172 
fxhw60 phfo 220135 
afdhfo 


Area forecast discussion 
National Weather Service Honolulu HI 
400 PM HST sun Mar 21 2010 


Synopsis... 
winds will become light and variable Monday as a weak trough forms 
near Oahu or Kauai. An upper trough will move over the islands 
Tuesday causing the weather to become somewhat unstable. 
Consequently there may be greater shower activity Monday evening and 
Tuesday with a chance of heavy showers. Moderate trades will return 
by the middle of next week. 


&& 


Discussion... 
a southeasterly low level flow is apparent in satellite 
imagery...the most prominent area of clouds just north of the 
islands now directed northward. Skies were mostly clear south of the 
smaller islands. Showers over central Kauai have decreased this 
afternoon while the usual diurnal clouds have spread over the slopes 
of The Big Island. Clouds and showers will likely continue moving 
onto the windward Big Island tonight and may also affect windward 
east Maui. Otherwise weak shower activity over the smaller islands. 


Over the next 12 hours a rather deep west-southwesterly flow will 
become established. High clouds presently passing over the islands 
are from a minor upper atmospheric short wave and should move 
off by Monday. 


There is a front about 700 miles to the northwest associated with a 
large low far to the north. The front will not reach our islands 
but the surface ridge just to the north has weakened. Models are 
now in better agreement depicting a weak surface trough which will 
extend roughly across Oahu or Kauai by Monday afternoon. A 
strengthening upper trough near the international dateline will 
progress eastward crossing our islands Tuesday. The resulting 
unstable atmosphere during the Monday evening Tuesday period should 
produce increased shower activity with locally heavy showers. 
Presently models are not depicting excessive precipitation...but the 
ingredients for some heavy showers will be present. 


More stable conditions and trade winds return on Wednesday although 
the high that builds to the north is somewhat weakened by a portion 
of a weak upper trough hanging back over the islands. Consequently 
returning trades will be only moderate strength Wednesday through 
the remainder of the week. Trade wind weather will exhibit the 
familiar pattern of showers mainly affecting the windward and 
mountain areas but may be slightly wetter due to the upper trough 
remaining over the area. 


&& 


Marine... 
the current south swell has decreased slightly...near 3.5 feet at 17 
seconds on buoys immediately to the south of Oahu and Lanai. The 
high surf advisory will be extended through Monday afternoon 
although with continued lowering of the swell surf may drop just 
below the 8 foot advisory before Monday afternoon. The wave 
generating fetch area persisted about two and a half days. Although 
it is not unusual for the first south swells to begin arriving in 
March or even February...swells this large usually begin arriving in 
may with the peak of south swell activity occurring around the end 
of June or beginning of July corresponding with winter in the 
southern hemisphere. 


The next large northwest swell is expected Tuesday with surf above 
advisory level on north facing and also west facing shores of some 
islands. Surf may even approach the warning threshold of 25 feet on 
north facing shores. 


The downward trend in wind speed has allowed the Small Craft 
Advisory to be cancelled. 


&& 


Hfo watches/warnings/advisories... 
high surf advisory until 6 PM HST Monday for south facing shores of 
all islands. 


&& 


$$ 


Hoag