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Hawaiian Islands Synoptic Discussion and Guidance

832
FXHW60 PHFO 182011
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1011 AM HST Thu Jul 18 2019

.SYNOPSIS...
High pressure far to the north will support locally breezy trade
winds into next week, although winds will be a little less gusty by
the weekend. Showers riding in with the trades will favor the
windward and mountain areas, with showers most frequent during
nights and mornings. An area of especially showery clouds will
arrive later today, and bring numerous windward showers to all
islands through tonight, with a few showers spreading leeward. The
leeward side of the Big Island will have clouds and a few showers
every afternoon and evening.

&&

.DISCUSSION...
High pressure will remain entrenched far N and NE of the islands
over the next week, supporting a seasonable trade wind weather
pattern. The high will support locally strong ENE trade winds
initially, with wind speeds easing slightly over the weekend as it
weakens a little. With trade winds blowing, we look for potential
moisture sources and/or destabilizing features to anticipate shower
coverage and intensity. While most showers affect windward areas,
strong trades and instability can bring occasionally bring rainfall
to leeward areas of the smaller islands. On the Big Island, this is
the wet season for the leeward slopes, where increased instability
usually leads to greater shower coverage and intensity.

After a few morning showers that affected primarily Oahu and Maui,
an area of mostly clear skies lies immediately upstream of the
islands from Maui to Kauai, and windward showers are expected to be
minimal the next couple of hours. The leading edge of an area of
open-celled showery cumulus clouds is approaching from the E, and
will soon arrive over windward Big Island, bringing an increase in
shower coverage, especially as compared to the current mostly dry
conditions. This area of showery low clouds will traverse the island
chain later today and overnight, bringing categorical rain chances
to windward areas, with a few showers occasionally spreading leeward
with the gusty trade winds. This moisture will move W of the islands
on Friday, with models depicting another slug of low level moisture
(now near 27N 143W) arriving from the ENE late Saturday into Sunday.

Current soundings indicate a strong subsidence inversion keeping a
lid on low clouds at around 8000 feet or so, with elevated
instability above about 12000-15000 feet. This instability is due to
a slow-moving persistent low aloft centered several hundred miles N
of the islands. Although the low aloft will drift N over the couple
of days, the associated E-W oriented trough will become N-S oriented
over the islands by tomorrow. Models do not depict significant
destabilization of the lower portions island atmosphere, but
incoming cumuli will be able to grow sufficiently tall to drop
showers as the subsidence inversion is weakened into early next
week. Additionally, the low will bring periods of high clouds,
potentially becoming thick over parts of the state at times. A ridge
aloft is expected to bring increased stability by the middle of next
week.

&&

.AVIATION...
A high about 1100 miles northeast of the main Hawaiian islands will
remain nearly stationary. The high will maintain locally strong east
winds over the islands through Friday. AIRMET Tango is in effect for
moderate turbulence below 7,000ft and this AIRMET will likely
continue into Friday.

The low level winds will carry some clouds and showers over east
sections of the islands. The incoming clouds are currently rather
sparse and stable and VFR conditions prevail. An area of showery
broken low clouds just upstream of the Big Island of Hawaii will
move over the islands later today and will likely produce MVFR
conditions over east sections.
Localized MVFR conditions are also possible over the west slopes of
the Big Island this afternoon. AIRMET SIERRA for MTN OBSC could be
required. The area of showery low clouds should clear the islands
Friday afternoon.

&&

.MARINE...
High pressure north of the state will maintain fresh to strong trade
winds through Friday. The high is expected to weaken some over the
weekend, which will help the winds ease slightly. Trade winds may
drop to moderate speeds early next week for a brief day or two.

The Small Craft Advisory (SCA) continues through Friday afternoon
for all waters in response to the trade winds. The forecast calls
for the winds to start to weaken Friday night, which will drop the
SCA back to the typical windier areas around Maui and the Big Island.
This SCA will likely be extended through the remainder of the
weekend.

Rough and choppy surf is expected to hold steady along east facing
shores at heights just below the advisory threshold through Friday.
As the winds ease over the weekend, surf is expected to drop
slightly. Some small background south and southwest swells are
expected through the weekend. An increase in surf along south facing
shores is possible early next week as a couple southwest swells
arrive in the islands.

&&

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

Small Craft Advisory until 6 PM HST Friday 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-Big Island Windward
Waters.

&&

$$

DISCUSSION...Birchard
AVIATION...Donaldson
MARINE...Kino

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