FXUS63 KLOT 260935
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
335 AM CST THU NOV 26 2009

.DISCUSSION...
335 AM CST

FORECAST ISSUES WILL CONTINUE TO BE MIXED PRECIP TODAY...AND THEN 
RESOLVING MODERATING TREND FOR FIRST PART OF THE WEEKEND...FOLLOWED 
BY NEXT CHANCE OF PRECIP WITH NEXT SYSTEM FOR LATTER PARTS OF THE 
WEEKEND.

WATER VAPOR IMAGERY EARLY THIS MORNING SUGGESTING CENTER OF UPPER 
LEVEL LOW SHIFTING JUST EAST OF CHICAGO. FAIRLY STRONG VORT MAX WILL 
CONTINUE TO WRAP AROUND BASE OF THIS TROUGH. MOST OF THE FORCING
WITH THIS SYSTEM IS TIED IN WITH UPPER LEVEL FEATURES AND
DEFORMATION FORCING NORTH OF THE MID/UPPER LEVEL HEIGHT MINIMUM.
ONE THING THIS SYSTEM APPEARS TO BE LACKING IS A PRONOUNCED
LOW/MID LEVEL BAROCLINIC ZONE. THIS LACK OF A STRONG MID/UPPER
LEVEL FRONT SHOULD TEND TO MAKE DEFORMATION ZONE A LITTLE MORE
BROAD IN NATURE. ALMOST MORE OF A RECYCLED TROWAL FEATURE FROM
PREVIOUS SYSTEM TO AFFECT AREA PAST 24 TO 48 HOURS WRAPPING AROUND
BACKSIDE OF THIS UPPER LOW HAS BEEN WHERE MOST SIGNIFICANT
REFLECTIVITIES HAVE BEEN OBSERVED. RUC 00HR CROSS SECTIONS CUTTING
ACROSS THIS TROWAL WRAPPING BACK INTO NORTH CENTRAL ILLINOIS DO
INDICATE A FAIRLY LARGE LAYER OF WEAK STATIC STABILITY IN THE MID
LEVELS...BUT AGAIN ABSENCE OF STRONG LOW/MID LEVEL FRONT SEEMS TO
BE A LIMITING FACTOR FOR GREATER PRECIP RATES. 

PRECIP TYPE REMAINS QUITE TRICKY. REPORTS FROM OUR NEIGHBORS TO
THE NORTH AT WFO MKX HAVE INDICATED SOME INSTANCES OF SLUSHY HALF
INCH TO INCH SNOW ACCUMULATIONS OVERNIGHT...MAINLY ACROSS GRASSY
AREAS. SURFACE OBSERVATIONS INDICATING RAIN HAS NOW BECOME MIXED
WITH OR CHANGED OVER TO WET SNOW ACROSS FAR NORTHWEST AND NORTH
CENTRAL ILLINOIS. NAM/GFS HAVE BEEN INDICATING SUBTLE DIFFERENCES
PAST FEW DAYS WITH TEMPS/DEW POINTS IN LOWEST 50 HPA OR SO WHICH ARE
VERY CRITICAL FOR PRECIP TYPE IN THIS INSTANCE. NAM BUFFER
SOUNDINGS THIS MORNING EVEN KEEP THIS LOWEST 50 HPA LAYER AT OR
BELOW FREEZING SUPPORTING ALL SNOW...BUT INLINE WITH PREVIOUS
FORECAST DOES APPEAR SLIGHTLY WARMER/MOISTER GFS MIGHT HAVE A
SLIGHTLY BETTER HANDLE ON THINGS. SURFACE TROUGH ROTATING THROUGH
BACKSIDE OF THIS SYSTEM ACROSS SOUTHERN WISCONSIN/EASTERN IOWA IS
ACCOMPANIED BY SLIGHT INCREASES IN SFC DEW POINTS BACK INTO THE
MID 30S AND LIKELY MARKS OLD OCCLUDED AIR MASS WRAPPING SOUTHWARD
BACK INTO THE AREA. GIVEN THIS...DID STAY MORE WITH GFS IDEA OF
CATEGORICAL POPS WITH RAIN/SNOW MENTION. DID DRAW UP JUST SOME
VERY MINOR SNOW ACCUMULATIONS IN FORECAST GRIDS THIS
MORNING...GENERALLY LESS THAN A HALF INCH ACROSS FAR NORTH CENTRAL
ILLINOIS AND EXTENDING EASTWARD INTO FAR NORTHERN SUBURBS.
HOWEVER...WOULD EXPECT MOST AREAS WILL NOT SEE ANY
ACCUMULATION...AND AS HAS BEEN THE CASE IN SOUTHERN WISCONSIN THIS
WOULD ONLY OCCUR ON GRASSY/COLDER SURFACES. DEFORMATION PRECIP
SHOULD GRADUALLY END FROM WEST TO EAST LATER THIS MORNING INTO
THIS AFTERNOON...ALTHOUGH SKIES SHOULD REMAIN MOSTLY CLOUDY
THROUGH THE DAY. HIGHS GENERALLY IN THE UPPER 30S TO AROUND 40
STILL SEEM TO BE IN THE BALLPARK AND NO CHANGES WERE MADE TO MAX
TEMPS TODAY.

LAKE EFFECT SNOW SHOWER POTENTIAL TONIGHT STILL LOOKS RATHER
UNIMPRESSIVE WITH NORTHWEST WINDS ACTUALLY ADVECTING SLIGHTLY WARMER
LOW LEVEL TEMPS ACROSS SOUTHERN LAKE MICHIGAN LATE THIS
AFTERNOON/EARLY THIS EVENING. MID LEVEL WAA ALSO A BIT MORE
IMPRESSIVE BY LATE EVENING WHICH WILL KEEP INVERSION HEIGHTS
FAIRLY LOW. HAVE ONLY KEPT A SLIGHT CHANCE OF SNOW SHOWERS GOING
FOR THIS EVENING ACROSS NORTHEAST PORTIONS OF PORTER COUNTY BUT
GREATER POTENTIAL OF SCATTERED SNOW SHOWERS WILL REMAIN FURTHER UP
THE MICHIGAN LAKESHORE. WITH CLEARING SKIES TONIGHT WAS TEMPTED TO
GO A BIT COLDER THAN GUIDANCE...BUT HAVE GENERALLY FOLLOWED
GUIDANCE MINS FROM UPPER 20S TO LOWER 30S TONIGHT GIVEN FULL
DECOUPLING MAY NOT OCCUR WITH SFC RIDGE AXIS REMAINING WEST OF THE
MS RIVER.

UPPER LEVEL RIDGING ACROSS WESTERN CONUS WILL BEGIN TO SHIFT 
EASTWARD FRIDAY INTO CENTRAL/NORTHERN PLAINS...AND WHILE LOW LEVEL 
WARM ADVECTION SHOULD BECOME MORE PRONOUNCED FRIDAY...EXPECTING MORE 
SIGNIFICANT LOW LEVEL WAA FOR FRIDAY NIGHT INTO SATURDAY. 
THUS...FRIDAY STILL SHAPING UP AS FAIRLY COOL DAY WITH TEMPS LIKELY 
STRUGGLING TO REACH LOWER 40S ACROSS NORTHEAST IL/NWRN IN. LOW LEVEL 
THERMAL RIDGING SHOULD BE MORE FIRMLY IN PLACE ON SATURDAY...WITH 
SATURDAY SHAPING UP TO BE THE MILDEST DAY OF THE PERIOD. GIVEN TIME 
OF YEAR...MIXED LAYER WONT BE ALL THAT DEEP AND ALL THIS WAA WONT BE 
REALIZED AT THE SURFACE BUT MIXING UP CLOSE TO 900 HPA SHOULD YIELD 
HIGHS FROM AROUND 50 NORTH TO MID 50S SOUTH.

FOCUS THEN SHIFTS TO NEXT SYSTEM TO AFFECT AREA LATE WEEKEND INTO 
EARLY NEXT WEEK. 12Z ECMWF WAS A SLOW/STRONG OUTLIER WITH THE 
MID/UPPER LEVEL TROUGH AFFECTING MID MS RIVER VALLEY SUNDAY 
NIGHT/MONDAY MORNING...BUT NEW 00Z ECMWF IS TRENDING TOWARD WEAKER 
MORE PROGRESSIVE SOLUTION OF GFS/CANADIAN. KEEPING WITH GFS IDEA OF 
WEAKER TROUGH AND MORE OF A SFC TROUGH PASSAGE AS OPPOSED TO DEEPER 
LOW OF OLD ECMWF...HAVE JUST KEPT LOW CHANCE POPS FOR -RA
SUNDAY...TRANSITIONING TO LOW CHANCE POPS FOR -RA/-SN MONDAY. NO
CHANGES POST MONDAY TO THE FORECAST WITH NEXT AMPLIFIED TROUGH TO
AFFECT AREA TOWARD MID WEEK. ALTHOUGH THERE ARE SOME EXPECTED
TIMING DIFFERENCES...MEDIUM RANGE MODELS ARE IN GENERAL AGREEMENT
WE WILL BE TRANSITIONING TO A MORE WINTERY TYPE AIR MASS BY MID TO
LATE WEEK NEXT WEEK.

MARSILI

&&

.AVIATION...
1115 PM CST

0600 UTC TAFS...SATELLITE ALONG WITH VAD AND PROFILER WINDS
POSITION MID/UPPER LOW OVER FAR NE IL AT 04Z. LATEST MODEL RUNS
HAVE IT MOVING ENE ACROSS FAR SOUTHERN LAKE MI TO FAR SW LOWER MI
OVERNIGHT. -RA TO BEGIN TO MIX WITH -SN AS FREEZING LEVEL
CONTINUES TO DROP TO AROUND 1500 FT AGL OVERNIGHT. SIGNIFICANT SN
FALLING ACROSS SOUTHERN WI AND FAR NW AND N CENTRAL IL IN
DEFORMATION ZONE. WITH SYSTEM MOVING ENE DO NOT ANTICIPATE THIS TO
REACH LOCAL TERMINALS. -RASN MIX TO PERSIST THRU MIDDAY THU AS 
SYSTEM CONTINUES MOVING SLOWLY JUST N OF DUE E.

BESIDES AREA OF IFR CONDITIONS IN SOUTHERN WI INTO NW IL...MVFR
CEILINGS AND UNRESTRICTED VSBY PREVALENT SO HAVE KEPT MVFR IN
TAFORS WITH ONLY TEMPO HI END IFR DURING OVERNIGHT INTO THU
MORNING.

DEEPENING AND STRENGTHENING UNIDIRECTIONAL NW FLOW SPREADS ACROSS
THE AREA AS MID LEVEL CIRCULATION MOVES FROM OVERHEAD AND EXPECT
WINDS TO GUST 25-30KT BY MID MORNING. SIGNIFICANT DRYING FORECAST
TO OCCUR DURING THE LATER AFTERNOON SO OVC TO BECOME BKN.

TRS

&&

.MARINE...
335 AM CST

MAIN MARINE CONCERNS CONTINUE TO BE GUSTY NORTHWEST WINDS DEVELOPING 
THIS MORNING BEHIND DEPARTING LOW PRESSURE SYSTEM AND COLD FRONT.

ELONGATED AREA OF LOW PRESSURE ANALYZED FROM SOUTHERN ONTARIO TO 
CENTRAL/SOUTHERN LAKE MICHIGAN EARLY THIS MORNING...WITH COLD FRONT 
TRAILING SOUTHWEST INTO IL. TROUGH TO PULL OFF TO THE EAST/NORTHEAST 
THIS MORNING...WITH GRADIENT TIGHTENING ALONG WESTERN PERIPHERY 
ACROSS LAKE MICHIGAN. LATEST MODEL GUIDANCE CONTINUES TO BE IN GOOD 
AGREEMENT WITH WINDS INCREASING TO AROUND 30 KTS LATER THIS 
MORNING...BEFORE GRADUALLY DIMINISHING LATER TONIGHT AND FRIDAY AS 
LOW MOVES AWAY AND WEAK HIGH PRESSURE RIDGE SPREADS INTO THE REGION 
BY FRIDAY NIGHT. SMALL CRAFT ADVISORY IN EFFECT LATER THIS MORNING 
THROUGH TONIGHT FOR IL/IN SHORES...AND THROUGH FRIDAY AFTERNOON FOR 
IN SHORES EAST OF GARY AS WAVES WILL BE SLOWEST TO SUBSIDE THERE 
AFTER PERIOD OF STRONG NORTHWEST FETCH. WINDS TO BACK SOUTH AFTER 
PASSAGE OF RIDGE AXIS FRIDAY NIGHT...WITH ANOTHER WEAKER FRONT THEN 
SLIPPING DOWN THE LAKE SATURDAY NIGHT.

RATZER

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742-LMZ743...10 AM 
     THURSDAY TO 4 AM FRIDAY.

     SMALL CRAFT ADVISORY...LMZ744-LMZ745...NOON THURSDAY TO 4 PM 
     FRIDAY.

&&

$$