Small Craft Advisory 26, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
1010 AM HST Sat Apr 29 2017

.SYNOPSIS...
Unsettled weather will affect the island chain through early next
week, with locally heavy showers and a few thunderstorms expected. A
front will stall near Maui county today as a potent low aloft moves
toward the islands. The most unsettled weather is expected over Maui
county today, potentially spreading to the other islands the next
couple of days as the low aloft remains nearby. A return to a more
typical trade wind weather pattern is expected toward the end
of the upcoming week.

&&

.DISCUSSION...
A potent closed low aloft has developed about 500 miles N of the
islands, centered near 28N 159W. This deep-layer low extends to the
surface as a 1009 mb low, thereby making it a Kona low, although the
system is not expected to bring widespread Kona winds to the
Hawaiian Islands. In fact, a cool and somewhat brisk northerly flow
prevails over Kauai and Oahu this morning, bringing stable clouds
and a few light showers to N and E facing slopes of Oahu while Kauai
has recently become completely free of low clouds. This cool air has
arrived in the wake of a gradually stalling cold front that is
straddling the island of Maui, where abundant moisture is pooled.
The front is quite shallow, with the low-level N winds turning to
the S and SW by 4-5 kft above the surface. Although the front is
drifting slowly SE, showers within the boundary are moving toward
the NE and E, and have been streaming over normally dry leeward
portions of Maui, in addition to dropping some showers over windward
areas. The Big Island is mostly sunny and dry except for isolated
showers moving N over Puna and adjacent waters. A thick canopy of
high clouds has recently developed on the S side of the low aloft,
and is located just N of the islands.

The forecast over the next 24 hours calls for the low aloft to dive
SSW toward Kauai while the surface front stalls over Maui county.
Greatest low-level moisture convergence is expected to remain
centered over Maui county today, and the forecast has been updated
to account for latest observations and trends, and no longer
indicates the potential for heavy rain or thunderstorms on Oahu
today. Although a Flash Flood Watch (FFA) remains in effect for the
islands from Oahu to the Big Island, the primary short-term threat
for flooding is in Maui county. The FFA for Oahu and the Big Island,
and the Winter Storm Watch for Big Island summits, are due to the
potential for heavy rain/snow that comes as the low aloft moves
closer to the islands and the moisture convergence band shifts E on
Sunday - as indicated by latest GFS/ECMWF guidance. This same
guidance indicates that any heavy rain threat for Oahu will be
later, perhaps not until Monday or Tuesday, and the need for an FFA
will be re-evaluated for the next forecast update.

As the low aloft moves SSW, it will drag it's surface reflection
along with it, and the cool brisk N winds over Kauai and Oahu will
weaken overnight. By Sunday, with the surface low NW of Kauai, low-
level winds will SE to S over all islands, and it is this flow that
will likely be more conducive for the heavy rain threat to spread
away from Maui county. The low aloft is forecast to stall about 200-
300 miles S of Kauai Sunday night through Monday night as the
surface low decouples, heads W, and gradually dissipates. A front
passing N of the islands will keep winds light and generally from
the SE through Tuesday even as the surface low dissipates. A new
high building N of this front will bring gradually building trade
winds Wednesday into Thursday as the associated moisture remains N
of the islands. The presence of the low aloft will keep the island
atmosphere unstable through Tuesday, and the FFA will be tailored
accordingly.

The low aloft is expected to bring increasing winds to the Big
Island summits as it nears, and although winds are currently near
Wind Advisory speeds, the expectation is for winds to increase to
High Wind Warning levels by tonight. These conditions are expected
to persist through Sunday.

&&

.AVIATION...
A weakening front continues its slow crawl across the state. At 8am
HST, the front was near Maui, tracking slowly eastward. Showers,
heavy at times, and isolated thunderstorms along and immediately in
advance of the front will continue to impact Maui County and spread
to the Big Island later today. A solid band of lower clouds extends
along and to the lee of the front...impacting Maui County
and Oahu.

The rear flank of this low cloud band has already begun to erode as
cooler drier air behind the front begins to cut down to the surface.
AIRMET Sierra for Mountain obscuration has been lifted for Kauai but
remains in place for Oahu and Maui County. Conditions are expected
to gradually improve over the western third of the state this
afternoon, and the AIRMET could be lifted for Oahu towards evening.

An active weather day is expected this afternoon for Maui County
when both a vigorous upper level disturbance and daytime heating
will come into play. Thunderstorm activity is expected to increase
across Maui County and spread to the Big Island by early afternoon
and persist overnight.

Brisk summit level winds on the Big Island, and a strengthening
inversion aloft near Kauai, are both expected to persist. AIRMET
Tango for mountain wave turbulence anticipated to remain in effect.

Judging from the 12Z soundings, there is a layer of speed shear
between FL200 and FL270 feet, enough to cause tempo mod turbulence.
Conditions may improve later today as the shear area pushes further
east.

&&

.MARINE...
With a stalled front and upper trough in the area, the chance for
thunderstorms over the coastal waters will persist through at least
Monday night.

A series of long-period, south swells will affect the south facing
shores into next week. The first of these swells will continue to
bring advisory level surf (8 ft) to exposed south facing shores
today. A High Surf Advisory remains in effect for these areas.
Please refer to the Coastal Hazard Message, CFWHFO, for more
information. A reinforcing south swell will fill in Monday and
Tuesday, and keep surf borderline for advisory conditions.

Surf along north facing shores, especially on Kauai and Oahu, has
been increasing somewhat from a short-period, choppy swell
associated with stronger winds behind the front, but remains below
advisory conditions. A series of small, longer-period northwest
swells are expected next week, but also expected to remain below
advisory levels.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through Sunday afternoon for Oahu-Molokai-
Lanai-Kahoolawe-Maui-Big Island.

High Surf Advisory until 6 PM HST this evening for South facing
shores of Niihau, Kauai, Oahu, Molokai, Lanai, Maui, and the Big
island.

High Wind Warning until 6 AM HST Monday for Big Island Summits.

Winter Storm Watch from this evening through Sunday afternoon
for Big Island Summits.

&&

DISCUSSION...Birchard
AVIATION...Bedal
MARINE...TS