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Area forecast discussion 
National Weather Service Des Moines Iowa 
1251 am CDT Sat Sep 6 2008 


Short term /today/... 
surface high pressure centerd over southeast Nebraska early this morning 
will move slowly east today. Airmass saturated or nearly saturated 
in lowest levels this morning...and with clear sky and calm 
winds...dense fog is once again forming and becoming more 
widespread. Feel there is a window of opportunity for a dense fog 
advisory to be effective before higher clouds move in from the 
west and bring visibility back up. The far southeast slower to respond 
to fog...so may leave them out until they start to go down. 


The rest of today shaping up similar to Friday. The next in a series 
of shortwaves dropping into broad base of upper level trough over 
the central Continental U.S. Is coming out over the Central High plains and 
causing various clusters of showers/thunderstorms to develop. NAM and GFS 
handle the speed of this feature a little differently...with NAM 
slower. It appears GFS may have a slight edge given current 
trends...and will expect isentropic lift to thicken clouds over 
central Iowa by early afternoon. Will once again be fighting 
relatively dry lower levels...so there is still some uncertainty 
about the extent of shower activity this afternoon. Have maintained 
slight chances of showers for the southwest half of the area...and 
will need to watch later trends carefully...as there is a chance 
that higher probability of precipitation will be needed. 


Long term /tonight through Friday/... 
little has changed in overall forecast thinking...with cool and 
fairly wet pattern in place beneath broad cyclonic flow aloft. A 
series of long wave troughs passing through this flow will provide 
several rounds of rain chances through the period. The first such 
impulse will traverse our area from this evening through early 
Sunday...with rainfall generally being light given low level dry 
air and lack of surface support. The second deeper trough will 
move through late Sunday night and Monday...bringing higher rain 
chances and the possibility of thunder especially across the 
south. Return flow on the back side of a departing surface high 
Sunday will allow for an increase in low level moisture...and a 
weak surface frontal boundary will provide surface convergence. 


An unseasonably cold night is forecast Monday night behind the 
front as skies will clear in wake of the trough and surface high 
pressure will rapidly build in overhead weakening winds. Some 
locations in the north will fall into the upper 30s and patchy 
frost is a possibility in these areas. 


A stronger long wave trough will approach the area by 
midweek...providing a chance for a prolonged period of rainfall. 
With good southerly and southwesterly flow ahead of this 
system...there is the possibility for severe weather and heavy 
rain Wednesday through Thursday. 


&& 


Aviation...06/18z 
fog has lifted with only a few patches of MVFR clouds. Conds 
expected to be VFR over all of the area before middle afternoon with scattered -shra 
moving in from the west. Do not expect to see any ceilings below 030 or visibility 
restrictions with these. Fog likely to develop after 09z...however it is not 
expected to be as widespread or dense as this morning. 


&& 


Dmx watches/warnings/advisories... 
dense fog advisory until 9 am CDT this morning for the entire County Warning Area. 


&& 


$$ 


Short term...MS Sep 08 
long term...Lee 
aviation...Moyer