FXUS63 KLOT 261131
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
531 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...
1200 UTC TAFS...AVIATION CONCERNS THIS MORNING FOCUS ON
PRECIPITATION TRENDS AND ASSOCIATED CEILING AND VISIBILITY
CONDITIONS THIS MORNING...ALONG WITH GUSTY NORTHWEST WINDS
DEVELOPING.
OCCLUDED LOW PRESSURE SYSTEM OVER SOUTHERN LAKE MICHIGAN THIS
MORNING PER GOES VAPOR IMAGERY AND SFC ANALYSIS. AREA OF DEFORMATION
PRECIPITATION ACROSS NORTHERN IL/SOUTHEAST WI WILL CONTINUE TO PIVOT
ACROSS FORECAST AREA THIS MORNING...WITH MIX OF RAIN/WET SNOW.
CHANGE TO WET SNOW HAS OCCURRED MAINLY WHERE DEEPER MOISTURE/HIGHER
RADAR REFLECTIVITY HAS BEEN JUXTAPOSED WITH SLIGHTLY DRIER LOW LEVEL
AIR NOTED WITH SUB-FREEZING SFC DEW POINT TEMPS. WARMER/MORE MOIST
LOW LEVEL AIR WRAPPING AROUND OCCLUDED SYSTEM ACROSS SOUTHERN WI
HELPING TO CHANGE PRECIP BACK TO RAIN FROM THE NORTHWEST...SO EXPECT
RELATIVELY SHORT WINDOW FOR SNOW OR RAIN/SNOW THIS MORNING AS
FORCING DECREASES AND SFC DEW POINTS CREEP BACK UP ABOVE FREEZING.
WHILE SOME LIFR CEILINGS HAVE BEEN NOTED ACROSS NORTH CENTRAL IL...
EXPECT MOST AREAS TO REMAIN IN IFR RANGE DURING PERIOD OF MIXED
PRECIP. IMPROVEMENT TO MVFR SHOULD BEGIN BY MID-MORNING AS SYSTEM
PULLS SLOWLY AWAY TO THE EAST. LARGE EXPANSE OF MVFR CLOUDS UPSTREAM
SHOULD GRADUALLY LIFT AND SCATTER BY EVENING...AS DRIER LOW LEVEL
AIR SPREADS IN FROM THE NORTHWEST.
AS FOR WINDS...SURFACE TROUGH/DEVELOPING COLD FRONT WILL MOVE ACROSS
TERMINALS THIS MORNING...WITH GRADIENT TIGHTENING AND MIXED LAYER
DEEPENING IN LOW LEVEL COLD ADVECTION IN ITS WAKE. WRF/BUFKIT
FORECAST SOUNDINGS INDICATE NORTHWEST WINDS GUSTING INTO MID 20 KT
RANGE BY LATE MORNING. GRADIENT RELAXES THIS EVENING WHICH ALONG
WITH DECOUPLING OF NOCTURNAL BOUNDARY LAYER SHOULD ALLOW WINDS TO
SUBSIDE.
RATZER
&&
.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.
&&
$$
