Drought in Hawaii, September 12 to 30, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
830 PM HST Mon Sep 11 2017

.SYNOPSIS...
A weak pressure pattern will remain in place through Thursday,
with daytime sea breezes and overnight land breezes expected.
Showers will be most prevalent during the afternoon and evening
hours across interior and leeward areas, with most of the shower
activity remaining offshore during the late night and early
morning hours. The trade winds will return late in the work week,
with moderate to breezy trades expected over the upcoming weekend.
This will bring a return of more typical trade wind weather, with
showers favoring windward and mauka areas, particularly during
the overnight and early morning hours.

&&

.DISCUSSION...
Currently at the surface, a weakening cold front is located around
350 miles northwest of Kauai, with a weak pressure pattern in
place over the State. This has resulted in a daytime sea breeze
and overnight land breeze pattern across the island chain.
Meanwhile aloft, a large and highly amplified upper level trough
is in place over the central Pacific, with the trough axis just to
the west of the islands. Infrared satellite imagery shows partly
to mostly cloudy skies in place across the state, with radar
imagery showing some lingering convective showers, particularly
over leeward sections of the Big Island. Main short term concern
revolves around rain chances over the next several days.

Tonight through Thursday,
The cold front northwest of the State will push southward tonight
through Tuesday night, before stalling out around 100 miles north
of Kauai Wednesday through Thursday. This will keep a weak
pressure pattern in place across the island chain, with daytime
sea breezes and overnight land breezes expected Statewide. Aloft,
upper level troughing will remain near or over the islands through
the period, keeping the airmass less stable than normal.
Precipitable water values are not expected to be particularly
high, generally remaining at or below 1.5 inches through Thursday.
Overall, we should see a convective weather pattern featuring
mainly afternoon and evening showers over interior and leeward
sections of the islands. Given the upper level trough in place, a
few of these showers could be locally heavy. During the late
night and early morning hours, showers should generally remain
offshore, with a few showers possibly hugging coastal areas.

Thursday night through next Monday,
High pressure surface and aloft will build in from the east,
allowing the trade winds to make a return by the end of the work
week. A new surface high will then build north of the State over
the weekend into early next week, bringing a return of moderate to
locally breezy trade winds to the island chain. Expect a more
typical trade wind weather pattern through the period, with mainly
windward and mauka showers, particularly during the overnight and
early morning hours.

&&

.AVIATION...
AIRMET SIERRA continues for mountain obscuration for the Koolau
Mountains on Oahu, and leeward Big Island. Clouds and showers are
beginning to fade over the Koolau range, while conditions remain
relatively unchanged over the Big Island. Expect conditions to
improve on the Big Island later tonight.

Elsewhere, clouds and showers that developed in the afternoon sea
breezes are beginning to fade, a trend that should continue
overnight as the land breezes take over. Light south to southeast
winds tomorrow will bring another round of afternoon clouds and
showers with the afternoon sea breeze. Anticipate the need for
another AIRMET SIERRA for some areas tomorrow afternoon.

&&

.MARINE...
The trade wind flow will remain light across the region through
Wednesday as a cold front approaches the islands from the
northwest. This will allow local nighttime land and afternoon
sea breeze circulations to develop through mid-week. Winds across
the coastal waters will remain well below the Small Craft
Advisory (SCA) criteria during this time. The trades are expected
to gradually strengthen later this week, and continue through the
weekend, as the front to our north eventually dissipates and
surface high pressure slowly builds far northwest and north of the
state. Trade wind speeds may approach the SCA threshold over the
typically windy waters adjacent to the Big Island by this weekend.

Surf along north and west facing shores will continue to decline
tonight as the current small northwest swell gradually fades.
Another small north swell is expected to reach the islands
Tuesday, and cause a slight increase in surf heights along most
north facing shores through Thursday. This north swell will
slowly subside by late Friday.

Small background swells from southern hemisphere sources will
maintain small surf along south facing shores through early
Tuesday. A small to moderate southwest (220 deg) swell that was
generated by a system in the Tasman Sea is forecast to arrive
starting Tuesday. This swell will continue to increase by mid-
week, causing a noticeable bump in surf heights along south and
west facing shores Wednesday and Thursday. This swell will
gradually subside Friday, but another small reinforcing southwest
swell arriving Saturday may cause another slight bump in surf
along south and west facing shores this weekend.

Surf along east facing shores will remain small during most of
the work week due to the light trade wind flow. Choppy surf will
likely increase by this weekend as the trades gradually
strengthen.

&&

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

&&

DISCUSSION...Jelsema
AVIATION...M Ballard
MARINE...Houston