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RH26 %
WindNE 20 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

882
FXHW60 PHFO 171945
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
945 AM HST Tue Jul 17 2018

.SYNOPSIS...
High pressure north of the state will maintain moderate to breezy
trade winds through the week, with the strongest winds holding
through the next couple of days. Clouds and showers will favor
windward and mauka areas, as well as the kona slopes of the Big
Island each afternoon. A brief increase in shower activity is
expected for the Big Island and possibly Maui tonight and
Wednesday.

&&

.DISCUSSION...
A breezy and stable trade wind flow remains in place this
morning. A broad area of surface high pressure parked roughly
1,200 miles north of the islands is driving the gusty trade winds,
and a strong mid level ridge sitting overhead is maintaining
stable conditions with an inversion between 5,000 and 6,000 ft. A
mix of stable stratocumulus and shower-bearing cumulus clouds is
moving along the trade wind flow, leading to generally a quarter
inch or less of rainfall across most windward slopes during the
past 24 hours. Expect little change today, along with typical
summertime clouds and scattered showers across the South Kona
slopes of the Big Island during the afternoon hours.

Breezy trade winds will hold tonight and Wednesday as portions of
the state see an uptick in rainfall. The surface ridge to the
north will remain nearly stationary as a weak tropical disturbance
passes a couple hundred miles south of the state. As a result, the
pressure gradient over the islands may tighten slightly, causing
the trades to increase a bit. Enhanced moisture along the northern
fringe of the tropical disturbance will affect the Big Island and
east Maui. Shower activity will likely increase along windward
slopes and along leeward slopes Wednesday afternoon. Although the
mid level ridge will maintain stable conditions across the
southern half of the island chain, we cannot rule out localized
heavy showers flaring over and downwind of the active lava flow in
the southeast Puna District. In addition, an upper level trough
currently sitting several hundred miles northwest of Kauai will
drift closer to Kauai. This feature is produce spotty
thunderstorms about 275 miles northwest of Kauai, and as it shifts
closer, thunderstorms could pop up closer to the state.

Trade winds will back off a notch on Thursday as drier air settles
back over the state. The result will be a moderate trade wind flow
with modest showers activity. Another increase in rainfall is
possible on Sunday.

&&

.AVIATION...
Breezy trade winds will continue over the next 24 hours with a
persistent high pressure ridge far to the north of the Hawaiian
Islands. While mostly VFR conditions will prevail today, low
clouds and scattered showers moving through the trade winds will
bring periods of MVFR conditions along some of the windward slopes
tonight. The windward slopes of the Big Island will see persistent
MVFR conditions and increasing showers tonight as deeper moisture
moves in from the east.

AIRMET Sierra is no longer in effect for mountain obscuration.
Extensive mountain obscuration is not expected through this
afternoon.

AIRMET Tango remains in effect for tempo moderate turbulence over
mountains and south through west sections of each island
including coastal waters. Breezy trade winds will keep these
conditions through this evening.

&&

.MARINE...
High pressure to the north of the islands will maintain a locally
breezy trade wind pattern over the islands through tomorrow. A
trough to the southeast of the islands will continue to move west,
passing south of the islands later this week. As it moves to the
west, the pressure gradient particularly over the southern end of
the islands will increase, causing another uptick in the winds. A
Small Craft Advisory remains posted for a number of zones, and the
SCA could be expanded to additional zones as the situation
develops.

This trough will also bring an increase in seas late tonight and
tomorrow, that will bring seas in excess of 10 feet to some areas
south of the Big Island.

The locally breezy trade winds combined with a small short-period
northeast swell may cause choppy surf along east facing shores to
approach the High Surf Advisory criteria from Wednesday into
Thursday. Surf along along east facing shores is expected to
gradually subside from Friday through this weekend as the trade
winds weaken.

A series of reinforcing south and southwest swells will maintain
small to moderate surf along south facing shores across the state
through Friday. Slightly larger and longer period south-southwest
swells spreading across the state this weekend may cause surf to
approach the High Surf Advisory along south facing shores from
Saturday into Monday.

See the latest Oahu Surf Discussion (SRDHFO) for additional
details on surf and swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 AM HST Thursday for Kaiwi Channel-
Maui County Windward Waters-Maui County Leeward Waters-Maalaea
Bay-Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-
Big Island Southeast Waters.

Small Craft Advisory from 6 PM this evening to 6 AM HST Thursday
for Oahu Leeward Waters.

&&

$$


DISCUSSION...Wroe
AVIATION...TS
MARINE...M Ballard

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