🚀 go-pugleaf

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1148 AM CST MON MAR 4 2013

DISCUSSION...
1126 AM CST

WE HAVE UPGRADED THE REMAINING COUNTIES IN THE WINTER STORM WATCH TO
A WARNING. WE DECIDED TO HOLD OFF ON ANYTHING FROM OUR FAR SOUTHERN
COUNTIES UNTIL THIS AFTERNOON...ALTHOUGH IT IS QUITE POSSIBLE THAT A
WARNING MAY BE WARRANTED FOR THESE COUNTIES AS WELL.

THE LATEST 12 UTC GUIDANCE CONTINUES TO HIT THE AREA HARD WITH HEAVY
SNOW...ESPECIALLY DURING THE DAY ON TUESDAY. IT CURRENTLY APPEARS
THAT MOST OF THE AREA WILL RECEIVE AT LEAST 5 TO 6 INCHES...BUT MANY
AREAS WILL LIKELY RECEIVE MUCH MORE THAN THIS. THE THREAT FOR LAKE
ENHANCEMENT LATER TUESDAY AFTERNOON AND EVENING COULD RESULT IN A
FOOT OR MORE ACROSS SOME ISOLATED LOCATIONS NEAR THE LAKE. FARTHER
INLAND...SNOW AMOUNTS WILL STILL BE FAIRLY IMPRESSIVE...WITH 5 TO 8
INCHES LIKELY.

THE STRONG DYNAMICS WITH THIS SYSTEM AND THE LIKELIHOOD OF LOW STATIC
STABILITY IN THE MID LEVELS WILL LIKELY LEAD TO SOME IMPRESSIVE SNOW
RATES...OF 1 TO 1.5 INCHES PER HOUR. SOME ISOLATED
AREAS...ESPECIALLY NEAR THE LAKE COULD SEE SOME PERIODS OF 2 INCH
PER HOUR RATES. I DID BUMP UP SNOW AMOUNTS SOME ACROSS NORTHEASTERN
ILLINOIS DUE TO THE LIKELIHOOD OF LAKE ENHANCEMENT.

WE WILL ASSESS THE NEED FOR EITHER A WARNING OR ADVISORY FOR THE FAR
SOUTHERN COUNTIES FOR THE AFTERNOON PACKAGE.

KJB

//PREV DISCUSSION...
332 AM CST

THE FORECAST CONCERNS CENTER ON THE UPCOMING WINTER WEATHER EVENT
WHICH AT THIS TIME LOOKS TO HAVE THE MOST WIDESPREAD SIX PLUS INCHES
OF SNOW IN THE FORECAST AREA OF ANY EVENT THIS COOL SEASON...AND
IRONICALLY COMES IN METEOROLOGICAL SPRING. HAVE UPGRADED A PORTION
OF THE WATCH TO A WINTER STORM WARNING WITH START TIME OF
APPRECIABLE SNOW BEING LATER TONIGHT IN THIS AREA...ALONG WITH HIGH
CONFIDENCE IN RECEIVING SIX PLUS INCHES OF SNOW WITH EMBEDDED
HEAVIER RATES TO HAVE SIGNIFICANT TRAVEL IMPACTS.

TODAY...
SURFACE LOW PRESSURE LIES OVER THE TEXAS/OKLAHOMA PANHANDLE REGION
EARLY THIS MORNING WHILE A CLOSED UPPER LOW IS DIGGING SOUTHEAST
ACROSS MINNESOTA. H85 BAROCLINIC ZONE EXTENDS FROM THE DAKOTAS
SOUTHEAST ACROSS IOWA AND INTO SOUTHERN ILLINOIS WITH MODERATE SNOW
ALREADY FALLING ACROSS PORTIONS OF IOWA. GUIDANCE IN PRETTY GOOD
AGREEMENT REGARDING THIS AREA TRANSLATING EAST INTO NORTHERN
ILLINOIS TODAY BUT UNDERGOING A WEAKENING TREND AS IT DOES. GFS IS
AN EXCEPTION AND MOST GUNG-HO WITH BRINGING PRECIP ALL THE WAY EAST
INTO THE CHICAGO AREA...AND ALSO HAS SOME OF THE HIGHEST QPF TOTALS.
NAM ON THE OTHER HAND SEEMS LIKE IT IS HOLDING ONTO THE ANTECEDENT
DRY AIR TOO LONG AND KEEPS MUCH OF THE FORECAST AREA DRY TODAY.
WILL LEAN TOWARDS THE ECMWF/GEM SOLUTIONS WHICH SEEM TO BE A GOOD
COMPROMISE. EXPECT SNOW TO MOVE INTO NORTH CENTRAL ILLINOIS THIS
MORNING AND EARLY AFTERNOON WITH LIGHT ACCUMULATIONS OF UP TO AN
INCH POSSIBLE BY EARLY THIS EVENING WEST OF THE FOX RIVER VALLEY.
MAY SEE LIGHT SNOW MOVE INTO THE CHICAGO AREA MID TO LATE
AFTERNOON...BUT BY THEN EXPECT A LIGHT DUSTING IF ANYTHING TO
ACCUMULATE.

DEUBELBEISS

TONIGHT THROUGH TUESDAY...
THE PRIMARY UPPER LOW TO THE NORTHWEST OF THE AREA BECOMES NEUTRAL
TO EVENTUALLY NEGATIVELY TILTED AS IT DIGS SOUTHEAST INTO THE CORN
BELT...AIDED BY A STRONG NORTHERLY JET COMPONENT. SURFACE
CYCLOGENESIS OCCURS IN RESPONSE UP THE OHIO RIVER VALLEY TUESDAY
AFTERNOON AND NIGHT. PACIFIC AND GULF MOISTURE IS TOGETHER TAPPED
WITH THIS SYSTEM...ESPECIALLY COME TUESDAY. PLACEMENT OF THESE
FEATURES IS CLOSE ENOUGH FOR CONFIDENCE TO BE HIGH THAT THE AREA
WILL SEE AT A MINIMUM OF NUMEROUS INCHES OF ACCUMULATING SNOW
WITH HEAVIER RATES PROBABLE AT TIMES IN THE FORECAST AREA.
HOWEVER...THE DIFFERENCE IN GUIDANCE PLACEMENT OF KEY MASS FIELDS
/ABOUT 150 MI SPREAD IN THE 850MB LOW FOR INSTANCE/ IS ENOUGH TO
PRESENT AN APPRECIABLE SPREAD IN THE HEAVIEST SNOW LOCATION...AND
THUS PRESENTS A CHALLENGE. HAVE BASICALLY WENT WITH A BLEND OF THE
04.00 GFS AND EC TRACKS...WITH THE LATTER BEING CONSISTENTLY
SLIGHTLY FURTHER SOUTH THAN MANY OF THE NCEP RUNS.

DURING TONIGHT...UPGLIDE ENVELOPS THE AREA AS THE LOW DIGS SOUTH
AND BROAD SCALE WARM AIR ADVECTION DEVELOPS OVER THE REGION.
WHILE THE GFS IS SLIGHTLY MORE ROBUST WITH THIS...ANY SNOW PRIOR
TO MIDNIGHT SHOULD BE MAINLY LIGHT. OVERNIGHT RATES SHOULD PICK UP
FROM NORTHWEST TO SOUTHEAST AS DEEPER HEIGHT FALLS AND MORE
FOCUSED CONVERGENCE SPREADS OVER THE REGION. EASTERN AREAS
INCLUDING THE HEART OF CHICAGO MAY STRUGGLE TO SATURATE AT FIRST
AND THE 00Z EC REALLY PAINTS THAT PICTURE. IN ADDITION...IT
APPEARS SOME CONVECTION MAY DEVELOP ACROSS THE MID-
MISSISSIPPI/LOWER OHIO VALLEYS NEAR DAYBREAK TUE WHICH MAY INHIBIT
DEEP MOISTURE TRANSPORT INTO THE REGION FOR MORE EFFICIENT SNOW
ACROSS EASTERN AREAS THROUGH MID-MORNING TUE. HAVE EASED SNOW
AMOUNTS IN EASTERN AREAS BUT HAVE NOT CHANGED THE START TIME OF
THE HEADLINE AT THIS TIME...AS THERE REMAINS UNCERTAINTY. FOR
SOUTHERN AREAS...A MIX IN PRECIPITATION TYPES REMAIN POSSIBLE AS
THE WARM NOSE ALOFT BECOMES NEAR THE THRESHOLD FOR DEEP ENOUGH TO
PRESENT SLEET OR EVEN POTENTIALLY SOME LIGHT FREEZING RAIN. HAVE
CONTINUED MENTION OF THIS IN SOUTHERN AREAS WITH BASICALLY NO SNOW
ACCUMULATION.

CONFIDENCE REMAINS HIGH IN THE ENTIRE AREA SEEING WIDESPREAD SNOW
THROUGH TUE AND INTO TUE EVE. THE LONE EXCEPTION REMAINS SOUTHERN
AREAS...MAINLY SOUTH OF THE KANKAKEE RIVER THAT COULD SEE A MIX
THROUGHOUT THE MORNING...OR AT LEAST MARGINAL BOUNDARY LAYER
TEMPERATURE A COUPLE DEGREES ABOVE FREEZING TO LIMIT RAPID SNOW
ACCUMULATION. AREAS ALONG AND NORTH OF INTERSTATE 80 THOUGH LOOK TO
BE IN A FAVORABLE AREA FOR MODERATE WARM AIR ADVECTION SNOW
THROUGHOUT THE MORNING...CHANGING IN TIME THROUGH THE AFTERNOON TO A
MORE DEFORMATION/TROWAL FORCED SNOW INTO THE EVENING. THE AFTERNOON
AND EARLY EVENING PERIOD LOOK THE BEST...ESPECIALLY ON THE EC...FOR
HEAVY SNOW RATES IN EXCESS OF ONE INCH PER HOUR AT TIMES. THIS IS
DUE TO STRONG QG FORCING...LOW STATIC STABILITY AS SEEN ON
FORECAST CROSS SECTIONS...AND INCREASING THICKNESS OF THE
DENDRITIC GROWTH ZONE WHILE COOLING THE OVERALL THERMAL PROFILE.
IN ADDITION...IMPACTS LOOK TO INCREASE AS WIND SPEEDS THROTTLE UP
AROUND THE DEVELOPING LOW...WITH SOME BLOWING SNOW LIKELY BY
EVENING IN OPEN AREAS.

AGAIN THERE REMAINS UNCERTAINTY ON THE HEAVIEST SNOW AXIS. QPF
AMOUNTS OF OVER A HALF AN INCH ARE ADVERTISED BY ALMOST ALL
GUIDANCE MEMBERS...WITH SNOW-TO-LIQUID RATIOS INCREASING AFTER
NOON ON TUESDAY AS COLDER AIR WRAPS INTO THE MID-LEVELS. GUIDANCE
GENERALLY FAVORS 5 TO 10 INCHES ACROSS THE AREA...WITH THE GARCIA
TECHNIQUE FAVORING AROUND SIX INCHES JUST IN THE WARM AIR ADVECTION
PART OF THIS SYSTEM /DUE TO PERSIST 3-3.5G/KG OF INFLOW MIXING
RATIOS/. THE NORTHWESTERN THIRD OF THE FORECAST AREA LOOKS HIGHLY
LIKELY TO GET INTO A CORRIDOR OF HEAVY SNOW AT TIMES TUESDAY.
WITH THE EARLY START TIME OF APPRECIABLE SNOW BEING LATER
TONIGHT IN THAT AREA...FEEL CONFIDENT IN THE WARNING ISSUANCE.
TOWARD CHICAGO...THE UNCERTAINTY IN TOTAL SNOWFALL AMOUNTS
GROWS...PARTIALLY DUE TO WHETHER OR NOT LAKE INFLUENCE AIDS ON
AMOUNTS INTO TUESDAY EVENING. STILL...THE INCREASING POTENTIAL FOR
SOME AREAS TO SEE NEAR EIGHT INCHES IN 24 HOURS ALONG WITH THE
IMPACT TIMING OF THIS SNOW STRONGLY SUGGESTS A WARNING IS COMING.
HAVE THUS COLLABORATED THAT HEADLINE FOR A LARGE PART OF
CHICAGOLAND. CONFIDENCE IN SNOWFALL AMOUNTS AND JUST START TIME
DIMINISHES QUICKLY IN AREAS SOUTH AND ESPECIALLY OVER NORTHWEST
INDIANA...SO MAINTAIN THE WATCH THERE.

WRAPAROUND FLOW OFF THE LAKE WILL PRESENT THE POSSIBILITY FOR LAKE
EFFECT SNOW INTO WEDNESDAY ACROSS NORTHEAST IL AND NORTHWEST IN.
GIVEN THE FINER DETAILS NEEDED TO FULLY ASSES THIS...AND SOMEWHAT
MARGINAL DELTA TS /11C-13C/...HAVE JUST CONTINUED WITH CHANCE
POPS...BUT COULD SEE SOME MINOR ACCUMULATIONS FOR WHERE THIS AREA
ESTABLISHES ITSELF...EVEN IF JUST BRIEFLY. FOR BEYOND HAVE MADE
VERY LIMITED CHANGES TO THE FORECAST DUE TO EMPHASIS IN THE SHORT
TERM. WITH CONFIDENCE HIGH IN FRESH SNOW COVER GOING INTO THE
WEEK...MAINTAIN A TEMPERED WARM-UP FOR NORTHERN AREAS...ALTHOUGH
MARCH SUN WILL DO ITS WORK ESPECIALLY AS UPPER RIDGING AND LOW
LEVEL SOUTHERLY FLOW BUILD INTO THE AREA THIS UPCOMING WEEKEND.
HIGHS TO OR POSSIBLY ABOVE EARLY MARCH NORMALS ARE EXPECTED BY
THAT POINT.

MTF

&&

AVIATION...

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

* EAST TO SOUTHEAST WINDS...BECOMING GUSTY LATE THIS AFTERNOON.

* POTENTIAL FOR LIGHT ACCUMULATING SNOW LATE THIS AFTERNOON AND
  EARLY EVENING.

* CEILINGS FALLING TO MVFR THIS EVENING...THEN IFR OVERNIGHT OR
  TUESDAY MORNING.

* HEAVY SNOW MID MORNING TUESDAY THROUGH THE EVENING...VISIBILITY
  LESS THAN A MILE AT TIMES.

PAW

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

A SURFACE LOW PRESSURE CENTERED IN OKLAHOMA MONDAY AFTERNOON WILL
PHASE WITH AN UPPER LEVEL LOW COMING OUT OF THE DAKOTAS OVERNIGHT
TO BRING SIGNIFICANT SNOW TO NORTHERN ILLINOIS AND NORTHWEST
INDIANA ON TUESDAY. A PRELIMINARY BAND OF LIGHT SNOW WILL MOVE
ACROSS THE AREA LATE THIS AFTERNOON AND EARLY THIS
EVENING...LOWERING CONDITIONS TO MVFR. THEN A RELATIVE LULL
THROUGH THE NIGHT WITH LIGHT SNOW AND FLURRIES CONTINUING UNTIL
THE MORE SIGNIFICANT SNOW ARRIVES TUESDAY MORNING WITH IFR
CONDITIONS...AND BECOMES HEAVIER BY EARLY AFTERNOON. SNOW
ACCUMULATION RATES UP TO AN INCH OR MORE PER HOUR ARE LIKELY AT
TIMES TUESDAY...REDUCING VISIBILITY TO A HALF-MILE OR LESS.

PAW


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

* HIGH CONFIDENCE IN WIND TREND.

* MEDIUM CONFIDENCE IN CEILING TREND AND DETAILS.

* LOW CONFIDENCE IN SNOW ACCUMULATING LATE THIS AFTERNOON AND
  EARLY EVENING.

* HIGH CONFIDENCE IN HEAVY SNOW TUESDAY.

PAW

//OUTLOOK FOR ORD/MDW FOR 00Z WEDNESDAY-12Z MONDAY...UPDATED 12Z...

TUESDAY NIGHT...HEAVY SNOW. IFR/LIFR.
WEDNESDAY...MVFR.
THURSDAY...VFR.
FRIDAY...VFR.
SATURDAY...SLT CHC RAIN.
SUNDAY...SLT CHC RAIN.

BMD

&&

MARINE...
302 AM CST

MAIN CONCERN FOR LAKE MICHIGAN WILL BE WINDS RAMPING UP TUESDAY AND
TUESDAY NIGHT AS AREA OF LOW PRESSURE MOVES FROM THE MID MISSISSIPPI
VALLEY TONIGHT THEN EAST ALONG THE OHIO VALLEY TUESDAY. PRESSURE
GRADIENT TIGHTENS ESPECIALLY ACROSS THE SOUTHERN HALF OF THE LAKE
TUESDAY AND TUESDAY NIGHT...WHEN WINDS ARE EXPECTED TO INCREASE TO
AROUND 30 KT. GUIDANCE CONTINUES TO ADVERTISE WARM MID LEVEL AIR
LIMITING THE DEPTH OF THE MIXED LAYER WHICH SHOULD HELP KEEP WINDS
BELOW GALES FOR THIS EVENT. WINDS SHOULD GRADUALLY TREND DOWN
THROUGH THE DAY WEDNESDAY AND THURSDAY AS HIGH PRESSURE BUILDS
ACROSS THE WESTERN GREAT LAKES. NO SIGNIFICANT WEATHER EXPECTED FOR
THE LATTER HALF OF THE WEEK. BMD

&&

LOT WATCHES/WARNINGS/ADVISORIES...
IL...WINTER STORM WARNING...ILZ005-ILZ006-ILZ012-ILZ013-ILZ014-ILZ020-
     ILZ021-ILZ022-ILZ023...3 AM TUESDAY TO MIDNIGHT WEDNESDAY.

     WINTER STORM WARNING...ILZ003-ILZ004-ILZ008-ILZ010-ILZ011-
     ILZ019...MIDNIGHT TUESDAY TO MIDNIGHT WEDNESDAY.

IN...WINTER STORM WARNING...INZ001-INZ002-INZ010-INZ011...3 AM
     TUESDAY TO MIDNIGHT WEDNESDAY.

LM...NONE.
&&

$$