598 
fxus63 klot 182104 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
404 PM CDT Friday Jul 18 2008 


Discussion... 
403 PM CDT 


Forecast problem is magnitude of rain showers and ts threat tonight and 
Sat...as well as potential for significant rainfall. 


A weak surface boundary has been located across far northern Illinois 
all day...while a lake enhanced outflow has settled into far NE Illinois. 
Dew points have been pooling S of these boundaries with 20z 
readings in the lower 70s common. Cap that had been in place 
across the forecast area the last couple days in now marginal or non existent 
based on acars temperature profiles allowing scattered rain showers and isolated ts to 
develop between 19z and 20z as surface temperatures reached middle 80s to 
lower 90s. With middle levels warmer positive area not as large so do 
not expect excessively vigorous updrafts though with precipitable 
water vales approaching 2 inches convective cells likely to be 
efficient rainfall producers. 


GFS and NAM models have caught on to the presence of both a 
compact middle level low on the far eastern NE-Kansas border...as well as 
an 800 mb circulation in far northwest MO per profiler and 88d networks along 
with satellite derived winds. These models move the middle level low 
NE to eastern Iowa by 06z Sat and continue lifting across northwest Illinois to 
southern WI during Sat morning. 


Enhanced low level convergence across northern Illinois ahead of the 
low level circulation as well as increasing upward vertical velocity as the middle level 
feature continues to approach from the west. This expected to 
continue development of rain showers and ts across NE MO and southeast Iowa across 
northern Illinois tonight. Do not anticipate excessive training of cells 
but only locally heavy downpours so no flash flood/flood watches 
issued at this time. Continued chances of rain showers and ts into Sat 
afternoon as middle level trough slowly progresses eastward to Southern 
Lake Michigan by 00z sun. 


Some uncertainties for Sat night through Monday night with prospects 
for more rain. Operational GFS fits well with GFS ensemble mean in 
bringing a short wave east-southeast across the upper MS valley and across 
southern WI and northern Illinois Sat night and Sun morning while NAM 
has axis of quantitative precipitation forecast far removed to the north across far northern WI and 
the u.P. Of Michigan. European model (ecmwf) and UKMET also show the more southern track 
so given majority of solutions bringing rain across the forecast area Sat 
night into sun will maintain chance probability of precipitation as upper trough develops over 
the Great Lakes and likely to see periodic impulses rotating southeast across 
the upper Midwest and into the western lakes. 


Trs 


&& 


Aviation... 


1800 UTC tafs...VFR conds prevailing with main focus of concern on 
thunderstorm development. Timing is first question. Analysis would indicate 
a weak diffuse trough from grb area SW to mkc. But outflow boundaries 
lay across area from last nights convective outflows which are 
probably responsible for enhancing showers in NE Illinois at present. 
This convection is north of ohare and playing havoc with takeoffs. 
This also producing outflows southeastward toward ohare with tempo winds from 
the northwest next hour. Winds have picked up briskly from the 
SW...altering plans from expecting lake breeze. Yet winds diminish 
with sunset to l/v or lite SW. Based on dewpt depression forecast 
overnight along with ups fog program output and weaker wind flow 
..probable to see MVFR fog after midngt. This lifting slowly Sat 
morning to 6 in haze for the day...turning out to be a hot hazy humid 
Summer day. A middle level trough approaches overnight and have inserted 
prob30 for thunderstorms at rfd after 09z. However prospects for any 
overnight complex losing energy in the predawn hours too much of a 
temptation to advect storms any further east to dpa or Ord. So 
have not attemped even a thunderstorms in the vicinity for terminals east of rfd. Will take 
a wait and see approach instead of disrupting aviation concerns 
for something losing likelihood with time. 


Rlb 


&& 


Marine... 
1245 PM CDT 


Still remaining in a weak low pattern extending from east Ontario 
back SW to the northern plains. A frontal boundary extending west from 
the first low pressure in Ontario will sag south tonight...setting 
up northeast winds across the northern half of Lake Michigan tonight. 
The front should stall over the southern half of Lake Michigan Saturday. 
Then a second low pressure area will deepen along this front 
Saturday night and track across Southern Lake Michigan. This allows 
the front to drop south of the lake on Sunday. Northwest winds 
are expected across Lake Michigan Sunday with weak flow leaving speeds 
of only 10 knots or less. Wind directions for the early part of 
next week conflict with model guidance with regards to location 
and intensity of surface high moving into the western Great Lakes. 
For now have continued idea of maintaining west winds Monday and Tuesday 
as ridge slowly builds into the western Great Lakes. The weak 
surface gradient across the area should not promote much wave 
action with generally 2 feet or less from tonight through the 
early part of next week. 


Rlb 


&& 


Lot watches/warnings/advisories... 
Illinois...none. 
In...none. 
Lm...none. 


&& 


$$