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Hawaiian Islands Synoptic Discussion and Guidance

244
FXHW60 PHFO 200335
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
535 PM HST Tue Feb 19 2019

.UPDATE...
Radar and satellite trends indicate that the Flash Flood Watch for
Maui county is no longer necessary, and it has been cancelled.

&&

.SYNOPSIS...
Widespread light to moderate rain will affect the Big Island into
Wednesday, with a chance of thunderstorms and heavy rainfall due to
a nearby surface trough. The trough will linger near the Big Island
into Friday, acting as a continued focus for cloud and shower
development. A mostly dry and quiet weather pattern now over Kauai
and Oahu will spread to Maui county overnight, with light winds and
limited rainfall expected for the next couple of days. Light to
moderate trade winds will prevail statewide by the weekend, with
brief windward showers favoring nights and mornings.

&&

.DISCUSSION...
A complex weather pattern remains in place over the islands this
afternoon, with a weakening mid- and upper-level trough over the
islands fueling widespread light to moderate rain over the area from
Molokai to the Big Island. Strong to severe thunderstorms have been
noted over Big Island SE waters this afternoon, and over offshore
waters to the E and SE, but little in the way of thunderstorms have
been noted elsewhere.

A weak 1012 mb surface low is centered about 450 miles NW of Kauai,
a very strong 1043 mb high is centered far NNE of the islands, while
a developing surface trough is analyzed near and NE of the Big
Island. The swath of mid- and high-level clouds moving over the area
from the S is reducing confidence as to the location and strength of
the surface trough.

The forecast calls for the mid- and upper-level trough to move E and
NE across the islands as the parent low lifts N over the next 12-24
hours. Rain chances over the area from Kauai to Oahu have decreased
dramatically as the trough axis has moved E, and a similar
drying/clearing trend is expected over Maui county this evening. The
ongoing Flash Flood Watch (FFA) for Maui county will expire this
evening, but has been extended overnight for the Big Island as the
trough aloft will combine with a nearby surface trough to bring the
threat of heavy rainfall on already wet grounds. Additionally,
temperatures on the summits of Mauna Kea and Mauna Loa are hovering
near the freezing mark, with a mix of snow and rain (and potentially
ice) prompting a Winter Storm Warning through tonight. The potential
for winter weather diminishes significantly tomorrow as temperatures
at summit levels warm. The upper trough is also bringing strong
winds to the summits that should also diminish Wednesday, and the
current Wind Advisory should be allowed to expire.

After light to moderate rain diminishes over Maui county this
evening, light winds and isolated showers are expected to prevail
from Kauai to Maui into Thursday. Meanwhile locally heavy showers
will continue near and E of the Big Island as the surface trough
lingers. Late in the week, a weak high pressure cell passing N of
the islands will bring light NE trade winds to most of the island
chain, with these trade winds expected to deliver brief windward
showers that will likely favor nights and mornings. The surface
trough is expected to linger near the Big Island, continuing to act
as a focus for cloud and shower development until it finally moves E
over the weekend.

Long range outlook features NE trade winds that will bring windward
showers, but high-impact weather is not expected at this time.

&&

.AVIATION...
An upper level trough axis just north of Kauai will move away from
the area tonight. Meanwhile, a north to south surface trough axis is
stretched over the Big Island. These features will bring widespread
layered clouds and showers, mainly over Maui and the Big Island
tonight. Isolated thunderstorms will also be possible over the Big
Island tonight.

AIRMET Sierra is in effect for tempo IFR conditions in the showers
across Maui and eastern sections of the Big Island. AIRMET Sierra
for tempo IFR conditions or mountain obscurations will likely
continue into tonight for Maui and the Big Island.

AIRMET Tango is in effect for moderate turbulence between FL250 and
FL350 across the entire state. Turbulence will likely decrease as
the trough lifts away later tonight.

Light icing is also expected across the eastern half of the state
due to the layered clouds and showers associated with the upper
trough.

&&

.MARINE...
A surface trough will continue to develop near the Big Island
tonight. Strong thunderstorms with gusty and erratic winds can be
expected near the trough across Big Island waters tonight. Outside
of thunderstorms, fresh southeasterly winds may briefly develop to
the east of the Big Island, while gentle and variable winds are
expected over the rest of the island chain.

The surface trough will meander around the Big Island from Wednesday
into the weekend. Variable winds will prevail to the west of the
trough, with gentle to locally moderate northeasterly winds
potentially developing late Wednesday or Thursday. To the east of
the trough, moderate southeasterly winds are expected, with gale
force winds over the offshore waters to the north of the Big Island
into Wednesday.

During the next 24 hours, small moderate northwest and north swells
will fade as a short-period east swell builds. A High Surf Advisory
remains in effect for east facing shores of the Big Island and Maui,
and was just expanded westward for Molokai, Oahu, and Kauai. This
east swell is being generated by a fetch of east to southeast winds
produced by a strong 1043 mb high sitting 1500 miles north-northeast
of the state. These winds have also been enhanced just east of the
state by the surface trough developing near the Big Island. Winds in
the fetch aimed at Hawaii should slowly ease beginning tonight,
leading to a peak in the swell locally on Wednesday or early
Thursday. East shore surf will decline late Thursday and Friday. No
significant northwest swells are due through the weekend, though an
increase in northwest swell is likely heading into next week.

A Small Craft Advisory (SCA) is in effect for waters around the Big
Island and Maui exposed to the building, short-period east swell,
which should cause combined seas to reach 10 ft. The SCA may nee to
be expanded westward to windward Oahu and Kauai waters as the east
swell peaks on Wednesday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Thursday for East facing shores
of Kauai Oahu Molokai Maui and the Big Island.

Flash Flood Watch through late tonight for 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 Thursday for Maui County
Windward Waters-Alenuihaha Channel-Big Island Windward Waters-
Big Island Southeast Waters.

&&

$$

DISCUSSION...Birchard
AVIATION...Foster
MARINE...Wroe

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



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