000 FXUS63 KILX 232051 AFDILX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE LINCOLN IL 251 PM CST MON NOV 23 2009 .DISCUSSION... ISSUED 251 AM CST MON NOV 23 2009 ANOTHER...AND LAST FOR A WHILE...PLEASANT LATE NOVEMBER DAY CONTINUES TO UNFOLD ACROSS CENTRAL AND SOUTHEAST ILLINOIS. SOUTHERLY FLOW IS DEVELOPING ACROSS THE REGION AHEAD OF A STORM SYSTEM THAT IS CURRENTLY DEEPENING ACROSS THE PLAINS. THIS STORM SYSTEM...AND THE PRECIPITATION CHANCES/TEMPERATURE CHANGES ASSOCIATED WITH IT...IS THE PRIMARY FORECAST CONCERN OF THE NEXT SEVERAL DAYS. 12Z MODELS IN PRETTY GOOD OVERALL AGREEMENT WITH THE APPROACHING SYSTEM...AND HAVE STAYED REASONABLY CLOSE TO THE TIMING SHIFT ADVERTISED YESTERDAY. ONE NOTABLE DIFFERENCE THAT REMAINS IS THE LOW LEVEL THERMAL FIELD AS A SECONDARY SHOT OF FORCING/COLD AIR WRAPS IN BEHIND THE SYSTEM ON THANKSGIVING. PREFER THE COOLER LOW LEVEL THERMAL PROFILE OF THE NAM...DESPITE THURSDAY FALLING AT THE END OF ITS FORECAST RANGE...CONSIDERING THE BETTER RESOLUTION OF THE MODEL AND THE STRENGTH OF THE LOW LEVEL CAA EXPECTED. THE LOW LEVEL THERMAL PROFILES ARE NOT TERRIBLY DIFFERENT...BUT THE DIFFERENCES ARE ENOUGH TO TIP THE SCALES FROM A RAIN PROFILE TO A MIX OR SNOW PROFILE. HOWEVER...IN ANY CASE...THE PRECIPITATION ON THANKSGIVING DAY IS NOT LIKELY TO BE SIGNIFICANT IN QUANTITY. SHORT TERM...TONIGHT THROUGH THURSDAY...THE EFFECTIVE WARM FRONT FROM THE APPROACHING SYSTEM CURRENTLY LIES ACROSS THE FORECAST AREA AND IT IS NOT EXPECTED TO MOVE MUCH UNTIL LATER TUESDAY. WHILE THE MODELS INDICATE FAIRLY STRONG ISENTROPIC ASCENT ASSOCIATED WITH THIS THERMAL GRADIENT INTO EARLY TUESDAY...THE ATMOSPHERIC PROFILE APPEARS TOO DRY TO SUPPORT PRECIPITATION INTO EARLY TUESDAY. OUR PRECIPITATION RISK WILL INCREASE...FROM WEST TO EAST...FROM LATE TUESDAY MORNING INTO TUESDAY EVENING. THIS WILL OCCUR AS THE OCCLUDED SYSTEM DRAGS A TRAILING COLD FRONT THROUGH THE FORECAST AREA AS IT MAKES ITS SLOW TREK NORTHEAST TOWARD THE GREAT LAKES REGION. THE FORCING WILL BE DECENT...AS WE WILL BE AT LEAST PARTIALLY IMPACTED BY THE EXIT REGION OF A 100 KT JET STREAK... THOUGH THE BETTER FORCING WILL STAY CLOSER TO THE TRACK OF THE SURFACE LOW TO OUR NORTHWEST. WHILE SOME WRAP AROUND RAIN REMAINS POSSIBLE LATE TUESDAY NIGHT INTO WEDNESDAY NIGHT AS THE SYSTEM CONTINUE TO SPIN OVER THE GREAT LAKES...A RELATIVE LULL IS LIKELY AS WE WILL BE BETWEEN SURFACE TROFS AND UNDER MORE NEUTRAL FLOW ALOFT. A STRONG SECONDARY SHORT WAVE WILL DIG DOWN THE BACKSIDE OF THE SYSTEM OVERNIGHT WEDNESDAY INTO THANKSGIVING DAY. THIS FEATURE...AND THE ASSOCIATED SURFACE TROF...WILL BRING A RENEWED CHANCE FOR PRECIPITATION TO THE FORECAST AREA. THE FAVORED NAM THERMAL PROFILES SUPPORT ANY PRECIPITATION FALLING AS SNOW OR A RAIN/SNOW MIX. HOWEVER...THE PRECIPITATION SHOULD BE LIGHT AND LITTLE/NO ACCUMULATION IS LIKELY. LONG TERM...THURSDAY NIGHT THROUGH MONDAY...MODELS/ENSEMBLES CONTINUE TO SHOW QUITE A BIT OF SPREAD AND RUN-TO-RUN VARIABILITY OVERALL. FAIRLY QUIET WEATHER IS EXPECTED INTO SATURDAY NIGHT... WITH MODERATING TEMPERATURES...AS UPPER/SURFACE RIDGING BUILD THROUGH THE REGION. WHILE THIS FIRST PORTION OF THE FORECAST IS RELATIVELY WELL AGREED UPON BY THE MODELS...A PATTERN SHIFT THAT WILL IMPACT THE SECOND HALF OF THE EXTENDED RANGE OF THE FORECAST SHOWS MUCH MORE VARIABILITY. THE BIGGEST PROBLEM REVOLVES AROUND HOW MUCH PHASING THERE WILL BE BETWEEN THE NORTHERN AND SOUTHERN STREAMS OF THE WESTERLIES. A FRONT MAY PASS THROUGH THE REGION SUNDAY INTO MONDAY...BUT THE BULK OF THE FORCING/MOISTURE IS EXPECTED TO BE TIED UP FURTHER TO OUR SOUTH. SO...HAVE MAINLY GONE WITH SLIGHT CHANCES OF RAIN WITH THIS FROPA AT THIS TIME. BAK && .AVIATION... ISSUED 515 AM CST MON NOV 23 2009 TWO AREAS OF CONCERN THIS FORECAST PERIOD. THE FIRST BEING WITH THE PATCHY AREAS OF FOG THIS MRNG...WITH THE SECOND CHALLENGE BEING WITH THE MVFR CIGS TRACKING NWD OUT OF SOUTHEAST MO AND WRN KY THIS MRNG. LIGHT WIND FLOW COUPLED WITH ABUNDANT LOW LEVEL MOISTURE FROM WET GROUND HAS LED TO SOME PATCHY MVFR/IFR FOG...ESP ACRS THE EAST. WL CARRY TEMPO GROUPS FOR THE FOG THRU AT LEAST 15Z AND THAT MAY NOT BE LONG ENOUGH AS THE EXTENSIVE MID AND HIGH LEVEL CLOUD COVER MAY LIMIT THE AMT OF MIXING THIS MRNG AND KEEP THE LOWER VSBYS IN FOR A BIT LONGER COMPARED WITH THE PAST FEW MRNGS. ONCE THE FOG DISSIPATES LATE THIS MRNG INTO EARLY THIS AFTN...WL SEE A BRIEF PERIOD OF VFR CONDITIONS BEFORE OUR ATTENTION TURNS TO THE MVFR AND LOCAL IFR CIGS TO OUR SOUTH FOR THIS EVENING AND OVERNIGHT. BASED ON THE PRESENT MOVEMENT OF THE LOWER CIGS TO OUR SOUTH...IT APPEARS THEY SHOULD BEGIN TO AFFECT SPI AND DEC EARLY THIS EVENING...SHORTLY AFTR 00Z...AND POINTS FURTHER NORTH AN HOUR OR TWO LATER. SEEING A LOT OF CIGS AOB 1000 FEET ACRS ARKANSAS AND SW KY EARLY THIS MRNG WITH THE LOW LEVEL FLOW EXPCTD TO BRING THAT NWD DURING THE DAY. MAY EVEN SEE SOME PATCHY DRIZZLE MOVE IN OVERNIGHT AS MODELS DO INDICATE SOME WEAK LIFT PUSHING ACRS THE AREA DURING THE EARLY MRNG HOURS...WHICH MAY ACTUALLY BRING CIGS AND VSBYS DOWN FURTHER THAN CURRENTLY INDICATED. WL CONTINUE TO MONITOR FOR THOSE TRENDS DURING THE DAY. SMITH && .ILX WATCHES/WARNINGS/ADVISORIES... NONE. && $$