FXUS63 KLOT 260223
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
823 PM CST WED NOV 25 2009

.DISCUSSION...
738 PM CST

UPPER LOW/STRONG VORT MAX EMBEDDED WITHIN BROADER LARGE SCALE
TROUGH CONTINUES TO TRACK EAST-SOUTHEASTWARD ACROSS IA TOWARD
NORTHERN/CENTRAL ILLINOIS. COLDER CLOUD TOPS ASSOCIATED WITH WARM
CONVEYOR BELT ARE WRAPPING AROUND THE NORTH SIDE OF THE UPPER LOW
THIS EVENING WITH BEGINNINGS OF TROWAL (TROUGH OF WARM AIR ALOFT)/
DEFORMATION ZONE DEVELOPING. THE WARMER AIR WRAPPING AROUND THE
NORTH SIDE OF THIS SYSTEM THAT IS SUPPORTING THE PRECIPITATION IS
ALSO RESPONSIBLE FOR SLOWING THE COOLING OF THE COLUMN AND KEEPING
TEMPS WARM ENOUGH FOR PRIMARILY RAIN WITHIN THE DEFORMATION
PRECIP THUS FAR...WITH STRONGEST COLD AIR ADVECTION TAKING PLACE TO
THE SOUTH OF THE PRECIP AXIS.

AS VORT MAX TRACKS EAST ACROSS NORTHERN PORTIONS OF CENTRAL
ILLINOIS LOOK FOR SOMEWHAT OF A BREAK IN PRECIP OR AT LEAST A
TAPERING TO DRIZZLE ACROSS SOUTHERN AND POSSIBLY EVEN CENTRAL
PORTIONS OF THE CWA LATER THIS EVENING INTO THE START OF THE
OVERNIGHT. BY LATER TONIGHT AS THE UPPER LOW CONTINUES EASTWARD
TROWAL WILL BEGIN TO WRAP MORE AROUND THE UPPER LOW AND INTO THE
REGION OF STRONG COLD AIR ADVECTION. THAT...PLUS THE TIME OF
DAY...WOULD BOTH FAVOR AN INCREASE IN COVERAGE OF SNOW LATE
TONIGHT INTO EARLY THURSDAY MORNING. 

AS PREVIOUS DISCUSSION ALLUDED TO...LOW LEVEL THERMAL PROFILES
WILL BE MARGINAL FOR SNOW AND LIKELY TOO WARM FOR ANY APPRECIABLE
ACCUMULATIONS TO OCCUR. WHILE CHANCES OF ANY ACCUMULATING SNOWFALL
APPEAR SMALL...IT DOES APPEAR AS THOUGH THERE WILL PROBABLY BE A
PERIOD OF SOMEWHAT STEADIER PRECIPITATION THAT WILL PROBABLY FALL
AS MOSTLY SNOW AS DEFORMATION BAND BECOMES MORE N-S ORIENTED AND
MOVES ACROSS THE AREA VERY LATE TONIGHT AND THURSDAY MORNING. 

ONLY VERY MINOR COSMETIC TWEAKS HAVE BEEN MADE TO GRIDS
TONIGHT...HOWEVER HAVE TRENDED WX GRIDS A BIT MORE TOWARD SNOW FOR
THURSDAY MORNING. UPDATED GRIDS/TEXT PRODUCTS WILL BE ISSUED
SHORTLY.

IZZI

&&

.PREV DISCUSSION...
306 PM CST

FOCUS FOR THIS FORECAST PACKAGE IS ON THE PRECIP TRENDS TONIGHT
THROUGH THANKSGIVING AND WHEN THE RAIN WILL CHANGE OVER TO SNOW.

SECONDARY VORT MAX OVER NORTH CENTRAL IOWA IS EXPECTED TO
PROPAGATE INTO NORTHERN ILLINOIS LATER TONIGHT. COLDER LOW LEVEL
AIR IS EXPECTED TO ADVECT IN ON THE BACKSIDE OF THIS SYSTEM
WITH 850MB TEMPS DROPPING TO -2 TO -5C BY THIS EVENING. SOMEWHAT
OF A COMPLEX SETUP TAKING SHAPE BUT THINK THAT THERE WILL BE THREE
MAIN PHASES TO OUR WEATHER THIS AFTERNOON THROUGH TOMORROW
AFTERNOON.

PHASE 1 WILL TAKE PLACE THIS AFTERNOON INTO THIS EVENING AS THE
VORT MAX ENTERS EASTERN IOWA AND THE CURRENT BAND OF LIGHT RAIN
AND DRIZZLE ACROSS EASTERN IOWA/NORTHWEST ILLINOIS PIVOTS
NORTHEAST INTO NORTHERN IL/IN IN ASSOCIATION WITH UPPER LEVEL
DIFFLUENCE/PV ADVECTION. TEMPERATURES ARE EXPECTED TO REMAIN IN
THE UPPER 30S TO LOW 40S KEEPING ANY PRECIP IN LIQUID FORM.
RAINFALL AMOUNTS WILL BE ON THE LIGHT SIDE AT ABOUT A TENTH OF AN
INCH OR LESS.

PHASE 2 WILL OCCUR OVERNIGHT AS THE CORE OF THE UPPER LOW
POSITIONS ITSELF OVER THE TOP OF THE FORECAST AREA. EXPECT THAT
SHOWERY PRECIP WILL CONTINUE BUT SHOULDN'T HAVE ANY MODERATE TO
HEAVY RAIN FALLING. STILL SHOULD REMAIN WARM ENOUGH FOR RAIN AT
THIS POINT.

PHASE 3 WILL START TAKING SHAPE IN THE EARLY MORNING HOURS
THURSDAY AND CONTINUE THROUGH THE AFTERNOON IN THE FORM OF A
DEFORMATION BAND POSITIONED NORTHWEST OF THE UPPER LOW AND SFC
TROF. THIS SW TO NE ARCHED BAND IS EXPECTED TO DRIFT ACROSS
NORTHERN ILLINOIS THROUGH THE MORNING AND EARLY AFTERNOON. LOW
LEVEL TEMPERATURES SHOULD BE COLD ENOUGH AT THIS POINT THAT SNOW
WILL START MIXING IN WITH BETTER CHANCES OF CHANGING ALL OVER TO
SNOW DEEPER INTO THE COLD AIR ACROSS NORTHWEST ILLINOIS. AS THE
DAY PROGRESSES ON THURSDAY...TEMPERATURES SHOULD CREEP UP BACK
INTO THE UPPER 30S TO AROUND 40 WHICH SHOULD KEEP MOST OF THE
PRECIP A MIX.

THE FRUSTRATING PART OF THIS PORTION OF THE FORECAST IS THE
SUBTLE DIFFERENCES BETWEEN THE NAM AND GFS IN REGARDS TO LOW LEVEL
TEMPS. THE NAM IS MUCH COOLER AND SWITCHES EVERYTHING OVER TO SNOW
QUICKER WHILE THE GFS WRAPS SOME WARMER AIR IN...MUCH LIKE THE
ECMWF...AND KEEPS IT MUCH MORE BORDERLINE. THIS DIFFERENCE IN
MODEL SOLUTIONS IS ENOUGH TO KEEP SOME UNCERTAINTY IN WHAT WILL
ACTUALLY OCCUR OVERNIGHT INTO TOMORROW. GIVEN THE UPSTREAM
TEMPERATURES NOT BEING SIGNIFICANTLY COOLER...HAVE TRENDED TOWARD
THE WARMER GFS/ECMWF SOLUTIONS AND THEREFORE HAVE FAVORED THE MIX
AND NO SNOW ACCUMULATIONS TO THE ALL OUT SNOW WITH SOME MINOR
ACCUMULATIONS.

AFTER THIS SYSTEM MOVES OUT OF THE REGION LATE THURSDAY
AFTERNOON...CONDITIONS ARE LOOKING NICE INTO THE
WEEKEND...PARTICULARLY ON SATURDAY. SHOULD REMAIN DRY TIL SUNDAY
WITH HIGHS ON SATURDAY REACHING THE 50S.  

UNCERTAINTY LINGERS THROUGH THE EXTENDED PORTION OF THE FORECAST
PERIOD...BUT THE TREND APPEARS TO BE GOING TO A DRIER...COOLER
PATTERN WITH HIGH PRESSURE BUILDING IN ACROSS THE REGION AFTER A
COLD FRONT MOVES THROUGH ON SUNDAY.  

HALBACH

&&

.AVIATION...
627 PM CDT 

0000 UTC TAFS...LOOKS LIKE AREA OF HEAVIER RAIN OVER NE IL WILL
MOVE OFF AROUND 01Z FOLLOWED BY A PERIOD OF LIGHTER RAIN.
EXTRAPOLATION OF BACK EDGE HAD BACK EDGE TO ORD AND MDW AROUND
06Z. PREVAILING CONDITIONS MVFR DUE TO CEILINGS BUT VISIBILITY
UNRESTRICTED. 

AREA OF SHRA AND A LITTLE SHSN UNDER MID/UPPER LOW OVER CENTRAL 
IA SHOULD MOVE OVER TERMINALS BY 06Z AS MODELS HAVE H7 AND H5 
LOW CENTERS OVER NE IL ATTM. WSHFT TO WNW-NW WITH ASSOCIATED
TROF/SECONDARY COLD FRONT TO OCCUR DURING 09Z-12Z WINDOW.

SHRASN TIL MID-LATE MORNING LIKELY TO TURN TO ALL SHSN BEFORE
ENDING AS FREEZING LEVEL DROPS TO 1500 FT AGL BY 18Z. MVFR
CEILINGS AND VISIBILITIES PREVAIL UPSTREAM BEHIND SECONDARY COLD 
FRONT SO KEPT IN THAT CATEGORY RANGE FOR THU...WITH VSBY BECOMING
UNRESTRICTED AROUND MIDDAY AS PRECIP ENDS.

SOME NW GUSTS 25-30KT THU WITH STRONG CAA AND BASED ON UPSTREAM
OBS AND FORECAST WIND PROFILES.

TRS

&&

.MARINE...
1146 AM CST

A LOW PRESSURE LOCATED NEAR TRAVERSE CITY MICHIGAN WILL LIFT
NORTHEAST ACROSS THE SOO THIS EVENING...WHILE TRAILING A TROUGH
SOUTHWARD ACROSS INDIANA. THIS LOW IS THEN FORECAST TO DRIFT
ACROSS NORTHERN LAKE HURON ON THURSDAY AS A REINFORCING PUSH OF 
COLD AIR IS DRAWN EASTWARD ACROSS THE SOUTHERN GREAT LAKES BY
THURSDAY MORNING. AS THE LOW SLOWLY MOVES EASTWARD ACROSS
ONTARIO...A PERIOD OF GUSTY NORTHWEST WINDS WILL DEVELOP BEHIND
THURSDAY AND THURSDAY NIGHT. WINDS WILL DIMINISH ON FRIDAY AS HIGH
PRESSURE RIDGING SPREADS ACROSS THE WESTERN GREAT LAKES.

MERZLOCK

&&

.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.

&&

$$