Drought in Hawaii, July 18 to 31, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
840 PM HST Mon Jul 17 2017

.SYNOPSIS...
Trade wind weather will persist into the weekend, with passing
clouds and showers favoring windward and mauka areas, occasionally
spreading to leeward areas of the smaller islands. Although it's too
early to be certain, increased moisture associated with the remnants
of Fernanda may affect the state this late this weekend and early
next week.

&&

.DISCUSSION...
A relatively strong surface high centered about 1100 miles N of the
islands is supporting moderate to locally breezy trade winds across
most of the tropical central N Pacific, including the Hawaiian
islands. Low cloud clusters embedded within the trade wind flow are
supplying periods of showery weather as the more organized clusters
pass through, such as the area that impacted most of the islands
today. This showery patch is in the process of clearing the islands
of Maui county, but Oahu and Kauai county still have some clouds and
showers immediately upstream to keep windward showers active, at
least through the evening on Oahu.

Wispy high clouds adding red and pink to island skies the last
couple of mornings/evenings hint at weak troughing aloft associated
the subtropical jet that is near and over the island chain.
Afternoon soundings indicate that the subsidence inversion is weaker
than normal, and elevated to near 10 thousand feet, likely due to
the presence of this trough. So, even with reduced moisture
immediately upstream of Maui county and the Big Island, windward
areas can expect a few passing showers as the island atmosphere is
moderately unstable. Radar shows a few small heavy rain cores that
are unlikely to remain over any one area for more than a couple of
minutes.

The center of the surface high is expected to drift slowly W the
next couple of days with little change in central pressure, with
little change in the gradient near the islands. As a new high
develops due N of the islands within the associated ridge this
weekend, still not much change expected to the persistent moderate
to fresh trade wind flow. This flow is expected to be somewhat
showery through Wednesday night per latest GFS guidance, which is
when the weak trough aloft is expected to diminish and slide E of
the area.

Fewer trade showers are possible Friday and Saturday per latest
model guidance, but late in the weekend and early next week is when
moisture associated with the remnants of Fernanda could potentially
bring a period of wet weather. The latest forecast from the National
Hurricane Center has Fernanda steadily weakening; crossing 140W into
the central Pacific on Thursday, and dissipating to a remnant low
about 600 miles E of the Big Island on Saturday. Global models
differ on the evolution thereafter and there is significant
uncertainty, but there are indications that a significant slug of
moisture associated with the remnants may move near/over the
islands, with recent model runs trending slower with the timing.

&&

.AVIATION...
Expect VFR conditions to prevail overnight, although the scattered
passing showers will briefly lower ceiling and vis to MVFR values.
These passing showers, riding in with moderate to locally strong
trade wind flow, will favor mostly windward and mountain areas of
the larger islands. Tops of these showers are up to 15k feet in the
heavier showers.  Expect the frequency of the showers to diminish
after 19z.

AIRMET Tango for low level turb around the islands will continue
beyond 1600z as the forecast models point to no significant change.

&&

.MARINE...
A high far northwest of the area will maintain moderate to locally
strong trade winds. A Small Craft Advisory (SCA) remains in effect
for waters around Maui County and the Big Island through at least
Tuesday afternoon.

Only very small swells are expected along north, south, and west
facing shores for the next several days.

Hurricane Fernanda, about 1530 miles east-southeast of Hilo, has
been moving toward the northwest around 10 mph. An east swell from
Fernanda is expected to reach the Hawaii coastal waters tonight,
then build through the week. The size and period of this swell will
be dependent on the exact track and intensity of Fernanda.

&&

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

&&

DISCUSSION...Birchard
AVIATION...Lau
MARINE...Kinel