777 FXUS63 KILX 150536 AFDILX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE LINCOLN IL 1136 PM CST TUE DEC 14 2010 .DISCUSSION... ISSUED 901 PM CST TUE DEC 14 2010 LOW PRESSURE IS STILL WELL OFF TO THE WEST...BUT THE WAA AND ISENTROPIC LIFT REGIONS ARE ALREADY CREATING CLOUDS ACROSS THE MID MISSISSIPPI RIVER VALLEY AND INTO WESTERN ILLINOIS. THE CLOUDS HAVE NOT REACHED ALL THE WAY ACROSS OUR CWA YET...AND THE ORIENTATION OF THE FORCING INDICATES WE MAY HAVE SOME OF OUR NE CWA WITHOUT CLOUD COVER UNTIL LATE TONIGHT. THAT WILL ALLOW LOW TEMPS TO BE DRAMATICALLY DIFFERENT FROM EAST TO WEST. WILL TRY TO ADJUST THE LOWS SLIGHTLY DOWN IN AREAS WHERE CLEARING MAY REMAIN LONGER. SOME SCATTERED FLURRIES MAY DEVELOP LATE TONIGHT IN EASTERN MISSOURI AND SW IL. HOWEVER...THAT FORCING WILL WEAKEN AFTER SUNRISE IN FAVOR OF A MORE FOCUSED AREA OF LIFT DEVELOPING ACROSS IOWA AND INTO WEST CENTRAL IL BY TOMORROW AFTERNOON. OUR AREA IS STILL ON TRACK FOR A COUPLE INCHES OF SNOW WED EVE...WITH FREEZING RAIN STILL POSSIBLE IN OUR SE CWA...ESPECIALLY SE OF I-70 WED NIGHT. SHIMON && .AVIATION... ISSUED 1136 PM CST TUE DEC 14 2010...FOR THE 06Z TAFS A BAND OF MID LEVEL CLOUDS CONTINUE TO COVER THE SW HALF OF THE CWA...IN RESPONSE TO WAA FLOWS INCREASING WELL AHEAD OF THE NEXT WEATHER DISTURBANCE. THE BACK EDGE OF THE MID CLOUDS IS HARD TO DISCERN ON SATELLITE WITH THE CIRRUS OVERCAST...BUT IT APPEARS TO BE ACROSS WESTERN IOWA AND WESTERN MISSOURI. FORECAST SOUNDINGS SHOW THAT THE MID CLOUDS WILL REMAIN NEARLY STATIONARY OVERNIGHT...WITH SOME SLOW ADVANCEMENT EASTWARD. LIGHT SNOW IS EXPECTED TO DEVELOP DURING THE AFTERNOON...ESPECIALLY FOR THE NORTHERN 3 TAFS OF PIA/BMI/CMI...CLOSEST TO THE AXIS OF MAX FORCING FOR PRECIP. FARTHER SOUTH...LIGHT SNOW WILL ALSO BE POSSIBLE IN THE AFTERNOON...BUT NOT NEARLY AS MUCH ACCUM IS EXPECTED THERE. THE HEAVIEST SNOWFALL SHOULD COME IN THE EVENING...AS THE WARM LAYER AT 850MB PUSHES INTO OUR SOUTHERN CWA. THAT WARM LAYER WILL WORK TO MELT THE ICE CRYSTAL ALOFT...CAUSING IT TO FALL INTO THE COLD LAYER BELOW IT. THE DROP MAY REMAIN WATER AND FREEZE ON CONTACT WITH SUB FREEZING SURFACE TEMPS. THE DROP MAY ALSO REFREEZE AND FALL AS SLEET...DEPENDING ON HOW FAR NORTH YOU GET OF I-70. FOR NOW WILL INCLUDE A TEMPO FOR FZRA/PL FOR THE SOUTHERN TAFS OF SPI/DEC. WILL DROP CIGS AND VIS TO IFR AFTER 00Z-02Z WITH THE ONSET OF THE HEAVIEST PRECIP...AND KEEP THAT PREVAILING THROUGH THE REMAINDER OF THE TAF PERIOD AT 06Z. SHIMON && .PREV DISCUSSION... ISSUED 244 PM CST TUE DEC 14 2010 QUIET BUT CHILLY CONDITIONS PERSIST ACROSS CENTRAL AND SOUTHEAST ILLINOIS TODAY WITH ARCTIC HIGH PRESSURE CENTERED OVER THE AREA. TEMPERATURES WILL MODERATE HEADING INTO WEDNESDAY AS RETURN FLOW DEVELOPS ON THE BACK SIDE OF THE SURFACE RIDGE. MAIN FORECAST CONCERNS TODAY REVOLVE AROUND PRECIPITATION CHANCES/TYPES/AMOUNTS WEDNESDAY INTO THURSDAY AS A FEW DISTURBANCES IMPACT THE REGION IN THE WAKE OF THE DEPARTING ARCTIC HIGH. 12Z MODELS IN DECENT AGREEMENT THROUGH THE SHORT TERM OF THE FORECAST WITH RESPECT TO FEATURES EXPECTED TO IMPACT THE FORECAST AREA. THERE ARE STILL DIFFERENCES IN THE EXPECTED VERTICAL THERMAL PROFILES FOR OUR MIDWEEK WEATHER MAKERS. DO NOT REALLY HAVE A PREFERRED SOLUTION...BUT SUFFICE IT TO SAY THAT THE FORECAST AREA WILL CONTAIN THE TRANSITION LINE FROM SNOW TO FREEZING RAIN. THE BIG QUESTION IS EXACTLY WHERE IT WILL BE. SHORT TERM...TONIGHT THROUGH FRIDAY...A WEAKENING UPPER RIDGE AXIS EXTENDING FROM THE SOUTHERN ROCKIES INTO THE CANADIAN PRAIRIE PROVINCES WILL CONTINUE TO BREAK DOWN HEADING INTO MIDWEEK...AND IT SHOULD EFFECTIVELY BE GONE BY THURSDAY. THIS BREAK DOWN WILL ALLOW SEVERAL PIECES OF ENERGY TO MOVE THROUGH THE MIDWEST THE NEXT COUPLE OF DAYS...PROVIDING A RISK OF PRECIPITATION. THE FIRST PIECE OF ENERGY WILL APPROACH FROM THE NORTHWEST TONIGHT. HOWEVER...AS COMPARED TO RECENT DAYS...IT IS WEAKER AND PRIMARILY TRACKS TO THE SOUTH OF THE FORECAST AREA. DO NOT EXPECT ANY PRECIPITATION TONIGHT OR EARLY WEDNESDAY AS A RESULT OF THIS DISTURBANCE. A STRONGER WAVE WILL APPROACH FROM THE NORTHWEST LATE WEDNESDAY INTO THURSDAY. AT THE SAME TIME...ANOTHER WAVE WILL TRACK DUE EAST TOWARD THE REGION FROM THE PACIFIC COAST. THE PACIFIC WAVE WILL HAVE A WEAK SURFACE LOW ASSOCIATED WITH IT...WHICH IS EXPECTED TO TRACK SOUTH OF THE FORECAST AREA. THE TRACK OF THE LOW SHOULD KEEP SURFACE TEMPERATURES BELOW FREEZING AND RESULT IN ANY PRECIPITATION FALLING IN THE FORM OF SNOW...SLEET OR FREEZING RAIN. THE TWO WAVES WILL COLLIDE IN THE VICINITY OF THE FORECAST AREA...ALONG WITH THEIR RESPECTIVE AREAS OF FORCING. THE WARM AIR BEING FLUNG NORTH ALOFT AHEAD OF THE PACIFIC SYSTEM WILL RESULT IN ROUGHLY THE SOUTHERN 1/3 TO 1/2 OF THE FORECAST AREA HAVING A SMALL ELEVATED WARM LAYER WEDNESDAY NIGHT AND EARLY THURSDAY...RESULTING IN ANY PRECIPITATION CHANGING OVER TO SLEET OR FREEZING RAIN. ALL SNOW IS EXPECTED FURTHER NORTH. ONE THING THESE SYSTEMS DO NOT APPEAR TO HAVE GOING FOR THEM IS A PROLONGED PERIOD OF DEEP MOISTURE. THE DEEPEST MOISTURE APPEARS LIKELY TO BE AROUND FROM LATE WEDNESDAY AFTERNOON INTO THE OVERNIGHT HOURS. THE MOIST LAYER SHRINKS CONSIDERABLY AFTER 06Z THURSDAY AND LINGERS THROUGH MUCH OF THE DAY. THE MOISTURE IS PREDOMINANTLY WARMER THAN -10C AFTER 06Z THURSDAY...SUGGESTING ICE CRYSTALS WILL BE LACKING. ONCE THE MID LEVELS DRY OUT...FREEZING DRIZZLE MAY BE AS COMMON A PRECIPITATION TYPE AS ANYTHING ELSE. IN ANY EVENT...OVERALL PRECIPITATION TOTALS SHOULD BE RATHER LIGHT AND THE UNCERTAINTY WITH HOW FAR NORTH THE MIXED PRECIPITATION WILL EXTEND PRECLUDES ME FROM ISSUING ANY WINTER HEADLINES AT THIS TIME. ONCE THE ABOVE MENTIONED WAVES DEPART THURSDAY AND FRIDAY...HIGH PRESSURE WILL BUILD BACK INTO THE AREA. THIS HIGH SHOULD PROVIDE QUIET WEATHER AND A CONTINUATION OF BELOW NORMAL TEMPERATURES. LONG TERM...SATURDAY THROUGH TUESDAY...MODELS/ENSEMBLES CONTINUE TO BE IN REASONABLE AGREEMENT IN DEPICTING A QUASI-ZONAL FLOW ACROSS THE LOWER 48 STATES. HOWEVER...THEIR HANDLING OF A YET TO DEVELOP HIGH LATITUDE BLOCKING PATTERN STILL LEAVES MUCH TO BE DESIRED. AN ANOMALOUSLY STRONG UPPER RIDGE/HIGH SHOULD BE ACROSS CENTRAL CANADA AT THE START OF THE PERIOD...WITH TROFFING DEVELOPING OFF THE PACIFIC AND ATLANTIC COASTS OF CANADA. THE DETAILS IN HOW STRONG THE TROFFING WILL BE AS WELL AS ANY OTHER WEAKER UPPER LOWS OR SHORT WAVES THAT MAY BE PINCHED BETWEEN THE BLOCKING HIGH AND ZONAL FLOW TO THE SOUTH IS VERY UNCERTAIN. THE IMPLICATIONS LOCALLY ARE HOW MUCH COLD AIR WILL BE ABLE TO SINK INTO THE MIDWEST AND/OR IF ANY OF THE LOWER AMPLITUDE WAVES SINK FAR ENOUGH SOUTH INTO THE ZONAL FLOW TO PRODUCE PRECIPITATION. FOR NOW...MODEL CONSENSUS SUPPORTS KEEPING A DRY FORECAST...WITH AT LEAST SOMEWHAT BELOW NORMAL TEMPERATURES. BAK && .ILX WATCHES/WARNINGS/ADVISORIES... NONE. && $$