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RH26 %
WindNE 16 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 180132

Area Forecast Discussion
National Weather Service Honolulu HI
332 PM HST Tue Jul 17 2018

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


A breezy and stable trade wind flow remains in place this
afternoon as patches of high clouds pass overhead. 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 6,500 ft. An upper level trough parked
several hundred miles north of the state is sending patches of
high clouds over the state, and a mix of stable stratocumulus and
shower-bearing cumulus clouds is moving along the trade wind
flow, leading to modest rainfall across windward slopes and a few
showers over the South Kona slopes of the Big Island.

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 should 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, the upper
level trough to the northwest will drift closer to Kauai. This
feature is producing spotty thunderstorms about 275 miles
northwest of Kauai, and as it shifts closer, thunderstorms could
pop up closer to the state, while high clouds continue to stream

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 shower activity. Another increase in rainfall is
possible on Sunday and Monday.


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 into this
evening, 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
are expected to see more persistent MVFR conditions and increasing
showers developing tonight through Wednesday morning as deeper
moisture moves through from the east. Conditions are expected to
improve throughout the afternoon as the moisture moves off to the
west and passes south of the state.

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 tonight and likely Wednesday as well.


High pressure to the north of the islands will maintain fresh to
strong trade winds through the week. The strongest winds expected
on Wednesday as a trough passing south of the state tightens the
local pressure gradient. Starting tonight, the Small Craft
Advisory (SCA) has been expanded from the typically windy waters
around the Big Island and Maui County to cover waters around Oahu.
Combined seas around the Big Island will also rise to near 10 ft.
Winds will decline a notch on Thursday and Friday, and the SCA
will likely be scaled back to the typically windy waters through
the weekend.

The fresh to strong trade winds will generate rough seas during
the next couple of days. Local PacIOOS buoys are registering seas
of 7 to 8 ft at 8 seconds this afternoon, and with an increase of
a foot or two due tomorrow, rough surf along east facing shores
will approach the High Surf Advisory (HSA) level. As winds
decline on Thursday and Friday, east shore surf will drop back to
seasonable levels.

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 HSA threshold along south facing shores from
Saturday into Monday.

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


Small Craft Advisory until 6 AM HST Thursday for waters around the
Big Island, Maui County, and Oahu.



MARINE...M Ballard/Wroe

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