Weather

185 
fxus63 kdmx 280433 aab 
afddmx 


Area forecast discussion 
National Weather Service Des Moines Iowa 
1130 PM CDT Wednesday Aug 27 2008 


..updated aviation discussion... 


Short term /tonight/... 
west-northwest counties remain in slight risk for tonight and this looks 
reasonable with initial threat along front and moisture/instability 
axis. Corfidi vectors would suggest main severe potential would be 
limited to western counties this evening. The arx WRF run also 
suggesting potential for late night mesoscale convective system development over southern 
Minnesota and possibly northern Iowa. This seems plausible given 
expected strength of low level Theta-E advection and bears watching. 


Long term /Thursday-Wednesday/... 
fairly good model agreement Thursday with respect to upper level and 
surface features. Best upper level dynamics are expected to remain 
north of the area...but with convergence in the vicinity of the 
surface low moving from southeast Nebraska through southern Iowa and 
along advancing front...qg and frontogenetical forcing...and even a 
bit of upper level divergence...expect high likelihood of showers 
and thunderstorms County Warning Area wide during the day. Should have a dying mesoscale convective system 
in the morning moving across the northwest half of the County Warning Area...then 
thunderstorms redeveloping in the pre-frontal airmass across the 
southeast third of the County Warning Area during the afternoon. Moderate 
instability and deep layer shear all favorable for supercell and 
multicell thunderstorms and attendant risk of severe 
weather...assuming debris clouds from morning activity does not 
completely shut down the surface-based convective potential. Low 
level shear...lapse rates and low LCLs southeast of Des Moines 
Thursday afternoon even suggest the potential for isolated 
tornadoes...so cannot completely rule those out. Precipitable water 
values of 1.5 to 2 inches will result in locally heavy rain. Much of 
the area should receive at least a quarter of an inch of rain...if 
not more...with local amounts in excess of 2 inches. 


Front exits the area Thursday evening and a chunk of Pacific 
maritime high pressure builds east into Iowa under increasingly 
zonal flow aloft. Will have essentially no chance of rain Friday 
through Sunday unless something new develops. Will have comfortably 
cool nights but increasingly warmer days. By Sunday...a 590+ dm 500 
mb ridge builds over the region and this will help boost temperatures above 
normal. It will stay this way until the next cold front passage 
which is a bit uncertain at this time. The slower European model (ecmwf)/gefs 
solution is preferred...which brings a front through early 
Wednesday. There will be thunderstorm chances with its passage and 
then there may be moisture or remnants to deal with from soon to be 
Hurricane Gustav making its way north from the lower Mississippi 
Valley region late next week. Right now...we begin small chances of 
thunderstorms in the west as early as Monday night...though that may 
be a bit ambitious. 


&& 


Aviation...28/06z 
a warm frontal boundary is being maintained by low level wind speeds 
of 30-40 kts. Meanwhile an upper level short wave is pushing across 
the High Plains accompanied by a very strong jet. The frontal 
boundary is providing the surface convergence while the upper level 
system is providing upper support for thunderstorm development. 
This should mainly affect taf's north of the Highway 30 corridor 
overnight but development is certainly possible towards kdsm by 12z. 
Thursday a stronger upper level system pushes in with a second 
stronger front that is currently moving through Montana and North 
Dakota. A hail threat will accompany the storms tonight. Surface flow 
will become nwrly after the passage of the second front middle day 
Thursday. If there is enough sun Thursday afternoon...storms will 
redevelop in the afternoon with another severe threat...mainly 
affecting Kalo and kotm. 


&& 


Dmx watches/warnings/advisories...none. 


$$ 


Short term...Kinney 
long term...Moyer 
aviation...fab