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Hawaiian Islands Synoptic Discussion and Guidance

234
FXHW60 PHFO 181956
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
956 AM HST Mon Feb 18 2019

.SYNOPSIS...
Generally fair weather can be expected across most areas today
with just a few showers expected across the area. A wet weather
pattern is setting up over Maui County and the Big Island over the
next couple of days as tropical moisture moves up from the
southeast. An increase in showers, some heavy at times along
scattered thunderstorms is expected over the eastern islands
tonight through Tuesday. Showers will be less likely and lighter
over the west end of the state. Decreasing shower trends are
forecast for most of the island chain late in the week and on into
the weekend.

&&

.DISCUSSION...
A broad upper level low is centered just to the northwest of Kauai
with an associated surface low centered a few hundred miles north
northwest of Kauai. Bands of showers associated with a weak low
level troughs are seen on satellite imagery in the island vicinity.
One band is generally centered to the south and southwest of the
western islands. This has produced a few light showers across the
area this morning with most of the showers associated with it
remaining over water. A broader area of showers is currently
located east and southeast of the Big Island. This area of enhanced
moisture will be the main weather player for the next couple of
days.

For the eastern half of the state, shower activity is expected to
increase today with wet weather expected over Maui County and the
Big Island starting tonight. This wet weather pattern developing
over the eastern islands will arrive in the form of a deep
tropical moisture plume moving in from the southeast. Expect
frequent showers, locally heavy at times, with scattered
thunderstorms developing under this band from tonight through
Tuesday. A Flash Flood Watch is in effect from tonight through
Tuesday night. Temperatures aloft will be cold enough for heavy
snow showers on the highest summits of Mauna Kea and Mauna Loa
where a Winter Storm Warning is in effect beginning tonight. A
Wind Advisory is also in effect for the Big Island summits.

For the western end of the state, showers are possible under a
light and variable wind regime through Tuesday night. Showers
would tend to favor interior and mauka areas but other areas could
see some shower activity as well. Most of these showers are
expected to be on the lighter side.

On Wednesday the upper low will weaken and lift out to the
northeast and be replaced by more of a zonal flow across the area.
The low level trough however is expected to linger near or just
east of the Big Island.

Wednesday night through Friday, some minor differences show up in
model solutions. Both models show weak high pressure building in
to our north and northwest. This will allow light northeast trades
to fill back in from Maui County westward. However the position
of the low level trough is closer to the Big Island than the ECMWF
which shows the trough axis just east of the Big Island. Thus the
GFS solution keep abundant moisture over the Big Island while the
ECMWF is a tad drier over them with the higher moisture being to
the east.

For next weekend, both models show a slighter stronger high
building in northwest of the area with a slight uptick in trade
wind speeds expected especially over the west end of the state.
The GFS remains the wetter solution with leftover moisture from
the low level trough getting picked up in the trades providing for
increased windward and mauka shower activity. The ECMWF remains
the drier solution. Thus a typical trade wind shower regime is
currently forecast with details on shower frequency still in question.

&&

.AVIATION...
The cool damp northerly flow over Kauai and Oahu will give way to
light and variable winds today but remain mostly stable
throughout the coming night. To the east, the airmass will become
increasingly moist and unstable. A band of layered clouds with
embedded deep convection is expected to approach the Big Island
from the southeast late this afternoon and spread across Maui
County by mid morning Tuesday. Lightning, heavy downpours, and IFR
conditions are associated with this new approaching feature.

An AIRMET for mountain obscuration will likely become necessary
for eastern slopes of the Big Island late this afternoon or early
evening and the whole of the Big Island and Eastern Maui by
morning.

In the meantime, A band of mid to higher clouds, associated with
an upper level trough, is approaching Kauai from the south.
Increasing clouds and scattered of showers are expected across
Kauai and Niihau.

Elsewhere, A weak sea breeze regime is expected to establish
itself across the remaining islands today. Expect clouds and
showers to favor island interiors late this afternoon and early
evening then east and southeast slopes and coasts by late evening
and throughout the night.

On Tuesday, deep tropical moisture should encompass the whole of
the state. However, the most vigorous convective activity will
remain focused over the Big Island and Maui.

&&

.MARINE...
An area of low pressure well north of the coastal waters will
track westward over the next couple of days. Light and variable
winds will overspread much of the coastal waters this morning,
with light to moderate east to southeast winds moving over the
eastern waters this afternoon. A new trough will then move into
the waters around the Big Island tonight and Tuesday, then linger
in this vicinity through late this week. Winds may increase to
moderate and strong levels along and to the east of this trough
early this week, otherwise generally light winds are expected
across the marine area through most of the week.

The current north-northeast swell will continue to lower and
become more northerly through the first half of the week as the
low north of the area shifts westward. In addition, the current
west-northwest swell should also gradually decline through mid
week. A High Surf Advisory (HSA) is in effect for east facing
shores through this afternoon, but will likely need to be
extended at least through tonight and possibly through Thursday as
another reinforcing east swell begins to build on Tuesday. A HSA
remains in effect for north facing shores of Kauai through this
evening and will likely be expired.

A Small Craft Advisory (SCA) is in effect through tonight for
most Hawaiian Waters, due to seas at or above the 10 foot SCA
threshold. Although many marine zones will drop below SCA
thresholds by daybreak 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 PM HST this evening for Niihau-Kauai
Windward-Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-
Windward Haleakala-South Big Island-Big Island North and East.

Flash Flood Watch from 6 PM HST this 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 Warning from 6 PM this evening to 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-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Pailolo Channel-Alenuihaha Channel-Big
Island Windward Waters-Big Island Southeast Waters.

&&

$$

DISCUSSION...Burke
AVIATION...Bedal
MARINE...Kino

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



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