092 
fxus63 klot 220429 
afdlot 


Area forecast discussion 
National Weather Service Chicago/Romeoville Illinois 
1129 PM CDT sun Mar 21 2010 


Discussion... 
330 PM CDT 


Looks like a fairly quiet week is in store...with the current 
forecast pretty well on track. Had to shave just a tad off the 
highs for this afternoon...mainly for the southeast third of the County Warning Area 
which remains under extensive cloud cover. This is the same area 
most likely to see rain redeveloping overnight as the surface low over 
the Ohio Valley wraps north around its upper level parent. 
Certainly not expecting any significant rainfall with this 
system...so rivers should continue their downward trend. 


Any lingering rainfall in our southeastern counties should be gone 
by tomorrow night as the upper level low pulls away to the east. 
Highs tomorrow will be limited by cool northeast winds...especially 
near the lake...and remaining cloud cover in our southeast. Tuesday 
looks a bit milder as light southwest flow develops ahead of the 
next cold front...which is anticipated to move through sometime 
during Tuesday evening. This weak front does not appear to have 
sufficient synoptic scale support for precipitation development...but 
Wednesday will be a bit cooler under the influence of northerly flow 
behind the front. Thursday appears milder again under a surface ridge 
and partly to mostly sunny skies. 


An upper level shortwave and associated surface low moving across the 
Midwest Thursday and Friday are anticipated to remain just south of 
the area. Forecast guidance begins to diverge somewhat by late in the 
week...but deterministic models and ensembles are all showing a 
stronger upper level wave and vigorous surface low moving out of the 
Southern Plains toward the area by next weekend. We may end up on 
the dry northern edge of this system...but for now have trended 
cooler and wetter for Friday through Sunday. 


Lenning 


&& 


Aviation... 


0600 UTC tafs...little change to going tafs with ceiling and visible 
remaining VFR through period. 


Deep closed low pressure center over lower MS valley will continue 
to slowly lift slowly east-northeastward across the Tennessee Valley overnight and 
Monday...with most of northern Illinois and northwestern Indiana remaining along 
periphery of high cloud shield off northwestern flank of this storm 
system. Only impact with this low will be to maintain a northeasterly wind 
flow through 00z Tuesday. Winds will remain northeasterly at around 10 kts 
overnight and then increase again during the late morning hours Monday 
as ample sunshine allows strong diurnal low level mixing...with 
frequent gusts to 20-25kt. Winds in the ugn- pwk- Ord- mdw areas 
will likely veer a bit into the east-northeast during the afternoon as well. 


Krein 


&& 


Marine... 


239 PM...complex pressure pattern to continue as low pressure 
over Arkansas and Tennessee slowly lifts northeast across the Ohio 
Valley tonight and Monday and then across New England Tuesday. A 
ridge of high pressure over Central Lake Michigan will merge with 
a stronger area of high pressure moving across Ontario overnight. 
This ridge will then stretch across Wisconsin and into the plains 
Monday...slowly shifting east Tuesday as it weakens. A weak area 
of low pressure will lift northeast across the upper lakes Tuesday 
afternoon...with a trailing cold front moving across the western 
lakes Tuesday night. High pressure builds in behind this front 
Wednesday morning and the gradient becomes rather strong...for a 
short time Wednesday morning and expect a period of 15-25kts. 
Winds diminish briefly Wednesday evening before a fast moving area 
of low pressure moves across the northern lakes Wednesday night. 
This will allow southerly winds to pick up some ahead of another 
cold front which will move across the lake Thursday morning. 
Northerly winds increase to at least 15-25kts behind this front 
and 30kts is certainly possible. 


In the short term...a tug boat operator reported 5-7 footers near 
Calumet Harbor this afternoon. As the low over Arkansas lifts 
northeast tonight and Monday...expect gradient over far Southern 
Lake Michigan to remain tight. Might be a period this evening or 
overnight when winds diminish some...but overall expect winds and 
waves to support Small Craft Advisory conditions through at least 
Monday evening. Cms 


&& 


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


&& 


$$