. Wetter trade wind weather expected Wednesday
night through Thursday night, then drier into the weekend, with
potentially another increase in trade wind showers Monday night
into Tuesday.

&&

.DISCUSSION...
A 1032 mb surface high is centered 1300 miles north northeast of
Hawaii and is driving fresh trade winds across the state. Water
vapor imagery shows a weak upper level trough encompassing much of
the area north of the islands, with the base of the trough passing
about 150 miles north of Kauai at it's closest point. The trough
appears to be having a little bit of influence at the low levels
as radar shows widely scattered moderate intensity showers passing
over Kauai and Oahu this afternoon, while the Big Island and Maui
County mostly dry. 00Z soundings from Hilo and Lihue show a
midlevel inversion over the area around 8000 feet with near to
below normal precipitable water (PW) values between 1 to 1.2
inches. Similar to the last 24 hours, anticipate clouds and
showers to fill back in a little bit late tonight into Wednesday
morning and dissipating through the day. Focus of the rain will be
over windward and mauka areas with Kauai and Oahu remaining the
highest chance due to the proximity of the upper trough.

The trough will meander north of the area over the next several
days with slight movements away and towards the islands. PW over
the islands will boost up to around 1.5 inches Wednesday night
through Friday morning as an area of moisture tracks westward
across the area. Thus, we expect an uptick in showers,
particularly windward slopes, during this time. Trade wind speeds
will also gradually strengthen through Friday and maintain
through early next week resulting in clouds and showers reaching
leeward sides of the smaller islands at times.

PWs will drop back to normal on Friday and through the weekend,
however, the upper trough will form a cutoff low and dig south
closer to state this weekend with a low level trough reflection
forming east of the islands. Both the upper and lower level
features will gain speed as they track westward on Monday with a
surge of trade wind showers pushing through Monday night into
Tuesday. Drier and more stable air will build in east to west as
the both features depart.


&&

.AVIATION...
High pressure well north of the state will generate moderate to
locally breezy trade winds over the next several days. Speeds will
ramp up through tonight and could require issuance of AIRMET Tango
for low-level turbulence later this evening.

Scattered showers will affect mainly the windward coasts and slopes,
though a few showers may be pushed over to leeward areas at times.
Periods of MVFR conditions are possible due to lowered ceilings and
decreased visibility in the showers, though widespread mountain
obscuration is not expected. Additional showers are likely over the
lee slopes of the Big Island during the next few afternoons.


&&

.MARINE...
Fresh to strong trade winds will prevail through the week as
strong high pressure holds far north and northeast of the
islands. A Small Craft Advisory (SCA) is in effect for the
typically windier waters around Maui County and the Big Island
through Friday and will almost certainly need to be extended
through the weekend and into next week.

No significant swells are due through much of the week. A mix of
mainly background swells from the southwest and southeast will
dominate, with a south-southwest swell potentially boosting south
shore surf to around summertime average on Thursday and Friday.
Trade wind swell will gradually build to around summertime average
by Thursday or Friday, then hold into early next week. Surf along
north facing shores will remain small.


&&

.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.


&&

$$


DISCUSSION...Foster
AVIATION...TS
MARINE...Wroe

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



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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
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