Heavy Rain in Hawaii, December 16, 2020

Area Forecast Discussion
National Weather Service Honolulu HI
1007 AM HST Wed Dec 16 2020

.SYNOPSIS...
Moderate to breezy trade winds will continue through Saturday,
becoming locally windy on Friday. Wet weather is on its way
especially over the eastern half of the state Thursday through
Friday. Heavy rainfall is expected with the potential for
localized Flash flooding Thursday afternoon through Friday over
portions of the Big Island and Maui. The atmosphere will slowly
stabilize over the weekend with more typical trade wind weather
returning over the western half of the state and a wet trade wind
pattern over the eastern half of the state.

&&

.DISCUSSION...
At the surface, we have high pressure far northeast of the state
producing moderate to locally breezy trade winds. In the upper
levels, we have a broad trough across the state and has brought
somewhat unstable conditions across the state today. For the most
part today, we are expecting scattered showers to continue to ride
in with the trade winds with some heavier showers possible this
afternoon. Isolated thunderstorms will be a possibility mainly
around the Big Island and Kauai this afternoon.

Starting Thursday, we will see a trough pass south of the state
and will bring a plume of moisture over the eastern half of the
state. Latest model guidance is showing precipitable water values
approaching 2 inches moving into the Big Island Thursday evening.
This will bring increasing rain throughout the day Thursday with
heavy rain possible Thursday evening through Friday. The main
focus of these showers will be over the Big Island and East Maui.
Elsewhere across the state, enhanced trade wind showers are
expected with brief downpours at times.

Localized flash flooding remains a threat for vulnerable areas
across the Big Island and Maui mainly between Thursday afternoon
through Friday. Confidence remains fairly high for multiple inches
of rain for the Big Island, however we are still uncertain on
rainfall rates. At this time, the conditions looks to remain
marginally unstable, thus confidence is not high enough to issue
a Flash Flood Watch at this time. We will continue to monitor this
weather event closely as it progresses, and may issue watch
products in the future if this weather pattern grows more
unstable. We are encouraging the public to closely monitor the
weather forecasts and conditions this week as island by island
weather impacts will likely evolve over time.

Also due to the deep moisture, the summits of the Big Island will
likely see a wintry mix of freezing rain and snow Thursday evening
through Friday. A Winter Weather Advisory will likely be issued
sometime during the next 24 hours.

Over the weekend, we should see the atmosphere slowly stabilize
as an upper level ridge builds over the state. But lingering
moisture will continue to produce a wet trade wind pattern
especially over the eastern half of the state.Shower activity will
trend drier on Sunday as an upper level ridge moves over the
islands. Wind speeds will decrease allowing local scale land and
sea breezes to form lasting through the first half of next week.

&&

.AVIATION...
VFR conditions prevail at most TAF sites with the exception of
PHTO where occasional MVFR is occurring in low clouds and shra.
Expect these conditions to persist through the day today. There
will likely be occasional MVFR conditions over east facing slopes
through the evening hours with the highest chances over windward
Big Island.

AIRMET Sierra is in effect for for mountain obscuration along
east facing slopes of Kauai and the Big Island. Conditions are
expected to persist across the Big Island through the evening
hours but may improve later today over Kauai.

AIRMET Tango is in effect for moderate upper level turbulence
associated with an upper level jet. Conditions are expected to
persist through the evening hours.

&&

.MARINE...
Moderate to fresh trade winds will persist through tonight, then
strengthen Thursday into the weekend as the pressure gradient
tightens over the area. Winds will trend down Sunday into early
next week as a weakness in the ridge forms in response to a cold
front passing to the north. A Small Craft Advisory will likely be
needed for the typically windier marine areas between Maui County
and the Big Island Thursday, then expanded to most waters by or
come Friday due to a combination of strong trades and seas
climbing to the 10 ft threshold.

Surf along exposed north and west facing shores will steadily
lower today as the current northwest swell declines. As this
source eases today, a new, long-period west-northwest swell should
begin filling in by tonight, then peak Thursday into Thursday
night (potentially requiring an advisory for north and west facing
shores of the smaller islands). Although confidence is low this
far out in time, guidance does support the active pattern across
the northern Pacific continuing with another northwest swell
arriving by Saturday night and building to near advisory levels by
the end of the weekend.

Surf along east facing shores will steadily trend up each day as
the trade winds strengthen, potentially exceeding the advisory
threshold Friday through the weekend with rough conditions
expected. A downward trend is possible Sunday into next week as
the winds relax.

Surf along south facing shores will remain small each day through
Friday, then potentially rise over the weekend as a small south-
southwest swell arrives.

Nuisance coastal flooding will remain possible each night through
Thursday morning around the peak daily high tides. Daily tidal
ranges will lower thereafter. See the Special Weather Statement
at www.weather.gov/hfo for more information on localized impacts.

&&

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

&&

DISCUSSION...Kino
AVIATION...Burke
MARINE...Foster