Area forecast discussion National Weather Service Chicago/Romeoville Illinois 301 am CDT Sat may 25 2013 Discussion... 300 am CDT Below normal temperatures and periodic chances for showers or thunderstorms through the weekend will set the stage for a rapid warming trend for next week. For the remainder of the night and into Saturday morning...a weak shortwave overtopping a building ridge over the central Continental U.S. Will bring increasing cloudiness and a chance for some light showers or sprinkles...particularly for locations west of the Fox Valley and the I-55 corridor. Latest radar imagery shows scattered showers crossing the MS river into northwestern Illinois...but as they move into an environment noted by dry low level conditions...the showers show a diminishing trend. However...repeated shower development will eventually moisten up the lower levels enough for some light precipitation to reach the ground. Temperatures have already reached a minima across the area and should begin to slowly rise during the remainder of the overnight hours as denser cloud cover overspreads the region and shutting off any further radiative cooling. For the remainder of Saturday and into Saturday night...will continue to focus on precipitation chances. The upper level pattern remains generally unchanged over the past 24-36 hours with a high amplitude...slowly progressive pattern continuing. The main culprit is a deep upper low making very low progress lifting northeastward through New England. This is helping build the upper ridge over the central Continental U.S. While a series of system move into the Pacific northwest. While the upper low remains over New England...and the upper ridge axis remains to the west of the local area...persistent weak cold advection through deep levels of the atmosphere will help keep temperatures on the cooler side. Also...at the surface...high pressure extends from Hudson Bay...through the eastern Great Lakes to the Tennessee Valley. This has kept northeasterly flow off of Lake Michigan...with the relatively Cold Lake waters helping to keep Lakefront temperatures in the 50s. There will be little change today as temperatures will still remain in the 50s along the Lakefront. The increasing cloud cover will also help to limit daytime heating...with highs remaining in the lower to middle 60s. A warm front is beginning to develop southeastward from a surface low over the Western Plains. This warm front will increasingly become the focus of further precipitation development through the remainder of the weekend and into early next week. The warm front will gradually become more focused...and extend eastward...through Nebraska...along the Iowa/MO border to southern Illinois/in by Sunday morning. While the boundary remains to the south...increasing warm advection and isentropic lift through the lower levels will help to spread precipitation from the southwest to the northeast across the County Warning Area through the weekend...though precipitation should generally be precipitation should begin to become more widespread across the County Warning Area by Sunday and Sunday night as the warm front slowly lifts northward. It is a little tough to say if the weekend and the Memorial Day Holiday will be a total washout...but periods of precipitation will likely impact much of area. And while thunderstorms will...similarly likely not be widespread...there will be a persistent chance for elevated convection in the persistent isentropic lift. While Memorial Day will likely not be dry...at least temperatures will be on the increase. By Monday...the upper low will finally lift northeastward into New Brunswick and the upper ridge will begin to progress eastward. The models are beginning to loose focus and continuity in handling the evolution of the upper ridge as there is little consensus on the timing and strength of additional shortwaves overtopping the ridge...which will in turn...lead to some uncertainty to the precipitation forecast. The one general certainty is that there will be a series of shortwaves tracking across the region...helping to focus the precipitation in vicinity of the warm front. By Monday night...the warm front should lift well north of the area...initiating the rapid warming trend as persistent southwesterly flow develops at the surface while the upper ridge builds across the Midwest. However...with a surface ridge extending through the southeastern Continental U.S....Gulf moisture will be limited through Tuesday. But by Wednesday...the ridge will retreat to the east...opening up the Gulf and allow more moisture to stream northward. So...by middle week...expect that dewpoints should reach into the middle to upper 60s to accompany the higher temperatures which should reach into the middle 80s. The GFS/European model (ecmwf) are still bringing 850mb temperatures around 18c across the Midwest...which would imply that temperatures in the upper 80s are not out of the question given enough sunshine to allow for deep deep mixing. As usual...the increase in Gulf moisture accompanying the increased heat will also keep the chances for thunderstorms over the area. So...the uncertainty in the cloud cover will be the major question mark right now as to whether the region will see a real taste of Summer next week. Krein && Aviation... //Ord and mdw concerns...updated 06z... * east-southeast winds generally 10 kts or less this morning and early afternoon becoming more east with lake influence middle afternoon. * Low probability of showers from VFR middle level cloud deck overnight and through the day...higher probability rain showers will remain west/south of chi area terminals. Mdb //discussion...updated 06z... High pressure is centered just east of Lake Michigan with weak low pressure out across the High Plains. Light and variable low level winds are in place and will gradually become more focused from the southeast from west to east through early morning as the high departs. Bands of showers continue to move southeastward across northeast Iowa and southwest Wisconsin with some embedded thunder further west across north central Iowa. Have included tempo for some light rain at rfd over for a few hour period but the rain should have an increasingly difficult time making much eastward progress with very dry low and middle level air across much of the area. Will leave the remaining terminals dry for now. Expect that ceilings and visibility will remain VFR though ceilings may lower to 5000-7000 feet at times overnight...with more widespread 6000-8000 feet ceilings expected during the day. The shower chances will remain highest west and south of the terminals through the day but rfd may have a higher chance this afternoon and early this evening as a wave passes to the southwest. Cannot completely rule out some sprinkles or light rain at the other terminals today however. Southeast winds will likely turn more easterly at Ord/mdw and eventually dpa thanks to a lake breeze that is expected to develop this afternoon. Mdb //Ord and mdw confidence...updated 06z... * high confidence in wind trends...except medium confidence with details of shift to East/Lake breeze this afternoon. * Low confidence in potential for light VFR showers to reach Ord/mdw...though high confidence in conditions remaining VFR even if they do. Mdb //outlook for Ord/mdw for 12z Sunday-12z Friday...updated 00z... Sunday...chance of rain showers and a few thunderstorms and rain especially later in the day/night. Mainly VFR. Monday through Thursday...VFR. Low probability for periodic rain showers/thunderstorms and rain especially Monday and again Thursday. Breezy south winds Wednesday and Thursday. Ratzer && Marine... 242 am CDT Quiet conditions persist across Lake Michigan with an axis of high pressure centered just east of the lake. The high will be slow to move to the east which will keep light and somewhat variable winds across the lake today...with an onshore component developing along the shorelines. It will likely take until Monday for a more persistent southeast flow to set up as low pressure gradually organizes across the plains and the high shifts to the east. Warmer air will be moving into the region which will limit mixing and gust potential but the pressure gradient may be strong enough at times to allow for periods of 15-25 knots winds by Wednesday. Wind direction will shift from southeast to south then southwest through the week. Mdb && 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