Weather Service Report


560 
FXHW60 PHFO 280703
AFDHFO

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

.SYNOPSIS...
A cold front and 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
northern Big Island this weekend with cooler and drier air reaching
Kauai and Oahu Saturday. A pool of cold air aloft will move over 
the state on Sunday and bring the potential for heavy showers and 
thunderstorms area wide Sunday and possibly Monday. 

&&

.DISCUSSION...
The approaching front and upper level trough have cause the trades
to become light and variable over the western half of the island
chain this evening. Scattered showers, aided by daytime heating,
resulted in some scattered showers, primarily on Oahu and Lanai.
These showers will be dwindling down in the next couple of hours 
over land but they won't be going away totally during the
remaining overnight hours as the air mass continues to destabilize
mainly in part of the upper trough. The forecast was updated to 
reflect that for especially Maui County. The forecast was also 
updated for the Big Island, firstly lowering the POPs for the 
windward side to isolated showers, and included some clouds and 
isolated showers for the leeward coastal sections. Lastly, a
slight chance of thunderstorms is now part of the overnight
forecast for the waters west of Molokai.

The front that will be affecting the islands is currently 180
miles NW of Kauai. It is still on tap to reach Kauai Friday 
afternoon, and Oahu in the evening. It will not be a clear cut 
frontal passage as light N and NE winds will be run ahead of the 
front as it reaches Kauai. The upper level trough is about 380
miles NW of Kauai. It is forecast to be near Kauai at day break
Friday.  

Breezy north winds will fill in behind the front across Kauai and
Oahu Friday night initially but diminish slightly on Saturday 
while the front inches to as far as northern Big Island Saturday 
afternoon. A cool dry air mass with dew point to as low as the 
upper 50's will accompany the brisk northerly winds that reach 
only as far as Oahu. East of Oahu, the weather will be of
scattered showers, a few locally heavy, and there will be a 
slight chance of thunderstorms.

The nice weather over Kauai and Oahu will only be brief. The 
weather after Saturday evening, mainly after midnight, will start 
to turn sour across Kauai and Oahu as the next upper level 
disturbance, a low with a pool of very cold air of minus 19 
degrees C at 500 mb, comes into play. The models are in sync with 
this feature settling over Kauai and Oahu by Sunday daybreak.

This is an messy picture synoptically speaking. The surface front
remains stalled over Maui County on Sunday which may act as a 
enhancer to the development of the heavier showers. In short, the 
active weather favors Maui County and the Big Island over the 
weekend. A Flash Flood Watch for at least Maui is still not out of
the question at this point. We will see how persistent the models
are with this unsettle scenario.

The front weakens to a trough over the central islands Sunday
night with a moisture southerly wind flow east of the trough. The
upper low stalls out some 200 miles SSW of Kauai, and lingers
there through Monday. The ECMWF and GFS solutions differs thereafter
in the placement of the upper low. The GFS has it just to the 
south of Oahu Monday night while the EC still has it 200 miles SSW
of Kauai. With either of these scenarios, the unsettle weather is
likely to persist into Monday night.



&&

.AVIATION...
A front will approach from the northwest tonight, then move
through Kauai on Friday. Light winds ahead of the front will allow
for overnight land breezes, with daytime sea breezes expected in
most areas from Oahu eastward on Friday. Across Kauai, winds will
pick up out of the north Friday morning behind the slow moving
front. Moisture ahead of the front along with an approaching 
upper level disturbance will keep conditions somewhat unsettled 
over the next 24 hours, with isolated showers and MVFR cigs/vsbys
possible in most areas. More widespread MVFR cigs/vsbys are 
expected across Kauai on Friday along and behind the slow moving 
front, with the most unsettled conditions expected over windward 
and mauka areas. Across the other islands, shower coverage will 
increase during the afternoon and evening hours on Friday, likely 
resulting in temporary MVFR cigs/vsbys as well.

AIRMET Sierra remains in effect for mountain obscuration across
Oahu. Conditions should improve here later tonight. 

&&

.MARINE...
The forecast has been updated to include a slight chance of
thunderstorms for the western waters tonight to account for 
convergence ahead of an approaching front. The front will near the
waters west of Kauai Friday, introducing Small Craft Advisory 
winds and seas to the waters around Kauai at least into Saturday. 
The front and an upper level trough will help to further 
destabilized the atmosphere, and the slight chance for 
thunderstorms will spread across all waters Friday. Thunderstorm 
chances will decrease behind the front, before spreading across
all waters late Saturday as an upper low and developing surface
low slide over the island chain. The forecast confidence remains
low on the exact placement of thunderstorm activity, but mariners
should stay tuned to the updated forecast Friday and this 
weekend.

Surf along north facing shores will see a small bump Friday night
into Saturday from a short period swell associated with the
stronger winds behind the front. Surf will stay below advisory 
conditions. A series of small, longer period northwest swells are
then expected for next week. 

A series of long period south swells will grace the south facing
shores into next week. The first swell is expected to peak Friday
night, brining near- or just below advisory level (8 ft) surf to 
the south facing shores Friday and Saturday. Followed by another 
similar sized swell Monday. The surf will then ease up by 
midweek. 


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

Discussion...Lau 
Aviation...Jelsema 
Marine...Eaton

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Western Regional Climate Center, wrcc@dri.edu