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Hawaiian Islands Synoptic Discussion and Guidance

272
FXHW60 PHFO 181439
AFDHFO

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

.SYNOPSIS...
A few showers may develop over the western half of the state today
as surface winds decrease and become more light and variable. An
upper low north of the state drifts away from the islands, and a deep
tropical moisture plume drifts into Hawaii and Maui Counties from the
southeast today. This tropical moisture band will produce frequent
showers, heavy at times, and scattered thunderstorms across the eastern
islands tonight through Tuesday. Decreasing shower trends are forecast
for most of the island chain from Thursday through Sunday.

&&

.DISCUSSION...
Satellite imagery this morning shows a deep plume of tropical
moisture with convective showers and thunderstorms just southeast
of the Big Island moving northwestward. Scattered showers are also
developing just south of Kauai and Oahu this morning.

The upper level low will finally drift west today and then northward
away from the islands on Tuesday. As this cold core low drifts away
from the islands a deep tropical moisture plume will move into the
eastern half of the state from the southeast. Wet and unsettled weather
spreads across the eastern half of the state later today through Tuesday
with only scattered shower activity forecast for the same time period
over the western islands. Temperatures will warm by a few degrees and
surface wind speeds weaken becoming more variable with land and sea
breezes forecast through Wednesday.

For the eastern half of the state, isolated showers during the early
morning hours will rapidly transition into numerous showers by afternoon
with extremely wet weather and heavy showers forecast over both Maui
and Hawaii Counties 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 for both
counties was issued from tonight through Tuesday night. Temperatures
aloft will be cold enough for heavy snow showers on the highest summits
of Maunakea and Maunaloa on the Big Island, and a Winter Storm Watch
was issued for these summits for the same time period. Temperatures
aloft will be warmer than the last event with snow levels at or above
11,000 feet elevation. A Wind Advisory also remains in effect for both
Big Island summits due to increasing upper level wind speeds. 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 on through
the upcoming weekend.

For the western islands of Kauai County and Oahu, isolated to scattered
showers are possible with light and variable surface winds developing
across the state from Monday through Wednesday initiating offshore land
breezes at night and onshore sea breezes each day. 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 isolated to 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 over time. 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 morning forecast grids continue to blend differences
between these solutions, in order to minimize model errors, by bringing
scattered showers to all windward and mountain areas while keeping
slightly higher showers over the eastern Big Island.

&&

.AVIATION...
The cool but damp northerly wind flow over the western part of
the island chain, continues to be nudged westward by a surface
trough located in Maui County. East of the trough is a warmer,
but juicier and unstable air mass. All islands should be under
this air mass by this evening. With the lighter winds in place by
this afternoon, isolated showers are expected over the interior
and lee areas, while a background east to southeast flow carries
in a few showers into the windward areas. In essence, isolated
showers for all parts of the smaller islands this afternoon. But
models are pointing to an area of layered clouds with embedded
convection, currently located between 90 nm and 600 nm east of
the Big Island, for this area to be the source and the beginning
of a period of unsettled, heavy rain, event. Models have this
area of deep tropical moisture reaching the Big Island this
afternoon, and continuing into Maui County later tonight through
Tuesday.

Currently, the low clouds and associated showers continues to
thin and dissipate across the windward and mountain areas of the
smaller islands. Thus, AIRMET Sierra, for mountain obscuration,
for the smaller islands, will likely be lowered for the 1600z
package.

AIRMET Sierra, is likely for the east facing slopes of the Big
Island as early as this afternoon, lasting through at least
tonight. This moisture will be enhanced by an strong upper level
trough situated west of Kauai, leading to the possibility of
thunderstorms as well. There are some signs this upper trough is
becoming active, with the flare up of CB's and TCU's some 200 nm
south-southwest of Kauai. Some of this debris, in the form of
layered clouds, will reach Kauai later this morning.

&&

.MARINE...
An area of low pressure north of the state will track westward
over the next couple of days well to the north of the coastal
waters. Light and variable winds will overspread much of the
marine area 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 next 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. Surf has dropped below
warning levels along east facing shores this morning, so the High
Surf Warning (HSW) has been cancelled here, and transitioned over
to a High Surf Advisory (HSA). This HSA for east facing shores is
now in effect through 6 PM this evening, but may need to be
extended into tonight for some areas, particularly over the
western end of the state. In addition, due to the swell remaining
a bit higher over Kauai and Niihau, the HSA for north facing
shores has been extended through 6 PM this evening. Elsewhere
across the state, the HSA for north facing shores has been
cancelled.

Surf is expected to drop below advisory levels along all
shorelines by daybreak Tuesday. A new easterly swell will build
from east to west across the island chain tonight through Tuesday
night, 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 night
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 this evening 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 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...Bohlin
AVIATION...Lau
MARINE...Jelsema

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



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