Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 280217

Area Forecast Discussion
National Weather Service Honolulu HI
417 PM HST Thu Apr 27 2017

A cold front and associated upper trough will approach the state
tonight and will bring the potential for heavy rain and
thunderstorms across the state on Friday. The front will stall
over Maui County or near the northern Big Island this weekend with
cooler and drier air reaching Kauai and Oahu Saturday. A cold
upper low will dig southward over the state on Sunday and bring
the potential for heavy showers and thunderstorms Sunday and
possibly Monday.


Light southeast flow over the islands have allowed sea breezes to
bring afternoon clouds and showers over the island interiors this
afternoon. Visible satellite and radar show moderate to briefly
heavy showers with mostly cloudy skies. These clouds and showers
are expected to decrease this evening around sunset. Land breezes
will fill in over most of the state tonight.

The southeast flow is the result of an approaching cold front 250
miles northwest of Kauai this afternoon. As associated trough
aloft is also approaching the state which will begin to destabilize
the atmosphere over the course of the next 12 hours. 00Z soundings
at Hilo and Lihue show a weakening and rising inversion height of
9000 to 10000 feet. Soundings showed precipitable water values of
around 1.2 inches.

The atmosphere will become more unstable as the upper trough
brings colder temperatures aloft over the state by Friday
morning. Models depict a cold pool with 500 mb temperatures around
-12 celsius temperatures by morning.

The resulting instability will tap into the low level moisture along
and ahead of the incoming front. Thus, a slight chance of thunderstorms
and possible locally heavy rain is in the forecast for most of
the state on Friday and Friday evening. The front itself will
reach Kauai Friday afternoon and, Oahu late Friday, then stall
over eastern Maui County or the northern Big Island Saturday
afternoon. Breezy north winds will build in through the day over
Kauai and Oahu Friday night.

Cool and dry air will follow behind the front, but will likely
only reach as far as Oahu and bring more pleasant conditions over
both islands on Saturday. The dry air will provide a brief reprieve
from the active weather for Kauai and Oahu with dew points dipping
into the upper 50s Saturday and Saturday night. Maui County and
the Big Island will remain under the stalled front and will keep
the higher humidity and the potential for heavy rain and
thunderstorms Saturday.

A very cold upper level low will cut off and drop southward
toward the islands, bringing more instability over the state
beginning around Sunday morning into early next week. The GFS,
ECMWF and NAM depict 500 mb temperatures dropping to -16 celsius.
While the models are coming into a little better alignment with
the core of the low passing between Kauai and Oahu on Sunday,
there are still some subtle timing differences between the passing
of the low over Hawaii.

Essentially, we are looking at the upper low tapping into the low
level moisture set up over Maui County and the Big Island and will
bring the potential for heavy rain and thunderstorms for Sunday
and Monday. While most of the focus will be over the eastern half
of the state, the greatest instability will pass over the Kauai
and Oahu Sunday. We cannot rule out the potential for heavy rain
and thunderstorms statewide on Sunday. Some showers and
thunderstorms may continue for the eastern half of the state into
Monday. There is also the chance for winter weather on the Big
Island summits this weekend.

Gentle trade winds and much more stable weather should return by
the middle of next week, as the upper trough weakens and moves
east of the area.


An approaching cold front has weakened the trade winds and shifted
winds from the southeast. Widespread leeward sea breezes are
expected in the afternoon with convective cloud build ups and
ISOL MVFR over mountain and interior sections. Skies will
partially clear overnight with downslope flow expected. The front
will reach Kauai Friday with TEMPO MVFR expected in showers and
moderate north winds. There will be a chance for scattered
prefrontal showers over the rest of the state.

AIRMET Sierra is in effect for mountain obscuration across all
mountains of Oahu through this afternoon.


Winds will remain light and variable today as a north to south
oriented trough forms over the central islands ahead of the
advancing front. Satellite imagery shows the front is just
beyond the NW offshore waters this afternoon, Kauai Friday
morning, and by Oahu Friday afternoon, before stalling over Maui
County through the weekend. Breezy NE winds behind the front will
warrant a Small Craft Advisory for the coastal waters around and
west of Oahu. The front is forecast to inch slowly eastward to the
Big Island late in the weekend with the NE winds weakening below
advisory level. The arrival of the front will be enhanced by a
strong upper level trough resulting in an unsettled weather period
this weekend, including a chance of thunderstorms over the
surrounding waters through Monday of next week.

Swell-wise, the currently NW swell will continue to subside today.
A moderate NW swell with an 8 second period arrives Friday night
and peaks Saturday. A similar size swell from the north follows
for Sunday night and Monday. A small NW swell with a 14 second
period is on tap for Tuesday night and Wednesday.

The south facing shores will be impacted by a couple of south
swells during the forecast period, the first of which is due to
arrive early Friday morning. Surf from this swell could be large
enough to warrant a high surf advisory for the south facing
shores. A gradual lowering will follow early Saturday morning.
Another southerly swell is expected to arrive Monday afternoon,
but the majority of this energy appears to stay east of Oahu,
focusing instead on eastern Maui County and the Big Island.

Smaller surf can be expected along the east facing shores over
the weekend as the trades take a breather.





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