Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 300234
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
433 PM HST Sat Apr 29 2017

.SYNOPSIS...
Unsettled weather will continue into early next week as a Kona low
passes close to the island chain, with locally heavy showers and a
few thunderstorms expected. Some of the thunderstorms could be
strong over Maui county and the Big Island tonight, with gusty winds
and small hail possible. A return to a more typical trade wind
weather pattern is expected around the middle of the upcoming week.

&&

.DISCUSSION...
A potent closed low aloft near 27N 160W (about 300 miles N of Kauai)
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 lies over Kauai and Oahu. 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
3-5 kft above the surface. S winds to the E of the stalling front
have been accelerating around portions of the Big Island, especially
near Keahole Point and Cape Kumukahi, to the point that a Wind
Advisory became necessary. These conditions are expected to continue
through Sunday. 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. Clouds and showers have
increased in coverage over the Big Island and Oahu in the last
couple of hours as the low has drawn closer.

The forecast over the next 24 hours anticipates a dynamic weather
pattern as the Kona low moves S toward Kauai while the surface front
stalls near Maui county and gradually transitions to a convergence
band. A Flash Flood Watch (FFA) remains in effect for the islands
from Oahu to the Big Island, as forecast guidance indicates a
possible E shift toward the Big Island in the short-term,
potentially spreading back toward Oahu on Sunday. With the moisture
likely spreading toward the Big Island overnight, a Winter Storm
goes into effect at midnight.

As the Kona low aloft moves S, the cool brisk N winds over Kauai and
Oahu will weaken overnight. By Sunday, with the low NW of Kauai,
light low-level winds will become SE to S over all islands, and it
is this flow that will likely be more conducive for the heavy rain
threat to spread toward Oahu as the low-levels begin to moisten. 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.

In addition to all the other hazards, the GFS and ECMWF are also
showing a potential for some thunderstorms to become severe. The
combination of increasing instability due to very cold temperatures
aloft associated with the upper low and a lingering surface boundary
stretched across Maui County, and increasing winds in the lower- to
middle-levels of the atmosphere will give the potential for a few
thunderstorms to become organized. Deep layer (0-6 km) shear and
lower level storm relative helicity will be sufficient for supercell
development once the upper low begins to impact the low-level
boundary. The threat appears to begin near Maui County late tonight,
and then spreads toward the Big Island by Sunday. It`s possible the
threat may continue into Monday, but this is highly dependent on the
motion and intensity of the upper low, as well as what actually
happens with the low level wind field in the wake of deep convection
that develops.

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.

&&

.AVIATION...
A ragged front continues its slow crawl across the state. At 2pm
HST, the front was located near Maui and tracking eastward at about
5 kt. Showers, heavy at times, and isolated thunderstorms along and
immediately in advance of the front are currently impacting much of
Maui county and are expected to spread across the Big Island by late
evening. A solid band of lower clouds extends along and to the lee
of the front...impacting Maui County...Oahu...and portions of Kauai.

The rear flank of the lower 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 remains in place for Oahu and
Maui County. Conditions are expected to gradually improve behind the
front, and AIRMET Sierra could be lifted for Oahu late this evening.

Convective activity is expected to increase overnight with the
approach of a vigorous upper level disturbance. Thunderstorm
potential will exist for most of the state after 10 pm.

Brisk summit level winds and a strong inversion aloft both
characteristic of the airmass behind the front. AIRMET Tango for
mountain wave turbulence in effect for Kauai and Oahu.

Judging from the PHTO 00Z sounding, there is a layer of speed shear
between FL180 and FL280 over the eastern half of the state, enough
to cause TEMPO MOD turbulence. AIRMET TANGO in effect for Oahu Maui
County and Big Island.

&&

.MARINE...
Southerly winds, on the east side of a stalled front over Maui
County, will increase around the Big Island this afternoon and
evening to 25 to 30 kt. This has prompted the issuance of a Small
Craft Advisory for the coastal waters around the Big Island for this
afternoon through Sunday. Winds will begin to decrease Sunday
evening, falling below advisory level overnight, into Monday
morning.

With a stalled front and upper trough in the area, the chance for
thunderstorms will persist over all coastal waters 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. The High Surf Advisory has been extended for these areas
through tonight. Please refer to the Coastal Hazard Message, CFWHFO,
for more information. A reinforcing south swell will fill in Monday
and Tuesday, keeping surf at borderline 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 AM HST Sunday evening for South facing
shores of Niihau, Kauai, Oahu, Molokai, Lanai, Maui, and the Big
island.

Wind Advisory until 6 PM HST Sunday for Kona-South Big Island-Big
Island North and East-Kohala-Big Island Interior.

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

Winter Storm Warning from midnight tonight to 6 PM HST Sunday for
Big Island Summits.

Small Craft Advisory until 6 PM HST Sunday for Alenuihaha Channel-
Big Island Windward Waters-Big Island Leeward Waters-Big Island
Southeast Waters.

&&

$$

DISCUSSION...Birchard/R Ballard
AVIATION...Bedal
MARINE...TS


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