397 
fxus63 kdvn 212002 
afddvn 


Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois 
300 PM CDT Thursday Aug 21 2008 




Synopsis... 
the upper level cut off low is now moving across the forecast 
area...generating a very nice area of rain...which has generated up 
to six tenths of an inch of rainfall so far today...though some 
locations in the northeast portions of the forecast area have only 
received a trace...and chances are diminishing fast for them to get 
much more. Main area of lift is rapidly moving up into southern 
Wisconsin and Minnesota...and the main area of rainfall has also 
moved north. Temperatures today have been sitting in the upper 60s 
to around 70...though some low 70s may sneak into the far south this 
evening before sunset now that the rain has moved away. ... 


&& 


Short term...tonight and Friday...tonight as the better lift for 
the precipitation continues to move north and weakens...expect that 
the rainfall will slowly end from south to north during the first 
half of the evening. Cannot rule out some stray showers over the 
eastern half for the remainder of the night...so have left those 
chances in. Friday we remain in the plume of moisture and with some 
marginal instability in the area we may get some scattered showers. 
So...despite being outside any decent lift area...have left in a low 
chance of showers and isolated thunder for the day. Fog is a 
distinct possibility tonight...especially if we can lose some of the 
clouds to allow some radiational effects to kick in. Have left out 
of the weather grids for now as most visibilities should be down 
around a mile...but later shifts can re-evaluate. Temperatures 
tonight will be in the middle and upper 60s. We may get enough of a 
clear slot on Friday for some temperatures to rise quite nicely...so 
have gone with at least middle 80s...but would not be too surprised to 
see a few upper 80s. ... 




Long term...Friday night through next Thursday... 
indications of long term regime change to a drier pattern into this 
fall. Chance of showers and thunderstorms through Saturday...then 
dry and seasonably cool... 


Long term signals suggesting our abnormally wet pattern over the past 
year or more has/is ending. Local tools suggesting favorable long 
term upper level ridging into fall. Model initializations good with 
GFS and hi-res European model (ecmwf) best with NAM-WRF and UKMET convective feedback 
issues beyond day 2. 


Friday night and Saturday...lowered probability of precipitation all but northwest 1/3 where 
advancing cool front to allow some lighter showers and some storms to 
arrive after midnight. As ts "fay" pushes into northern Gulf and 
intensifies...will result in weaker low level forcing and middle level 
shear. This will limit any rainfall coverage amounts with cool front 
passage and subsequently almost none chance of severe suggested due 
to elevated wbz/S at or above 12.5k above ground level and semi-moist conditions through 
the column. Hence...kept probability of precipitation at 30 to 40 percent. Saturday...front 
to pass with modest instability despite daytime heating and weak 
forcing and shear for scattered to possibly likely coverage of 
showers and storms with amounts generally below a half inch except in 
any stronger storms for later shifts to reconsider. Temperatures with 
heating to push back into low/middle 80s Saturday with mins Friday night 
upper 60s to near 70f. Saturday night...clearing with some lingering 
showers south of Highway 34 in evening as cooler and drier air moves 
into region with middle to upper 50s north 1/2 as lower 60s south. 


Sunday through Thursday...nearly ideal late Summer weather as a 
moderate strength Canadian surface high pressure settles over Great 
Lakes to keep temperatures slightly below normal. Still suggestion 
lows Monday...Tuesday in north 1/2 to fall into upper 40s for later 
shifts to assess. Highs upper 70s to lower 80s with low dewpoints. 
Thursday...possible front to approach from west but left dry at this time 
due to uncertainty of when ts "fay" lifts northeast. ..Nichols.. 


&& 


Aviation...MVFR clouds and visibilities have been affecting all taf 
sites much of the day...with some IFR conditions in the heavier 
precipitation areas. As the rainfall moves out...at least 
visibilities will improve...though ceilings may fluctuate around a 
thousand feet for a few more hours...before rising to MVFR. During 
the early morning hours fog should develop with the recent 
rainfall...and have put in IFR visibilities and ceilings through 
approximately 13z Friday morning. This should clear quickly by 
18z. Expect that there may also be some showers in the area during 
the morning...but with MVFR ceilings and visibilities. Do not expect that 
great of coverage so have left out of taf locations for now. 
... 


&& 


Dvn watches/warnings/advisories... 
Iowa...none. 
Illinois...none. 
MO...none. 


&& 


$$ 


Le/Nichols