Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 220648
AFDHFO

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

.SYNOPSIS...
Another round of unsettled weather is upon the islands once more.
The wet weather will increase over the area especially after
midnight and through Thursday. Unsettled conditions, with heavy
rains and thunderstorms, will persist through Friday and likely
into the weekend. Possibly some improvement is expected early
next week.

&&

.DISCUSSION...
A Flash Flood Watch now encompass all of the major Hawaiian
Islands. Also, a Winter Weather Advisory is in effect for the Big
Island summits. These weather bulletins are good until late Friday
afternoon, with the possibility of an extension.

The shower activity this afternoon has been spotty. It tried to
get going along the windward side of the Koolau`s on Oahu, over
various parts of Maui and the Big Island. Interior Kauai had a
brief heavy shower earlier in the afternoon. There are a couple
of showers spread out across Molokai now that bears watching.
Otherwise, it has been an o.k. kind of a day. Dense layered high
clouds continue to harass the Big Island summits, but the winds
have completely shut down there which is near calm this hour.

As noted earlier, shower activity is slated to increase toward day
break Thursday. This comes as the upper level low settles in
around 26n170w or 750 miles west-northwest of Kauai. This low is
expected by both the old ECMWF and the new GFS runs, to linger in
that area through Friday. The low is in a ripe location to pick
up deep tropical moisture aloft and carry it to and over the
islands. At the same time, the low level wind flow will be getting
more moist in the next 12 to 24 hours across the islands, and
this will add to the higher level moisture. The current surface
wind is light easterly, and low level moisture appears to be
limited.

The cold air associated with this upper low is not as
cold as the previous upper low from a couple of days ago, where
temps at 500 MB got to minus 17/18 degrees C. We are looking a
modest minus 12/13 C with this low, favoring the area west of
Maui. So, the cold temp aspect is not to bad, but there is still
other dynamics present such as shear that will support a few
thunderstorms with gusty winds. These storms are expected to be
spotty and can occur anywhere around the islands.

It is difficult to pinpoint areas of concern with this event.
Certainly the Big Island and the windward side of Oahu are top of
the list for especially this evening, due to the recent heavy
rains. From Thursday on, the GFS appears to be lock on Oahu and
Kauai.

Back to the surface winds, the E to SE winds will be gaining
strength in the next 24 hours where the latest GFS hints at Small
Craft Advisory (SCA) level winds of 25 kt. Most favorable area
will be the exposed windward waters mainly east of Oahu and
waters south of the Big Island. We will be reassessing this notion
overnight. Also under this SE wind flow, there will be some
blocking by the islands resulting in lighter winds across the
smaller islands.

Looking further down, the upper low opens up to a deep trough and
nudges a bit further away from the main Hawaiian Islands on Sunday.
The latest GFS shows moderate to locally strong trades building
back into the area that will hopefully bring some improvements as
early as Sunday.

&&

.AVIATION...
A low aloft about 650nm NW of Kauai is forecast to move SE
slowly. SW flow aloft is carrying middle and high clouds over the
main Hawaiian islands and the approaching low is making the
atmosphere increasingly unstable. Light icing is possible in the
middle and high clouds. This possibility has been mentioned in the
AIRMET bulletin, but conditions are expected to remain below the
AIRMET threshold.

Near the surface, a trough to the west and a ridge to the east are
producing SE flow. The low level SE flow has produced MVFR
ceilings over the SE side of the Big Island of Hawaii and AIRMET
SIERRA will remain in effect for MTN OBSC affecting this area.

As the low aloft moves closer on Thursday. MTN OBSC may spread to
the islands from Maui to Kauai and thunderstorms will be possible
across the area.

&&

.MARINE...

A northwest 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
will likely be needed for these areas by Thursday night or Friday
and could potentially reach warning levels by the middle of next
week. The combination of the elevated seas and increased winds
will produce Small Craft Advisory conditions starting tomorrow,
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.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch from 6 AM HST Thursday through Friday
afternoon for Niihau-Kauai-Oahu-Molokai-Lanai-Kahoolawe-Maui.

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
Summits.


&&

$$

DISCUSSION...Lau
AVIATION...Donaldson
MARINE...TS


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