Heavy Rain in Hawaii, December 25, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
759 PM HST Mon Dec 24 2018

.SYNOPSIS...
A wet trade wind weather pattern is setting up across the area
over the next couple of days as abundant moisture and instability
associated with an upper low move into the islands from the southeast.
Winds will diminish on Wednesday and Thursday as a cold front
approaches the islands from the northwest. This cold front will
sweep across the islands on Friday spreading showers from west to
east across the state through Saturday. Unsettled showers will
linger across the region through Sunday. Drier trends are forecast
for the first half of next week.

&&

.DISCUSSION...
Shower bands will continue drifting into the windward and
mountain areas of each island tonight as an upper level low near
the Big Island continues to destabilize the atmosphere. Expect
numerous showers to continue across the region into Christmas Day
with higher rainfall amounts favoring windward mountain areas and
the eastern half of the state. Cloud heights will rise to around
25000 feet over the Big Island by tomorrow morning, high enough
to produce snow showers over the highest peaks of Maunakea and
Maunaloa on Christmas Day. Elevated instability may allow
isolated thunderstorms to develop near the Big Island over the
next 12 to 24 hours.

Wind speeds will diminish on Wednesday and Thursday as a cold
front and upper trough break down the ridge to the north of the
state. Light large scale surface winds will allow smaller scale
terrain driven land and sea breezes to develop over each island.
Onshore sea breezes will build clouds and showers over interior
sections of each island during the day with offshore land breezes
will producing clearing each night.

A cold front will move into the western islands from the
northwest on Friday morning. Elevated moisture and instability
created by the previous upper level low will cause shower bands to
develop ahead of the frontal zone. The cold front will then sweep
from west to east across the island chain through the day on
Friday with frequent rainfall, becoming heavy at times, and good
potential for scattered thunderstorm activity. The mid range model
guidance has improved over the past 24 hours with the weather
impacts from this frontal passage. Some differences remain in the
timing of the frontal passage between the more progressive
American (GFS) model and the slower European (ECMWF) model.
However, both models are showing a trend towards significant
rainfall developing in the warm sector ahead of the front. A
strong upper level trough will also help to enhance convection
across the state. The latest model runs are showing the forward
motion of this cold front stalling over Maui County on Saturday
morning. Some of these showers may drift back westward at this
point towards Oahu and Kauai by Sunday. Either way this unsettled
weather pattern continues across the state through the upcoming
weekend.

A stronger high pressure ridge builds back in north of the islands
from Sunday into Monday. Expect breezy trade winds with drying
trends from Monday onward due to stronger subsidence aloft and
drier air moving in from the east under this ridge of high
pressure.

&&

.AVIATION...
Moderate trades continue, with a downward trend over the afternoon
hours. VAD wind profile shows winds 5 to 15 kts, which matches
well with surface observations across the state. Winds are
expected to remain near their current levels through Tuesday, with
further weakening possible Tuesday night. Have dropped AIRMET
Tango for low level turbulence, and removed mention of strong
surface winds in the aviation area forecast as a result.

Clouds are plentiful over the mainly windward areas of the
islands, with some showers mixed in. An uptick in showers is
possible tonight as increased moisture moves in from the
southeast. An upper level low southeast of the Big Island will
move towards the islands and could enhance shower activity, mainly
over the eastern end of the state. AIRMET Sierra is in place for
mountain obscuration over the north and east sides of the islands.
This may need to be expanded with shower development overnight.

&&

.MARINE...
Trades will steadily trend down into midweek as a cold front
passes to the north and a second front approaches the region from
the northwest. Land and sea breeze conditions will be likely
Wednesday through Thursday and possibly into Friday, depending on
when the second front moves into the area. Once the boundary moves
into the area, guidance suggests it will stall and weaken over
the central portion of the islands Friday into the weekend. The
front will bring increasing clouds and showers along with a few
thunderstorms beginning as early as late Thursday for the
northwest waters. Rain chances may linger into the weekend over
portions of the waters as the boundary stalls.

Overlapping northwest-to west-northwest swells will continue to
support moderate surf along exposed north and west facing shores
through midweek. Recent observations from buoys 51001 and 51101
reflected a small reinforcement beginning earlier today. Surf
should respond and rise overnight into Tuesday, then potentially
near advisory levels Tuesday night through Wednesday as a larger
reinforcement fills in out of the west-northwest. This source
will fade through the day Thursday.

A larger northwest swell will is expected by Friday from a
hurricane-force low that is currently tracking east over the far
northwest pacific. Notable arrival time differences are shown
between the various wave model solutions. The GFS/WAVEWATCH III
depicts forerunners filling in all day Thursday with corresponding
surf heights exceeding advisory levels by late Thursday
afternoon/early evening. The ECMWF-Wave solutions, however,
suggests a later arrival time through the overnight hours Thursday
night. The swell should peak through the day Friday, then slowly
ease over the upcoming weekend. Seas will likely exceed small
craft advisory levels around the peak of this event Thursday night
through Friday night.

A small to moderate west-northwest long-period swell will be
possible early next week from a broad gale- to storm-force low
potentially setting up over the upcoming weekend across the far
northwest Pacific.

&&

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

&&

DISCUSSION...Bohlin
AVIATION...M Ballard
MARINE...Gibbs