🚀 go-pugleaf

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1155 AM CST TUE MAR 5 2013

DISCUSSION...
1006 AM CST

HAVE INCREASED SNOWFALL AMOUNTS ACROSS THE AREA THIS MORNING AS
REPORTS OF UP TO 2" PER HOUR RATES HAVE BEEN RECEIVED ACROSS
PORTIONS OF NORTH CENTRAL ILLINOIS. THESE VERY HEAVY SNOWFALL RATES
APPEAR TO BE TIED TO A BAND OF STRONG 850-700 MB FRONTOGENESIS
ASSOCIATED WITH THE APPROACHING 700 MB LOW. THE LATEST GUIDANCE
SUPPORTS THESE HEAVY SNOW RATES CONTINUING TO SPREAD ACROSS ALL OF
NORTHERN ILLINOIS AND INTO NORTHWESTERN INDIANA OVER THE NEXT FEW
HOURS.

12Z LOCAL HI-RES SIMULATIONS HAVE NUDGED UPWARDS WITH THE INTENSITY
OF OMEGA/FORCING IN THE LOW-LEVELS IN THE 18-00Z TIMEFRAME. MORE
FOCUSED BETWEEN I-80 AND I-88 CORRIDORS OF NORTHERN IL/NORTHWEST IN.
THE BEST FORCING BAND APPEARS TO PIVOT IN THE AFTERNOON ALLOWING FOR
A PROLONGED PERIOD OF MODERATE TO HEAVY SNOWFALL.

MODEL CROSS SECTIONS ACROSS THE AREA INDICATE VERY STRONG FORCING
FOR ASCENT IN THE DENDRITIC GROWTH ZONE THROUGH THE
AFTERNOON...ALONG WITH SOME -EPV IN THE MID LEVELS...INDICATING THE
PRESENCE OF STEEP MID LEVEL LAPSE RATES. THE INVERTED SURFACE LOW
PRESSURE TROUGH SHOULD PIVOT ACROSS THE AREA THROUGH THE AFTERNOON
AND INTO THE EARLY EVENING...LIKELY RESULTING IN SEVERAL HOURS OF
HEAVY SNOW RATES IN A BAND RIGHT ACROSS THE CWA. WITH THIS IN
MIND...IT APPEARS THAT 7 TO 10 INCHES AMOUNTS...WITH SOME AMOUNTS IN
EXCESS OF A FOOT WILL BE POSSIBLE BY THIS EVENING. UPDATED FORECAST
AMOUNTS WILL BE ISSUED SHORTLY.

KJB/DB

//PREV DISCUSSION...
322 AM CST

BIGGEST SNOW STORM OF THE YEAR FOR MANY AREAS LIKELY TO BLANKET THE
REGION WITH A FRESH COATING OF SNOWFALL TODAY INTO THIS EVENING.

WATER VAPOR IMAGERY THIS MORNING SHOWS VIGOROUS UPPER LOW OVER THE
NORTHERN PLAINS PUSHING SOUTHEASTWARD WITH IR IMAGERY SHOWING AN
AREA OF EXPANDING COOLING CLOUD TOP IN ADVANCE OF THE UPPER LOW FROM
MN SOUTH INTO IA/NORTHERN MO. REGIONAL RADAR IMAGERY HAS SHOWN A
CORRESPONDING INCREASE IN COVERAGE/ORGANIZATION OF AREA OF SNOW IN
THIS REGION. UPPER LOW IS PROGGED TO WEAKEN SOME TODAY AS IT MOVES
SOUTHEAST INTO THE MIDWEST...BUT THEN BEGIN TO RE-INTENSIFY THIS
EVENING AS THIS UPPER LOW BEGINS TO PHASE WITH SOUTHERN STREAM VORT
NOTED OVER SOUTHERN KS.

WHILE THIS IS GOING ON ALOFT...SURFACE LOW OVER THE BOOTHEEL OF
MISSOURI IS FORECAST TO MOVE ENE UP THE OHIO RIVER WHILE
INTENSIFYING. INVERTED TROUGH ARCING NORTH/NORTHWEST FROM THE LOW
INTO MINNESOTA WILL BECOME ORIENTED MORE WEST TO EAST AS THE LOW
MOVES/DEVELOPS QUICKLY EASTWARD TODAY. THE INVERTED TROUGH WILL
LIKELY BE THE FOCUS FOR THE BAND OF SNOW WITH THE ASSOCIATED
DEFORMATION BAND BECOMING MORE WEST-EAST ORIENTED BY THIS EVENING AS
IT PUSHES OUT OF THE CWA. WOULD EXPECT THE HEAVIEST SNOWFALL TOTALS
FROM THIS SYSTEM TO BE ALIGNED WITH WHERE THE PIVOT POINT FOR THE
INVERTED TROUGH END UP...WHICH APPEARS TO BE OVER CENTRAL/SOUTHERN
CWA.

FORECAST SNOWFALL TOTALS IS A BIT TRICKY THIS MORNING AS ALL 00Z
GUIDANCE CAME IN WITH NOTABLY LOWER QPF VALUES...IN SOME CASES
BARELY SUPPORTIVE OF REACHING WARNING CRITERIA (6"/12 HOURS OR 8"/24
HOURS). GENERALLY SPEAKING SET-UP LOOKS RATHER MARGINAL FOR INTENSE
SNOWFALL RATES WITH THIS SYSTEM AS INITIALLY STEEP LAPSE RATES OVER
THE AREA ADVECT EASTWARD BEFORE THE STRONGER FORCING ARRIVES THIS
AFTERNOON. RATHER WEAK LAPSE RATES/LACK OF SIGNIFICANT INSTABILITY
WOULD SUGGEST LOWER THREAT OF CONVECTIVE SNOWFALL RATES LIKE THE
NORTHERN SUBURBS SAW THE PREVIOUS COUPLE OF SYSTEMS. IN
ADDITION...SREF GUIDANCE SUGGESTS THAT DEPTH OF THE DENDRITIC GROWTH
ZONE WILL BE RAPIDLY SHRINKING AS THE STRONG FORCING MOVES INTO OUR
AREA THIS AFTERNOON WITH TIME/HEIGHT CROSS SECTIONS INDICATING
STRONGEST LIFT NOT COINCIDING WITH BETTER SNOW GROWTH TEMPS. WHEN
COMBINED WITH SFC TEMPS NOT FAR FROM FREEZING AND POOR TIME OF DAY
IN MARCH FOR SNOW...THINKING THAT SNOW:LIQUID RATIOS MAY END UP
BEING RATHER LOW...CLOSE TO OR A BIT BELOW 10/11:1. IN THE GRIDS
HAVE GONE WITH HIGHER RATIOS IN ORDER TO SUPPORT MAINTAINING HIGHER
SNOWFALL TOTALS WITH THE LOWER QPF AMOUNTS NOW FORECAST...BUT ALL
INDICATIONS ARE THAT MOST AREAS WILL END UP IN THE 4-8 INCHES RANGE
WITH 6"+ AMOUNTS POTENTIALLY BEING MORE THE EXCEPTION THAN THE RULE
AND CONFINED TO A NW/SE BAND ACROSS THE CWA.

WHILE WINDS WILL BE OFF THE LAKE THROUGH THE DURATION OF THE EVENT
VERY LOW INVERSION HEIGHTS AND MINIMAL CONVERGENCE WILL RESULT IN
VERY LITTLE LAKE ENHANCEMENT TODAY. BY THIS EVENING COLD AIR
ADVECTION INTENSIFIES STEEPENING UP THE LOW LEVEL LAPSE RATES AND
CREATING A MORE FAVORABLE SET-UP FOR SOME LAKE ENHANCEMENT INTO NE
IL THIS EVENING. FROM CHICAGO NORTH TO THE WI BORDER THINK THE
LIKELIHOOD OF REACHING 6" TOTALS WILL BE TIED CLOSELY TO THE LAKE
EFFECT THIS EVENING...WHICH STILL ONLY LOOKS MODERATELY FAVORABLE.
HAVE ADJUSTED THE TIMING OF THE WARNING BACK BY ABOUT 6 HOURS FOR
MOST OF THE AREA TO A START TIME OF 15Z TO BETTER CORRESPOND TO
START OF STEADIER SNOW. WAS TOO LATE TO PUSH BACK THE START TIME OF
WARNING WESTERN CWA AS IT HAD ALREADY GONE INTO EFFECT AT 06Z...SO
LEFT TIMING AS IS THERE. BASED ON UNANIMOUS CONSENSUS FOR LOWER QPF
AND RESULTANT SNOW AMOUNTS FROM ALL THE MODELS...INCLUDING A SREF
PLUMES FOR ORD SHOWING A 6" DECREASE IN MEAN SNOWFALL FROM THE 15Z
TO THE 03Z RUN...HAVE LOWERED SNOWFALL FORECAST RANGE TO 4 TO 8
INCHES...LEAVING MENTION OF ISOLATED 10" AMOUNTS FOR SAKE OF
CONSISTENCY.

AFTER THIS SYSTEM NOT MUCH TO TALK ABOUT WITH SOME LINGERING LAKE
EFFECT SNOW SHOWERS POSSIBLE WEDNESDAY INTO WEDNESDAY NIGHT INTO
NE IL...BUT VERY LITTLE ACCUMULATION EXPECTED. CLOUDINESS WILL
PROBABLY BE MORE THE RULE THAN THE EXCEPTION UNTIL THURSDAY WHEN
SOME CLEARING TAKES PLACE. HOPES OF A SUBSTANTIAL WARM UP THURSDAY
INTO THE WEEKEND HAVE BEEN DASHED A BIT BY THE HIGH LIKELIHOOD OF
A DECENT BLANKET SNOW OVER THE REGION. GIVEN THE HIGH MARCH SUN
ANGLE AND ABOVE FREEZING TEMPS WOULD STILL ANTICIPATE SIGNIFICANT
MELTING OF SNOW COVER TO TAKE PLACE...ESPECIALLY FRIDAY INTO THE
WEEKEND AS DEWPOINTS CREEP UP. MEDIUM RANGE MODELS HAVE LARGE
DEGREE OF VARIABILITY WITH HANDLING OF THE NEXT SYSTEM WITH THE
GFS MUCH WEAKER AND MORE PROGRESSIVE RESULTING IN BETTER RAIN
CHANCES THIS WEEKEND. THE TYPICALLY MORE RELIABLE ECMWF KEEPS CUT
OFF LOW HANGING BACK OUT WEST WITH MORE OF AN UPPER RIDGE OVER OUR
AREA RESULTING IN A DRIER WEEKEND FORECAST. TRIED TO NUDGE THE
FORECAST IN THE DIRECTION OF THE MORE CONSISTENT ECMWF BY BACKING
OFF ON PRECIP CHANCES A BIT.

IZZI

&&

AVIATION...

//ORD AND MDW CONCERNS...UPDATED 18Z...

* SNOW CONTINUING THROUGH THE AFTERNOON AND EVENING. POTENTIAL FOR
  PERIODS OF HEAVY SNOW THROUGH THE AFTERNOON.

* IFR CEILINGS AND VISIBILITIES WITH THE SNOW. LOWEST VISIBILITIES
  WITH THE HEAVIEST SNOW THIS AFTERNOON.

* NORTHEAST WINDS BECOMING GUSTY THIS AFTERNOON/EVENING. POTENTIAL
  FOR BLOWING SNOW TO IMPACT VISIBILITY.

PAW

//DISCUSSION...UPDATED 18Z...

SNOW BEGAN FALLING OVER NORTHERN ILLINOIS AND NORTHWEST INDIANA
DURING THE MORNING. RADAR IS SHOWING EMBEDDED BANDS OF HEAVIER
SNOW WITH VISIBILITY AT OR BELOW A QUARTER MILE...CEILINGS AT
3-5 HUNDRED FEET AND ACCUMULATION RATES OF AN INCH OR MORE PER
HOUR. MODELS INDICATE THESE HEAVIER BANDS OF SNOW SHOULD BE EAST
OF RFD BY MID AFTERNOON AND EAST OF THE EASTERN TERMINALS BY LATE
AFTERNOON...THOUGH SOMEWHAT LIGHTER SNOW WILL CONTINUE THROUGH THE
EVENING. BY MID TO LATE EVENING THERE WILL BE SOME LAKE
ENHANCEMENT TO THE SNOW OVER NORTHEAST ILLINOIS. THE QUESTION IS
WHETHER IT WILL GET INTO ANY OF THE TERMINALS. IN ADDITION...NORTH
TO NORTHEAST WINDS WILL BE INCREASING THIS AFTERNOON...GIVING THE
POTENTIAL FOR BLOWING SNOW TO IMPACT VISIBILITY.

PAW

//ORD AND MDW CONFIDENCE...UPDATED 18Z...

* HIGH CONFIDENCE IN SNOW TREND. MEDIUM CONFIDENCE IN TIMING OF
  HEAVIEST SNOW AND ASSOCIATED VISIBILITIES AND CEILING.

* MEDIUM TO HIGH CONFIDENCE IN WIND TREND.

PAW

//OUTLOOK FOR ORD/MDW FOR 00Z THURSDAY-12Z TUESDAY...UPDATED 12Z...
12Z...

WEDNESDAY NIGHT...SLT CHC SNOW. MVFR.
THURSDAY...VFR.
FRIDAY...VFR.
SATURDAY...VFR.
SUNDAY...SLT CHC RAIN.
MONDAY...CHC RAIN.

BMD

&&

MARINE...
302 AM CST

MAIN CONCERN FOR LAKE MICHIGAN WILL BE WINDS PICKING UP TO AROUND 30
KT TODAY AS LOW PRESSURE DEVELOPS ACROSS THE MIDWEST. WINDS ARE
EXPECTED TO PEAK THIS EVENING AS THE GRADIENT BECOMES TIGHTEST BEHIND
THE DEPARTING AREA OF LOW PRESSURE OVER THE UPPER OHIO VALLEY AND
MODEST PRESSURE RISES MOVE ACROSS SOUTHERN LAKE MICHIGAN. THE
THREAT FOR LOW END GALES SEEMS INCREASINGLY UNLIKELY FROM THIS
EVENT. SNOW IS LIKELY OVER SOUTHERN PORTIONS OF LAKE MICHIGAN
TODAY...WITH SOME LAKE ENHANCEMENT/LAKE EFFECT SHOWERS THIS
EVENING AND TONIGHT. HIGH PRESSURE WILL MOVE ACROSS THE LAKE
THURSDAY INTO FRIDAY ALLOWING CONDITIONS TO IMPROVE...BUT WINDS
FRESHEN UP AGAIN LATE IN THE WEEKEND AS ANOTHER AREA OF LOW
PRESSURE MOVES INTO THE MIDWEST. BMD

&&

LOT WATCHES/WARNINGS/ADVISORIES...
IL...WINTER STORM WARNING...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ011-ILZ012-ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-ILZ022-
     ILZ023-ILZ032-ILZ033-ILZ039 UNTIL MIDNIGHT WEDNESDAY.

IN...WINTER STORM WARNING...INZ001-INZ002-INZ010-INZ011-INZ019 UNTIL
     MIDNIGHT WEDNESDAY.

LM...SMALL CRAFT ADVISORY...NEARSHORE WATERS
     UNTIL 3 AM THURSDAY.

&&

$$