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Hawaiian Islands Synoptic Discussion and Guidance

221
FXHW60 PHFO 290154
AFDHFO

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

.SYNOPSIS...
An unsettled weather pattern will prevail through the weekend and
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 after tonight, and the front will likely focus the heaviest
rainfall over Maui county, thunderstorms will be possible statewide
through most of the weekend. Wet weather is expected to continue
early next week, with a trend toward a more typical trade wind
weather pattern expected by mid-week.

&&

.DISCUSSION...
The current weather pattern over the area is rather dynamic, and is
expected to remain that way over the weekend into early next week.
While forecast models continue to indicate a period of unsettled
weather in our near-term future, with a special emphasis on Maui
county this weekend, there remains some uncertainty as to the
specifics. A Flash Flood Watch (FFA) has been issued for all islands
except Kauai county, and a Winter Storm Watch has been issued for
the summits and upper slopes of the Big Island peaks. The forecast
reasoning follows.

Latest water vapor imagery shows a sharp N-S oriented trough aloft
with an axis along 161W that extends from near the islands to about
45N. A N-S oriented surface trough is analyzed near Maui County,
while a diffuse frontal boundary is in the process of passing over
Kauai with a surge in N winds producing gusts near 30 kt over parts
of the island. North winds have been fairly strong at buoys
51001/51101 NW of Kauai, with sustained winds between 20 and 25 kts
shunting a cooler and drier low-level air mass toward Kauai and
Oahu. To the E of the trough axis, light to moderate E to SE winds
prevail over most of Maui county and the Big Island, delivering a
moister air mass. Afternoon soundings show a dramatic increase in
CAPE at PHLI to near 2500 J/kg, while a low-level subsidence
inversion was detected at PHTO.

Forecast models continue to depict a cutoff low aloft hatching out
of the sharp trough near 30N161W tonight, then moving straight S
toward and over the islands through the weekend before stalling SW
of the islands early next week. Guidance agrees that the coldest air
aloft, associated with the core of the low, will pass over Kauai on
Sunday. Models differ dramatically as to the evolution of the low
early next week, and the focus for now will be on the impacts of the
low through the weekend due to the increased uncertainty thereafter.

In addition to the low bringing unusually cold mid-level
temperatures - leading to a significant destabilization of the
island atmosphere - the surface trough is expected to sharpen over
the area as a diffuse and poorly-defined cold front near Kauai moves
SE over the islands and stalls. Low-level moisture convergence will
be greatest over Maui county, but the Big Island, and to a lesser
degree, Oahu, will be close enough to warrant being included in the
Flash Flood Watch. The greatest potential for heavy rain appears to
be from Saturday night into Sunday, but the latest ECWMF indicates
that weather could become active as early as Saturday.

With a cool and dry air mass streaming over Kauai (at least
initially), the potential for flooding rain is diminished, and Kauai
county has been excluded due to this forecast philosophy. If the low-
level boundary pushes further E than expected, then Oahu could be
sufficiently dry to limit the flooding rain potential as well.
Uncertainty remains high as to the details in the sensible weather,
including surface winds, as a slight shift in the position of the
features will lead to dramatic differences in the weather that
occurs. A surface low is expected to develop in response to the
strong low aloft, and as it moves slowly S and SW Saturday, winds
will weaken and become fairly light and variable by Sunday.

With a pool of cold air associated with the low overlying warmer
than normal water temperatures, lapse rates will become quite steep,
increasing the potential for thunderstorm development, and
increasing the potential for briefly strong storms that could
produce small hail. Although their coverage is expected to be
limited in the cool dry air over the W end of the island chain this
weekend, this is where the coldest air aloft is expected, and any
showers that develop may quickly develop into thunderstorms. The
forecast has been modified to increase thunderstorm chances through
Saturday night. The low will also bring the threat of wintry weather
and strong winds to the Big Island summits, where a Winter Storm
Watch is in effect, but confidence is not sufficiently high to issue
a High Wind Watch at this time.

The general forecast philosophy for Sunday into next week is for the
low aloft and the surface trough to gradually weaken. However, the
potential for unsettled weather will remain high with enhanced low-
level moisture in place over most of the chain, and the low
continuing to provide instability. A more settled trade wind weather
regime is in the long range guidance for the middle to latter part
of next week.

&&

.AVIATION...
A weak front along with a trough aloft will cause occasional MVFR
conditions tonight especially over interior and mountain areas.
Locally heavy showers can be expected along with a slight chance of
thunderstorms. AIRMET Sierra is currently in effect for portions of
Kauai and Oahu.

AIRMET Tango is currently in effect for moderate turbulence between
flight level 270 and flight level 360 over and around Kauai and Oahu.

&&

.MARINE...
A complex weather pattern is expected for the next several days,
with a chance for thunderstorms across most of the coastal waters
through Monday. A front and upper level trough, currently just east
of Kauai will continue to move southeast before stalling over the
central islands Saturday. Small Craft Advisory winds and seas will
fill in over the waters surrounding Kauai this evening and tonight
as the front passes. A small break in thunderstorm chances should
occur behind the front, before spreading across all waters again
late Saturday as the upper low and developing surface low move over
the island chain. The forecast confidence remains low on the exact
placement of thunderstorm activity, but mariners should stay tuned
for the latest updates.

Surf along north facing shores will bump up 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 for next week, also remaining below advisory level surf.

A series of long-period south swells will impact 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 morning. 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...
High Surf Advisory until 6 AM HST Saturday for South facing shores
of Niihau, Kauai, Oahu, Molokai, Lanai, Maui, and the Big island.

Flash Flood Watch through Sunday afternoon for Oahu-Molokai-Lanai-
Kahoolawe-Maui-Big Island.

Winter Storm Watch from Saturday morning 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...Birchard
Aviation...Burke
Marine...TS

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



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