718 
fxus63 kdmx 191136 aaa 
afddmx 


Area forecast discussion...updated 
National Weather Service Des Moines Iowa 
536 am CST Wednesday Nov 19 2008 


..updated aviation discussion... 


Short term.../today through Friday night/ 
surface low swinging across northern Minnesota and over Lake Superior will 
bring a cold front from northwest to southeast across Iowa today. The front will 
cross most of our County Warning Area by noon and the far southeast in early 
afternoon. With north-northwest wind surge and some low clouds behind frontal passage...as 
well as slightly delayed cold air advection...biggest forecast difficulty is 
progging maximum temperatures. Obviously will be coldest in the north where 
temperatures are currently lowest and least heating will take place due to 
earlier frontal passage. No precipitation expected with the front as dry air is 
firmly in place. 


Outside of potential for some flurries/light snow showers in 
relatively steep low level lapse rates and higher low level relative humidity 
Thursday...main concern will be temperature trends tonight through Friday. Vorticity 
lobe swings through late Thursday as strong high pressure becomes 
entrenched in the MO River Valley. Airmass remains mixed into 
Thursday after cold front passage tonight resulting in relatively high confidence 
mins and maxes. NAM/GFS soundings in good agreement. This results 
in going above guidance tonight and near compromise Thursday. Mixing 
does not really subside until Friday so have gone above guidance Thursday 
night with mixing seeming to keep temperatures up and radiational cooling 
down. Little decoupling noted Thursday night either so lows remain above 
even the warmest guidance outside of kest and Nishnabotna valley 
areas closer to ridge. 


Much like prognosticated yesterday...short wave ejects out of Gulf Alaska low 
later today reaching MO valley late Friday. 00z GFS/European model (ecmwf) solutions 
have apparently converged on a solution with the GFS less aggressive 
than previous runs and the European model (ecmwf) more so. NAM is an outlier to the 
south and west and will not be followed. Bucking GFS/European model (ecmwf) 
agreement in favor of the NAM in later periods is rarely the way to 
go. This favored scenario results in fairly deep layer of 
coincident kinematic and thermodynamic forcing with noted fn 
response. GFS Cobb data depicts typical 13:1 ratios with the event 
and could result in 1-2 inches over northern and possibly into central Iowa 
so have boosted probability of precipitation well into chance category...especially northwestern 
sections before forcing begins to weaken as system heads southeast. 
Although temping...did not want to go with likely wording quite 
yet. That would be a big change from our current forecast...and GFS 
MOS as well as sref/gefs precipitation probs have yet to respond too much 
and only suggest low end chances for now. Although transient nature 
of the system should keep things from getting out of hand...decent 
moisture transport and low static stabilities above 700 mb on the southern 
fringe could produce short lived but moderate/heavy snows and low 
visibilities. Dendritic layer uvm also suggests higher probability of precipitation may have to be 
pulled southward if trends continue...but soundings also show drier low 
levels which could reduce its effectiveness. 


Long term.../Sat through Tuesday/ 
not many changes in this period with GFS now trending toward deeper 
European model (ecmwf) solution. System early in the week the primary concern...but 
with closed low appearing to be passing to our north Iowa should not 
see too much in the way of precipitation. Backed off to slight wording sun 
and switched precipitation more toward snow outside of central sections 
Monday during peak heating. 


&& 


Aviation...19/12z 
primary aviation concern today will be cold front advancing from northwest 
to southeast across the area. The front should pass through the northern 
sites around 15-17z...kdsm around 18z...and kotm around 19-20z. 
Immediately behind the front winds will shift to north-northwest and increase 
substantially...especially at kmcw and Kalo. Also some concern 
regarding MVFR ceilings in cold air advection behind the front. This 
appears most likely at the northern sites after sunset tonight...but 
there is also a chance of some low ceilings around fl020 in the few 
hours after frontal passage this morning/afternoon. This will be closely 
monitored. No precipitation is expected. 


&& 


Dmx watches/warnings/advisories...none. 


&& 


$$ 


Jwl/bss/klp