Drought in Hawaii, July 1 to 31, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
854 PM HST Fri Jun 30 2017

.SYNOPSIS...
Breezy trade winds will continue tonight, before easing to
moderate levels over the weekend as a weak surface trough passes
through the island chain. Periods of showery weather are expected
for the next couple days, with showers favoring windward and
mauka areas. Showers will also reach leeward areas more regularly
due to the somewhat unstable conditions associated with an upper
level disturbance passing over the island chain. Breezy
conditions along with drier and more stable conditions will then
return on Monday and linger through much of next week.

&&

.DISCUSSION...
Currently at the surface, a 1028 mb high centered around 950 miles
north-northwest of Kauai, is driving breezy trade winds across the
island chain this evening. Meanwhile, a weak surface trough of low
pressure is located around 300 miles east of Hilo. Aloft, an upper
level low is evident in water vapor imagery around 600 miles
east-northeast of Hilo, with upper troughing extending westward
just to the east of the Big Island. Infrared satellite imagery
shows partly to mostly cloudy skies in place across the State,
with cloud cover most prevalent over Kauai, Maui County and the
Big Island. Radar imagery shows scattered to numerous showers
moving into windward areas, with the highest coverage across
Kauai, the Big Island and Maui County. Additionally, there are
some lingering convective showers over leeward section of the Big
Island. Main short term concern revolves around rain chances.

Tonight through Sunday night,
High pressure north of the State will keep the trade winds
blowing right through the weekend. The trades will remain breezy
tonight, before easing into the moderate range over the weekend as
a weakening surface trough of low pressure shifts westward and
over the island chain. MIMIC total precipitable water (PW) imagery
shows values in the 1.3 to 1.7 inch range across the State, and
this lines up well with the 00z observed soundings at PHLI and
PHTO which came in at 1.56 and 1.26 inches respectively. Inversion
heights at both sites were between 7 and 9 kft. Model solutions
show PW values remaining in the 1.3 to 1.7 inch range through the
weekend, which is above normal for this time of year.
Additionally, as the upper level low shifts westward to the north
of the islands, a trailing upper trough will move westward over
the island chain, and this will de-stabilize the airmass and
elevate inversion heights. As a result, we expect periods of
showery weather right through the weekend. Showers will continue
to focus over windward and mauka areas, but will also spread
leeward more regularly due to the elevated inversion heights and
persistent trade winds.

Monday through next Friday,
The weak surface trough or what's left of it, will shift west of
the islands Monday morning, allowing surface high pressure north
of the State to exert more of an influence on the area. As a
result, we expect the trade winds to ramp back up to breezy levels
by Monday afternoon and continue through at least the middle of
next week. For the end of next week, the ECMWF keeps breezy trade
winds in place, while the GFS shows the trades easing due to a
frontal boundary shifting southward towards the islands. For now
will utilize a blend of the two solutions, and show a slight
easing of the trades for the end of next week. Aloft, upper level
ridging will build in from the east, stabilizing the airmass over
the island chain through mid-week. Upper level heights are then
expected to lower slightly for the end of next week.

Overall, drier and more stable conditions are expected to move
into the State on Monday, although some showery weather may
continue into the morning hours due to convergence associated with
the return of breezy trade winds. The dry and stable airmass will
then remain in place through mid-week, with an increase in trade
wind showers expected for the end of next week. Showers will favor
windward and mauka areas through the period, with a stray shower
reaching leeward areas mainly during the overnight and early
morning hours.

&&

.AVIATION...
Breezy northeast to east trade winds will continue across all
islands with periods of passing showers. These stronger winds will
produce low level turbulence over and south through west of the
mountains. AIRMET Tango remains in effect for TEMPO MOD TURB below
10000 FT.

VFR conditions will prevail with TEMPO MVFR ceilings occurring
over mountain areas and north through east sections of all islands
tonight and tomorrow morning.

&&

.MARINE...
Fresh to strong trade winds associated with high pressure north of
the islands will continue through Saturday, then slightly weaken
through the second half of the weekend as a weak surface trough
approaches and moves through the islands. As a result, the small
craft advisory will likely hold through the day Saturday across the
typically windier channel waters, Maalaea Bay and south of the Big
Island before dropping just below advisory levels Sunday. Trades
will surge back Sunday night through the first half of the upcoming
week as the trough weakens and shifts west of the area and high
pressure strengthens to the north.

Rough surf along east facing shores will continue through Saturday,
then lower temporarily Sunday as the trades slightly back off. This
will be short-lived as the trades are forecast to surge back
across the waters and upstream of the state through the first
half of the upcoming week. This will generate rough surf along
east facing shores once again Tuesday through midweek.

Surf along southern shores will gradually build overnight, peak
Saturday below advisory levels then slowly ease Sunday into the
upcoming week as a south-southwest (210-220 deg) swell fills in from
a storm- to hurricane-force low several days ago across the
Tasman Sea. This source peaked at the Samoa buoy a few days ago
and has begun to show within the 17 to 19 second bands at the
offshore buoys south of the islands over the past several hours.
Looking ahead into next week, no significant changes are
anticipated along southern shores due to a persistent blocking
pattern that remains well established east of New Zealand, where
the typical swell generation region for Hawaii is located. This
blocking high east of New Zealand is shunting large storm systems
farther east across the south Pacific, where much of that energy
is focusing toward the Americas and only sending small,
background, south-southeast swell toward Hawaii. The other
generation source for Hawaii is the Tasman Sea, where another gale
is currently being analyzed this evening. WAVEWATCH III picks up
on this south- southwest swell (220 deg) and indicates it arriving
across the Hawaiian waters late next week. This source will
likely come in smaller than the current Tasman swell filling in
this weekend.

Up north, outside of a small north-northwest source (2 ft at 10
seconds) expected to persist through the weekend, mainly wrap into
the typically exposed locations from the strong trades locally and
upstream will be expected for surf. Otherwise, nothing significant
is anticipated through the upcoming week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Saturday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

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

&&

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Gibbs