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fxus63 kdmx 231726 aaa 
afddmx 


Area forecast discussion...updated 
National Weather Service Des Moines Iowa 
534 am CST Monday Nov 23 2009 


..updated aviation discussion... 


Short term /today/... 
moist southeasterly flow continue over the state...however drier air is being 
drawn nwwd into the southeastern zones. Precipitation area will continue to lift northward this 
morning and with combo of upper trough passage and somewhat drier air advance 
expect precipitation to taper off from the S between 12z and 15z. This fits best 
with Gem-regional/WRF-ex 4.5/fim solutions. For the east half of the 
state...the rest of the day should be dry. Next system will push clouds 
and precipitation into western Iowa this afternoon. Temperatures will be a bit tricky with the 
thinning clouds. Do not expect to see the low clouds to prevail and 
would not be surprised to see broken conds over much of the east half of 
Iowa. For that reason...raised maximum temperatures today a few degree from going 
forecast. 


Long term /tonight through Sunday/... 
00z GFS/ec/Canadian/sref...and now the 06z NAM are in good agreement 
regarding the system set to affect the region through Tuesday night. 
At 00z tonight the upper level low should have closed off over the 
western Central Plains...and a surface low should be over central Kansas 
with old stalled frontal boundary angling north into western Minnesota. With 
the upper level system finishing its dig tonight and convergence 
near the surface low to our SW and along surface trough/front over western 
Iowa...have adjusted probability of precipitation tonight to be highest...nearly 100 percent 
across western zones...but only chance probability of precipitation farther east. Min temperatures tonight 
will be mild...warmer than normal maximum temperatures...as dewpoints in the 
lower to middle 40s remain in place ahead of the system. 


Tuesday promises to be wet as the upper level and surface system migrate 
across central Iowa. Strong kinematic forcing moves over Iowa...but 
the frontogenetical forcing oriented north-south over western Iowa will 
dissipate. Meanwhile a secondary but weaker band of frontogenesis 
aligned under the deformation zone sets up more west-east across 
central Iowa. Essentially...with a low tracking from northwest MO into southeast 
Iowa...expect comma head precipitation shield to affect most of the area 
through the day. There is the potential for some decent rainfall in 
the one half inch to one inch range. Temperatures will show little recovery 
during the day...but could rise a few degrees. Expect some afternoon 
cooling in the west as cold advection starts to kick in. 


Thunder chances still appear high enough to mention tonight and 
Tuesday morning across the southern half or so...and the area may 
need to be expanded. Middle level lapse rates not as steep as shown 24 
hours ago...but tq index values support embedded convection and 
there is still some negative epv showing up. 


While the first system pulls out Tuesday night...another upper level 
low dives out of southern Canada and crosses Iowa Wednesday. The 
cold front associated with this system is what will usher in the 
colder air for the end of the week. Have maintained likely probability of precipitation in 
the northeast Wednesday...but much lower probability of precipitation farther souhthwest as 
moisture pulls away to the northeast. Thermal profiles still only 
support a rain or rain/snow mix through the day Wednesday...except 
in the far northwest where what does fall should be snow. 
However...another surge of colder air...deeper relative humidity...and some upper 
level forcing Wednesday night and Thursday morning will yield a decent 
chance for light snow across mainly the northeast third of the 
area...but could see flakes just about anywhere. Thanksgiving day 
looks to be cold and breezy with scattered snow showers or 
flurries...at least early in the day. Thinking that at worst...1 to 
2 inches of snow is possible northeast of a line from Estherville to 
Marshalltown by Thanksgiving morning...with possibly a dusting 
farther south. 


No changes to the forecast made beyond Friday. An upper level ridge 
builds across the upper Midwest Friday into Saturday...but gives way 
to the next longwave trough pressing across The Rockies Sunday. 


&& 


Aviation... 
23/18z...isentropic lift continues to maintain light rain over NE 
corner of County Warning Area. Expect this area to lift NE over the next few hours. 
More synoptic scale forcing entering western Iowa this 
evening...then progressing into eastern Iowa overnight. A period of 
somewhat stronger Theta-E advection is expected over central Iowa in 
the early morning hours. Not sure this will be enough to spark 
thunderstorms...but will keep cumulonimbus mention in dsm taf. As far as 
ceiling/visible GOES...went mainly with MOS guidance and adjusted with sref 
probabilistics. Most terminals should see IFR ceilings overnight 
into tomorrow. 


&& 


Dmx watches/warnings/advisories...none. 
&& 


$$ 


Short term...MS Nov 09 
long term...Moyer 
aviation...hinsberger