FXUS63 KLOT 260946
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
346 AM CST SAT DEC 26 2009

.DISCUSSION...
346 AM CST

ATTENTION REMAINS FIXED ON LARGE CUTOFF UPPER LOW OVER THE MIDWEST 
AND ON PRECIPITATION TRENDS AND SNOWFALL AMOUNTS ACROSS FORECAST 
AREA INTO SUNDAY AS SYSTEM VERY SLOWLY DRIFTS EAST.

00Z UPPER AIR ANALYSIS DEPICTS APPROXIMATE CENTER OF LARGE SCALE LOW 
ALONG IA/MO BORDER...WITH CURRENT GOES VAPOR LOOP SHOWING SEVERAL 
SMALLER SCALE SHORT WAVES/VORT CENTERS ROTATING COUNTER-CLOCKWISE 
ABOUT. THESE SMALLER EMBEDDED DISTURBANCES TO GOVERN PERIODS OF 
PRECIP ACROSS THE AREA THROUGH SUNDAY...BEFORE UPPER TROUGH AXIS 
FINALLY SHIFTS EAST OF THE FA. ONE SUCH WAVE LIFTED ACROSS NORTHERN 
IL/NORTHWEST IN LAST EVENING...AND HAS LIFTED INTO SOUTHERN WI ALONG 
WITH ASSOCIATED MID LEVEL COLD POOL. NEXT IS NOTED DIGGING ACROSS 
SOUTHWEST IA/NORTHWEST MO...AND GUIDANCE INDICATES THIS FEATURE WILL 
ROTATE ACROSS IL THROUGH THIS AFTERNOON. LIGHT SNOW HAS ALREADY 
PICKED UP IN ADVANCE OF THIS FEATURE FROM WEST CENTRAL INTO 
NORTHEAST IL...THOUGH THIS MAY BE MORE ASSOCIATED WITH IMPRESSIVE 
MID LEVEL WARM ADVECTION IN WAKE OF INITIAL COLD POOL WITH LAST 
EVENINGS WAVE...AND CONVERGENCE NOTED IN LOWER/MID LEVEL PROFILER 
WINDS ACROSS MO/WESTERN IL. 03Z SREF APPEARS TO CAPTURE TREND WELL 
WITH INCREASING 3 HOUR POPS SHOWING GOOD AGREEMENT WITH RADAR 
DEPICTED SNOW AREA. SREF WOULD SUGGEST THIS AREA OF LIGHT SNOW TO 
CONTINUE TO DEVELOP NORTHEAST ACROSS NORTHERN IL TODAY...BEFORE VORT 
LIFTS NORTHEAST OF AREA THIS EVENING AND PRECIP DIMINISHES FROM THE 
WEST. GOOD AGREEMENT IN QPF OUTPUT FROM 06Z WRF/00Z GFS/03Z SREF 
WITH 0.10-0.15 INCHES TODAY WHICH BASED ON SNOW/LIQUID RATIOS OF 
APPROXIMATELY 18 TO 1 FROM WRF BUFFER OUTPUT WOULD YIELD 2-3 INCHES 
OF SNOW ACCUMULATION THROUGH 00Z...AND PERHAPS ANOTHER 1 INCH THIS 
EVENING. APPEARS THAT NORTHWEST 2/3 OR SO OF CWA WOULD BE IN 
POSITION FOR BEST ACCUMULATION...WITH AMOUNTS TAILING OFF TO AN INCH 
OR LESS FAR SOUTHEAST. FLUFFY NATURE OF SNOW...COMBINED WITH SOUTH 
WINDS GUSTING AROUND 20 MPH MAY CAUSE SOME DRIFTING ON ESPECIALLY 
RURAL ROADS...THOUGH FORECAST AMOUNTS A BIT ON THE LOW SIDE OF 
ADVISORY CRITERIA. GIVEN POTENTIAL FOR SOME TRAVEL IMPACT WILL  
HIGHLIGHT IN HWO THIS MORNING...THOUGH WILL MONITOR AND DEFER TO DAY 
SHIFT FOR NOW FOR ANY POTENTIAL HEADLINE ISSUANCE.

SNOW SHOULD DIMINISH FROM THE WEST EARLY THIS EVENING AS VORT 
CONTINUES TO LIFT NORTHEAST OF FORECAST AREA...THOUGH OCCASIONAL 
LIGHT SNOW WILL CONTINUE OVERNIGHT AS FRIDAY EVENINGS VORTEX WRAPS 
BACK ACROSS THE AREA LATE TONIGHT/EARLY SUNDAY AS IT BECOMES MAIN 
UPPER LOW CIRCULATION. SREF POPS...AS WELL AS THOSE FROM MAV/MET 
GUIDANCE...DECREASE AFTER THE EARLY EVENING HOURS WITH MODEL QPF 
ALSO DECREASING ACROSS FA OVERNIGHT. WITH THIS IN MIND HAVE TRENDED 
FROM CATEGORICAL POPS ACROSS FA TODAY...TO SCATTERED SNOW SHOWERS 
BEYOND THIS EVENING AND SUNDAY. MINOR ADDITIONAL ACCUMULATIONS 
GENERALLY AN INCH OR LESS EXPECTED LATE TONIGHT/SUNDAY. UPPER TROUGH 
THEN FINALLY OPENS UP AND MOVES OFF TO THE EAST SUNDAY NIGHT/MONDAY. 
MID LEVEL DRYING BEHIND TROUGH SUGGEST NO MORE THAN SOME LINGERING 
FLURRIES MAINLY SUNDAY NIGHT.

GUIDANCE COMING INTO BETTER AGREEMENT IN DIGGING ANOTHER SHORT WAVE 
SOUTH OUT OF MANITOBA/WESTERN ONTARIO MONDAY...WITH REINFORCING PUSH 
OF COLDER AIR ARRIVING LATE MONDAY. BASED ON THIS HAVE LOWERED MIN 
TEMPS INTO THE SINGLE DIGITS AND LOWER TEENS MONDAY NIGHT/EARLY 
TUESDAY ESPECIALLY ACROSS NORTH CENTRAL IL. AT THIS TIME...MODEL 
TIME SECTIONS INDICATE SIGNIFICANT AMOUNT OF DRY AIR ABOVE BOUNDARY 
LAYER THUS HAVE REFRAINED FROM ADDING ANY PRECIP WITH COLD FRONT.

NO CHANGES TO EXTENDED PORTION OF FORECAST BEYOND DAY 4 AT THIS 
TIME. 

RATZER

&&

.AVIATION...
1045 PM CST

0600 UTC TAFS...MAIN FORECAST FOR THE OVERNIGHT HOURS WILL FOCUS
ON SNOWFALL COVERAGE AND IMPACT ON VISIBILITY. LATEST RADAR
IMAGERY SHOWS THAT THE SNOW HAS BECOME RATHER SPOTTY...BUT FEEL
THAT RADAR MAY BE UNDER REPRESENTING SNOWFALL AS MOST OF THE
METARS INDICATE SOME FORM OF SNOW IN THE PRESENT WEATHER
INDICATOR. THUS FEEL THAT THE RADAR RETURNS REPRESENT THE HEAVIER
SNOW AND LIGHT SNOW MAY BE ESCAPING DETECTION. SO...TIMING OF
INDIVIDUAL BURSTS OF SNOW REDUCING VISIBILITY WILL BE QUITE A
CHALLENGE OVERNIGHT. GIVEN THE SPOTTY NATURE OF THE HEAVIER
SNOW...HAVE GONE WITH PREVAILING 6SM -SN TO START OUT THE 06Z
UPDATE...WITH A TEMPO 2SM. CONFIDENCE IS LOW AT THIS TIME FOR
TIMING AND DURATION OF IFR VIS RESTRICTIONS...BUT FEEL THAT THERE
WILL BE PERIODS OF IFR VIS DURG THE REMAINDER OF THE NIGHT.
CEILINGS SHOULD BE LESS PROBLEMATIC. WHILE CIGS HAVE GENERALLY
IMPROVED TO VFR ACROSS NRN IL...SHORT TERM GUIDANCE AND UPSTREAM
OBS SHOW SOME TENDENCY FOR MVFR CIGS...SO HAVE GONE WITH MVFR CIGS
IN THE TEMPO AND THEN RETURNED TO PREVAILING MVFR DURG THE MORNING
HOURS. 

GUIDANCE STILL INDICATES A MODEST MID LEVEL IMPULSE ROTATING
AROUND THE MAIN UPPER LOW...SO WILL KEEP A PERIOD OF 2SM VIS
REDUCTION FROM LATE MORNING TO LATE EVENING. EVENTUALLY...THE
UPPER LOW WILL BEGIN TO EJECT NEWD TOMORROW AFTERNOON OR
EVENING...AND WINDS WILL EVENTUALLY SHIFT FROM PREVAILING SLY TO
MORE WLY. DECREASING MOISTURE SHOULD ALLOW FOR PCPN INTENSITY TO
TAPER OFF...BUT CYCLONIC CURVATURE SHOULD BE SUFFICIENT TO ALLOW
SOME LIGHT SNOW TO CONTINUE INTO THE NIGHT.

KREIN

&&

.MARINE...
327 AM CST

EVOLUTION OF WEAKENING SFC LOW ACROSS IOWA OVER NEXT 36 
HOURS AND ARCTIC FRONT APPROACHING THE REGION EARLY MONDAY BRINGING 
STRONGER WINDS WILL BE THE MAIN FORECAST ISSUES THIS MORNING.

SURFACE LOW ACROSS SOUTHERN IOWA CONTINUES TO FILL THIS MORNING AND 
WILL BE APPROACHING CENTRAL PORTIONS OF LAKE MICHIGAN LATE THIS 
EVENING. SOUTH WINDS WILL CONTINUE AND OBS INDICATING GUSTS TO 
AROUND 25 KNOTS CONTINUE ACROSS SOUTHERN LAKE MICHIGAN THIS MORNING. 
AS LOW CONTINUES TO FILL AND SFC PRESSURE GRADIENT SLACKENS OFF 
LATER THIS MORNING SHOULD SEE GUSTS OF THIS MAGNITUDE BEGIN TO 
SUBSIDE BACK INTO THE 15 TO 20 KT RANGE ACROSS SOUTHERN PART OF THE 
LAKE. LOW VERY SLOW TO DEPART CENTRAL/NORTHERN LAKE MICHIGAN 
SUNDAY...AND FOCUS FOR SUNDAY NIGHT WILL BE APPROACH OF ANOTHER 
TROUGH ROTATING THROUGH BACKSIDE OF THIS LOW BRINGING SHOT OF STRONG 
COLD AIR ADVECTION. SOME CONCERN FOR GALES DURING THIS PERIOD GIVEN 
DEEP MIXING AND PRESENCE OF 40 KNOT 850 HPA FLOW. AT THIS TIME 
BELIEVE NORTHWEST GALES WOULD REPRESENT JUST THE HIGHER END GUSTS 
AND DO NOT HAVE ENOUGH CONFIDENCE TO GO WITH A GALE WATCH AT THIS 
TIME FOR MONDAY...BUT WILL CONTINUE TO MENTION IN GLF MAINLY ACROSS 
NORTHERN HALF OF LAKE WHERE MIXING HEIGHTS WILL BE DEEPER. WINDS 
THEN BEGIN TO SUBSIDE MONDAY NIGHT AND TUESDAY AS STRONG HIGH 
PRESSURE BEGINS TO BUILD IN FROM THE WEST.

MARSILI

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&

$$