Weather

327 
fxus63 kdmx 071745 cca 
afddmx 


Area forecast discussion 
National Weather Service Des Moines Iowa 
627 am CDT sun Sep 7 2008 


Short term /today/... 
weak high pressure is over the Central Plains and extends into 
western and southern Iowa early this morning...while a weak cold 
front stretches from northwest Minnesota to north central Nebraska. For the 
third morning in a row we are dealing with departing clouds...low 
surface dewpoint depressions and calm winds...which will once again 
result in fog through the middle morning hours. Dense fog will be a a 
hazard for many places this morning. Will initially issue an 
advisory for areas that look like they are already or are on their 
way down to low visibility. Southeast central Iowa likely will see 
dense fog too...but have noticed a drop in dewpoints that is keeping 
many places fog free for now. Will expand the advisory later if 
conditions warrant. 


Qg forcing and weak isentropic lift ahead of weak cold front will 
slide mainly east...north of our border today. However...cannot rule 
out a few showers across the north. The front will move across 
central Iowa during the afternoon...but will have little to work 
with as better forcing north and southwest of the area. After the 
morning fog/low clouds burn off...should see a little more sunshine 
than recent days across the southern half...and that should yield a 
good 20 to 25 degree diurnal rise...putting much of the southern 
half of Iowa into the lower to possibly middle 70s. 


Long term /tonight through Saturday/... 
forecast thinking continues more or less on track...with two 
rounds of active weather expected as shortwave troughs move 
quickly through broad cyclonic flow aloft. First wave will 
approach late tonight...with good model consensus indicating an 
area of strong isentropic lift moving across most of the state 
just ahead of the trough on Monday. With this broad area of lift 
supported by middle/upper dynamics...have raised probability of precipitation to the top of 
likely and would have gone categorical except that there is some 
uncertainty regarding the exact path of strongest lifting. Expect 
probability of precipitation to be raised further for at least part of the area by 
tonight. There is also a good chance of moderate to heavy rain 
with accumulations possibly over an inch in a some 
locations...but the rainfall will be spread out enough in time 
that flooding will not be a concern. Forecast soundings also 
indicate possibility of isolated thunder during the day 
Monday...mainly across the south. 


Once the middle level trough and weak surface front pass through late 
Monday we will clear out rapidly with surface high pressure 
building in from the west and southwest. Strong radiational 
cooling will result in a cold night across the state...and have 
maintained low temperatures in the 39-41 range across most of the County Warning Area. 
The possibility of patchy frost continues to be a concern...but we 
are right on the edge so will hold off on any related statements 
for now. 


With the high departing Tuesday and Tuesday night decent return 
flow will increase low level moisture ahead of the next 
trough...which will approach late Wednesday. This will promote 
another round of high probability of precipitation Wednesday night and Thursday with heavy 
rain and thunder once again possible. 


Aviation...07/18z 
weak frontal boundary movt through the state this afternoon will set off isolated 
showers in vicinity of frontal boundary. These will move out of the state by early evening 
with fnt becoming quasistationary just to the south of the taf sites. Strong upper 
level system will swing into the central Continental U.S. Later tonight into Monday. Strong warm air advection 
wil produce large area of rain and some thunderstorms and rain by snrs Monday. Conds will 
become IFR in the S and MVFR in the north Monday morning. 


&& 


Dmx watches/warnings/advisories...none. 


&& 


$$ 


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