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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 282020

Area Forecast Discussion
National Weather Service Honolulu HI
1020 AM HST Fri Apr 28 2017

An unsettled weather pattern will prevail through the weekend and
early next week, 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 the
weekend. Unsettled weather is expected to continue early next week,
with a trend toward a more typical trade wind weather pattern
expected by mid-week.


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, 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 roughly along 160W that extends from just W of Kauai to
about 45N. A surface trough has developed over the islands in the
last 24 hours, and this N-S oriented boundary is currently analyzed
to be over Molokai, Lanai and Kahoolawe and adjacent waters. North
winds to the W of the trough axis have been fairly strong at Buoys
51001/51101, 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

Forecast models continue to depict a closed 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. Models differ
dramatically as to the evolution of the low after that point, 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.

With a pool of cold air associated with the low overlying warmer
than normal water temperatures, lapse rates will become quite steep,
and there is a high potential for thunderstorm development. Although
their coverage is expected to be limited in the cool dry air over
the W end of the island chain, this is where the coldest air aloft
is expected, and any showers that develop may quickly develop into
thunderstorms. The forecast will be changed by this afternoon to
increase the weekend thunderstorm coverage.

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.


A weak front along with a trough aloft will cause occasional MVFR
conditions today 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, Oahu and The Big Island.

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


A complex weather pattern is expected for the next several days,
introducing a slight chance for thunderstorms across most of the
coastal waters today through Monday. A front and upper level trough
just west 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 today 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 to the updated forecast today and this weekend for updates.

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

A series of long-period south swells will impact the south facing
shores into next week. The first of these swells will bring advisory
level surf (8 ft) to exposed south facing shores today into 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, but keep surf borderline for advisory conditions.


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 from Saturday morning through Sunday afternoon for
Oahu-Molokai-Lanai-Kahoolawe-Maui-Big Island.

Small Craft Advisory until 6 AM HST Saturday for Niihau-Olomana.

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.




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