Weather Service Report

FXHW60 PHFO 260153

Area Forecast Discussion
National Weather Service Honolulu HI
353 PM HST Sat Feb 25 2017

Cloudy and somewhat cool conditions are expected into Sunday as
weak easterly trade winds focus modest shower activity over the
central portion of the state tonight then near Kauai and Oahu on
Sunday. The atmosphere will become increasingly moist and unstable
Monday and Tuesday as a strengthening trough to the west of
islands pulls moisture up from the deep tropics. Chances for
heavy flooding rainfall and thunderstorms will increase on Tuesday
and will remain rather high into Wednesday as the trough slowly
moves across the area. A ridge will build over the islands
Thursday and Friday, bringing drier conditions.


Cloudy and somewhat cool conditions prevail this afternoon under
a gentle to locally moderate easterly trade wind flow. A weak
surface high sitting about 500 miles northeast 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 7500 ft according
to the afternoon soundings. These dry and stable conditions are
disrupted along a narrow band of low level moisture that has been
stalled across Maui County, where only a few tenths of rainfall
have fallen along windward slopes. Otherwise, little to no rainfall
has been observed. A deepening upper level trough several hundred
miles to the west is generating a thick shield of high clouds
over and southwest of the state. This, along with the cool airmass
in place, has caused afternoon high temps to struggle to get much
higher than the mid 70s.

Weak easterly trade winds and cloudy skies will prevail tonight
into 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 to Kauai on Sunday. As the mid level ridge
further erodes and high clouds thin somewhat, there is a chance
for some afternoon convection with a heavy shower or two over
interior Kauai, but since odds are low, heavy showers have been
left out of the forecast for now.

The atmosphere will trend increasingly unstable on Monday as the
mid to upper level trough edges closer to the islands. High clouds
that may have thinned somewhat Sunday and Sunday night will
thicken again, and deep tropical moisture will approach the
islands from the south. This enhanced moisture may reach the Big
Island and windward Maui by afternoon, where heavy showers have
been added to the forecast along with increasing chances for snow.
Some of this moisture may get caught up in the easterly trade
winds and fuel heavy shower development along windward slopes
across the rest of the state Monday night.

Moist and unstable conditions will prevail Tuesday into Wednesday,
raising the possibility of flash flooding as well as winter storm
conditions on the high summits of the Big Island. The ECMWF and
GFS models show a strong west Pacific jet stream diving into the
nearby mid to upper level trough and driving it over the islands.
This trough will tap tropical moisture surging from the south to
create conditions conducive for heavy flooding rainfall and
thunderstorms. Details remain unclear as to which portion of the
state is most at risk, with the models hinting at surface low
development somewhere over the islands Tuesday night or early
Wednesday. Although the freezing level may be somewhat elevated
during this time, chances are high that the upper reaches of the
Big Island summits could see winter storm conditions with strong
winds and significant snowfall.

Drier and increasingly stable conditions will develop on Thursday
and Friday as the deep trough lifts northeast of the state and low
level ridging builds in from the east. This will result in south
to southwest surface winds with pockets of showers passing
overhead. Another front may approach the state next weekend.


Gentle background east southeasterly flow near the surface will
allow for variable winds across much of the state tonight. A weak
shower band straddled over Maui County this afternoon will slowly
push north up the island chain tonight. Isolated MVFR ceilings
and mountain obscurations will be possible near these light
showers, but no AIRMET Sierra is in effect for this.

A 130 knot subtropical jet over the state will bring moderate
turbulence aloft over Kauai and Oahu tonight. 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 Sunday. The band of high clouds is
also expected to deepen during the day Sunday and may cause some
moderate icing problems near the Big Island.


The long-period northwest swell that caused advisory level surf
along north and west facing shores of the smaller islands
yesterday through this morning has decreased sufficiently to
warrant the cancellation of the High Surf Advisory. 

The small, short-period north swell from earlier today, will shift
out of the north-northeast tonight. A reinforcing modest, longer-
period swell from the north-northeast will filter in Sunday night
and Monday. 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 rest of 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.






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