000 FXUS63 KILX 042333 AFDILX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE LINCOLN IL 533 PM CST THU DEC 4 2008 .DISCUSSION... ISSUED 258 PM CST THU DEC 4 2008 LATEST SURFACE ANALYSIS HAD A LARGE/COLD AREA OF HIGH PRESSURE CENTERED OVER THE CENTRAL PLAINS. ALOFT...WEAK IMPULSES IN THE WEST TO NORTHWEST FLOW CONTINUE TO SPREAD PATCHES OF CLOUDS OVER THE MIDWEST. MAIN FORECAST CONCERNS THE NEXT FEW DAYS REVOLVE AROUND PRECIPITATION...MAINLY SNOW...CHANCES AS IMPULSES CONTINUE TO TRACK ACROSS THE FORECAST AREA EVERY COUPLE OF DAYS OR SO. 12Z MODELS IN REASONABLE AGREEMENT WITH RESPECT TO FEATURES EXPECTED TO IMPACT REGION OVER THE NEXT FEW DAYS. AS A RESULT...DO NOT REALLY HAVE A FAVORED MODEL SOLUTION IN THE SHORT TERM. GUIDANCE TEMPERATURES SHOWING A FAIR AMOUNT OF SPREAD. WILL TEND TO FAVOR A TEMPERATURE FORECAST ON THE COOL SIDE OF A MODEL BLEND CONSIDERING THE UNSEASONABLY COOL AIRMASS IN PLACE AND SNOW COVER OVER NORTHERN SECTIONS OF THE FORECAST AREA. IN FACT...HAVE UNDERCUT ALL GUIDANCE TONIGHT OVER THE SNOW PACK DUE TO THE LIGHT WINDS AND PARTLY CLOUDY SKIES EXPECTED. THERE WERE SOME SINGLE DIGIT LOWS OVER THE SNOW LAST NIGHT AND SEE NO REASON THIS WILL NOT REPEAT TONIGHT. SHORT TERM...TONIGHT THROUGH SUNDAY... TONIGHT THROUGH FRIDAY EVENING...QUIET BUT COLD CONDITIONS EXPECTED ACROSS FORECAST AREA AS HIGH PRESSURE BUILDS ACROSS AT THE SURFACE UNDER CONTINUED NORTHWEST FLOW ALOFT. OVERNIGHT FRIDAY AND SATURDAY...A CLIPPER SYSTEM...CURRENTLY TOPPING A PACIFIC COAST RIDGE OVER NORTHERN BRITISH COLUMBIA...WILL DIVE TOWARD THE REGION. CURRENT PROGS HAVE THE STRONGEST FORCING WITH THIS SYSTEM STAYING NORTH OF THE FORECAST AREA OVER THE GREAT LAKES. HOWEVER...THE ISENTROPIC LIFT AHEAD OF THE WAVE IS IMPRESSIVE...ABOUT 40 KNOTS STRAIGHT UP THE PRESSURE SURFACES. THE FORECAST AREA WILL ALSO BE UNDER THE LEFT EXIT REGION OF AN UPPER JET AT THE SAME TIME. COINCIDING WITH THE BEST LIFT OVERNIGHT FRIDAY INTO MIDDAY SATURDAY ARE CONDENSATION PRESSURE DEFICITS BELOW 10 MB AND SPECIFIC HUMIDITIES AROUND 2 G/KG. THE PERIOD OF STRONG LIFT IS BRIEF AT ANY ONE POINT IN THE FORECAST AREA AND THE AIRMASS IS INITIALLY ON THE DRY SIDE. STILL...IT WOULD APPEAR THAT MINOR ACCUMULATIONS AROUND AN INCH ARE LIKELY. IF THIS SYSTEM TRENDS STRONGER...SLOWER...OR WITH BETTER MOISTURE PROFILES...ACCUMULATIONS/POPS WILL NEED TO BE BOOSTED. THIS THREAT WILL NEED TO BE WATCHED CLOSELY. SATURDAY NIGHT INTO SUNDAY...A SHOT OF COLD AIR SURGES IN BEHIND THE CLIPPER AS ANOTHER CHILLY CANADIAN HIGH BUILDS IN. A BROAD AREA OF WAA/ISENTROPIC LIFT WILL BECOME ESTABLISHED OVER THE PLAINS AS ADDITIONAL NORTHERN STREAM IMPULSES APPROACH FORECAST AREA FROM THE NORTHWEST. THESE IMPULSES WILL SPREAD CLOUD COVER INTO THE REGION BY SUNDAY...BUT THE MOISTURE DEPTH IS MARGINAL FOR ANY PRECIPITATION TO WORK THIS FAR SOUTHEAST. SO...WILL LEAVE FORECAST AREA DRY AT THIS TIME AND CONTINUE TO MONITOR MODEL TRENDS. LONG TERM...SUNDAY NIGHT THROUGH THURSDAY...MODELS/ENSEMBLES INITIALLY IN REASONABLE AGREEMENT WITH THE CONTINUED CHILLY/ACTIVE PATTERN. HOWEVER...MODEL SPREAD BECOMES SIGNIFICANT AROUND MIDWEEK. A FRONTAL SYSTEM WILL CROSS THE REGION MONDAY INTO TUESDAY WITH THE APPROACH OF A NORTHERN AND SOUTHERN STREAM WAVE. UNFORTUNATELY...THE DEGREE OF PHASING BETWEEN THE STREAMS AND THE EXACT SHAPE THE SYSTEM TAKES IN UNCLEAR AT THIS POINT. THIS ALSO ADDS UNCERTAINTY WITH RESPECT TO PRECIPITATION TYPE. FOR NOW...WILL GO WITH A CHANCE OF SNOW OR RAIN AND WILL REFINE THE FORECAST AS CONFIDENCE GROWS IN THE EVOLUTION OF THE SYSTEM. BEYOND THIS FRONTAL SYSTEM...THE MODEL SPREAD INCREASES. THE ONLY THING REALLY AGREED UPON IS THAT THE WEATHER WILL TREND EVEN COLDER. THERE ARE SIGNIFICANT DISAGREEMENTS IN WHEN THIS COOLING WILL OCCUR AND TO WHAT DEGREE. HAVE FOLLOWED A MORE MIDDLE OF THE ROAD SOLUTION...IN LINE WITH THE HPC FAVORED GFS/ECMWF ENSEMBLE MEANS. BAK && .AVIATION... ISSUED 533 PM CST THU DEC 4 2008 CLOUD COVER AND ISOLATED SNOW SHOWERS WILL QUICKLY MOVE THROUGH AND OUT OF THE AREA THIS EVENING. BASED ON SATELLITE LOOPS...CLOUDS APPEAR TO BE DIURNAL IN NATURE. BMI AND DEC COULD SEE SOME TEMPO BKN CIGS AT 2.5KFT UNTIL 01Z AND ALSO AT CMI UNTIL 02Z. COULD BE SOME LIGHT SNOW WITH THIS...BUT DON'T BELIEVE WORTH MENTIONING IN TAFS. ANOTHER AREA OF LOWER CLOUDS IN SE IA/NE MO APPEAR TO DISSIPATE AS THEY MOVE INTO THE AREA...SO WILL NOT TIME THEM INTO THE AREA AT ALL TONIGHT. WINDS WILL LIGHTEN THIS EVENING AND THEN SWITCH TO MORE WESTERLY OR WSW TOMORROW. DON'T SEE ANY SIGNIFICANT MOISTURE FOR CLOUD COVER TOMORROW AS SFC RIDGE MOVES ACROSS THE AREA. AUTEN && .ILX WATCHES/WARNINGS/ADVISORIES... NONE. && $$