660 
fxus63 klot 240845 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
345 am CDT Thursday Jul 24 2008 


Discussion... 


335 am CDT 


Precipitation chances from later today through Saturday are the main 
forecast issues this morning. 


An unseasonably large amplitude upper pattern continues across eastern 
half of Continental U.S. This morning with eastern longwave trough and broad 
central Continental U.S. Upper ridge being maintained. Regional radar mosaics 
this morning indicating increasing coverage to showers and 
thunderstorms across much of western and southern Iowa at nose of 
southwest low level jet and fairly impressive low level moisture 
convergence. With this large amplitude pattern...low/mid level 
moisture gradient remains just off to the west this morning as 
evident in 00z radiosonde observation sampling from last evening with kmpx sounding 
indicating precipitable waters  of around 0.75 inches and koax with 1.50 inches. 
Current position of low level ridge axis appears to be a bit 
further west than models have been advertising for this time 
period the past few days...and given amplitude of low level ridge 
axis would expect that this moisture gradient off to the west will 
not really move in with full force into northern Illinois until 
this evening. With not much advective change in low level moisture 
profile today...expecting surface dew points to again be in the middle 50s 
this afternoon and with fairly strong middle level subsidence 
inversion showing in last evenings ilx/dvn radiosonde observations...precipitation 
chances look to be minimal even across far western portions of the 
forecast area through this afternoon. Have trimmed back probability of precipitation to 
slight chance probability of precipitation for -tsra across the far southwest this 
afternoon. 


Profiler network does suggest presence of upstream middle/upper level 
short wave trough across eastern Colorado which will be a feature 
to watch for tonight. Low level winds will back to the west in 
advance of this short wave tonight as low level ridge axis continues 
to track off to the east allowing for more impressive low level 
moisture advection. Still expecting convection to increase across 
southern Iowa/northern Missouri this evening in response to 
approaching short wave and low level Theta-E advection forcing...but 
corfidi vectors/thickness pattern hinting at propagation perhaps 
just south of the forecast area. Given this have backed off a shade 
on probability of precipitation but kept highest chance probability of precipitation across the far south tonight. 
May even end up being more rain than thunder given rather limited 
elevated instability/marginal middle level lapse rates. 


Although widespread severe weather still does not appear to be on 
the horizon...perhaps best chance of any isolated severe would be on 
Friday as larger scale upper trough currently across Pacific 
northwest moves into the Great Lakes region. Forcing will be rather 
weak however...with surface trough/cool front not approaching northern Illinois 
until after peak heating. Nonetheless...forecast soundings still 
indicate around 1500 j/kg of mixed layer cape and some enhancement 
to shear profiles across the far north during the afternoon as an 
upper jet streak moves through the base of the approaching upper 
trough. Other focus area possible across the far south with slow 
moving or stationary boundary likely draped across central 
Illinois during this time frame. Overall severe threat looks on 
the low side for Friday. 


The surface cold front is expected to stall out somewhere across far 
southern portions of the County Warning Area on Saturday or just to the south. Did 
include an isolated thunderstorms and rain mention across southern half of the area 
Saturday given expected proximity of this front and some moderate 
afternoon instability. Another surface high will build into the Great 
Lakes late Sunday into Monday and expecting additional precipitation 
chances to be confined well south of the forecast area with main surface 
boundary sinking to the south. Have not made any changes to extended 
forecast at this time with European model (ecmwf)/GFS completely out of phase by 
middle of next week across western Continental U.S.. 


In terms of temperatures...highs today in the low 80s are expected most 
locations although upper 70s still look on track for northern 
Cook/Lake counties in northeast Illinois where lake breeze has 
potential of making some inland progress today. Temperatures warm a bit for 
the end of the week in advance of northern tier upper trough...and 
then generally lower to middle 80s remainder of the period with some 
minor Lakeside cooling possible on Sat-Mon. 


Marsili 


&& 


Aviation... 


1251 am CDT 


For 06z tafs...main concern today is with weak lake breeze potential 
for kord/kmdw. Otherwise just vrf middle/high cloud cover and high 
level convective debris cloudiness through next 18-24 hours as pair 
of short waves move into upper ridge and drop southeast across 
MO/southern Illinois. 


Surface ridge axis across northeast Illinois at 05z...with light pressure 
gradient allowing calm or light/variable wind conditions across 
forecast area this morning. Surface high prognosticated to slowly shift 
south across central/southern Illinois and Ohio Valley by midday... 
resulting in light south/southwest surface winds by afternoon. 
Various MOS guidance do not indicate lake breeze today...though 
with weak gradient this morning...high resolution NAM/RUC and 
locally run WRF all suggest southeast lake breeze to develop by 
midday...moving through kmdw and just making it to kord before 
slightly stronger south winds push it North/East of these terminals 
later in the afternoon. With this in mind have introduced southeast 
winds at kord/kmdw 16-17z...with a more southerly shift by middle 
afternoon. Return flow expected to be rather weak...though middle/high 
cloud cover expected to help limit Land/Lake temperature differential and 
keep lake breeze push lighter than it otherwise would be. Gradient 
wind becomes light south/southwest tonight as high weakens and 
continues to drift off across Ohio Valley. 


Cloud wise...cirrus already streaming across region...and as low 
level warm advection becomes better established into the Mississippi 
Valley today forecast soundings from both NAM/GFS indicate 
moistening of middle/upper levels to continue...aided by convective 
blow off from storms in Nebraska/Iowa and eventually MO. Soundings/time 
sections indicate some high based cumulus or middle cloud in 6-7 kft range 
by afternoon... though VFR conditions to prevail with relatively dry 
air below that level. Appears any real threat of 
precipitation/thunderstorms to hold off until Friday morning with better 
low level jet forcing staying off to the west of forecast area until 
then. 


Ratzer 


&& 


Marine... 


345 am CDT 


High pressure over the far western Great Lakes will continue to 
move south today...into the Ohio Valley tonight. This will allow 
light northerly winds to back to the south by this afternoon... 
though the relatively weak pressure gradient on the south portion 
of the lake will likely result in lake breezes which may result in 
variable wind directions for a time this afternoon. The high will 
continue to drift southeast across the Ohio Valley on Friday... 
while low pressure traverses northern Manitoba and Ontario with a 
south/southwesterly gradient wind developing across Lake Michigan. 
A weak cold front trailing from this low will push southeast 
across the western lakes Friday night into Saturday...as the low 
continues toward James Bay. This will shift winds back to the 
northwest...though a secondary surface trough is expected to move 
across the lake early Sunday with winds briefly backing west or 
southwest ahead of it. Weak high pressure will then spread across 
the upper Midwest and western lakes by the end of the weekend. 


Ratzer 


&& 


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


&& 


$$ 

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