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Hawaiian Islands Synoptic Discussion and Guidance

806
FXHW60 PHFO 180701
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
901 PM HST Sun Feb 17 2019

.SYNOPSIS...
Northerly winds and showers will diminish over the western islands
tonight as the low pressure system that brought days of cold and
unsettled weather to the state moves west on Monday and then
northward away from the island chain on Tuesday. As the upper low
moves northward, a deep tropical moisture plume drifts into
Hawaii and Maui Counties from the southeast. Expect frequent
showers, heavy at times, and scattered thunderstorms for the
eastern islands from Monday night through Tuesday. Decreasing
shower trends are forecast for most of the eastern island from
Thursday through Sunday.

&&

.DISCUSSION...
The upper level low that has brought cold wintery showers to the
state over the past week will finally drift west on Monday and
then northward away from the islands on Tuesday. As this cold
core low drifts northward a deep tropical moisture plume will move
into the eastern islands from the southeast. Wet and unsettled
weather returns to the eastern half of the state from Monday night
through Tuesday with less shower activity forecast for the same
time period over the western islands. Temperatures will warm by a
few degrees and surface wind speeds will weaken becoming more
variable with land and sea breezes through Wednesday.

For the eastern half of the state, drier conditions are forecast
to start the week on Monday with extremely wet weather returning
for both Maui and Hawaii Counties starting late Monday night. This
wet weather for the eastern islands will arrive in the form of a
deep tropical moisture plume advecting into the islands from the
southeast. Expect frequent showers, locally heavy at times, with
scattered thunderstorms developing under this band impacting both
Hawaii and Maui Counties from Monday night through Tuesday. A
Flash Flood Watch for both counties was issued for the Monday
night through Tuesday night time period. Temperatures aloft will
be cold enough for snowfall on the highest summits of Maunakea and
Maunaloa on the Big Island, and a Winter Storm Watch was issued
for this same period. Temperatures aloft will be warmer than the
last event with higher snow levels in the 11,000 to 12,000 foot
range above the slopes of Haleakala on Maui. A Wind Advisory also
remains in effect for both Big Island summits due to increasing
upper level wind speeds starting later tonight. Shower activity
will decrease a bit over both Maui and Hawaii Counties from
Wednesday into Thursday as the tropical moisture band weakens.
Wind directions will shift towards a light trade wind regime and
showers will tend to favor windward and mountain areas from
Thursday through the upcoming weekend.

For the western islands of Kauai County and Oahu, northerly winds
with cold wintery showers will diminish later tonight as the low
begins to drift northward away from the state. Drier conditions with
light and variable surface winds will develop across the state
from Monday through Wednesday initiating offshore land breezes at
night and onshore sea breezes each day. Isolated to scattered
showers are possible with interior clouds developing over the
islands each afternoon. A west to east moving upper trough may
enhance shower activity on Thursday with some of these showers
moving into leeward areas. Precipitation chances may need to go
higher on Thursday if this upper trough persists in the next
series of model runs. Variable surface winds will become light
trades from Thursday onward under the influence a weak high
pressure ridge north of the state. As the trade winds return
expect scattered showers to favor the typical windward and
mountain areas from Thursday through Sunday.

Global models are in fair agreement with this weather pattern
through the weekend. Slight differences remain from Wednesday
onward in how the tropical moisture band weakens across the state.
The American (GFS) model spreads shallow showers across the state,
while the European (ECMWF) model shows a deeper moisture band
holding together and moving eastward with continued elevated
showers developing along the eastern side of the Big Island and
less showers forming over the smaller islands to the west. The
current forecast grids split the difference between these solutions
by bringing scattered showers to all windward and mountain areas
with slightly higher shower activity over the eastern Big Island.

&&

.AVIATION...
A surface trough near Maui will be moving westward through Monday.
This will change the wind pattern, from a northerly direction
currently over Oahu and Kauai, to a southerly direction after 18z
Monday. The trough will also be ushering in a different airmass,
from a cool and shallow one, to a unstable, deep warm and humid
tropical one. We expect deteriorating conditions over the Big
Island Monday afternoon and spread progressive westward Monday
night through Tuesday to the remaining islands. This area of
weather is currently between 160 and 530 nm east of the Big
Island.

In the mean time, the showery weather will gradually cease over
the smaller islands as the night progresses. So, as for AIRMET
Sierra for the mountain obscuration of the smaller islands, we
anticipate Sierra to continue through 10z but be lifted by 16z.

No other AIRMETs expected.

&&

.MARINE...
An area of low pressure north of the state will track westward
over the next couple of days, dragging a weak surface trough
westward through the marine area. Fresh to strong northerly winds
will continue across the waters around Kauai tonight, with light
to moderate northerly winds holding in place over the waters
around Oahu, and light and variable winds elsewhere. Light and
variable winds will overspread the entire marine area on Monday.
A new trough will then move into the waters around the Big Island
Monday night and Tuesday, then linger in this vicinity through
late next week. Winds may increase to moderate and strong levels
along and to the east of this trough early in the week, otherwise
generally light winds are expected across the marine area through
late next week.

The current north-northeast swell will begin to lower and become
more northerly tonight through the first half of the week as the
low north of the area shifts westward. A High Surf Warning (HSW)
remains in effect for east facing shores tonight, and surf should
drop to advisory levels here by Monday. A High Surf Advisory (HSA)
remains in effect for north facing shores through tonight, and
surf should lower below advisory thresholds here overnight. The
one exception may be Kauai where advisory level surf may hold in
place through the day on Monday.

Surf is expected to drop below advisory levels along all
shorelines Monday night. A new easterly swell will build Monday
night and Tuesday as southeasterly winds increase to the east of
the island chain. This swell may bring advisory level surf to east
facing shores Tuesday through Thursday. No significant northwest
swells are expected this week, and surf along south facing shores
is expected to remain small.

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

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Monday for Niihau-Oahu North
Shore-Molokai Leeward-Maui Central Valley.

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

Flash Flood Watch from Monday evening through Tuesday afternoon
for Molokai-Lanai-Kahoolawe-Maui-Kona-South Big Island-Big
Island North and East-Kohala-Big Island Interior.

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

Winter Storm Watch from Monday evening through late Tuesday
night for Big Island Summits.

Small Craft Advisory until 6 PM HST Monday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui County
Windward Waters-Pailolo Channel-Alenuihaha Channel-Big Island
Windward Waters-Big Island Southeast Waters.

&&

$$

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

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



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