Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 180647

Area Forecast Discussion
National Weather Service Honolulu HI
847 PM HST Sat Feb 17 2018

Moist and unstable conditions will spread westward across the
entire state overnight and Sunday as a strong upper level trough
approaches and a weak surface trough currently near Maui drifts
westward. The resulting unstable conditions will bring a threat of
heavy, flooding rainfall and thunderstorms through the Monday
holiday. A somewhat wet, though more stable, southeasterly flow
will likely develop by Tuesday.


Scattered heavy showers and isolated thunderstorms have flared
across parts of Maui County and the Big Island along with the
adjacent coastal waters early this evening. Recently, showers have
also flared up along the Oahu windward coast. Thus far, most of
the activity has not persisted very long at any particular
location, with cells tending to intensify, peak and die out rather
quickly. Recent aircraft data from Kona and Kahului continue to
show little or no inversion present, while a recent aircraft out
of Lihue still shows a significant although weakening inversion
based near 6000 feet. The 00Z Hilo sounding measured PW of 1.26
inches, but recent MIMIC-TPW satellite imagery shows higher PWs
of 1.5 to 1.7 inches across the eastern half of the state, with a
significantly drier airmass over Oahu and Kauai. The 00Z Lihue
sounding was consistent with the more recent aircraft data, with
PW of only 0.67 inches and a strong inversion based at 5500 feet.
The moist airmass is found along and east of a surface trough
located near Maui. Aloft, water vapor imagery depicts a strong
upper trough continuing to dig west of the state.

Overnight, expect the digging upper trough to continue eroding
the lingering inversion over western sections, while the surface
trough near Maui continues to drift slowly westward. This will
allow the deeper moisture to slowly spread westward as well. The
Flash Flood Watch remains posted for the entire state through
Monday afternoon. The highest potential for heavy rains and
thunderstorms during the night would appear to exist from around
western parts of the Big Island through Oahu. We will also
continue to monitor for possible strong thunderstorms capable of
generating small hail and gusty winds. In addition, a Winter
Weather Advisory remains posted for the Big Island summits through
Monday. Although little snow has been observed at the summits
thus far, there remains the potential for significant snowfall as
deeper moisture moves across the area and summit-level
temperatures decrease well below freezing during the next 24 to
36 hours.

The threat for heavy showers and thunderstorms is expected to
continue for Sunday through the Monday holiday. The GFS and ECMWF
models both show a sharp and negatively-tilted upper trough,
possibly cutting off fully into an upper level low developing near
Kauai by Sunday evening, which then slides eastward over the
islands on Monday. This will provide significant instability with
cold mid-level temperatures resulting in no inversion, which will
combine with adequate although not copious moisture to support
heavy downpours and 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 convergence zone to the
northeast of the state, there should still be enough moisture to
trigger scattered heavy showers and thunderstorms over the
islands, especially during the afternoon hours.

The mid/upper level trough lifts to the northeast Monday night
and Tuesday, allowing the atmosphere to become more stable with a
mid level dry slot moving over the islands. This should result in
less intense shower activity with the threat for thunderstorms
lessening as well. East to southeast winds are expected along
with lingering scattered shower activity for the midweek period.
Longer range guidance shows yet another deep mid/upper trough
digging later in the week, but further to our west. This would
maintain moist east/southeast flow over the islands through the
rest of next week, with trade winds remaining absent.


A weak surface trough near Maui will gradually shift west over the
next few days. Conditions will remain unstable as an upper level
trough starts moves over the state tonight. Pop up isolated
thunderstorms should be expected tonight, especially over the
central and eastern islands. Areas of heavy rain can bring a
reduction in CIGs/VIS. AIRMET SIERRA for mountain obscuration is
posted over the Big Island, Maui, Molokai, and Lanai, and the
north through east sections of Oahu. This will be tailored as

Upper level turbulence may become more of a factor as the upper
trough moves over the state and AIRMET TANGO for upper level
turbulence may become necessary by early tomorrow morning.

Any of the layered cloud areas have the potential for light icing,
mainly between 140-FL240.


Unstable atmospheric conditions combined with pockets of
enhanced moisture will continue to produce isolated strong
thunderstorms and locally heavy downpours through Monday. Expect
reduced visibilities, locally gusty winds, increased seas,
frequent lightning, and the possibility of small hail in the
strongest thunderstorms. If a Special Marine Warning due to
strong thunderstorms is issued for your location while boating
this weekend, be prepared to return to safe harbor immediately.

A surface trough in the vicinity of the islands will maintain
moderate southeasterly winds east of the trough, and light to
moderate northeasterly winds to the west of the trough. Expect
mainly light to moderate east to southeast winds to prevail from
Tuesday through the middle of next week. Therefore, no Small Craft
Advisory conditions are forecast.

Small northwest swells will continue through early next week, with
surf remaining well below the High Surf Advisory (HSA) thresholds
along north and west facing shores of the islands through Tuesday.
However, a longer period west-northwest swell is expected to
spread down the island chain late Tuesday, which may cause surf to
approach the HSA criteria along some north and west facing shores
on Wednesday, especially Kauai and Niihau. This swell will
gradually subside Thursday, but another reinforcing west-
northwest swell will arrive Thursday night. This swell will
gradually fade from Friday into next weekend.

The long fetch due to the flow around a surface high over the
northeastern Pacific will send a short-period east swell into the
islands early next week. This swell will continue to build Monday
night, with surf likely reach the HSA threshold of 8 feet along
most east facing shores on Tuesday. This east swell will subside
slightly Wednesday, but may increase again on Thursday.


Flash Flood Watch through Monday afternoon for all islands.

Winter Weather Advisory until 6 PM HST Monday for Big Island



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