591 
fxus63 klot 181744 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
1244 PM CDT Friday Jul 18 2008 


Discussion... 


359 am CDT 


Main focus remains on short term and thunderstorm potential/timing. 


Upper pattern characterized by fast fairly zonal flow over 
northern Continental U.S. With several embedded short waves propagating 
through. Profiler network and RUC analysis indicates one middle level 
disturbance across the upper Mississippi Valley early this 
morning...which will move along northern periphery of Ohio Valley 
upper ridge and across WI/northern Illinois this morning. Despite very 
warm/moist boundary layer conditions...convection associated with 
this feature/mesoscale convective system remnants from Iowa thunderstorms last evening... 
radar mosaic indicates precipitation continues to erode along/east 
of Mississippi River due to capping from very warm middle level 
temperatures and resulting poor lapse rates. Have maintained a minimal 
pop across far northern through the morning as this wave traverses 
the region...though expect precipitation will continue to have a tough 
time working east/southeast across region through midday. Surface 
pressure falls across northern lakes prognosticated to develop/deepen 
surface low as it pushes into the St Lawrence Valley and New 
England this afternoon/tonight...with weak high pressure across 
the upper Mississippi Valley expected to help nudge stationary 
front which is currently stretched across northern lakes/Minnesota 
south toward Illinois/WI border later today. Forecast soundings from 00z 
NAM depict 800-1000 j/kg MLCAPE in low level moist/thermal axis 
along and south of the front across northern Illinois this afternoon... 
though suspect remaining weak capping may be difficult to break 
given otherwise weak low level wind field and marginal moisture 
convergence depicted ahead of front. Slater Iowa wind profiler and 
RUC vorticity analysis suggests weak vorticity center or mesoscale convective vortex 
associated with Thursday evening thunderstorms over Iowa will 
propagate across northern Illinois this afternoon...which may help ease 
the cap enough for thunderstorm development. Confidence not very 
high with this scenario at this time...though feel it is prudent 
to maintain chance probability of precipitation mainly north of Interstate 88 this 
afternoon shaded higher toward the WI state line. 


Appears greater thunderstorm threat will be late tonight/Saturday 
as front sags into northern Illinois...and another middle level trough 
approaches from the plains. 00/06z NAM depict more focused low 
level southerly flow developing in advance of this feature...and 
developing convection eastward into forecast area through Saturday 
morning. Will maintain likely probability of precipitation during this period...with 
northern Illinois portions of the County Warning Area most likely to see more organized 
precipitation. 


Large scale pattern change develops later in the weekend as upper 
ridge builds westward across central/southern rockies...and broad 
trough develops across the eastern Continental U.S.. this to eventually allow 
stationary front to sag south across forecast area late Sunday. 
However...models having trouble with just how far south the front 
makes it as fairly vigorous short wave drops southeast across 
upper Midwest/Great Lakes region on Monday. 00z European model (ecmwf) has come 
more into line with other guidance in depicting this slower 
southward progression...and lingering threat for thunderstorms 
Monday/Monday night as front slow pushes to the south. High 
pressure then builds across area Wednesday with cooler/drier 
weather for middle week. 


Ratzer 


&& 


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. 


&& 


$$ 

National Weather Service Glossary of Abbreviations

Non-Expanded Version (with abbreviations)