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RH95 %
WindSW 43 mph
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Hawaiian Islands Synoptic Discussion and Guidance

748
FXHW60 PHFO 200138
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
338 PM HST Mon Feb 19 2018

.SYNOPSIS...
A gradual improvement in our weather pattern is forecast over the
next few days as an area of low pressure moves away from the area.
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. Daytime
heating has caused for some clouds and showers along with a few
thunderstorms to develop, mainly over The Big Island. These
weather conditions are expected through the evening hours with
showers and thunderstorms activity diminishing through overnight.

Both the upper low and low level trough will move away to the
north northwest, as high pressure builds 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 and weaken 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 has kept the atmosphere unstable, so thunderstorms
remain possible during the evening over the central and E section
of the Big Island. AIRMET SIERRA is in effect for MTN OBSC on the
Big Island through early evening. Land breezes are expected to
clear away the afternoon convection with VFR prevailing tonight.

&&

.MARINE...
Scattered showers and isolated thunderstorms are still possible
over the Hawaiian coastal waters this evening. 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 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 later tonight and Tuesday, with resulting surf likely
reaching the advisory threshold of 8 feet along most east facing
shores Tuesday. A High Surf Advisory has been issued for these
areas and will go into effect Tuesday morning. 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...
High Surf Advisory from 6 AM Tuesday to 6 AM HST Wednesday for
Kauai Windward-Oahu Koolau-Olomana-Molokai Windward-Maui
Windward West-Windward Haleakala-Big Island North and East.

Winter Weather Advisory until 6 AM HST Tuesday for Big Island
Summits.

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

&&

$$

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

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