Small Craft Advisory 9, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
958 AM HST Thu Feb 22 2018

.SYNOPSIS...
Low pressure at the surface and aloft to the west of the islands
will continue to draw moist tropical air over the islands the next
few days. This will lead to periods of 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...
A broad surface trough is located about 300 mi W of Kauai, and a
large, strong 1041 mb surface high is centered about 1800 mi to
the NNE of the islands. Neither of these systems is budging much,
keeping us stuck in an increasingly moist and unstable ESE flow at
the surface. An upper low out to our NW is maintaining relatively
cold air aloft over the islands, and the tropical moisture
undercutting the cold air aloft is making the local airmass quite
unstable. This is fueling 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 west.
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.

The surface trough is expected to sharpen with time as the high to
the NE shoves it's way south. This will eventually pull the axis
of heaviest rain to the W of the islands. When exactly this
happens is still very uncertain, and the models continue to
struggle mightily with this. 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.

We will remain in relatively moist SE flow into the middle of next
week. Total precipitable water remains above normal in the
guidance but not quite as moist as the next couple of days are
showing. The models are showing the strong NE Pacific high
building southward and increasing the pressure gradient over the
islands, with rather strong background winds from a fairly
unusual ESE or SE direction by the middle of next week.

&&

.AVIATION...
The atmosphere will be moist and unstable through at least
tonight. A strong mid to upper level trough to the west of state
is gradually drawing up an area of deep moisture toward the
islands from the southwest. Meanwhile, east to east-southeast low
level winds continue to focus rainfall along windward and SE
facing slopes.

Multiple AIRMETS are in place and will likely remain posted
through at least tonight. 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. As a result, AIRMET SIERRA is in place for
mountain obscuration for portions of all islands. Light icing will
be possible in the middle and high clouds. This is mentioned in
the AIRMET bulletin, but conditions are expected to remain below
the AIRMET threshold. Also, AIRMET TANGO remains posted for mid
level turbulence. Guidance shows the likelihood of this turbulence
decreasing, and without any supporting observations, this AIRMET
will probably be cancelled today.

&&

.MARINE...
Ample moisture and an upper level low just west of the state will
aide in the development of heavy showers and thunderstorms today.
Most activity will 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 today. The threat for heavy showers and
thunderstorms will gradually shift north and west into Saturday.

The current northwest swell will hold today, with surf heights
below advisory levels. After this swell fades tonight into the
weekend, long term guidance does not favor another west-
northwest/northwest swell through at least next week.

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 are expected to start to
increase around the state today, and remain elevated through most
of next week. 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 likely be needed for these areas by early Friday
morning 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 this
evening, mainly for the channels and exposed windward waters.

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 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 Friday for Big Island
Summits.

&&

R Ballard/Wroe/EATON