Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 172000

Area Forecast Discussion
National Weather Service Honolulu HI
1000 AM HST Sat Feb 17 2018

Wet and unstable conditions will pose a threat for flooding over
the eastern end of the state today. After a dry start, afternoon
sea breezes will produce spotty showers over the western half of
the island chain today. The moist and unstable conditions will
spread to the entire state overnight as a potent upper level
trough moves in from the northwest. The resulting unstable
conditions will bring a threat of heavy, flooding rainfall and
thunderstorms across much of the state through the Monday holiday.
A somewhat wet, though more stable, southeasterly flow will
likely develop by Tuesday.


Moist and unstable conditions have developed slightly faster than
anticipated over the eastern end of the state. Deep moisture
(precipitable water of around 2 inches) is pooled around a slow-
moving surface trough between the Big Island and Maui, as a strong
upper level digs over the state from the northwest. The resulting
instability has triggered quite a bit of heavy shower and
thunderstorm activity around the Big Island and just east of Maui
this morning, where a Flash Flood Watch goes into effect shortly.
The threat of heavy showers and thunderstorms will rise through
the day as the atmosphere becomes increasingly unstable. Snow
showers, some heavy, are expected to develop on the high summits
of the Big Island, where a Winter Weather Advisory will be in
effect by noon.

Rather dry conditions are hanging on over the western half of the
island chain this morning, though instability will increase this
afternoon. The overnight Lihue sounding, recent aircraft data out
of HNL, and satellite imagery showed a low level inversion near
5,000 ft and precipitable water values of less than an inch. As
the upper level trough advances, the low level inversion will
erode quickly this afternoon, but moisture will increase only
slightly. As a result, expect sea breezes to produce spotty
afternoon showers over the interior with an outside shot at a
heavy shower or thunderstorm popping up.

Moist and unstable conditions will spread to the entire island
chain tonight and Sunday. The approaching upper level trough will
wipe out the inversion everywhere this evening, and by early
Sunday, 500 mb temps will drop to -13 to -16C, leading to mid
level lapse rates greater than 7C/km. Meanwhile, the surface
trough and associated moisture currently over the eastern end of
the state will spread westward. As a result, it`s looking
increasingly likely that we will be expanding the Flash Flood
Watch to all islands, and the Winter Weather Advisory for the Big
Island summits will continue. In addition, we will have to monitor
the potential for strong thunderstorms capable of generating hail
and gusty winds.

The threat for heavy showers and thunderstorms is expected to
continue into the Monday holiday. The GFS and ECMWF models both
show a compact upper level low developing just southwest of Kauai
Sunday night then swinging over the islands on Monday. Although
500 mb temperatures will be on an upward trend, significant
instability will persist and will be easily strong enough to
support thunderstorms. The models are in fairly good agreement in
showing a sharpening of the surface trough over the western end
of the island chain during this time. While this could shift the
focus of heavy showers to a zone of convergence to the northeast
of the state, there should still be enough moisture to trigger
heavy showers and thunderstorms over the islands, especially
during the afternoon hours.

The upper level trough lifts to the northeast on Tuesday,
allowing the atmosphere to become more stable through midweek.
East to southeast winds and lingering shower activity is expected.


A surface trough of low pressure will remain stalled near the Big
Island today but shift westward tonight. Meanwhile, an upper
level trough approaching from the northwest will increase air
mass instability. Broken to overcast low clouds and embedded
heavier showers will linger across the Big Island through this
evening then shift westward. Slight chance of thunderstorms over
the eastern third of the state will spread statewide by tonight.
AIRMET Sierra for tempo mountain obscurations along windward and
southeast portions of the Big Island may need to be expanded this
evening to include the remainder of the Big Island or east Maui.


An upper level disturbance will produce unstable weather with the
potential for thunderstorms today near the Big Island and Maui,
and then across the state tonight through Monday.

A series of small west-northwest swells are expected over the
next few days. Resultant surf will remain below advisory levels. A
larger, long period swell is expected Tuesday night which will
produce near advisory level surf Wednesday. High pressure in the
East Pacific will send a short period swell into east facing
shores starting Monday which will continue into next week. This
could produce advisory level surf along east facing shores. Winds
and seas will remain below small craft advisory threshold through
the holiday weekend.

See the latest Oahu Surf Discussion (SRDHFO) for additional
details on surf and swell.


Flash Flood Watch from noon HST today through Monday afternoon
for the Big Island and Maui.

Winter Weather Advisory from noon today to 6 PM HST Monday for
Big Island Summits.



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