FXUS63 KLOT 220416
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1016 PM CST MON DEC 21 2009

.DISCUSSION...
829 PM CST

REGIONAL RADAR MOSAICS AT 02Z INDICATE EXPANDING AREA OF -SN
ACROSS EASTERN IOWA WITH HIGHER REFLECTIVITIES IN SN/FZRA BACK
ACROSS NORTHWEST IOWA AND FAR SOUTHWEST MINNESOTA. WATER VAPOR
IMAGERY OF APPROACHING VORT MAX TONIGHT ACROSS SOUTHEAST SOUTH
DAKOTA/EASTERN NEBRASKA IS RATHER IMPRESSIVE AND APPEARS TO BE A
BIT MORE ROBUST THAN PREVIOUS MODEL GUIDANCE HAD BEEN INDICATING.
LOW LEVEL FRONTOGENESIS CONTINUES TO STRENGTHEN THIS EVENING BACK
ACROSS NORTHWEST IOWA WHILE INCREASE IN WEAKER REFLECTIVITIES EAST
OF THIS AREA DUE TO BROAD AREA OF WEAKER ISENTROPIC ASCENT. STILL
EXPECTING THE MORE PRONOUNCED LOW/MID LEVEL FRONTOGENESIS TO TRACK
ACROSS FAR NORTHERN ILLINOIS LATE TONIGHT INTO EARLY THURSDAY
MORNING. NEW 00Z WRF/RUC HAVE SHIFTED LOW LEVEL FRONTOGENESIS AXIS
SLIGHTLY SOUTHWARD...AND FEEL THIS IS IN LARGE PART DUE TO MODELS
SLIGHTLY UNDERPLAYING STRENGTH OF UPSTREAM VORT. GIVEN
PROGRESSIVELY MORE NORTHWEST TO SOUTHEAST ORIENTATION OF LOW
LEVEL FRONT ACROSS NORTHERN ILLINOIS/NORTHWEST INDIANA...IT IS HARD
TO ARGUE WITH THESE TRENDS. TRENDS IN REGIONAL RADAR MOSAIC
ASSOCIATED WITH WEAK BROAD ISENTROPIC ASCENT ASSOCIATED WITH THIS
FRONT ALSO SUGGEST A SLIGHT SOUTHWARD SHIFT IN HIGHER POPS MAY BE
IN ORDER. MAY SEND A LATE EVENING UPDATE TO ACCOUNT FOR THIS
SLIGHT SOUTHWARD POP PLACEMENT. OTHER QUESTION IS JUST HOW STRONG
LOW LEVEL FORCING WILL BE AS FAR EAST AS WFO LOT CWA LATER
TONIGHT. LOW LEVEL FLOW EXPECTED TO BE STRONGEST WELL WEST OF
FORECAST AREA...BUT SHOULD SEE WEAK TO MODERATE LOW LEVEL
FRONTOGENESIS OVERSPREAD NORTHERN ILLINOIS OVERNIGHT. MODEL
TIME/HEIGHT SECTIONS DO SHOW ABOUT A 3 TO 6 HOUR SIGNAL OF GOOD
OVERLAPPING BETWEEN STRONGEST UVM AND DENDRITIC GROWTH ZONE
OVERNIGHT IN ASSOCIATION WITH FRONTOGENETIC FORCING SO HAVE A
LITTLE CONCERN ON SOME ISOLD LOCATIONS REACHING CLOSE TO 4 INCHES
BY TIME SNOW WINDS DOWN TUESDAY MORNING. GIVEN LOW CONFIDENCE AND
EXPECTED ISOLATED NATURE OF THESE TYPE OF SNOWFALL AMOUNTS HAVE
HELD OFF ON ADVISORY FOR NOW. WILL BE WORKING ON EVENING UPDATE
THAT WILL NOT CHANGE TOTAL SNOW ACCUMULATIONS BY THAT MUCH OF
TOTAL SNOW ACCUMS IN THE 1 TO 3 INCH RANGE...BUT MAY SHIFT HIGHER
POPS AND SNOW ACCUM MENTION A BIT FURTHER SOUTH. OTHERWISE...NO
SIGNIFICANT CHANGES TO FORECAST WITH TEMPS LIKELY NOT DROPPING
MUCH FROM CURRENT READINGS WITH WEAK WAA/CLOUD COVER.

MARSILI

&&

.PREV DISCUSSION...
328 PM CST

.SHORT TERM (TONIGHT THRU WEDNESDAY MORNING)...

SNOW CHANCES TONIGHT AND INCREASING PRECIP CHANCES FROM THE SOUTH 
LATE TUESDAY NIGHT CONTINUE TO BE FOCUS OF SHORT TERM FORECAST. 

MORNING RAOB/PROFILER ANALYSIS INDICATES FORECAST AREA SITUATED  
IN BRISK NORTHWEST FLOW ALOFT.  HEALTHY SHORTWAVE CURRENTLY OVER 
CENTRAL DAKOTAS IS EXPECTED TO MOVE SOUTH ACROSS THE AREA TONIGHT 
AND ON TUESDAY DURING THE DAY.  SURFACE ANALYSIS INDICATES 
STATIONARY FRONT/BAROCLINIC ZONE SITUATED FROM SOUTH DAKOTA ACROSS 
NEBRASKA AND IOWA...WITH A WEAKER...THOUGH STILL APPRECIABLE SURFACE 
TEMPERATURE GRADIENT ACROSS THE LOT CWA.  THIS BAROCLINIC ZONE IS 
EXPECTED TO TRANSITION ACROSS THE CWA LATER THIS EVENING...WITH 
MODEL GUIDANCE INDICATING DECENT LOW LEVEL FRONTOGENESIS TAKING 
PLACE NEAR THE WISCONSIN STATE LINE.  CURRENT LIGHT SNOWFALL OVER 
DAKOTAS AT THIS HOUR IN ASSOCIATION WITH AFOREMENTIONED SHORTWAVE 
SHOULD TRANSITION SOUTH TOWARD ROCKFORD LATE THIS 
EVENING...EVENTUALLY OVERSPREADING ENTIRE CWA DURING THE OVERNIGHT 
HOURS.  WHILE LIGHT SNOW SEEMS PROBABLE FOR THE ENTIRE FORECAST 
AREA...EXPECT THAT INTERACTION OF STRENGTHENING SURFACE/LOW LEVEL 
BAROCLINIC ZONE AND SHORTWAVE ALOFT WILL LIKELY LEAD TO A BAND OF 
SOMEWHAT HEAVIER SNOWFALL ACROSS NORTHWESTERN PORTIONS OF THE CWA.  
AS THE SHORTWAVE RIPPLES ACROSS THE AREA ON TUESDAY...EXPECT SNOW TO 
LIFT TO THE NORTH WHILE SIMULTANEOUSLY DISSIPATING DURING THE 
DAY...AND EVENTUALLY YIELDING JUST A FEW LIGHT SNOW SHOWERS OR 
FLURRIES BY EVENING.  QPF ISN'T PARTICULARLY IMPRESSIVE WITH THIS 
SYSTEM...JUST ON THE ORDER OF A FEW TENTHS OF AN INCH...YIELDING 
TOTAL SNOWFALL AMOUNTS IN THE 1 TO 3 INCH RANGE.

ONCE THIS SHORTWAVE MAKES ITS WAY OUT OF THE AREA...ATTENTION THEN 
TURNS TO WARM AIR ADVECTION PRECIP EXPECTED TO DEVELOP JUST TO OUR 
SOUTHWEST.  MODELS INDICATE PRETTY IMPRESSIVE 850 WAA ADVECTION 
VALUES...BUT IT ALSO APPEARS THAT ANY RESULTING RISING MOTION
WILL BE FIGHTING A BATTLE...AND PERHAPS A LOOSING ONE AT
THAT...WITH RISING HEIGHTS ALOFT. MODEL SOUNDINGS SUGGEST THAT
ANYTHING THAT FALLS WOULD BE IN THE FORM OF EITHER VERY LIGHT
SNOW/ICE CRYSTALS OR FREEZING DRIZZLE. THIS PRECIP SHOULD WANE
LATE TUESDAY NIGHT...BEFORE MUCH MORE SIGNIFICANT QPF BEGINS TO
MOVE IN WITH THE MID/LATE WEEK STORM SYSTEM.

TEMPERATURES NOT TOO TRICKY THROUGH THE PERIOD.  GENERALLY FOLLOWED 
COOLER MAV NUMBERS...AS FRESH INCH OR TWO OF SNOW ON THE GROUND 
TOMORROW SHOULD ACT TO SUPPRESS HIGH TEMPERATURES.  ALSO OF 
NOTE...NOT TOO MUCH IN THE WAY OF A DIURNAL TEMPERATURE TREND 
EXPECTED TUESDAY NIGHT WITH WAA CRANKING UP ALOFT...WITH GENERALLY 
STEADY TEMPS EXPECTED. 

BOXELL

.MEDIUM/LONG TERM (WEDNESDAY THROUGH MONDAY)...

EVOLUTION OF WINTER STORM...AND POTENTIAL FOR MIXED PRECIP FOR A
BUSY HOLIDAY TRAVEL PERIOD ARE FOCUS OF LONGER RANGE FORECAST THIS
AFTERNOON. 

MEDIUM RANGE GUIDANCE CONTINUES TO BE IN REASONABLY GOOD AGREEMENT
WITH HANDLING OF LARGE UPPER TROUGH/UPPER LOW WHICH IS EXPECTED TO
DEVELOP ACROSS THE SOUTHWESTERN CONUS WEDNESDAY...AND LIFT ACROSS
THE MIDWEST THURSDAY INTO SATURDAY. GUIDANCE OVERALL HAS CONTINUED
TO TREND TOWARD A SOLUTION WHICH BRINGS BOTH UPPER/SURFACE LOWS
ACROSS MISSOURI CHRISTMAS EVE...THEN ACROSS NORTHEAST IOWA OR
NORTHWEST IL ON CHRISTMAS MORNING. REASONABLE SUPPORT/CLUSTERING
FROM MEDIUM RANGE ENSEMBLES AT THIS TIME...WITH SOME INDICATION OF
FAVORING SLOWER/FARTHER WEST ECMWF/WRF TRACK. EITHER WAY...LOT
FORECAST AREA EXPECTED TO BE ON WARM SIDE OF STORM SYSTEM...THOUGH
THIS PRESENTS CHALLENGING PRECIPITATION TYPE FORECAST DURING THE
PERIOD.

ANALYSIS OF MODEL THERMAL/MOISTURE PROFILES INDICATES THAT PRECIP
TO BEGIN WEDNESDAY INITIALLY AS MIX OF SNOW...SLEET AND FREEZING
RAIN AS WARM AIR RIDES OVER SHALLOW COOL AIR ACROSS THE AREA FROM
THE SOUTH. WRF/GFS BUFFER SOUNDINGS ACROSS CWA INDICATE WARM LAYER
ALOFT DEVELOPS WHICH SHOULD POSE MORE OF A SLEET/FREEZING RAIN
THREAT WITH TIME ESPECIALLY ACROSS THE NORTHERN PARTS OF THE
FORECAST AREA WHERE TEMPS LESS LIKELY TO CLIMB ABOVE FREEZING.
TEMPS EXPECTED TO WARM ABOVE THE FREEZING MARK ACROSS FAR SOUTHERN
COUNTIES WEDNESDAY AFTERNOON...WITH PRECIP CHANGING TO JUST RAIN
FAR SOUTH. PUSH OF LOW LEVEL WARM ADVECTION SPREADS NORTH ON
THURSDAY/CHRISTMAS EVE DAY...WHICH IS EXPECTED TO PUSH TEMPS ABOVE
FREEZING OVER MOST OF CWA AND SHIFT PRECIP OVER TO RAIN AS LOWER
SEVERAL THOUSAND FEET OF TROPOSPHERE WARMS ABOVE 0 DEG C. RAIN
THEN EXPECTED TO CONTINUE THURSDAY NIGHT INTO FRIDAY MORNING
BEFORE LOW LIFTS NORTH OF THE AREA AND THICKNESSES AGAIN BEGIN TO
FALL...AND CHANGE PRECIP BACK TO SNOW FROM WEST TO EAST THROUGH
FRIDAY AFTERNOON. FLURRIES OR LIGHT SNOW SHOWERS MAY THEN LINGER
SATURDAY AS MODELS HINT AT SECONDARY WEAKER TROUGH LINGERING ACROSS
THE AREA.

FOR NOW...WEDNESDAY INTO THURSDAY MORNING LOOK TO BE THE PERIOD OF
WEATHER WITH THE GREATEST POTENTIAL FOR MESSY TRAVEL...WITH
SNOW/SLEET AND FREEZING RAIN DURING THIS PERIOD. SHOULD ALSO NOTE
THAT GUIDANCE PUMPS OUT 1.5+ INCHES OF LIQUID QPF DURING THIS
EVENT...WHICH COULD PERHAPS RESULT IN SOME FLOODING CONCERNS GIVEN
FROZEN GROUND CONDITIONS.

RATZER

&&

.AVIATION...

0600 UTC TAFS...SNOW DEVELOPMENT IN BAROCLINIC ENVIRONMENT
STRETCHING FROM THE NRN PLAINS INTO NRN IL. SNOW SHOWERS MADE GOOD
PROGRESS IN LAST 6 HOURS AND ARE AHEAD OF SCHEDULE. IFR CIGS IN
RELUCTANT TO REACH NERN IL AS YET BUT SHUD BE COMING DOWN NOW THAT
SNOW HAS BEGUN. THIS ALL LASTING THRU THE MONDAY MRNG WITH VSBY
LOWERING TO IFR IN SOME OF THE HEAVIER SNOW SHOWERS OVERNIGHT.
FIRST IMPULSE MOVING THRU BAROCLINIC ZONE TONIGHT SHUD BE EAST OF
TERMINALS BEFORE NOON MONDAY AND SHUD SEE SOME IMPROVEMENT IN VSBY
FROM IFR TO MVFR. DOUBT WE WILL SEE SNOW COMPLETELY STOP AND SHUD
EXPECT TO SEE FLURRIES ON AND OFF THRU THE AFTN AND EVENING
TUESDAY. TEMPS HOLDING IN UPR 20S OVERNIGHT AND RISING TO LOWER
30S TUESDAY SHUD CAUSE SOME MELTING AND ADD SOME SATURATION TO
LOWER LVLS FOR CONTINUED FOG FORMATION. LIGHT AND VARIABLE WINDS
EARLY TONIGHT VEERING MORE TO THE EAST BY MORNING AND PICKING UP
MONDAY AFTN TO 10 KTS OR MORE AS PRESSURE GRADIENT TIGHTENS IN
RESPONSE TO DEEPENING LOW PRESSURE IN THE SRN PLAINS. THIS SYSTEM
WILL SPREAD MORE SNOW...THEN FRZG RAIN AND SLEET ON WEDNESDAY.

RLB

&&

.MARINE...
235 PM CST

DURING THE NEXT 24 HOURS...WINDS ACROSS THE LAKE WILL UNDERGO A
GRADUAL TURNING FROM WEST TO NORTH AND THEN AROUND TO EAST AS A
WEAK LOW PRESSURE SYSTEM MOVES ACROSS THE GREAT LAKES AND SLOWLY
DISSIPATES. BEYOND TOMORROW MORNING THE TWO DOMINANT WEATHER
FEATURES FOR THIS WEEK WILL BE A LARGE AREA OF HIGH PRESSURE OVER
ONTARIO AND A LOW PRESSURE SYSTEM DEVELOPING OVER THE SOUTHERN
PLAINS. THE HIGH REMAINS PARKED IN THE NEIGHBORHOOD OF JAMES
BAY UNTIL LATE IN THE WEEK WHILE THE LOW PRESSURE SYSTEM STRENGTHENS
SIGNIFICANTLY AND TRACKS INTO WESTERN MISSOURI BY LATE THURSDAY.
EAST WINDS OVER THE LAKE WILL INCREASE CONSIDERABLY IN THE
GRADIENT BETWEEN THESE TWO SYSTEMS...ESPECIALLY ON THURSDAY. A 24
TO 48 HOUR PERIOD OF GALES IS POSSIBLE FROM LATE WEDNESDAY THROUGH
MUCH OF FRIDAY. A MESSY MIX OF PRECIP IS ALSO EXPECTED WITH THIS
STORM SYSTEM AS IT MOVES OUT OF MISSOURI AND THROUGH WISCONSIN.
RAIN...FREEZING RAIN...SLEET...AND SNOW ALL APPEAR POSSIBLE OVER
THE LAKE WITH THIS SYSTEM.

LENNING

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...MIDNIGHT WEDNESDAY TO 6 PM WEDNESDAY.

&&

$$