Area forecast discussion National Weather Service Chicago/Romeoville Illinois 904 am CDT Tuesday may 21 2013 Discussion... 321 am CDT Today through tonight... main concern for today revolves around the potential for severe weather...mainly along and east of a Pontiac to Chicago line. Line of showers and thunderstorms from early this morning continues to push east...stretching from lower Michigan south through southern in. This area of convection had brought severe weather to portions of northern Illinois last evening...associated with a tongue of anomalously high moisture. Precipitable water values within this channel of moisture hovered between 1.2-1.6" as some drying was noted across northern Missouri/southern Iowa in the wake of the evenings convection. Temperatures should hold up early this morning in the middle/upper 60s. Then the focus turns towards timing of convection and the intensify...possibly severe weather again. Guidance indicates another cap will develop and hold through most of the morning timeframe...then the question is when this cap will dissolve. Local guidance suggests the more favored zone for the strongest instability will be along and east of a Pontiac to Gary line...where lapse rates will steepen quickly by midday/afternoon. The overall system that has produced the widespread severe weather remains negatively tilted...which this type of a setup coupled with instability and abnormally high moisture...has a tendency to lead to severe weather occurrence. The challenge is trying to pin down the timing/placement...and once again the lack of a trigger. A weak lobe of vorticity does appear to be prognosticated to slide north through southcentral Illinois during the afternoon hours...which may aid in initiation of convection across the southeastern County warning forecast area during the afternoon/evening hours. Prognosticated wind fields are more uniform...which should limit The Hazards to a wind/hail threat. Expect any convection to form into a line segments shortly after generation. Temperatures will be difficult to pindown for this afternoon as well. Given the recent rainfall...coupled with slightly cooler air aloft to keep temperatures closer to the low 80s for much of the County warning forecast area. The far southeast County warning forecast area may touch the middle 80s. The Channel of moisture will continue to hover across the County warning forecast area through the overnight hours...as dew points remain in the low/middle 60s. Have maintained likely probability of precipitation through the overnight hours...with a continued thunderstorms mentioned. Feel that as time moves forward this will be able to be fine tuned...and possibly limit convection to the eastern County warning forecast area. Confidence in severe weather initiation...low/medium. Wednesday through Thursday... 500mb trough axis finally arrives across overhead...however remains in a negatively tilted state. It appears instability will be waining a bit throughout the day...so have trimmed back to a slight chance of thunder. This seems reasonable given the abundance of clouds. Temperatures will be much cooler Wednesday...mainly in the middle 70s. For Thursday the 500mb vorticity slows across the County warning forecast area...with cooler air slowly filtering into the near surface environment. Winds will slowly turn northerly behind a frontal boundary...then northeasterly early Thursday. This will allow the marine environment to filter into far northeast Illinois...keeping temperatures in the upper 50s Thursday. Anti-cyclonic flow is prognosticated to arrive across the region Thursday aftn/eve...bringing an end to the precipitation. Confidence...medium/high. Friday through Monday... ensembles have continued to suggest the departing 500mb trough will bump up against a western Atlantic ridge. This will slow the eastward progression of the trough...which should hold the anti-cyclonic flow across the Great Lakes region through Friday night with dry weather. Considerable differences on precipitation returning to the region amongst guidance for Sat/Sat night...which is most likely a reflection of the slowing trend and the middle-level ridge that is prognosticated to build across the central Continental U.S. Into the Canadian prairies Sat/sun. Much cooler air will remain in place for the start of the extended...as temperatures struggle to reach the middle/upper 60s...and for areas adjacent to Lake Michigan...temperatures may actually struggle to reach the upper 50s as the cooler marine environment flows inland from a northeast wind. Then temperatures will begin to warm for the weekend into the upper 60s to near 70 degree. Then by early next week temperatures should steadily warm into the low 70s. Have held onto mention of slight chance probability of precipitation in the later periods of the extended...however it is possible that as heights increase aloft that diffluent flow will inhibit the precipitation potential. Could see things trending drier for the final few periods of the extended. Confidence...medium/high. Beachler && Aviation... //Ord and mdw concerns...updated 14z... * strong gusty south to southwest winds through 00z. * Slight chance thunderstorms and rain this afternoon and evening. * Thunderstorms and rain likely after midnight. Paw //discussion...updated 12z... In the wake of last nights showers and storms...things have stabilized for now. Expect mostly clear skies through the morning hours. By middle day expect atmosphere to begin to destabilize again but dont see any real focus for thunderstorms to develop. Scattered thunderstorms are possible in the afternoon and evening but dont have enough confidence in timing or location to put thunder in the tafs at this time. Wave rounding the base of the plains trough over eastern nm this morning will lift northeast toward Illinois late tonight. This should be the focus for more widespread showers and thunderstorms late tonight into Wednesday morning. South winds will be increasing this morning and turning SW this afternoon. Winds will diminish and gustiness will end this evening. Winds will become light and variable late tonight as weak low pressure moves overhead. Allsopp //Ord and mdw confidence...updated 14z... * high confidence in wind trend. * Low confidence in thunderstorms this afternoon and early evening. * Medium-high confidence in thunderstorms after midnight. Paw //outlook for Ord/mdw for 00z Thursday-12z Tuesday...updated 12z... Wednesday night...chance rain showers. Possible MVFR. Thursday...chance rain showers in the morning. VFR afternoon. Friday...VFR. Saturday - Monday...slight chance thunderstorms and rain. Allsopp && Marine... 144 am CDT Low pressure was over the ND/Minnesota border with a warm front extending east across Northern Lake Michigan early this morning. This will maintain south winds over all but the far north end of the lake today. The low will move slowly east across the lake tonight through Wednesday. In the wake of the low Wednesday night north winds will increase over the Lake. A period of gale force north winds is possible Thursday before winds diminish Thursday night. High pressure will build into the upper Mississippi Valley and western lakes Thursday night and Friday. Allsopp && Lot watches/warnings/advisories... Illinois...none. In...none. Lm...none. && $$ Visit US at http://weather.Gov/Chicago (all lowercase) Follow US on facebook...twitter...and youtube at: www.Facebook.Com/US.Nationalweatherservice.Chicago.Gov www.Twitter.Com/nwschicago www.Youtube.Com/nwschicago