771 
fxus63 kdvn 090932 
afddvn 


Area forecast discussion 
National Weather Service quadrant cities Iowa Illinois 
332 am CST Friday Jan 9 2009 


Synopsis... 
elevated warm advection on nose of 35 to 40 knots low level jet 
resulting in widespread light to locally moderate snow over 
north and northeast third of the forecast area early this morning. 
This was occurring out ahead of an upper level shortwave moving 
across Montana and developing surface low pressure center over north 
central Nebraska. Current trends shift the axis of snow east of the 
area this morning...then focus turns to another round of snow to 
impact the area tonight with the passage of the upper shortwave. 


&& 


Short term...today... 
primary challenge is handling of the developing break in the 
event today with the on-going long duration advisory. Current 88d 
trends show the back edge of the accumulating snow roughly from 
the quadrant cities to near Independence moving east around 35 kts at 
09z. GFS convergence axis around 800 mb layer had a good handle on 
this elevated warm advection snow...showing it developing around 
06z...then shifting to along and NE of a line from dbq to vys by 
12z. Next round of isentropic lift and middle level qg forcing ahead 
of the upper shortwave will spread northwest to southeast across the area middle to 
late afternoon. This will leave a window of time with cloudy skies 
and slowly rising temperatures this morning. For now...have opted 
to leave advisory as is...and will evaluate changes to advisory 
configuration to better focus on periods of accumulating snow 
later this afternoon and tonight. Afternoon snow accumulations 
should be less than 1 inch as strongest forcing holds off until 
tonight. 


Only minor changes to temperatures...with east winds 
veering to the south along and south of Highway 34 boosting 
temperatures into the middle and upper 30s...while colder easterly 
flow in the north limits highs to the 20s. Elevated warm layer may 
allow some sleet to mix with snow as it redevelops this afternoon 
and kept wording in place. Potential for freezing drizzle looks 
limited and left out for simplicity. ..sheets.. 




Long term...Friday night through Thursday... 
the snow event will be back in full force tonight...with the primary 
upper system lift moving through the area. Models are still split 
on the exact position of the deformation zone...whether it focuses 
along and north of Highway 30...or generally along and 40 miles 
either side of Interstate 80. The 00z European model (ecmwf) is the northern 
outlier...and the NAM is by far the southern outlier. The GFS seems 
to have a good handle on the current snow bands...and its solution 
for tonight seems like a pretty good call. This would keep the main 
band of snow over central and north during the evening...and the 
central and south after midnight. The track of the 850mb low...is 
generally and eastward track along or just south of Interstate 
80...thus despite some differences in quantitative precipitation forecast...the over all dynamics 
are in generally agreement on categorical probability of precipitation for accumulating snow. 
Some mixed rain and snow...with potential sleet mixed in will 
persist through early evening in the southeast...then go over to 
snow. A broad 00z to 12z snow accumulation of 1 to 3 inches seems 
very likely...with some higher totals around 4 inches where the 
deformation zone is exactly centered. These would be fresh snow 
totals...above what falls through this morning. The current 
advisory area may well need to be expanded south some...but with the 
break in snow today...and the potential to handle that primary 
issue...we will not expand the advisory at this time. 


By 12z Saturday...light snow will be ending or ended in the east 
half...with a breezy morning followed by a calmer afternoon. Some 
drifting snow is likely during the morning hours...as cold advection 
keeps temperatures from rising much. Highs in the teens north...to middle 20s 
south are expected. 


The pattern throughout the extended is active and Arctic. Saturday 
night...brief short wave ridging should partially clear 
skies...allowing for a rapid temperatures drop in the northeast 1/2 
over the fresh snow cover. After midnight...the first of many 
clippers should spread clouds over the County Warning Area. Sunday and Sunday 
night...the first clipper should produce light snow showers over the 
northern 2/3rds. Not much temperature drop will occur in the wake 
of this initial clipper...as another strong wave is set for Monday. 
This system has the potential for a few inches of snow...but 
placement is varied in the models...so low probability of precipitation are forecast. Arctic 
air will spread in Monday night...and after dry night...over running 
snow appears a growing threat into Tuesday...and Tuesday night as 
another clipper moves down. This storm should bring extremely cold 
air south...and looks to possibly produce a winter storm event...as 
accumulating snow combines with blowing snow...strong winds...and sub 
zero temperatures. This Arctic air should persist through at least 
Thursday. 
Ervin 




&& 


Aviation... 
large axis of MVFR and local IFR conditions due to low ceilings and light 
snow impacting Cid...dbq and mli will exit the area near or 
shortly after sunrise this morning. Conditions will likely improve to 
VFR as ceilings rise this morning at all sites but dbq...where low 
ceilings and fog will likely hold MVFR conditions through the day. 
Upper level disturbance moving through tonight will result in widespread 
IFR conditions with accumulating snowfall through 06z tonight. 
.Sheets.. 


&& 


Dvn watches/warnings/advisories... 
Iowa...Winter Weather Advisory until 6 am CST Saturday for Benton- 
Buchanan-Clinton-Delaware-Dubuque-Jackson-Jones-Linn. 


Illinois...Winter Weather Advisory until 6 am CST Saturday for Carroll-Jo 
Daviess-Stephenson-Whiteside. 


MO...none. 
&& 


$$ 


Sheets/Ervin