Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 220209

Area Forecast Discussion
National Weather Service Honolulu HI
409 PM HST Wed Feb 21 2018

Wet weather will spread across the islands overnight and Thursday,
and last into the weekend. Abundant moisture from the south will
combine with cold upper level temperatures to provide an unstable
airmass that could lead to heavy showers and some thunderstorms
through the end of the week. Some improvement is expected early
next week.


As discussed earlier, the Flash Flood Watch was expanded to the
smaller islands, starting tomorrow morning. While there could be
some shower activity overnight over the smaller islands, the main
threat for flooding will come tomorrow as the colder air moves
overhead. Rain has been falling over the Big Island this afternoon
and don`t expect much dry weather over the next couple of days.
There have been a few thunderstorms outside the coastal waters
this afternoon, but so far they have held off over the immediate
area. Anticipate that to change as we head into the overnight
hours for the Big Island, and tomorrow for the smaller islands.

Winds on the Big Island Summits declined this afternoon and are
below advisory levels. The High Wind Warning was replaced with a
Wind Advisory at noon, and that advisory was cancelled with the
afternoon package. Winds are expected to remain below advisory
levels on the summits through the weekend.

The abundant, deep level moisture is reaching above the summits,
and with freezing temperatures already in place, winter weather
can be expected. The Winter Storm Watch was replaced with a Winter
Weather Advisory with this package.

Several modifications were made to the grids today, but the
general forecast remains in place for the next 3 days. The upper
trough to the west of the islands will move closer to the islands,
and an upper level low forms along the trough. This will drive
colder upper level temperatures over the islands from the
northeast. The coldest temperatures remain over the Kauai through
the period. The moisture source south of the islands will be drawn
northward over the islands, with the highest precipitable water
values remaining over the Big Island.

A number of factors are a concern with this upcoming event. Near
the Big Island, the pool of above normal PW combined with near
normal, upper level temperatures can still produce thunderstorms.
Near Kauai, the colder upper level temperatures can help to make
up for the PW not being quite as high. There will be enough shear
in the environment to support some thunderstorms having gusty
winds, but the placement of all pieces is still to be determined.

Although we are expecting rainfall to increase across all
islands, the biggest threat tonight will be over the Big Island.
As the ingredients spread to the smaller islands, Kauai will be a
big concern with their proximity to the upper level disturbance.
Recent rains over Oahu, Maui County and the Big Island will play a
larger factor for flooding threats in those areas.

The other concern remains the possibility of southeast flow that
could lead to training of showers, particularly along the eastern
side of Oahu, an area that was hit particularly hard during the
rain event last weekend. The direction of the wind flow and the
length of time that flow sets up will play a large part in any
training of showers over that area starting Thursday.

As we head into the new work week, we see some model differences
that could make a difference in what happens. The GFS keeps
surface troughs lingering just west of the islands, with a
surface high far northeast of the islands. The ECMWF has the
troughs a little further west, allowing for trade winds to start
to build in. In either case, the upper level disturbance should
have lifted out, which will help to stabilize the environment near
the islands, so we should at least see fewer showers.


MVFR conditions have developed across S and SE facing slopes of
the Big Island this afternoon and are expected to slowly spread up
the island chain tonight and Thursday as the atmosphere becomes
increasingly moist and unstable. AIRMET Sierra for mountain
obscuration has been issued for portions of the Big Island, as
well as AIRMET Zulu for light icing. Expect these AIRMETs to
remain in place through tonight and likely be expanded to
additional locations on Thursday, along with a threat of

At this time, VFR conditions dominate over most areas from Maui
to Kauai, though border line MVFR conditions are occurring on
portions of windward Maui and Oahu. Expect some deterioration
along mainly windward slopes tonight, then more widespread MVFR
conditions over all areas on Thursday.


The High Surf Advisory for the north and west facing shores of
Kauai has been cancelled. The swell will continue into tomorrow,
but surf heights should remain below the advisory level. After
this swell fades Friday into the weekend, long term guidance does
not favor another WNW/NW swell at least through next week.

Persistent high pressure NE of the state will continue to produce
an area of strong winds directed towards the area. We should
start to see an uptick in ESE/SE winds around the state starting
Thursday, and remain elevated at least through midweek next week.
The swell generated from the broad area of strong winds will
produce elevated surf along E facing shores. A High Surf Advisory
likely be needed by Thursday night or Friday and could
potentially reach warning levels by midweek next week. The
combination of the elevated seas and increased winds, starting
tomorrow, will produce Small Craft Advisory conditions, mainly for
the channels and exposed windward waters.

Small long period S swells could produce a small bump in surf
along S facing shores this weekend.

See the latest Oahu Surf Discussion (SRDHFO) for additional
details on surf and swell.

Moisture spreading up from the south and an approaching upper
level disturbance will lead to areas of heavy rains and
thunderstorms. Most significant weather is expected around the Big
Island tonight and should spread north through Friday night.


Flash Flood Watch from Thursday morning through Friday afternoon
for Niihau-Kauai-Oahu-Molokai-Lanai-Kahoolawe-Maui-Big Island

Flash Flood Watch through Friday afternoon for Kona-South Big
Island-Big Island North and East-Kohala-Big Island Interior.

Winter Weather Advisory until 6 PM HST Friday for Big Island



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