234 
fxus65 kboi 180354 
afdboi 


Area forecast discussion 
National Weather Service Boise Idaho 
950 PM MDT Wednesday Mar 17 2010 


Discussion...an old weak frontal zone continues to lie along the 
Idaho Nevada border with nothing more than broken to overcast middle 
and high clouds and a small temperature dewpoint discontinuity. 
Meanwhile...a potent middle level trough continues to dig southeast 
from western Washington and Oregon. This feature is forecast to 
interact with the hung up front early Thursday morning producing 
isolated showers over our southern border with Nevada with a better 
chance of probability of precipitation remaining well east of the Boise forecast area near 
Utah...northeast Nevada and eastern Idaho. The bigger story for US 
however is the gusty winds and much cooler temperatures arriving 
over the area with the passage of the upper level trough. Very windy 
conditions expected Thursday afternoon and early evening over 
southwest Idaho mainly from the lower Treasure valley of Idaho 
eastward to Mountain Home...as the upper level winds are well 
aligned with the surface and the terrain features. A Wind Advisory 
may be needed in this location with northwest sustained speeds 
ramping up to between 20 and 30 miles per hour with gusts to 40 miles per hour or greater. 


&& 


Aviation...VFR conditions through Thursday. Scattered to broken middle 
cloud forming off the mountains of eastern Oregon tonight. Northwest 
surface winds will be light to moderate tonight...increasing 
Thursday to 25g40kts after 18z. Winds aloft will be moderate west to 
northwest. 


&& 


Previous discussion...tonight through Friday...cold front is draped 
just to the south of the Idaho/Nevada border...although it is difficult to 
pinpoint exactly because the winds near it are so weak. It runs 
about parallel to the band clouds as depicted in the satellite 
imagery which stretches across northern Nevada and southern Idaho. There 
are some 88d radar returns within this band of clouds...but probably 
too weak for precipitation. A weak upper level trough near the pacnw coast 
will gradually strengthen and move into the area tonight. This 
trough will invigorate the front with showers possible in the 
southern portions of the County Warning Area late tonight through Thursday morning. 
The front and upper trough will drive south and east of the area 
late Thursday with a cooler...drier..and stable airmass to follow 
lasting into Friday. Winds will increase on Thursday...possibly 
reaching advisory speeds in parts of the upper Treasure valley for a 
time Thursday afternoon. Models have wavered on strength of the 
winds so am not confident at this time to issue a Wind Advisory. 
Temperatures will be back to near climatology levels Thursday and Friday. 


Long term...Friday night through Wednesday...models agree well 
Friday night through Sunday morning...with an upper level ridge over 
the Cascades decreasing in amplitude as it shifts eastward. Starting 
Sunday differences increase...especially offshore where the 
operational GFS and European model (ecmwf) bring a trough onshore while the GFS 
ensemble mean and Canadian have the trough 5 degrees further west. 
The ensemble means of the GFS...ECMWF...and Canadian show the best 
agreement...so we are relying on these solutions from Monday Onward. 
With the ensemble mean holding onto the ridge through Monday...any 
chance of showers that day will be confined to our northern 
periphery and higher mountains after a dry weekend. Temperatures 
will be a few degrees above normal through Monday...then cool as 
troughing develops over our area Tuesday and Wednesday...with 
temperatures averaging a few degrees below normal with precipitation 
probabilities near climatology. 


&& 


Boi watches/warnings/advisories... 
Idaho...none. 
Or...none. 
&& 


$$ 


Weather.Gov/Boise 


Discussion...gs 
previous discussion...bw 
long term....jt/bb 
aviation.....Js