Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 090713

Area Forecast Discussion
National Weather Service Honolulu HI
913 PM HST Thu Dec 8 2016

Unsettled weather is expected to continue through Saturday, with
considerable cloudiness and periods of showery weather. Some
improvement is expected Sunday and Monday, but showers will
remain possible as a surface trough remains in place across the
central and eastern islands. Drier weather is then expected
Tuesday through Wednesday as weak high pressure builds over the
state, with unsettled conditions perhaps returning once again for
the latter part of the upcoming work week.


Currently at the surface, a weak trough of low pressure is located
just to the west of the island chain, with a 1025 mb high off the
northern coast of Baja California. This places the state in a weak
southeasterly boundary layer flow. Meanwhile, water vapor imagery
depicts upper level troughing to the west of the island chain,
resulting in southwesterly flow aloft with embedded shortwave
disturbances moving overhead through the region. The instability
provided by the shortwave disturbances moving through combined
with a moist southeasterly boundary layer flow is resulting in
widespread rainfall across the smaller islands as seen in radar
imagery. Main short term concern for the overnight hours revolves
around rainfall trends.

The surface trough will remain nearly stationary to the west of
the island chain, while the high well east of the state remains
anchored off the northern Baja California coast. This will keep a
moist southeasterly boundary layer flow in place across the
region. MIMIC total precipitable water imagery showers values
between 1.4 and 2.0 inches across the state, highest across the
central islands. The combination of moist southeasterly boundary
layer flow and shortwave disturbances moving overhead in the
southwesterly flow aloft, will keep conditions unsettled through
the overnight hours. Widespread showers are expected over the
smaller islands, where the moisture and forcing for ascent will
be best aligned. The best instability has already lifted north of
the state, so thunder chances don`t appear all that good
overnight. That said, with strong forcing for ascent and plenty of
deep layer moisture in place, will continue to mention a slight
chance of thunderstorms through the night as they can`t be
entirely ruled out. In addition, will keep the flood watch intact
as is with an expectation that it will be cancelled with the
morning forecast package.

Friday through Saturday,
Unsettled conditions are expected to continue through the period
as the large scale synoptic pattern remains rather stagnant. The
surface trough just to the west of the state and high pressure off
the Baja California coast will remain nearly stationary. This will
keep a light southeasterly boundary layer in flow in place across
the islands, with plenty of lingering deep layer moisture, with PW
values in the 1.5 to 1.9 inch range. Meanwhile, upper troughing
will remain west of the islands, with periodic shortwave
disturbances passing over the state in the southwesterly flow
aloft. The combination of deep layer moisture and periodic
shortwave disturbances enhancing lift across the area, will result
in considerable cloudiness and periods of showery weather, with
perhaps some improvement during the afternoon hours on Saturday.

Saturday night through Sunday night,
Conditions are expected to improve as the flow aloft flattens out
over the state and shifts the best synoptic forcing for ascent to
the east of the island chain. With a lingering surface trough in
place across the state and plenty of lingering deep layer moisture
however, shower chances will remain in the forecast. That said,
with the flattening out of the upper level flow, we should see
more sunshine as mid and upper level cloud cover decreases. Winds
will remain light through the period with overnight land breezes
and daytime sea breezes expected in most areas.

Monday through next Thursday,
Medium range guidance is in decent agreement through the extended
periods, showing surface troughing remaining near the Big Island
Monday and Monday night. This should keep the highest rain chances
generally confined to the central and eastern islands. The surface
trough will then shift east of the state Tuesday through
Wednesday, with weak high pressure and drier conditions
overspreading the entire island chain. The models then show a deep
stacked low developing to the west of the state Wednesday night
and next Thursday, which would suggest a return of more unsettled
conditions for the latter part of the upcoming work week.


Clouds and showers remain over the islands this evening, with the
heaviest concentration over the smaller islands, including Maui.
Isolated thunderstorms have been occasionally igniting within the
layered clouds. Expect MVFR, with embedded pockets of IFR,
conditions to continue overnight over the smaller islands, where
AIRMET SIERRA continues for mountain obscuration.

AIRMET TANGO is in effect for upper level turbulence due to
strong jet stream winds. AIRMET ZULU is in effect for moderate
icing between 120 and FL220 for the area west of the Big Island.
These AIRMETS are expected to continue through tonight with
gradual improvement Friday morning.


Surf is expected to remain below advisory levels into early next
week. The current northwest swell will decline tonight, with
another low end moderate northwest swell due on Monday and
Tuesday. Pulses of small to occasionally moderate north swell are
expected through much of the week, especially across the western
end of the island chain. Please see the recently issued
Collaborative Nearshore Swell and Wind Forecast for Oahu (SRDHFO)
for more details.

Winds will remain rather weak through early next week, and Small
Craft Advisory conditions are not expected. Isolated thunderstorms
will produce lightning and gusty winds within close proximity of
storm cells tonight.


Flash Flood Watch until 6 AM HST Friday for Niihau-Kauai-Oahu-



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