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Hawaiian Islands Synoptic Discussion and Guidance

517
FXHW60 PHFO 182005
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1005 AM HST Wed Jul 18 2018

.SYNOPSIS...
High pressure will drive breezy trade winds into Thursday. Rather
wet conditions are expected over windward areas today, especially
on Maui and the Big Island. Showers will carry over into leeward
sections of the smaller islands at times today. Drier trade wind
weather will return on Thursday, with the winds easing a bit by
Friday. Moderate trade winds are then expected to continue through
the weekend, with showers focused mostly on windward areas. The
leeward slopes of the Big Island will continue to see the typical
summertime clouds and widely scattered showers each afternoon.

&&

.DISCUSSION...
Surface analysis depicts two 1033 mb highs about 1700 miles
northeast and northwest of Honolulu. Meanwhile, a surface trough
is located about 450 miles south of the Big Island and moving
west. The tight pressure gradient between these features is
producing a breezy trade wind pattern over the islands, which will
continue through Thursday as the trough weakens and moves away to
the west. 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 midlevel ridge overhead is maintaining stable
atmospheric conditions.

Moisture riding over the islands in the stable trades is
resulting in windward and mauka showers, particularly over Maui
and the Big Island. Although the midlevel ridge will maintain
stable conditions, localized heavy showers continue flaring over
and downwind of the active lava flow in the southeast Puna
District. The 12Z Hilo sounding was likely influenced by nearby
showers at release time, but did reflect the greater moisture
which has moved into eastern parts of the state last night, with
an elevated inversion near 10 kft and precipitable water (PW) of
1.83 inches. The Lihue sounding remained drier with PW of 1.52
inches, and found a lower inversion near 5500 feet. MIMIC-TPW
satellite imagery depicts the higher estimated PW values
associated with the tropical disturbance extending south from the
Big Island. The leading edge of a drier airmass with estimated PW
values below 1.4 inches is located about 150 miles east of the Big
Island and is moving steadily westward, and will push drier
conditions across the state tonight.

Elsewhere, 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.

The aforementioned drier airmass will spread east to west across
the state tonight into Thursday morning. This should result in
decreasing shower activity. Trade winds are forecast to weaken
somewhat by Thursday night into Friday, as the weak tropical
disturbance passes far southwest of the state allowing the
pressure gradient to relax a bit. Stable and fairly dry weather
with moderate trade wind flow is then expected to prevail through
the weekend. Passing mainly light showers will focus over
windward areas. Heading into early next week, the last couple of
GFS runs have come into much better agreement with the ECMWF in
keeping deeper moisture associated with the next westward moving
tropical disturbance suppressed mainly to the south of the state.
Therefore have lowered rain chances somewhat for Sunday and
Monday. Another wave of tropical moisture may approach the state
from the southeast by the middle of next week.


&&

.AVIATION...
The 12Z Hilo sounding was likely influenced by nearby showers at
release time, but did reflect the greater moisture which has moved
into eastern parts of the state last night, with an elevated
inversion near 10 kft and PW of 1.83 inches. The 12Z Lihue
sounding remained drier with PW of 1.52 inches, and found a lower
inversion near 5500 feet. MIMIC- TPW satellite imagery depicts the
higher estimated PW values extending south and east from the Big
Island, as the northern end of the westward-moving moisture axis
associated with the surface trough passing south of the state.
The leading edge of a drier airmass with estimated PW values below
1.4 inches is located about 150 miles east of the Big Island and
is moving steadily westward.


&&

.MARINE...
High pressure to the north of the islands will maintain fresh to
strong trade winds. The strongest winds are expected today 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 9 ft at 8 to 10 seconds. Thus, a High
Surf Advisory (HSA) is in effect for east facing shores through
tonight. 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...
High Surf Advisory until 6 AM HST Thursday for Kauai Windward-
Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

Small Craft Advisory until 6 PM HST Thursday for Kaiwi Channel-
Maui County Windward Waters-Maalaea Bay-Pailolo Channel-
Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

Small Craft Advisory until 6 AM HST Thursday for Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Maui County Leeward
Waters.


&&

$$


DISCUSSION...Foster/Brenchley
AVIATION...Chevalier
MARINE...Kino/M Ballard

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



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