658 
fxhw60 phfo 220604 
afdhfo 


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


Synopsis... 
winds will lighten into Monday as a weak trough forms near Oahu. A 
moister air mass...together with cold air in the middle 
atmosphere...will intensify showers through Tuesday. Moderate trades 
with more typical weather will return gradually by late in the week. 


&& 


Discussion... 
surface observations around the main Hawaiian islands showed 
variable wind conditions overall...though most sites still reported 
an easterly wind component. Pressure trends implied a significant 
decrease in wind speed over the past 24 hours...and cloud motions 
east of the state suggested a trend toward southeasterlies. Aloft... 
a gradually deepening trough developed west of the state. 


All of these changes agreed well with model predictions that the 
troughing aloft will develop a surface trough near or just south of 
the islands over the next 24 hours. While the weak frontal band 
approaching from the northwest will continue to play a role...the 
surface trough closer to the state likely will prevent the surface 
flow from developing too much of a southerly component. The 
resulting sharp pressure gradients may lead to some unusual wind 
interactions with the complex local topography though. 


Colder temperatures associated with the trough aloft...together with 
moisture convergence at lower levels...likely will destabilize the 
atmosphere from Monday into Tuesday. Although rainfall will become 
somewhat more widespread...locally heavier showers may be the more 
noticeable effect. Mechanisms of initial shower formation probably 
will not change much though...so typical scenarios might include 
trade-wind and diurnal sea-breeze showers developing into proper 
towering cumuli. The local tool for predicting thunderstorm 
development shows little chance when run off of GFS output...and 
only a modest bullseye north of Oahu on Tuesday morning using European model (ecmwf) 
output. In summary...showers will be heavier than usual through the 
first half of the week...but neither excessive rainfall nor severe 
weather are expected at this time. 


From Tuesday through the second half of the week...the middle-level 
trough will fill in gradually...though passing short waves may 
reinforce it at times. With reduced support from aloft...the surface 
trough will dissipate...allowing the subtropical ridge to rebuild 
somewhat...bringing back moderate trades. Thus...more typical 
trade-wind weather will return...though lingering moisture from the 
trough...and from the dissipated frontal band further north...will 
continue to make windward showers a bit more widespread than usual. 
Conditions are expected to trend toward climatology by late in the 
week. 


&& 


Marine... 
an unseasonable south swell is expected to keep the high surf 
advisory in effect through Monday afternoon. However...faster 
decreases in the swell may drop surf just below the 8-foot advisory 
level before this time. 


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


&& 


Hfo watches/warnings/advisories... 
high surf advisory until 6 PM HST Monday for Niihau-Kauai 
windward-Kauai leeward-Oahu South Shore-Waianae coast-Molokai- 
Lanai makai-Kahoolawe-Maui leeward west-Maui Central Valley- 
leeward Haleakala-Kona-south Big Island-Big Island north and 
east. 


&& 


$$ 


Ryshko