954 
fxhw60 phfo 210418 cca 
afdhfo 


Area forecast discussion...corrected 
National Weather Service Honolulu HI 
400 PM HST Sat Mar 20 2010 


Synopsis... 
trade winds will weaken Sunday as a surface high far north is 
displaced eastward by a strong low moving across the central 
North Pacific. Winds will be light and variable Monday. Wetter 
weather is expected Monday evening and Tuesday with a chance of 
heavy showers when the atmosphere becomes unstable. Light trades 
will return by the middle of next week. 


&& 


Discussion... 
clouds were rather sparse immediately upwind to the northeast of the 
islands...although during the day clouds clung to windward sides of 
the islands. There were scattered showers over mountains mainly 
on Maui and The Big Island. Shower activity over leeward Big Island 
was more widespread than in the past few days...but will fade at 
night with the subsidence that will accompany nighttime cooling of 
the land. Open cellular clouds farther northeast should bring an 
increase in clouds and accompanying scattered showers to windward 
and mountain sections tonight. Slightly more moist conditions were 
reflected in the afternoon balloon sounding over Hilo but it was 
rather dry on the Lihue sounding with precipitable water at three 
quarters inch...temperature inversions ranged from 5000 to 7500 
feet. Weak ridging aloft implies continued stable conditions and 
computer models also suggest slightly drier trade wind weather 
pattern Sunday. 


&& 


Previous discussion 10 am... 
a change in the synoptic weather pattern will begin on Monday. The 
surface high to the north will shift eastward in response to the 
movement of a large low presently in the central North Pacific. The 
front associated with the low will not however progress onto our 
islands. A surface trough will extend from this low to near or over 
main Hawaiian islands on Monday...the GFS depicts a sharper trough 
across the islands than other models. In any case...weak winds are 
expected Monday...possibly from the southeast with afternoon sea 
breezes likely. 


The more significant effect however will be the unsettled weather 
Monday evening into Tuesday. An upper trough just east of the 
dateline Monday will advance toward the main Hawaiian islands 
producing a somewhat unstable...though not particularly 
moist...southwest flow. The resulting unstable conditions will 
promote a convective weather pattern...enhanced afternoon clouds and 
showers Monday and Tuesday with a chance of some heavy showers. The 
air mass will be a little warmer than it has been lately. 


The upper trough is not especially strong and flattens 
somewhat as it moves over the islands Tuesday. The southwest flow 
ends Wednesday night although a weak upper trough hangs back over 
the islands after the main trough to the north has progressed 
eastward. Therefore a surface high is delayed in becoming 
reestablished to the north of the islands and we are left with only 
light to possibly moderate trades through Friday. 


&& 


Marine... 
this afternoon the big south swell registered near 4.5 feet and 
continued at a powerful long period of 20 seconds on the buoy near 
the South Shore of Oahu and the cdip buoy south of Lanai...the peak 
of the swell probably occurring tonight. Surf will slowly decrease 
Sunday afternoon through Tuesday. The high surf advisory for south 
facing shores of all islands runs through Sunday evening but may 
need to be extended into Monday. Another large northwest swell is 
expected Tuesday. 


Seas below 10 feet tonight except in the typically windier 
areas.,,Consequently small craft advisories for only the typically 
windy areas around Maui and southeast of The Big Island. 


&& 


Hfo watches/warnings/advisories... 
Small Craft Advisory until 6 PM HST Sunday for Maalaea Bay-Pailolo 
Channel-Alenuihaha Channel until 6 PM Sunday. 


High surf advisory until 6 PM HST Sunday for south facing shores of 
all islands. 


&& 


$$ 


Hoag