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Hawaiian Islands Synoptic Discussion and Guidance

731
FXHW60 PHFO 180634
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
834 PM 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 overnight into
Wednesday.

&&

.DISCUSSION...
Breezy trade wind weather continues this evening across the state,
with radar showing only widely scattered showers over some
windward areas, and scattered high clouds passing overhead. Rain
gauges report only isolated light rainfall amounts of a quarter
inch or less at some windward locations during the past six hours.
Surface analysis depicts an elongated east-west area of high
pressure located about 1200 miles north of the islands, with a
weak tropical disturbance located about 450 miles southeast of
the Big Island. The tight pressure gradient between these features
is producing the breezy trade wind pattern over the islands.
Aloft, water vapor imagery depicts a narrow but strong mid level
ridge sitting over the state and extending off to the northeast,
with a large upper level trough to our northwest and north. The
ridge overhead is maintaining stable conditions, as shown by the
00Z soundings which found strong inversions based between 5500 and
7000 feet. MIMIC-TPW satellite imagery shows estimated PW values
of around 1.5 inches surrounding the state, with an area of
greater moisture (estimated PW values up to 1.8 inches) just
southeast of the Big Island.

Overnight into Wednesday, breezy trade winds will continue, as
mainly eastern portions of the state see an uptick in rainfall.
The surface ridge to the north will remain nearly stationary as
the weak tropical disturbance passes a couple hundred miles to our
south. 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 of those islands, with some
showers also developing 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. Otherwise, the upper
level trough to the northwest will drift closer to Kauai. This
feature has been producing isolated thunderstorms about 275 miles
northwest of Kauai, and as the trough shifts closer, a few
thunderstorms could pop up in the offshore waters somewhat closer
to the state. The trough will also keep areas of high clouds
streaming overhead.

Trade winds will relax slightly beginning on Thursday as the
tropical disturbance passes southwest of the state allowing the
pressure gradient to relax a bit. Model guidance also brings a
drier airmass in from the east over the state, which should result
in decreasing shower activity. Stable and fairly dry weather with
moderate trade wind flow is then expected to prevail through at
least Saturday. Passing mainly light showers will focus over
windward areas. For Sunday and Monday, the latest GFS has come
more into agreement with the ECMWF in keeping deeper moisture
associated with the next westward moving tropical disturbance
suppressed to the south of the state. Will have to wait and see if
the previously anticipated rainfall increase for early next week
pans out.

&&

.AVIATION...
Breezy trade winds will continue over the next 24 hours, thanks
to persistent high pressure far north of the main Hawaiian
Islands. VFR conditions will prevail through tonight, but low
clouds and scattered showers embedded within trade flow will bring
periods of MVFR conditions along windward slopes. Windward slopes
of the Big Island are expected to see more persistent MVFR
conditions and increasing showers tonight through Wednesday
morning as deeper moisture moves through from the east. Conditions
are expected to improve throughout the afternoon as this moisture
moves off to the west.

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.

&&

.MARINE...
High pressure to the north of the islands will maintain fresh to
strong trade winds through the week. The strongest winds are
expected Wednesday as a trough passing south of the state tightens
the local pressure gradient. As a result, 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 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 have recently been
registering seas of 7 to 8 ft at 8 seconds, and with an increase
of a foot or two due Wednesday, rough surf along east facing
shores will approach the High Surf Advisory (HSA) level. As winds
decline 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.

&&

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

&&

$$

DISCUSSION...Jacobson
AVIATION...Powell
MARINE...Kinel

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

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