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Hawaiian Islands Synoptic Discussion and Guidance

229
FXHW60 PHFO 192004
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1004 AM HST Mon Feb 19 2018

.SYNOPSIS...
The potential for locally heavy showers and possible thunderstorms
remains in place today but is expected to be much less widespread
than on Sunday. A gradual improvement in our weather pattern is
forecast over the next few days. Light east to southeast winds
with local land and sea breezes can be expected with just a few
showers expected mainly during the afternoon hours. A return of
unsettled weather is possible later in the week and on into the
weekend as an upper low affects our weather pattern once again.

&&

.DISCUSSION...
An upper low along with a low level trough lie in the island
vicinity resulting in an unstable airmass across the area. The
lower levels of the atmosphere are beginning to dry out as the
bulk of the moisture that was over us yesterday has moved off to
the north and northeast of the state. However, the unstable
airmass caused by the upper low along with daytime heating will
likely allow for some cloud and shower development over the area
this afternoon. Some thunderstorms are also possible though they
are not expected to be a widespread as on Sunday. A Flash Flood
Watch remains posted as any heavy rains that do fall could lead to
flash flooding due to the saturated soil conditions in place.

As both the upper low and low level trough move away to the north
northwest, high pressure will be building in far to the north
northeast of the state. This will place the islands in a rather
light east to southeast wind flow. Daytime sea breezes and
nighttime land breezes can be expected over some areas under this
light wind regime. With a drier and more stable airmass in place,
clouds and some showers will likely favor interior areas during
the afternoon and evening hours both on Tuesday and Wednesday.

As we head into the second half of the week, a potential wetter
weather pattern is beginning to shape up. A strong upper low is
forecast to take up a position several hundred miles to the
northwest of the area. There are some slight differences between
the GFS and the ECMWF on the movement and position of the upper
low. The GFS has the low positioned a bit further to the north and
west than the ECMWF. However both models show a weak low level
trough in the island vicinity along with increasing moisture
moving up from the south over the area. The result will likely be
an increase in shower activity across much of the area. The low
level wind flow will continue to favor and east southeast
direction on through the weekend. Additional fine tuning of the
forecast for this period will be likely over the next couple of
days as forecast confidence rises.

&&

.AVIATION...
A NW to SE surface trough the Alenuihaha channel will remain
nearly stationary as a N to S trough aloft lifts NE.

The trough aloft will make turbulence likely. AIRMET TANGO remains
in effect for turbulence aloft.

The trough aloft will keep the atmosphere unstable today, so
thunderstorms remain possible. As the islands heat up, clouds and
showers will form over the islands with MVFR conditions likely
and isolated IFR conditions possible this afternoon and evening.
AIRMET SIERRA for MTN OBSC may need to be issued this afternoon.

After daytime heating ends and the trough aloft moves farther NE the
chance of thunderstorms will decrease late tonight.

&&

.MARINE...
Scattered showers and isolated thunderstorms will continue over
the Hawaiian coastal waters today. Reduced visibilities, gusty
and shifting winds, frequent lightning, and possibly small hail
and waterspouts can be expected in the stronger thunderstorms.
Thunderstorm activity is forecast to shift northeast of the area
tonight and Tuesday, with drier weather and only scattered lighter
showers through Wednesday.

Small to moderate northwest swells will continue through early
Tuesday, with surf remaining below advisory level along north and
west facing shores. However, a longer period northwest swell is
expected to spread down the island chain beginning Tuesday night
and peaking Wednesday. This may result in advisory level surf along
some north and west facing shores, especially Kauai and Niihau,
during the peak of the event. This swell will gradually subside
Thursday, but another west-northwest swell is expected Thursday
night into Friday, before fading into the weekend.

A long fetch due to the flow around a surface high over the
northeastern Pacific will send a short-period east swell into the
local waters during the next several days. This swell is expected to
increase tonight and Tuesday, with resulting surf likely reaching
the advisory threshold of 8 feet along most east facing shores
Tuesday. A further increase in this swell is expected Thursday,
remaining elevated into the weekend, with high surf likely
continuing along east facing shores through that time.

Combined seas may increase to around 10 feet over most windward
waters starting late Wednesday due to the combination of west-
northwest and east swells, and local wind waves. A Small Craft
Advisory may be needed for the latter portion of the work week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through this afternoon for Niihau-Kauai-Oahu-
Molokai-Lanai-Kahoolawe-Maui-Kona-South Big Island-Big Island
North and East-Kohala-Big Island Interior.

Winter Weather Advisory until 6 PM HST this evening for Big
Island Summits.

Wind Advisory until 6 PM HST this evening for Big Island Summits.

&&

$$

Synopsis/Discussion...Burke
Aviation...Donaldson
Marine...TS

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