Area Forecast Discussion
Issued by NWS Honolulu, HI

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074
FXHW60 PHFO 251955
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
955 AM HST Sat Feb 25 2017

.SYNOPSIS...
Cloudy and somewhat cool conditions are expected through tonight
as weak easterly trade winds focus modest shower activity over the
central portion of the state. The atmosphere will become
increasingly moist and unstable late Sunday into early next week
as a deepening trough to the west of islands pulls moisture up
from the deep tropics. Chances for heavy showers will increase on
Tuesday and Wednesday as the trough slowly moves across the area.
A ridge will build over the islands during the second half of next
week, bringing drier conditions.

&&

.DISCUSSION...
Cloudy and somewhat cool conditions prevail this morning under a
gentle to locally moderate easterly trade wind flow. A weak
surface high sitting about 375 miles north of the islands has
brought in a somewhat cool airmass with dew point temps hovering
around 60 degrees F and precipitable water values around 0.8 of an
inch. Conditions are rather stable as a weak mid level ridge is
producing a low level inversion between 4000 and 7000 ft according
to the overnight soundings. These dry and stable conditions are
disrupted along a narrow band of low level moisture stretching
across Maui County, where a few tenths of rainfall have fallen
along windward slopes. Otherwise, little rainfall has been
observed. A deepening upper level trough several hundred miles to
the west is generating a thickening shield of high clouds over
and southwest of the state.

The cloudy and somewhat cool conditions will continue today.
Thick high clouds will continue to stream overhead, and with dew
point temps remaining in the lower 60s, it is unlikely that high
temps will reach the 80 degree mark this afternoon. The mid level
ridge will be eroded by the strengthening trough to the west, but
the inversion should remain robust enough to inhibit day time
convection. Modest showers are expected to remain most active
along the above mentioned narrow band of low level moisture over
Maui County that should drift northward to Oahu during the
afternoon.

Weak easterly trade winds and cloudy skies will prevail tonight
and Sunday. Conditions will be somewhat cool with dew points in
the lower 60s, but thick high clouds will prevent overnight low
temps from dropping more than a few degrees below normal into the
low to mid 60s range. Weak mid level ridging is expected to
maintain somewhat stable conditions, while the main source for
rainfall will be along the diffuse band of low level moisture.
This feature will mainly affect windward Oahu and Molokai tonight
then lift northward on Sunday.

A trough aloft will deepen west of Kauai early next week bringing
increasing instability. Meanwhile, the low level flow near the
islands will shift out of the southeast. Precipitable Water (PW)
values will increase as tropical moisture streams in. The combined
instability and moisture will bring the threat of heavy showers,
especially for the Big Island and Maui, Tuesday and Wednesday.
The moisture will likely be deep enough and the freezing level low
enough for snow on the Big Island summits.

A surface ridge building over the islands during the second half of
next week will keep winds light. The atmosphere will stabilize as
the trough northwest of the area lifts north. Afternoon clouds may
continue, but rainfall is expected to become light. The light winds
may allow vog to spread over the smaller islands through the second
half of the week.

&&

.AVIATION...
Gentle background east southeasterly flow near the surface will
allow for variable winds across much of the state through tonight.
A weak shower band straddled over Maui County this morning will
slowly push north up the island chain today and tonight. Brief
MVFR ceilings and mountain obscurations will be possible near
these light showers, and AIRMET Sierra could be issued if sea
breezes focus more extensive clouds and showers onto the islands
this afternoon.

A 130-140 knot subtropical jet over the state will bring moderate
turbulence aloft over Kauai and Oahu today. AIRMET Tango is posted
for these areas. This jet stream is also producing broken to
overcast cirrus cloud cover over all islands. As the jet shifts
east, the focus of the turbulence will likely shift towards the
eastern end of the state by Sunday.

&&

.MARINE...
A storm east of the Kamchatka peninsula produced a large long-period
northwest swell that filled in overnight and peaked early this
morning at Buoys 51001 and 51101 northwest of the islands. A gale
north of the islands also produced a smaller and shorter-period
north swell. These two swells will combine to produce border line
advisory level surf along north and west facing shores of the
smaller islands today. However, the swell heights at the
aforementioned buoys are already declining at this time, and the
nearshore buoys should be peaking in the next hour or two, before
declining as well. The shorter period north swell is expected to
gradually shift out of the north northeast today, removing its
contribution to the surf as well. As a result, the High Surf
Advisory will likely be dropped with the afternoon forecast
package.

A reinforcing north-northeast swell will filter in Sunday night
and Monday, with a slightly longer period. This could result in
near advisory level surf for east facing shores through Monday
night. Elsewhere, a small, long-period south-southwest swell will
continue to provide a small bump to the relatively quiet surf
along the south shores, before gradually subsiding through the
weekend.

An east to west surface ridge near 25N will maintain gentle to
locally fresh trade winds through this evening. Winds will weaken
and shift out of the southeast starting tonight as a trough
deepens northwest of the area. Winds and seas will remain below
Small Craft Advisory (SCA) levels for the next several days.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Sunday for north and west facing
shores of Niihau, Kauai, Oahu and Molokai and north facing shores
of Maui.

&&

$$

Public...Wroe/Donaldson
Aviation...Foster
Marine...Stall



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