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Hawaiian Islands Synoptic Discussion and Guidance

428
FXHW60 PHFO 190627
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
827 PM HST Mon Feb 18 2019

.SYNOPSIS...
A tale of two very different weather patterns is developing
across the state through Wednesday. Over the western islands of
Kauai, Niihau and Oahu, expect isolated to scattered showers in a
light wind regime. The eastern islands of Maui and Hawaii Counties
will see frequent showers, locally heavy at times, with
thunderstorms due to a deep plume of tropical moisture moving into
the islands from the southeast. Light trade winds return to all
islands from Thursday into the weekend with isolated to scattered
showers favoring windward and mountain areas.

&&

.DISCUSSION...
An upper level low just west of Kauai this evening will drift
slowly northward over the next two days. A surface trough
associated with the upper low is moving into the Big Island this
evening. A deep tropical moisture plume surrounds this surface
trough and an upper level trough will enhance shower activity over
Maui and Hawaii Counties through Wednesday. Meanwhile a more
stable weather pattern will continue over the western half of the
island chain with light winds and isolated to scattered showers
across Kauai County and the island of Oahu.

Wet weather remains in the forecast across the eastern half of the
state as the surface trough drifts northwestward. Showers will
become more frequent tonight, some of these showers will become
locally heavy at times, with isolated to scattered thunderstorms
through at least Tuesday. On Wednesday, this trough and deep
moisture plume will change direction and drift back eastward with
decreasing showers from west to east over Maui County, and
continued showers over the Big Island.

A Flash Flood Watch remains in effect for Maui and Hawaii
Counties through Tuesday afternoon. This watch may be extended
into Wednesday if heavy rainfall is likely to continue.
Temperatures will remain cold enough to keep snow levels in the
11,000 to 12,000 foot range. This means heavy snowfall is likely
over the Big Island summits of Maunakea and Maunaloa through
Wednesday morning. Wind speeds at the summit level are currently
meeting advisory criteria and a Wind Advisory remains in effect
for these two mountain peaks. A combination of heavy snow, icy
roads, and strong winds will create treacherous winter driving
conditions with periods of near zero visibility in white out
conditions on the road to the Big Island summits. A Winter Storm
Warning remains in effect for the Big Island summits through
Wednesday morning.

Over the western islands of Kauai, Niihau and Oahu,the weather
conditions remain fairly stable with light winds allowing sea
breezes to develop during the day and land breezes to form
overnight. The heating of the land each day will produce sea
breeze convergence over each island producing interior clouds and
just a few showers through Wednesday. Temperature will warm up by
a few degrees and with lighter winds it will feel much warmer for
island residents as compared with last weeks wintery weather.

From Thursday through Sunday, the surface trough has moved just
to the east of the Big Island and some elevated showers will
continue to drift over the eastern and southeastern sections of
the island. At this point we see a divergence of model solutions
between the American (GFS) and the European (ECMWF) models. The
latest evening model run from the ECMWF are showing a drier
solution with the surface trough moving further eastward away from
the Big Island. The GFS model on the other hand shows a wetter
solution for the state as the surface trough will linger over the
eastern side of the Big Island and bands of showers will expand
westward increasing showers mainly over windward and mountain
areas into the upcoming weekend. The forecast grids will blend the
differences between both models to reduce errors between the two
solutions. Mostly fair weather conditions are forecast over
leeward areas during this time period.

&&

.AVIATION...
Abundant mid and high level moisture continues to stream across
the eastern islands from the tropical south, with the base of
this layer at around 12k feet, extending to fl250. There was
a light rime icing report, occurring at fl220. Conditions will
become more ripe for more of these report as the night progresses
and through Tuesday. Further more, there has been brief, isolated
instances of CB/thunderstorms south of the Big Island in the past
couple of hours. The air mass will become more supportive for
additional CB/thunderstorms later tonight through Tuesday for not
only the Big Island, but for Maui County as well. For tonight
though, we are confining the CB's and thunderstorms for eastern
Big Island. AIRMET Sierra, for mountain obscuration is in place
for the eastern part of the Big Island, from Laupahoehoe to
Mountain View to South Cape, and this AIRMET is likely to
continue through at least Tuesday.

This heavy weather is not expect to advance over to Oahu and
Kauai. So far, this area west of Molokai looks fine. There are
a couple of pockets of showers floating around, mainly over the
waters. No AIRMETs are expected for this area through tonight.

&&

.MARINE...
A trough will begin to move into the waters around the Big Island
tonight and Tuesday, then linger in this vicinity through late
this week. Light to locally moderate east to southeast winds will
continue through Tuesday, before becoming light and variable
Tuesday night through Wednesday night. The one exception, is for
the coastal waters east of the Big Island, where moderate to
strong south to southeast winds could develop Tuesday through
Tuesday night. Weak high pressure is expected to develop north of
the state Thursday through the weekend and we should see a return
of light to locally moderate northeast winds.

As the current north-northeast swell declines tonight along north
and east exposures, a new short-period east swell will begin to
build along east facing shores of the Big Island on Tuesday, and
late Tuesday into Tuesday night across the smaller islands. This
new swell is expected to peak on Wednesday and gradually decline
Thursday through rest of the week. Due to the new incoming east
swell, the High Surf Advisory (HSA) for east facing shores was
extended through Wednesday night. The HSA for north facing shores
of Kauai and Niihau has also been extended through tonight.

A Small Craft Advisory (SCA) is in effect through tonight for
most windward marine zones and channels due to seas at or above
the 10 foot SCA threshold. Although many marine zones will drop
below SCA thresholds by Tuesday, a new easterly swell may keep
SCA level seas in place for some eastern and windward waters
Tuesday through Wednesday night.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Tuesday for Niihau.

High Surf Advisory until 6 AM HST Thursday for Kauai Windward-
Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

Flash Flood Watch through Tuesday afternoon for Molokai, Lanai,
Kahoolawe, Maui and the Big Island.

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

Winter Storm Warning until 6 AM HST Wednesday for Big Island
Summits.

Small Craft Advisory until 6 AM HST Tuesday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters-
Alenuihaha Channel-Big Island Windward Waters.

&&

$$

DISCUSSION...Bohlin
AVIATION...Lau
MARINE...Jelsema

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office



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