Flash Flooding and Heavy Rain in Hawaii, December 27 to 29, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
832 PM HST Wed Dec 26 2018

.SYNOPSIS...
A wet pattern is expected through the second half of the week and
into the upcoming weekend as a cold front approaches and moves into
the area. The front is forecast to move over the islands and stall
Thursday night into the weekend, then gradually diminish. Locally
heavy rainfall along with a few thunderstorms will become a
possibility through this time, especially late Thursday through
Friday for the northwest islands. Conditions should gradually begin
to improve late in the weekend, with a return of drier trade wind
conditions early next week.

&&

.DISCUSSION...
The latest surface analysis and satellite imagery showed a cold
front extending southwest several hundred miles northwest of Kauai
tracking eastward (east of Midway). A weakness within the ridge has
developed north of the islands in response, which has led to light
east-southeast low-level wind flow locally across the state. This
will result in land breezes setting up overnight into Thursday,
which should lead to mostly quiet overnight conditions. Satellite-
derived precipitable water (PW) imagery, however, does depict above
average moisture in place over the state (PWs: 1.4"-1.7"). A
combination of this moisture and overnight land breezes could be
enough to trigger showers near/over windward coastal areas from
midnight through daybreak Thursday. Clouds and showers will favor
windward and leeward/interior areas through the afternoon hours
Thursday as sea breezes develop.

Guidance remains in decent agreement late Thursday through
Saturday and shows an amplifying upper pattern setting up over the
region featuring a sharp upper trough digging southeastward over
the state. The upper trough may be sharp enough for an upper low
to pinch off near or just south of the islands Friday into the
weekend. This combined with its attendant cold front moving over
the islands and stalling through this time will keep the chances
for heavy rainfall and flooding as the primary forecast concerns.
Additionally, upper height falls destabilizing the airmass
combined with the surface boundary and deep moisture will support
thunderstorms, especially over the northwest islands Thursday
night through Friday. Hi- resolution model reflectivities show
spotty pre-frontal showers developing over the northwest islands
as early as Thursday afternoon, then becoming more widespread in
coverage across Kauai and Oahu Thursday night through Friday.
Expect the wet pattern to persist into Saturday as trades
gradually begin to fill back in and the moisture associated with
the dissipating front lingers in place. Although the bulk of the
shower coverage will favor windward areas, some showers will
manage to spill over into leeward areas.

Improving conditions will be likely by the end of the weekend
through next week as high pressure builds to the north and drier
air fills in. Drier conditions along with a more typical trade
wind pattern will be likely by Monday.

&&

.AVIATION...
Light winds will continue over the island chain tonight and
Thursday as a cold front approaches from the northwest. As a
result, overnight land breezes and daytime sea breezes are
expected in most areas. Showers will favor the coastal waters
overnight and the interior and mauka areas of the islands on
Thursday. Can't rule out a thunderstorm Thursday afternoon on the
Big Island as well.

No AIRMETs are in effect and none are expected tonight.

&&

.MARINE...
Light to moderate east to southeast winds will continue across the
Hawaiian waters through Thursday as a front approaches the state.
Winds will be light enough for sea/land breezes to develop near
area shores. Starting Thursday night, the front is expected to
begin impacting the northwest waters around Kauai. Then rest of
the state Friday and Saturday, as the front stalls and weakens
somewhere between Oahu and the Big Island. Moderate to fresh
northerly winds are expected with the front and the threat of
thunderstorms.

A moderate northwest swell will continue to hold steady tonight
and into Thursday morning. Long-period forerunners from a large
northwest swell is expected to fill in through the day Thursday
and may reach advisory level by the evening hours. Swell will
continue to build Thursday night and will likely peak on Friday
near warning levels for north and west facing shores of the
smaller islands. This swell will gradually fade this weekend.
Another northwest swell is due in around Tuesday next week
producing surf below advisory levels along north and west facing
shores.

Strong, persistent trades east of the islands has generated an
easterly swell that will keep the surf up along east facing shores
even as the trades become light and variable over some parts of
the island chain through Thursday. Surf is likely to stay below
advisory levels along east facing shores. Moderate to strong trade
winds are on tap to return over the weekend, and will bump the
surf along east facing shores to near advisory levels.

There are no marine warnings at the moment. However, a Small Craft
Advisory (SCA) is likely for late Thursday or Thursday night
through Friday due to the incoming large northwest swell. SCA will
likely be in effect for the typical Maui County/Big Island waters this
weekend as the breezy trade winds return.

&&

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

&&

DISCUSSION...Gibbs
AVIATION...Jelsema
MARINE...Genki