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Hawaiian Islands Synoptic Discussion and Guidance

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FXHW60 PHFO 290707
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
907 PM HST Fri Apr 28 2017

.SYNOPSIS...
An unsettled weather pattern will prevail through the weekend and
into early next week, with thunderstorms and heavy showers in the
forecast. This will occur as a front stalls over the islands and
a potent low aloft significantly destabilizes the atmosphere.
While cool north winds will limit shower coverage and intensity
over Kauai county and the front will likely focus the heaviest
rainfall over Maui county, thunderstorms will be possible
statewide through early next week. A return to a more typical
trade wind weather pattern is then expected by the middle of
next week.

&&

.DISCUSSION...
Currently at the surface, a weak 1011 mb low is located around 500
miles north of Kahului, with a trough extending southward through
Maui County. Meanwhile, a front is located between Oahu and
Kauai, to the west of the surface trough. Aloft, broad upper level
troughing dominates the synoptic pattern over the central Pacific,
with a deep upper level low located around 550 miles north of
Honolulu, with the trough axis extending southward and over the
island chain. Additionally, a shortwave trough embedded within the
broad upper level trough is moving through Maui and approaching
the Big Island this evening. Infrared satellite imagery shows
mostly cloudy skies in place across most of the state, with some
pockets or reduced cloud cover over Oahu, Molokai and Lanai.
Meanwhile, radar imagery shows showers scattered about across the
island chain, with the main focus around Oahu and Maui County at
the moment. Main short term concern revolves around the potential
for heavy rainfall and flooding across the central and eastern
islands through the weekend.

Rest of tonight,
The surface trough running through Maui County will remain nearly
stationary through the overnight hours, while the front which
extends through the Kauai channel will shift slowly eastward
through Oahu later this evening and approach Molokai right around
daybreak Saturday. Meanwhile aloft, the shortwave trough currently
moving through Maui County will track quickly eastward through the
evening hours, moving east of the Big Island by midnight. The loss
of daytime heating as well as the best forcing for ascent provided
by the upper level shortwave trough exiting to the east of the
state later this evening, should act to have a stabilizing effect
on the weather across the islands overnight. Showers will likely
remain most prevalent over Oahu and Maui County through the
overnight hours, with reduced coverage expected across Kauai and
the Big Island. Across Kauai and Oahu, northerly winds behind the
front will keep most of the shower activity focused over areas
favored in a northerly flow regime, while the surface trough over
Maui County will result in a light wind regime with scattered
showers possible almost anywhere through the overnight hours.

Saturday through Monday night,
Model solutions are in good agreement showing the front and
surface trough consolidating over Maui County on Saturday, then
remaining nearly stationary through Sunday before slowly
dissipating heading into early next week. Meanwhile aloft, a deep
upper level low will shift southward toward Kauai on Saturday and
move over Kauai by late Saturday night or early Sunday morning.
The upper low will then pivot off to the south of Kauai during
the day on Sunday before stalling out around 200 miles south of
Kauai Sunday night through Monday night. The deepest moisture is
progged to remain over Maui County through the period, but it will
remain very close to both Oahu and even closer to the Big Island.
Shower activity appears to be more limited across Kauai where dry
surface dewpoint air will move in from the north, with dewpoints
dropping into the mid 50s through the weekend. That said, with the
core of the upper level low moving over Kauai, the instability
aloft will likely be enough to produce some scattered showers, the
coverage should just be reduced in comparison to the other
islands.

The potential remains for a significant hydrology event over the
weekend, particularly across Maui County, with the heaviest
rainfall appearing to focus over the area Saturday night and
Sunday when the best forcing for ascent will be in place. In
addition, the instability provided by the upper level low will
keep thunderstorms a possibility across the entire state, with
the best chances for thunder expected across Maui County and Big
Island. A Flash Flood Watch remains in effect from Oahu eastward
to the Big Island through Sunday, with the highest probabilities
for flooding rains expected over Maui County at this time.

Additionally, the proximity of the deep moisture to the Big Island
and the instability aloft may result in wintry conditions over the
Big Island summits, and the Winter Storm Watch remains in effect
Saturday through Sunday. Windy conditions are also expected to
develop over the Big Island summits Saturday afternoon, reaching
Wind Advisory levels possibly right through the day on Monday,
with Warning level winds not out of the question Saturday night
and Sunday.

Unsettled conditions are expected to continue Sunday night through
Monday night as the upper level low remains just to the south of
the islands and the lingering moisture from the dissipating
surface trough remains over the island chain. Due to the
continued instability, thunderstorms will need to remain in the
forecast, and the Flash Flood Watch may eventually need to be
extended into early next week.

Tuesday through next Friday,
The upper level low to the south of Kauai will begin to weaken
and open up on Tuesday in response to ridging aloft building in
from the west. This should have a stabilizing effect on the
weather over the islands, with a return to more of a typical trade
wind pattern expected Tuesday night through late next week.

&&

.AVIATION...
A diffuse front is in the Kauai channel as of 05z, moving e at 10
kt. The front is expect to cross Oahu between 08z and 12z and
continue eastward into Maui county where it will stall. A shallow
layer of moisture lies immediately west of the front, followed by
a dry and cool air mass. With this said, Kauai is expected to
improve early this evening, by 12z, with the lowering of AIRMET
Sierra, followed by Oahu by 16z. AIRMET Sierra is not likely to be
imposed on the remaining islands overnight.

Spotty heavy showers remain along and east of the front to near
Upolu Point on the Big Island. Tops of these showers are as high
as 25k feet. During the overnight hours, scattered showers are not
out of the question for the Kona coast and the northern part of
the Big island. Also, there is a slight chance of an isolated
thunderstorm, favoring the coastal waters.

An upper level trough, above 15kt feet is currently over Maui
moving east at 20 kt. So, it should clear the Big island by 10z.
Therefore, expect AIRMET Tango, for the high level turbulence to
be lowered with the 10z package.

AIRMET Tango was updated to include Kauai and Oahu for low level
turbulence below 8k feet caused by a brisk northerly wind flow.
Expect this flow to ease overnight, but it will be reevaluated
if it will be extended beyond 16z.


&&

.MARINE...
With a front and upper trough in the area, the chance for
thunderstorms over the coastal waters will persist through
Saturday and beyond. Winds in the wake of the surface boundary
are strong enough to warrant a Small Craft Advisory (SCA) for
waters around Kauai and Oahu Windward waters, and seas will ramp
up to 10 feet in select zones as well. The SCA is currently in
effect through early Saturday.

Surf along north facing shores will increase some tonight into
Saturday from a short-period, choppy swell associated with the
stronger winds behind the front, but will stay below advisory
conditions. A series of small, longer-period northwest swells are
expected next week, also expected to remain below advisory levels.

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
through early Saturday. A High Surf Advisory has been posted 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.

&&

.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 Saturday for Niihau-Kauai
Windward-Kauai Leeward-Oahu South Shore-Molokai-Lanai Makai-
Kahoolawe-Maui Leeward West-Maui Central Valley-Leeward
Haleakala-Kona-South Big Island.

Winter Storm Watch from 6 AM HST Saturday through Sunday
afternoon for Big Island Summits.

Small Craft Advisory until 6 AM HST Saturday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters.

&&

$$

DISCUSSION...Jelsema
AVIATION...Lau
MARINE...Kinel

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office