696 
fxus63 klot 070952 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
452 am CDT Tuesday Oct 7 2008 


Discussion... 
300 am CDT... 


The unusually early high amplitude pattern is causing unusually 
difficult forecast challenges. 


Typically...high amplitude patterns tend to have a high level of 
predictability for the short term. Unfortunately this has not been 
the case for the past few days and remains so tonight. The the 
local areas has been caught between the proverbial immovable object 
and irresistible force. This this case...a strong surface high parked 
over southeastern Canada/northestern Continental U.S....and a full-longitude trough over the 
Mississippi River valley. Even a strong upper level jet stream 
pattern has had trouble bringing much progression the the pattern. 
Thus...any precipitation associated with the trough has been very slow to 
cross the Mississippi River into Illinois. However...the latest 
radar trends indicate that precipitation is knocking on the County Warning Area door. The 
NAM/GFS/ECMWF all initialize very similarly...so the near term 
forecast...through 18z is not too problematic and have continued 
the trend of introducing precipitation into the southwestern portions of the County Warning Area. 
The main concern comes a little later with how quickly to spread 
the precipitation north and east. For choice of "model of the day"...have 
opted to trend closer to the GFS...which is the slower of the 
models...particularly with the northern stream shortwave...initially 
moving through central Iowa...into the upper Midwest. Given the latest 
satellite trends which show an almost northerly track to the 
circulation...this seems to be the better choice than the faster 
NAM/ECMWF. The slower progression will also have implications for 
the quantitative precipitation forecast forecast. The latest HPC guidance brings an areal average 
of almost 1 inch to the southern half of the County Warning Area by 00z this 
evening. While was originally was concerned that this was a bit 
high given the initially dry airmass over the area...the slow 
progression of the trough will likely contribute to potentially 
higher total precipitation amounts. Also...the amount of embedded ts within 
the general rain showers shield is a concern. Much of the ts should be 
confined to the vicinity of the upper level circulation...where the 
center of the cold core should maximize instability. There is also 
a split in the flow...seen in the satellite imagery...with a southern 
stream shortwave focusing ts activity over the lower Mississippi 
Valley...and moving into the Tennessee Valley. With the trend for the northern 
stream short wave taking a northeasterly path into the upper Mississippi 
Valley and the southern stream short wave moving into the Tennessee 
Valley...there should be a relative minima in ts activity over northern 
Illinois/northwestern Indiana. However...given some low level forcing from 
the associated surface frontal boundary...isolated ts is certainly 
possible...so will go with categorical probability of precipitation for more widespread 
rain showers with isolated embedded ts...rather than more widespread ts. 


The surface front should finally push east of the area by 
Wednesday evening...bringing an end to the precipitation potential. 
However...the models continue to show significant disagreement by 
early Thursday and little run to run consistency. So...have made 
little adjustment to the forecast beyond Wednesday...with the 
exception of dropping probability of precipitation over the eastern portions of County Warning Area for early 
Thursday morning with the passage of the surface frontal boundary. 


Krein 


&& 


Aviation... 


1200 UTC tafs...models have made a distinctive change since 
yesterday...forming an elongated trough over the MS valley by the end of this 
taf period instead of two separate streams with a closed off low in 
southern MO and Illinois. This closed low being delayed and not forming until 
Wednesday with the northern stream located across terminals instead of 
north of them. Onset of precipitation also on hold with New Run of data. 
Should remain VFR through the day with precipitation chances increasing in the 
late afternoon. Still carrying 20z as a good start time but this may be 
pushed back with next model run too. Best precipitation onset is after dark 
..say after 03z...With ceilings lowering to MVFR by that time. MOS 
guidance indicating LIFR ceilings by midngt but have backed off to low 
MVFR around 15 hundred through 06z. Winds not likely to change much 
in next 24 hours...still east to southeast 10-15kts. Surface low passing 
through terminals after 06z likely to veer winds to south to southwest. 
Precipitation diminishing after that time along with improving ceilings and visibility. 


Rlb 


&& 


Marine... 
157 am CDT 


Nearly stationary high pressure to the east of the lakes and 
approaching low pressure from the plains will maintain a rather 
brisk east to southeast flow over the lake today and tonight. A 
persistent strong east flow will also maintain Small Craft 
Advisory winds and waves for the Illinois nearshore waters through 
this evening. Low pressure passage over the lake on Wednesday 
will shift winds to the west and northwest and result in a looser 
pressure gradient within the low pressure trough as it passes. 
Building high pressure over the lakes Thursday will maintain 
rather light west to northwest flow before backing to the 
southeast once again Thursday night and Friday after ridge 
passage. 


Rlb 


&& 


Lot watches/warnings/advisories... 
Illinois...none. 
In...none. 
Lm...Small Craft Advisory...Illinois nearshore waters until 4 am Wednesday. 


&& 


$$