Drought in Hawaii, January 1 to 10, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
904 PM HST Sat Dec 31 2016

.SYNOPSIS...
A weakening front between Maui and the Big Island will maintain
the possibility of flooding over windward slopes of the Valley
Isle tonight. Some moderate showers will affect windward slopes of
the Big Island as well, while a cool and drier northeasterly flow
over Kauai and Oahu keeps showers light and limited to windward
slopes. A rather stable and dry trade wind flow will continue
across Oahu and Kauai through early next week, while the stalled
out front keeps windward sections of the Big Island and Maui
somewhat unsettled.

&&

.DISCUSSION...
Currently at the surface, a nearly stationary front runs through
the Alenuihaha channel between Maui and the Big Island this
evening, with a weak 1019 mb high centered around 630 miles
northwest of Kauai. Meanwhile aloft, an upper level trough is in
place over the island chain, with the trough axis running roughly
southwest to northeast through Maui County. Infrared satellite
imagery shows mostly cloudy conditions in windward areas, with
partly cloudy or mostly clear conditions in place in most leeward
locales. Meanwhile, radar imagery shows most of the shower
activity confined to windward sections of Maui and the Big Island,
with only light showers confined to windward slopes of Oahu and
Kauai. Main short term concern for tonight revolves around
rainfall trends.

Tonight,
The weak front between the Big Island and Maui will remain nearly
stationary overnight, while the area of high pressure northwest of
Kauai edges closer to the island chain. Meanwhile, the upper
level trough axis will shift eastward to a location east of the
Big Island around daybreak New Years morning. MIMIC total
precipitable water imagery shows the deepest moisture over Maui
and the Big Island in close proximity to the stationary front,
with values in the 1.3 to 1.5 inch range, while values across Oahu
and Kauai remain at or below 1.0 inch. As a result, a more moist
and unstable airmass will linger over windward sections of Maui
and the Big Island overnight, while a drier trade flow over Oahu
and Kauai will keep showers light and limited to windward slopes.
It does appear the threat for additional heavy rainfall and
flooding has largely ended across Maui, but due to the recent
heavy rainfall, the stalled front close by, and the upper level
trough over the central islands, we will keep the flood watch up
through the night with the expectation that it will be cancelled
with the 4 am forecast package.

New Years Day through Tuesday,
High pressure will slowly settle southward and over the island
chain by Tuesday, with light to moderate trades on New Years Day
giving way to a land and sea breeze set up by Monday night and
Tuesday. Meanwhile, the stalled out frontal boundary will weaken
on New Years Day and dissipate by early next week. The remnant
moisture associated with the weakening or remnant front will
continue to linger over the eastern end of the state through the
period however, keeping windward sections of Maui and the Big
Island somewhat unsettled. A drier airmass will remain in place
over Oahu and Kauai, with light showers mainly limited to the
windward slopes.

Tuesday night through next Saturday,
Medium range guidance is coming into better agreement showing a
weakening cold front shifting southward through the island chain
during the middle of next week, with high pressure then building
north of the state heading into next weekend. Winds will remain
light and mainly sea/land breeze driven in advance of the front,
with light to moderate trades expected to fill back in following
the frontal passage. The models are still showing some timing
differences, but overall we should see an increase in trade
showers during the middle of next week as the front shifts through
the islands, with a drier airmass then building in by the end of
the week and heading into next weekend. Showers are expected to
favor windward and mauka areas through the period.

&&

.AVIATION...
Conditions across Maui County and Windward Big Island will
gradually improve from the west in response to the movement of
an upper level trough. The upper level trough is expected to push
east of the Big Island around daybreak Sunday, but the passing of
the upper trough, will not be sufficient to clear out the low
level moisture however. So improvement will be slow, especially
for Maui. With this said, AIRMET SIERRA will likely be continued
beyond 10z for Maui, but will be likely be terminated at 10z for
Molokai.

Radar indicates enhanced trade wind shower activity persisting
across windward Molokai Maui and northeastern Big Island. The edge
of the exiting surface front is ragged but visible on satellite
as a ne to sw boundary extending out of the northeast pacific
through the Alenuihaha channel between the Big Island and Maui.
Airmet Sierra could become necessary over windward Big Island
later tonight.

AIRMET TANGO will continue for tempo moderate turbulence between
FL270 and FL350, associated with the upper level trough and the
area east of it. Significant low level turb is not expected.

&&

.MARINE...
A small craft advisory remains posted overnight for waters south
of Maui County and west of the Big Island, where NE flow is being
accelerated around and between the islands. High-resolution model
guidance suggests that the advisory may be needed for the
Alenuihaha Channel and possibly some other areas around Maui
County on Sunday and Sunday night. Will evaluate this further
prior to the early Sunday morning forecast issuance.

The current north-northeast swell will continue to decline through
Sunday. A new short-period north-northeast swell will arrive
Sunday and peak Sunday night into Monday morning, with resulting
surf expected to remain below advisory levels. Another north swell
will arrive Wednesday and Wednesday night, with resulting surf
also expected to remain below advisory levels. A small but long
period south-southwest swell will arrive Tuesday and continue into
Wednesday, producing a bump in the surf on south facing shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch until 6 AM HST Sunday for Maui.

Wind Advisory until 6 AM HST Sunday for Big Island Summits.

Small Craft Advisory until 6 AM HST Sunday for Maui County
Leeward Waters-Maalaea Bay-Pailolo Channel-Alenuihaha Channel-
Big Island Leeward Waters.

&&

DISCUSSION...Jelsema
AVIATION...Bedal
MARINE...Jacobson