Weather

213 
fxus63 kdvn 070829 
afddvn 


Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois 
329 am CDT sun Sep 7 2008 


Synopsis... 
latest surface analysis was indicating Central Plains ride complex to 
the southwest of cyclonic flow pattern over the upper MS River Valley 
and into the northwest Great Lakes. Another quasi-stationary front was 
located from the deep southwest plains...northeastward along and 
north of the Ohio River valley. Aloft...middle an high level moisture 
fields seen on west/v imagery was indicating broad scale cyclonic upper 
flow pattern-l/west trough nicely. This imagery was also indicating a 
rather complex vorticity structure/S right over the County Warning Area and 
extending/lobing back westward across the Central Plains. A larger wave 
was noted rolling southeastward out of the Lake Winnipeg region and 
toward northwest Minnesota. 


&& 


Short term...today and tonight...once again...with a clearing slot 
seen in the ac deck between forcing fields sweeping across central 
into eastern Iowa...will have to watch for fog formation in the west 
and southwest through sunrise and will add areas of fog to grids through 
9 am...and probably cover any dense development with another special 
statement as it should not be widespread enough for a headline. Will 
also have to walk out lingering light showers and sprinkles out of 
west central Illinois through 15z...but sctrd sprinkles moving across far 
northeast Iowa and into WI should not make it into the northern County Warning Area 
and will remove the ongoing morning probability of precipitation there. 


The rest of the day will be characterized by partly to mostly sunny 
skies...as the main forcing fields and deep enough saturation will 
remain to the north of the County Warning Area in view of the main upper low 
mentioned above that will continue to propagate across Minnesota and WI through 
Sunday evening. Trailing low level trough axis induced by these processes 
should skirt across the far northern County Warning Area by late afternoon and into 
the evening. Will keep low chance probability of precipitation in the vicinity of this feature 
/the northern third to quarter of the forecast area/ later this 
afternoon and into the evening...but Don/T expect much more than a 
few sprinkles possibly skirting the northern fringe. Low to middle 
level thermal parameters remain somewhat unchanged...and as long as 
we get some insolation...expect widespread highs today in the upper 
60s to lower 70s...with a few locations with longer duration 
sunshine pushing the middle 70s. 


Tonight...low level cyclogenesis takes off over the southern to Central 
Plains in response to digging wave dropping out of the northern 
rockies and across the northwest High Plains. Although best mass 
fields of lift develop and remain to the southwest of the County Warning Area through 
12z Monday...some wings of forcing/elevated bands do spread across 
southern third of the County Warning Area and possibly across the west after midnight 
Sunday night. Some low level saturation issues to overcome for any 
initial precipitation that gets generated to reach the surface...but conditions p-def 
forecasts and forecast soundings suggest the southern third to get rain 
reaching the ground late tonight...while mainly elevated hydrometeors 
/virga takes off across the central and northwestern County Warning Area. ..12.. 




Long term...Sunday through next Saturday... 
..early fall moderate to locally heavy precipitation event Monday 
main forecast issue... 


Average initialization tonight...blended hi-res European model (ecmwf) and UKMET 
forcing with secondary forcing inputs from 80 km NAM-WRF. 00z GFS 
convective scheme resulting in quantitative precipitation forecast and forcing too far into cool air 
and discarded due to Gravity wave and evaporative cooling issues. 


Monday...last few runs of solutions shows phasing of upper wave with 
Gulf moisture along front. This to produce wave with evidence of 
weak occlusion process over eastern Iowa. This will result in 
moderate to locally heavy rain amounts with heaviest amounts near 
triple point or roughly north of otm to near mli to sqi axis. Strong 
evaporative cooling to allow Mercury to fall into lower to middle 50s 
after rain starts. Along and ahead of triple point and 925-850 mb 
front to see thin axis of scattered non-severe convection with 
stratiform rains further into cool air from coupling of cold air 
advection and evaporative cooling. Forcing suggests ribbons of 120 
to 180 miles of rain for 3 to 6 hours of moderate to brief periods of 
heavy rain most locations for a cool and damp early fall day. 
Lowered maximum temperatures into upper 50s northwest sections with timing 
suggesting along and west of I-380 corridor maximum temperatures may need 
further lowering by up to 5 degrees for later shifts to assess. Rain 
amounts suggest all area to receive at least a half inch of rain with 
axis of .75 to 1.25 amounts ~50 miles either side of a Sigourney Iowa 
to quadrant cities to near Sterling line. Monday night...rain to end by 
late evening with clearing northwest to southeast by daybreak. Coolest air of the 
fall season suggests mins northwest to fall into upper to middle 30s and later 
shifts may need to trim mins a couple more degrees if clearing is 
faster than anticipated. 


Tuesday...cool 850 mb temperatures of +3 to +6c support with minor 
bl mixing as surface ridge will be overhead for highs in the middle 60s. 
Some fair weather cumulus to develop middle day for well below normal 
temperatures and crisp fall like day. Tuesday night...lows trimmed 
east 1/2 to 2/3 a couple degrees as return flow to not set up until 
after midnight. If surface high even stronger and slower progression 
east min temperatures may still be too mild many locations with clear 
skies for good radiational cooling. 


Wednesday and Thursday...delayed probability of precipitation until late PM and overnight 
with warming ahead from next disturbance. Not enough instability and 
forcing to support thunderstorms with scattered showers ahead of 
front. Next system to be impacted by hurricane "ike" as heads into 
northwest Gulf and near TX/la. Poor confidence in phasing at this time so kept 
chance probability of precipitation with clarity of timing and forcing better ascertained 
next 36-48 hours. Limited instability suggest marginal lower end 
severe risk Thursday if enough heating occurs in PM into early 
evening hours. 


Friday and Saturday...no changes made...but issue of how and if next 
front interacts with "ike" for later shifts to assess. Again...this 
will be hard to diagnose for next couple of days. ..Nichols.. 


&& 


Aviation... 
with clearing of ac and thinning of cirrus...the Cid/dbq and possibly 
the brl terminals may be impacted by MVFR to IFR fog through at 
least 14z. After that...expect a mainly VFR day with cumulus above 3k feet 
and some ac at times as the region will be aligned in between main 
forcing regions in cyclonic mean flow pattern across the Midwest. 
Sctrd high based VFR sprinkles or light showers may be in the dbq 
vicinity this afternoon and early evening...but better chance for this 
to the north. Overrunning rains off developing low pressure storm 
system in the central/Southern Plains may reach the brl terminal 
after 08z tonight...with ceilings and possibly visibilities dipping into the 
MVFR category. Expect mainly a west-southwest surface wind regime today 
with speeds of 6-11 kts through 00z tonight...then a decrease. 
.12.. 


&& 


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


&& 


$$ 


12/Nichols 

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