Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 091335
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
335 AM HST Fri Dec 9 2016

.SYNOPSIS...
A trough of low pressure west of the state will keep conditions
somewhat unsettled through Saturday, with considerable cloudiness
and periodic showers affecting the island chain. Some slow
improvement is expected from west to east across the state Sunday
through Monday, but showers will remain possible as a trough of
low pressure remains over 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 for the latter part of the upcoming work week.

&&

.DISCUSSION...
Currently at the surface, a weak trough of low pressure is located
just to the west of the island chain, with a 1026 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 and southwest 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 from Oahu to Maui, with rainfall
coverage more sparse across Kauai and to a lesser extent the Big
Island. Main short term concern for today revolves around
rainfall trends.

Today through Saturday,
The large scale synoptic pattern remains rather stagnant through
the period, with a surface trough just to the west of the state
and high pressure off the Baja California coast remaining 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 precipitable water (PW) values in the 1.5 to
1.9 inch range. Meanwhile, upper troughing will remain over or
just 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.

Shower coverage is expected to be the greatest from Oahu to the
western slopes of the Big Island this morning, with showers
becoming more sparse this afternoon as an upper level disturbance
lifts north of the island chain. In fact, some breaks in the
layered clouds will be possible across Kauai, particularly this
afternoon, but the remainder of the island chain will likely stay
mostly cloudy. Another upper level disturbance will approach from
the west tonight and move overhead on Saturday, keeping a chance
of showers in the forecast. The models due show some decrease in
layered cloud cover Saturday afternoon however, so perhaps some
more sunshine will be seen, particularly across Oahu and Kauai.

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
east of the island chain. With a lingering surface trough over
the islands along with plenty of deep layer moisture however,
shower chances will need to be kept in the forecast. That said,
with the flattening out of the upper level flow, we should see
more sunshine area wide, 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 to differing
degrees, 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.

&&

.AVIATION...
Thick layered clouds along with areas of light to moderate rain
and scattered showers continue over the state early this morning.
Areas of MVFR conditions with embedded pockets of IFR will remain
likely at least through the morning and possibly through the
afternoon as well. AIRMET SIERRA for mountain obscuration remains
posted for all of the smaller islands, and has recently been
posted for the leeward coast and slopes of the Big Island.

AIRMET TANGO is in effect for upper level turbulence due to
strong jet stream winds. AIRMET ZULU is in effect for moderate
icing between 125 and FL220. These AIRMETS are expected to
continue at least through this morning, with some improvement
possible this afternoon.

&&

.MARINE...
Surf is expected to remain below advisory levels into early next
week. A low end moderate northwest swell is expected to move in on
Monday and continue through 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.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION/MARINE...Jelsema
AVIATION...Jacobson



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