Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 230217
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
417 PM HST Thu Feb 22 2018

.SYNOPSIS...
Low pressure at the surface and aloft to our west will continue
to draw moist tropical air over the islands the next couple of
days. This will keep the weather unsettled, with heavy rain and
possible thunderstorms into the weekend. There may be some
improvement later in the weekend as the low pressure shifts very
slowly westward.

&&

.DISCUSSION...
The islands remain trapped in a moist southeasterly flow between
a broad surface trough about 300 mi W of Kauai, and a large,
strong 1041 mb surface high is centered about 1800 mi to our NNE.
Neither of these systems is in a big hurry to go anywhere. An
upper low to our NW is maintaining relatively cold air aloft over
the islands, and the tropical moisture undercutting the cold air
is making the local airmass rather unstable. This continues to
fuel a large area of showers and a few embedded thundershowers
south of the islands extending across Maui and the Big Island.

This pattern is only going to evolve very slowly with time. What
makes this a very tricky forecast is that we will be on the edge
of what is likely to be even more active weather out to our W and
SW. Through at least Friday, our concern for heavy rain and flash
flooding is maximized over the E and SE facing slopes and mauka
sections of each island where the low level flow will focus
orographic lift. In addition, these areas are starting off with
saturated antecedent conditions from last weekend`s heavy rains.
This is not to say that other areas won`t have heavy rain and
flash flooding, however, and the risk is high enough to maintain
the statewide Flash Flood Watch through at least Friday.

One complicating factor in the near term is the persistent, slow
moving thunderstorm complex about 130 miles south of Oahu. For
the time being, it appears to be limiting the activity closer to
the islands. Since none of the models are handling this activity
very well, it`s difficult to know how long it will last or where
it will go. Otherwise, the aforementioned ingredients are there
for heavy rain to develop overnight.

The surface trough is expected to sharpen with time as the high to
the NE shoves its way south. This will eventually pull the axis of
heaviest rain W of the islands. When exactly this happens is
still very uncertain, and the models continue to struggle with
this. The 12z ECMWF run is more pessimistic and lingers a threat
for heavy rain through Sunday, while the GFS is quite a bit more
optimistic. We may have to extend the Flash Flood Watch into the
weekend, but confidence is not yet high enough to do so at the
moment. One thing that we can say is that the threat will linger
the longest for Kauai.

&&

.AVIATION...
The atmosphere will remain moist and unstable into Friday. A
strong mid to upper level trough to the W of state continues to
draw up an area of deep moisture over the islands from the SW.
Meanwhile, E to ESE low level winds continue to focus showers
along windward and SE facing slopes. Expect widespread MVFR
conditions across island terrain and E to SE facing slopes, and
periods of IFR conditions in heavy showers and thunderstorms will
be possible anywhere.

AIRMET SIERRA remains in place for mountain obscuration across
interior terrain and E to SE facing slopes of all islands. Little
change is expected tonight, with portions of the state likely
being removed on Friday. Light icing will be possible in the thick
mid and high clouds blanketing the state. This is mentioned in
the AIRMET bulletin, but conditions are expected to remain below
the AIRMET threshold through tonight with improvement expected
Friday.

&&

.MARINE...
Ample moisture and an upper level low just west of the state will
aide in the development of heavy showers and thunderstorms tonight.
Most activity will continue to move into the area from the south.
Coastal water areas can expect to experience periods of reduce
visibility, gusty winds, and rough seas in any of these shower or
thunderstorm areas. Special Marine Warnings may need to be issued
on a case by case basis tonight. The threat for heavy showers and
thunderstorms will gradually shift north and west into Saturday.

Persistent strong high pressure northeast of the state will
continue to produce an area of strong winds directed towards the
area. A lingering surface trough, just west of the offshore
waters will help to force a more southeast wind pattern for the
state. The east-southeast/southeast winds will continue to
increase around the state tonight, and remain elevated through
most of next week. An ASCAT pass from this morning showed Small
Craft Advisory (SCA) already making it to the Alenuihaha Channel
and waters south of the Big Island. Model guidance continues to
show the potential of winds reaching gale force strength by the
middle of next week. The strongest winds will be around the Big
Island and windward waters of the smaller islands, continuing to
favor the more unusual southeast direction. The swell generated
from these winds will produce elevated surf along east- facing
shores.

A High Surf Advisory will go into effect for all exposed
east-facing shores at 6 am HST Friday morning. Surf could
potentially reach warning levels by the middle of next week. A
Small Craft Advisory is posted across windward waters and around
the Big Island for the combination of the elevated seas and
increasing winds. These conditions will continue into next week.

The current northwest swell will begin to fade tonight into the
weekend and long term guidance does not favor another west-
northwest/northwest swell through at least next week.

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


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Friday afternoon for all of the main
Hawaiian Islands.

High Surf Advisory from 6 AM Friday to 6 AM HST Monday for east
facing shores of Kauai Oahu Molokai Maui and the Big Island.

Winter Storm Warning until 6 PM HST Friday for Big Island
Summits.

Small Craft Advisory until 6 AM HST Monday for Kauai Windward
Waters-Oahu Windward Waters-Kaiwi Channel-Maui County Windward
Waters-Alenuihaha Channel-Big Island Windward Waters-Big Island
Leeward Waters-Big Island Southeast Waters.

&&

$$
R Ballard/Wroe/EATON


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