Issued by NWS Central Illinois
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000 FXUS63 KILX 292331 AFDILX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE LINCOLN IL 631 PM CDT SAT AUG 29 2009 .DISCUSSION... ISSUED 209 PM CDT SAT AUG 29 2009 MODELS IN DECENT AGREEMENT THIS AFTERNOON FOR THE PERIOD THROUGH MIDWEEK WITH SPREAD GROWING TOWARD THE END OF THE FORECAST IN THE DETAILS THOUGH IN AGREEMENT ON GENERAL WEAK FLOW PATTERN BETWEEN RETREATING TROUGH IN THE EAST AND A DIGGING TROUGH ALONG THE WEST COAST OF U.S. SHORT TERM...TONIGHT THROUGH MONDAY STRONG COLD ADVECTION CONTINUES TO OVERSPREAD ILLINOIS THIS AFTERNOON KEEPING TEMPS WELL BELOW SEASONAL NORMALS. TRAPPED LOW- LEVEL MOISTURE...BROAD CYCLONIC FLOW...AND COOL MID-LEVEL TEMPERATURES HAVE PRODUCED SCT-BKN SKIES. DIURNAL CUMULUS AND BREEZY NORTHWEST WINDS SHOULD DIMINISH QUICKLY TOWARD SUNSET. STRONG VORTICITY SPOKE NOW OVER WISCONSIN AND EASTERN MINNESOTA WILL LIKELY ROTATE INTO ILLINOIS LATE TONIGHT AND PRODUCE A PERIOD OF SCT-BKN SKIES AGAIN AFTER MIDNIGHT. CLOUDINESS ASSOCIATED WITH THIS ENERGY SHOULD KEEPS TEMPS FROM FALLING QUITE AS FAR AS THEY MIGHT UNDER CLOUDLESS SKIES THOUGH IT SHOULD BE WELL BELOW NORMAL...MOSTLY IN THE 40S NORTH OF INTERSTATE 70. THE FINAL IN A THE SERIES OF VORT LOBES ARE EXPECTED DURING THE AFTERNOON ON SUNDAY AND SHOULD ENHANCE THE DIURNAL CUMULUS DEVELOPMENT. WILL HIT CLOUD COVER HARD ONCE AGAIN. HOWEVER...MOISTURE SHOULD BE SHALLOW AND PRECIP IS NOT EXPECTED. ONCE THE CLOUDS DIMINISH SUNDAY NIGHT...NEARLY PERFECT RADIATIONAL COOLING CONDITIONS WILL DEVELOP AS THE ATMOSPHERE DECOUPLES. THIS WILL LIKELY BE THE COLDEST NIGHT IN THE FORECAST PERIOD WITH SOME NORMALLY COLDER LOCATIONS POSSIBLY FALLING INTO THE LOWER 40S. AS THE UPPER TROUGH CONTINUES TO SLOWLY PULL AWAY...LAPSE RATES SHOULD LEVEL SOMEWHAT AND THOUGH DIURNAL CUMULUS WILL BE LIKELY MONDAY...PERIODS OF BKN CONDITIONS SHOULD BE MORE INFREQUENT ALLOWING A BIT MORE SUN AND THE BEGINNING OF A GRADUAL WARM-UP. LONG TERM...MONDAY NIGHT THROUGH SATURDAY. THE MODELS TRANSITION TO A WEAK FLOW WITH THE POLAR JET RETREATING INTO CANADA. THE RIDGE IS NOT EXPECTED TO BE OVERLY INTENSE AND SEVERAL WEAK DISTURBANCES ARE FORECAST TO DRIFT INTO THE MIDWEST BY THE END OF THE WEEK. THERE IS SIGNIFICANT SPREAD IN THE OPERATIONAL MODELS AND GFS ENSEMBLES ON WHETHER ANY PARTICULAR DISTURBANCE CAN IMPACT THE REGION EAST OF THE MISSISSIPPI RIVER. GIVEN THIS UNCERTIANTY AND THE GENERAL PATTERN OF THE SURFACE AND UPPER RIDGE STILL IN THE NEIGHBORHOOD...PREFER TO KEEP THE FORECAST DRY ATTM. TEMPS WILL GRADUALLY WARM AS 850 TEMPS CLIMB STEADILY FROM NEAR 4C MONDAY MORNING TO NEAR 13C BY SATURDAY AFTERNOON. HIGHS AND LOWS BY THE BEGINNING OF THE HOLIDAY WEEKEND SHOULD BE NEAR SEASONAL NORMALS. && .AVIATION... ISSUED 625 PM CDT SAT AUG 29 2009 MAIN FORECAST CONCERN WL BE WITH EXTENT OF CLOUD COVER ASSOCIATED WITH SHORTWAVE SHIFTING SOUTH ACRS THE LOWER LAKES THIS EVENING. DIURNAL CU SHOULD DISSIPATE QUICKLY THIS EVENING BUT LEADING EDGE OF LOW END VFR CIGS PUSHING INTO NRN IL...WITH MVFR CIGS NOTED FURTHER NORTH ACRS SRN WI. SHORTWAVE FCST TO SHIFT ACRS OUR AREA LATE TNT INTO SUN MRNG ACCOMPANIED BY OCCASIONAL MVFR CIGS AND VSBYS AFTR 09Z. PREDOMINATE MVFR CIGS SHOULD GRDLY BECOME LOW END VFR CIGS BY EARLY AFTN BASED ON THE LATEST MODEL FORECAST SOUNDINGS. LIGHT NORTHWEST WINDS TNT SHOULD INCREASE FROM THE NORTH ON SUNDAY WITH SPEEDS OF 7 TO 9 KTS. IF THE CIGS SCATTER OUT SOONER THAN FCST...WE MAY SEE SOME GUSTS ARND 15 KTS DURING THE AFTN BEFORE WINDS DIMINISH QUICKLY AGAIN BY 00Z. SMITH && .ILX WATCHES/WARNINGS/ADVISORIES... NONE. && $$
