Heavy Rain in Hawaii, August 29, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
951 AM HST Tue Aug 29 2017

.SYNOPSIS...
Trade winds will continue in the light to locally moderate range for
the next few days as a weak ridge of high pressure lies a few
hundred miles north of the state. Clouds and passing showers will
favor windward and mauka areas with some afternoon clouds and
showers possible over interior areas. Winds will become lighter by
the weekend as a front approaches the area from the northwest.
Showers will tend to favor interior and lee areas during the
afternoon hours with still some passing showers possible along
windward areas.

&&

.DISCUSSION...
The 12Z surface analysis shows a 1021 mb high over 1200 miles
northeast, and a 1022 mb high over 1400 miles northwest of Oahu.
Ridges from both of these systems were about 325 miles north of
Oahu at that time. A weak surface trough is between the islands
and the 1021 mb high. This combination is helping to bring light
to locally moderate trade winds to the islands.

Water vapor imagery shows an upper level low developing about 500
miles northeast of Honolulu, along the surface trough mentioned
above. The GFS and ECMWF both depict this feature, and have it
nearly stationary for today, and then lifting northward. Dewpoints
are already in the low 70s in many locations, so muggy conditions
will continue today.

With the upper level feature lifting to the north tomorrow,
moderate trades will find their way into some more locations, but
dewpoints are still expected to exceed 70 in many locations.
Heading into Thursday and Friday, the models continue to show a
front approaching from the northwest. The GFS solution continues
to show a stronger, more defined front reaching the islands, while
the ECMWF solution shows a weaker system approaching. Both
situations will bring lighter, southeast to south winds to the
islands. Dewpoints will remain elevated so expect muggy conditions
to continue through the weekend.

&&

.AVIATION...
Moisture embedded in light to locally moderate trade wind flow is
producing some MVFR conditions over windward and mauka areas.
Windward TAF sites will continue to see some MVFR conditions today
while most leeward TAF sites should remain VFR. AIRMET SIERRA for
mountain obscuration is posted for northeast sections of the Big
Island, Maui and Kauai. Daytime heating will likely cause for clouds
to build up over interior and leeward slopes this afternoon with some
brief MVFR conditions possible.

&&

.MARINE...
Weak ridging north of the islands will maintain mainly moderate
trades through Thursday night. The ridge will shift southward over
the islands allowing winds in response to an approaching front. Winds
will weaken to light and variable Friday through the weekend.

A mix of small southeast and southwest swells will continue
through Wednesday. An above-average south swell is expected to
build beginning on Wednesday night. This swell will peak around
Friday morning and remain elevated into Saturday, and will likely
produce advisory level surf along south facing shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

DISCUSSION...M Ballard
AVIATION...Burke
MARINE...Foster