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AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1120 AM CST THU FEB 7 2013

DISCUSSION...
1117 AM CST

HAVE EXPANDED WINTER WEATHER ADVISORY TO INCLUDE WINNEBAGO AND BOONE
COUNTIES AND THE I-88 CORRIDOR.

RECENT AMDAR SOUNDINGS OUT OF ORD/MDW SHOW WARM LAYER UP TO JUST
UNDER +3C BETWEEN 950 AND 900 MB...WHICH LATEST RAP HAS A DECENT
HANDLE ON...RESULTING IN PLAIN RAIN FOR THE CITY AND POINTS SOUTH.
HOWEVER...TEMPERATURES AT THAT LEVEL ACROSS THE NORTHERN TIER OF
COUNTIES ARE DOWN BELOW 1C...ALREADY SUPPORTING A CHANGE TO SNOW
WITH THE CURRENT OB AT KUGN DOWN TO 1/2 MILE VSBY IN SNOW. ALSO
RECEIVED A REPORT OF SNOW STARTING TO ACCUMULATE IN WAUKEGAN AND
ZION IL AND A CHANGEOVER IN CHERRY VALLEY IL. WITH THIS IN
MIND...WILL START RIGHT OUT THE GATE WITH AN ADVISORY FOR WINNEBAGO
BOONE MCHENRY AND LAKE COUNTIES. ALSO BUMPED UP SNOW AMOUNTS TO 2 TO
5 INCHES FOR THE NORTHERN TIER GIVEN THE EARLIER START TIME TO THE
SNOW.

FARTHER TO THE SOUTH...FOR THE I-88 CORRIDOR...THE RAP ALSO SHOWS
THE 950 TO 900MB LAYER QUICKLY COOLING OFF BELOW 0C THROUGH THE
AFTERNOON INDICATIVE OF DYNAMIC COOLING. SO THESE AREAS SHOULD
CHANGE BACK OVER TO SNOW AND SEE DECENT SNOWFALL RATES GIVEN THE
FORCING AT HAND AND THE FACT THAT SURFACE TEMPERATURES EVEN NOW ARE
HOVERING JUST ABOVE FREEZING. FORECAST AMOUNTS WILL BE MARGINAL FOR
AN ADVISORY AT ROUGHLY 2 TO 4 INCHES...BUT SINCE THE RUSH HOUR WILL
BE IMPACTED...FELT IT WAS PRUDENT TO ISSUE ONE STARTING AT 20Z.
INCLUDED OGLE...DEKALB...KANE...DUPAGE AND COOK IN THIS NEW
ADVISORY...BUT KEEP IN MIND THAT THERE WILL LIKELY BE A GRADIENT
FROM NORTH TO SOUTH ACROSS DEKALB THROUGH COOK WHERE THE NORTHERN
PORTIONS WILL BE SEEING THE MORE SIGNIFICANT IMPACTS FROM THE SNOW.

WITH A GOOD FGEN BAND COINCIDENT WITH STRONG LIFT NOTED ON MUCH OF
THE GUIDANCE PIVOTING ACROSS THE CWA AND BEING LOCATED OVER
NORTHEAST ILLINOIS INCLUDING CHICAGO BY THE EARLY EVENING...ALSO
CONCERNED THAT HEAVY SNOWFALL RATES COULD LEAD TO HIGHER TOTALS THAN
THE 3 TO 5 INDICATED IN LAKE AND MCHENRY ALONG THE I-88 CORRIDOR IN
NORTHEAST ILLINOIS. WILL CONTINUE TO ASSESS FOR ANY FURTHER UPDATES
NEEDED. BUT IN SUM...THE AFTERNOON COMMUTE IS LIKELY TO BE PRETTY
MESSY...ESPECIALLY FOR THE NORTHERN PART OF THE METRO.

RC/KJB

&&

PREV DISCUSSION...
553 AM CST

PRECIPITATION HAS OVERSPREAD NORTHERN IL EARLY THIS MORNING WITH
TEMPERATURES INITIALLY ABOVE FREEZING. AS PRECIPITATION BECAME A
LITTLE STEADIER...TEMPERATURES HAVE FALLEN TO OR A DEGREE BELOW
FREEZING. THIS HAS RESULTED IN FREEZING RAIN BEING REPORTED ACROSS
MANY OF MY FAR NORTHEAST ILLINOIS AREAS...MAINLY ALONG AND NORTH
OF I-88. THUS HAVE OPTED TO HOIST A FREEZING RAIN ADVISORY UNTIL
10 AM...WHICH IS THE TIME THAT TEMPS SHOULD PUSH ABOVE THE
FREEZING POINT WITH RISING DEW POINTS. PRECIPITATION AT THAT POINT
SHOULD TRANSITION OVER TO ALL RAIN.

BEACHLER

//PREV DISCUSSION...
337 AM CST

VERY DYNAMIC AND CHALLENGING WEATHER SYSTEM CONTINUES TO SLIDE EAST
FROM THE CENTRAL PLAINS TODAY. SFC TEMPS HAVE REMAINED MILD
OVERNIGHT IN THE LOW/MID 30S...WITH DEW POINTS HOVERING AROUND 30
DEG IN MANY LOCATIONS. IR IMAGERY INDICATES CONSIDERABLE CLOUD
COVER...WITH THE COLDEST CLOUDS ORIENTED FROM CENTRAL IOWA NORTHEAST
THROUGH CENTRAL WISCONSIN INTO NORTHERN LOWER MICHIGAN. EARLY THIS
MORNING CONSIDERABLE LIGHTING WAS NOTED ACROSS KANSAS AND CENTRAL
MISSOURI...INDICATING THAT CONSIDERABLE LIFT/UPGLIDE WAS OCCURRING
IN THIS AREA. IN ADDITION TO THE THUNDERSTORM ACTIVITY UPSTREAM OF
NORTHERN IL/NORTHWEST IN...WAS TEMPS IN THE LOW/MID 40S ACROSS
SOUTH CENTRAL IOWA EARLY THIS MORNING. WARM/MOIST AIR CONTINUES TO BE
TRANSPORTED NORTH FROM THE WESTERN GULF OF MEXICO AHEAD OF THE
SHORTWAVE...WHICH WILL PLAY A LARGE ROLE IN KEEPING THE BULK OF THE
PRECIP AT THE ONSET AS LIQUID. HOWEVER...A FEW SITES CONTINUE TO
SHOW TEMPS AROUND THE FREEZING MARK. THUS IT IS CONCEIVABLE THAT A
BRIEF PERIOD OF FZRA MAY OCCUR EARLY THIS MORNING. REGIONAL RADAR
MOSAIC EARLY THIS MORNING SHOWS A SLUG OF PRECIP SLIDING EAST ACROSS
NORTH CENTRAL IL/FAR WESTERN IL...HOWEVER SFC OBS INDICATE THIS IS
VERY LIGHT AND IN SOME LOCATIONS NOT REACHING THE GROUND.

THE FOCUS TURNS TOWARDS WHAT WILL TRANSPIRE BY MID-MORNING
ACROSS THE CWFA. THE GREATEST FOCUS FOR P-TYPE CONCERNS CONTINUES TO
BE ALONG AND NORTH OF A ROCKFORD TO WAUKEGAN LINE. GUIDANCE HAS
STRUGGLED WITH THE ANALYSIS AND FORECAST FIELDS OF SFC TEMPS WITH A
COLD BIAS...THUS HAVE OPTED TO MAINTAIN A SLIGHTLY WARMER TREND.
SREF 2M TEMP PLUMES CONTINUE TO SHOW MINIMAL SPREAD WITH THE TEMPS
TDY...HOWEVER APPROXIMATELY 2-3 DEG TOO COOL. CONSIDERABLE
CORRECTIONS CONTINUE TO TAKE PLACE EVEN AT HOUR ZERO AHEAD OF THIS
SYSTEM...HOWEVER SOME CONSISTENCY HAS BEEN NOTED IN THE LACK OF AN
EASTERLY LLVL JET AT 850MB. THE U-WIND ANOMS HAVE CONTINUED TO BE
POSITIVE FROM THE SW AT 20-30KTS...INDICATING AN EFFICIENT TRANSPORT
OF PARCELS FROM SW TO NE. THE WAVE REMAINS OPEN...HOWEVER SHOULD
BEGIN TO AMPLIFY WITH TIME. THE PATH OF THE 850/925MB LOWS ARE
SIMILAR AMONGST THE EC/NAM IN TIME PUSHING FROM WEST TO EAST ACROSS
THE NORTHERN THIRD OF THE CWFA MIDDAY. JUST TO THE NORTH OF THIS
PATH APPEARS TO BE THE FAVORED REGION FOR P-TYPE CONCERNS AND
ACCUMULATING SNOW POTENTIAL. THE 540M THICKNESS LINE IS JUST NORTH
OF RFD...STRETCHING SLIGHTLY SOUTHEAST OF WAUKEGAN FOR MUCH OF THE
DAY.

AS MOISTURE CONTINUES TO FEED NORTHEAST AHEAD OF THE 500MB TROUGH
AXIS...AND PRECIP OVERSPREADS THE AREA...EXPECT DYNAMIC COOLING TO
SLOWLY OCCUR ACROSS THE FAR NORTHERN CWFA INTO SOUTHERN WISC. THIS
SHOULD ALLOW A SLOW TRANSITION OVER TO A RAIN/SNOW/SLEET MIX. AT
THIS TIME ACCUMS HAVE FOCUSED FURTHER NORTH ACROSS THE STATE LINE IN
SOUTHERN WISC. SREF PLUMES FOR RFD SUGGEST A 70% PROB OF FZRA/RA AT
ONSET TRENDING TO RA...THEN ARND 21-00Z 80% LIKELIHOOD OF SN.
FURTHER EAST AT UGN THE PROB OF SN IS SLIGHTLY HIGHER AND EARLIER.
THEN BY 00Z PRECIP TYPE SHUD SWITCH OVER TO ALL SN FROM WEST TO EAST
AS COOLER AIR STEADILY ADVECTS INTO THE CWFA. SOUNDINGS ARE NOT MUCH
ASSISTANCE AT THIS TIME...GIVEN THE POOR INITIALIZATION.

SNOW ACCUMS WILL ALSO BE A CHALLENGE...IF INDEED IT DOES SNOW AND
CAN ACCUMULATE. WITH THE HIGH DEW PTS HOVERING ARND THE FREEZING
MARK IT WILL BECOME A CHALLENGE. AT THIS TIME HAVE OPTED TO TRIM
BACK SNOW ACCUMS TO A 1-2" RANGE...WITH A FOCUS ACROSS MCHENRY/LAKE
COUNTIES. HOWEVER IF THE DYNAMIC COOLING CAN TAKE PLACE
EARLIER...AND THE FGEN BAND COULD CRANK OUT HIGHER ACCUMS. VERY
DIFFICULT TO PULL THE CURRENT ADVISORY...AND AT THE SAME TIME
DIFFICULT TO EXPAND THE ADVISORY FURTHER WEST. SO AT THIS TIME WILL
LEAVE HEADLINES AS IS.

CONFIDENCE IN TEMPERATURES...HIGH.

CONFIDENCE IN ACCUMULATING SNOWFALL...LOW.

CONFIDENCE IN PRECIP CHANGE TIMING...LOW.

TONIGHT THROUGH FRIDAY...

500MB TROUGH AXIS PIVOTS OVERHEAD AND PUSHES NORTHEAST TOWARDS THE
EASTERN GREAT LAKES EARLY FRI. 850MB/SFC LOW CONTINUES TO DEVELOP
DOWNSTREAM OF OUR AREA...WITH A DRY WEDGE ADVERTISED BY GUIDANCE
ARRIVING ARND 06-09Z. THIS SHUD AID IN SHUTTING OFF THE
PRECIP...WHICH BY 00Z FRI SHUD BE ALMOST ALL LGT SNOW. SOME
WRAP-AROUND MOISTURE CONTINUES TO BE NOTED BY THE OPERATIONAL
SOLUTIONS...THUS HAVE CONTINUED MENTION OF SOME LIGHT SNOW OR
FLURRIES ACROSS ALONG THE BACKSIDE OF THE BETTER PRECIP SHIELD.

ANTI-CYCLONIC FLOW QUICKLY BUILDS IN FOR FRI...ERODING ANY REMAINING
LLVL MOISTURE. WINDS TURN NORTHEAST FOR A PERIOD FRI WHICH COULD
PULL SOME MOISTURE OFF OF LAKE MICHIGAN...ALTHOUGH THIS LOOKS VERY
MARGINAL. THUS HAVE CONTINUE WITH A DRY FORECAST. 850MB THERMAL
TROUGH REMAINS OVERHEAD AT -4 TO -6 DEG C FRI KEEPING AFTN MAX TEMPS
MAINLY IN THE LOW/MID 30S. THE FLOW REMAINS PROGRESSIVE WITH HEIGHTS
INCREASING UPSTREAM OF THE CWFA FRI AFTN/EVE. PRIOR TO THE MID-LVL
RIDGING ARRIVING...THE SFC RIDGE WILL BECOME CENTERED OVER THE CWFA
FRI NIGHT. THIS SHUD ALLOW SFC TEMPS TO RADIATE INTO THE TEENS/LOW
20S. LIGHT WINDS AND RECENT RAINFALL FROM THURSDAY COULD SET THE
STAGE FOR PATCHY FOG DEVELOPING FRI NIGHT.

CONFIDENCE...HIGH.

SATURDAY THROUGH WEDNESDAY...
500MB RIDGING PUSHES OVERHEAD FOR SAT/SUN AS A DEEP TROUGH CONTINUES
TO DIG INTO THE FOUR-CORNERS REGION. AS THIS UPSTREAM TROUGH BECOMES
AMPLIFIED...THE WESTERN GULF OF MEXICO WILL OPEN UP WITH ABUNDANT
MOISTURE STREAMING NORTH ACROSS THE OZARKS INTO THE WESTERN GREAT
LAKES SUN. GUIDANCE HAS CONTINUED TO INDICATE THAT AFTER A DRY
SAT...SUN WILL BE WET. THIS SYSTEM ON SUN APPEARS TO BE VERY ROBUST
BASED ON LATEST GUIDANCE...WITH MINIMAL SPREAD BEING NOTED AMONGST
THE ENSEMBLE MEMBERS. AT THIS TIME HAVE HELD OFF ON INCLUDING
CONVECTION IN THE FORECAST FOR SUN...WOULDN/T BE SURPRISED TO SEE
THIS ADDED OVER TIME. TEMPS SUN SURGE AHEAD OF THE SYSTEM...WITH
MID/UPR 40S LOOKING PROBABLE. COULD EVEN SEE TEMPS TOUCH 50 DEG IN A
FEW LOCATIONS OF OUR SOUTHERN CWFA.

THEN COOLER AIR WILL RETURN FOR THE START OF NEXT WEEK...WITH TEMPS
RETURNING TO SEASONAL CONDITIONS IN THE 30S.

CONFIDENCE...HIGH.

BEACHLER

&&

AVIATION...

//ORD AND MDW CONCERNS...UPDATED 16Z...

* RAIN THROUGH THE REST OF THE DAY...MIXING/CHANGING TO SNOW
  EARLY TO MID EVENING.

* GRADUALY BACKING OF GENERALLY 7 TO 9 KT WINDS FROM SE TO NE
  TODAY AND TO N AND NW TONIGHT. WINDS TO INCREASE SOMEWHAT
  TONIGHT...AVERAGING 9 TO 12 KT BY LATE EVENING.

* VFR CIGS WITH AREAS OF MVFR LOWERING IFR BY EARLY
  AFTERNOON...WITH POSSIBLR LIFR CIGS LATER AFTERNOON INTO THE
  NIGHT.

* REDUCED VISIBILITY...GENERALLY 2 TO 4SM REST OF DAY AND THROUGH
  MUCH/ALL OF THE EVENING.

KREIN

//DISCUSSION...UPDATED 15Z...

THE NE TO SW ORIENTED UPPER TROUGH EXTENDED FROM WESTERN ONTARIO
TO N TX AT MID MORNING AND FROM WHAT NEW MODEL OUTPUT IS
AVAILABLE AT THIS TIME HAS THE TROUGH AXIS REACHING A LINE ACROSS
LAKE SUPERIOR SW TO OK. IR SATELLITE SHOWS THE CLOUDS ALONG AND
AHEAD OF THE UPPER TROUGH...WHERE ENHANCED UVV IS OCCURRING TO BE
OCCURRING...CLOUD TOPS WERE COOLING AND EXPANDING IN COVERAGE.
THE LOOP OF THE NATIONAL MOSAIC OF RADAR REFLECTIVITY WAS SHOWING
RAIN FROM EASTERN KS...NW MO AND SE IA TO W CENTRAL AND NORTHERN
IL AND SOUTHERN WI.

LOW LEVEL TEMPERATURES HAVE RISEN A COUPLE DEGREES F DURING THE
FIRST PART OF THE MORNING AS WAA CONTINUES AHEAD OF A SURFACE LOW
CURRENTLY IN CENTRAL...AND WILL PERSIST MUCH OF THE AS THE LOW
MOVES E ACROSS NORTHERN IL TO NW IN. THIS WILL KEEP PRECIP ALL
RAIN AT AND AROUND THE KLOT AND KMDW AREAS FOR THE THE REST OF
THE DAY.

AS THE SURFACE LOW MOVES ACROSS CENTRAL IL DURING THE AFTERNOON
WINDS WILL BACK FROM SE TO E AROUND MID AFTERNOON AND TO NE BY
08.00Z. MIXING/CHANGING TO SNOW WILL NOT START TO OCCUR LOCALLY
TIL EARLY EVENING WITH JUST SNOW OVERNIGHT AS THE MID AND UPPER
TROUGHS PASS E ACROSS THE LOCAL AREA AND DEEPER...STRONGER CAA
SETS IN ACROSS THE REGION.

TRS

//PREVIOUS DISCUSSION...UPDATED 12Z...

AT 11Z...TEMPERATURES ACROSS NRN IL/NWRN IN WERE 32-36F WITH EVEN
WARMER AIR OVER UPSTREAM OVER CNTRL IL AND INTO MO. WITH THE
TEMPERATURES HOVERING NEAR THE FREEZING POINT...IN LOCATIONS WHERE
PCPN HAS NOT ALREADY BEGUN...THE INITIAL ONSET OF PCPN WILL START
AS SOME LIGHT SLEET...BUT SHOULD QUICKLY CHANGE OVER TO RAIN AND
FREEZING RAIN.

THE WARMER AIR ALOFT OVER THE COOLER AIR AT THE SFC SHOULD WORK
DOWN TO THE SFC BY ARND 14Z...ENDING ANY ADDITIONAL FREEZING RAIN
POTENTIAL. THE RAIN SHOULD PERSIST THROUGH THE REMAINDER OF THE
AFTERNOON...BUT AS WINDS GRADUALLY BECOME EASTERLY...EXPECT THAT
CIGS WILL LOWER TO IFR...AND POSSIBLY LIFR...LEVELS BY THIS
AFTERNOON. THE RAIN AND IFR/LIFT CIGS WILL PERSIST UNTIL LATER
THIS EVENING WHEN A SFC LOW WILL PASS JUST SOUTH OF THE
TERMINALS...TURNING WINDS NLY-NWLY...DRAWING COOLER AIR INTO THE
AREA AND CHANGING THE RAIN OVER TO SNOW. LIGHT SNOW SHOULD PERSIST
FOR A SHORT TIME...UNTIL TAPERING OFF TO FLURRIES. AS THE COOLER
AIR WORKS INTO THE AREA...CIGS SHOULD SLOWLY IMPROVE TO MVFR
OVERNIGHT TONIGHT AND INTO TOMORROW MORNING.

KREIN

//ORD AND MDW CONFIDENCE...UPDATED 16Z...

* HIGH CONFIDENCE IN PRECIP OCCURRENCE AND TYPE THROUGH
  AFTERNOON.

* HIGH CONFIDENCE IN WIND TRENDS REST OF DAY AND THIS EVENING.

* MEDIUM CONFIDENCE IN TIMING OF CHANGE/MIX OF RAIN AND DURING
  FIRST PART OF EVENING...THEN TO ALL SNOW OVERNIGHT.

* MEDIUM CONFIDENCE IS VIS TRENDS.

* LOW CONFIDENCE IN TIMING OR EXTENT OF LIFR CIGS.

TRS

//OUTLOOK FOR ORD/MDW FOR 00Z SATURDAY-12Z THURSDAY...UPDATED 16Z...

FRIDAY NIGHT...VFR. WX NIL.

SATURDAY...VFR. WX NIL.

SUNDAY...RAIN LIKELY.  IFR CONDITIONS POSSIBLE.

MONDAY...CHANCE OF RAIN AND MVFR CIGS EARLY...THEN VFR.

TUESDAY...VFR. WX NIL.

WEDNESDAY...VFR. WX NIL.

KREIN

&&

MARINE...
320 AM CST

A RIDGE OF HIGH PRESSURE EXTENDS FROM WESTERN ONTARIO TO THE
NORTHERN PLAINS WHILE TWO LOW PRESSURE SYSTEMS DEVELOP...ONE OVER
IOWA AND ANOTHER OVER THE SOUTH HIGH PLAINS. AS THE 2 LOWS
CONSOLIDATE INTO A SINGLE LOW OVER INDIANA TONIGHT...THE HIGH WILL
BUILD INTO THE UPPER MISSISSIPPI VALLEY...LEADING TO A
STRENGTHENING NORTHERLY GRADIENT OVER THE LAKE...WITH WINDS UP TO
AROUND 30 KT DEVELOPING. ON FRIDAY THE INDIANA LOW WILL MERGE WITH
A RAPIDLY DEEPENING LOW OF THE MID ATLANTIC COAST WHILE THE HIGH
PRESSURE SPREADS OVER THE LAKE. THIS WILL LEAD TO RAPIDLY
DIMINISHING WINDS OVER THE LAKE FOR FRIDAY NIGHT AND SATURDAY.

KREIN

&&

LOT WATCHES/WARNINGS/ADVISORIES...
IL...WINTER WEATHER ADVISORY...ILZ003-ILZ004-ILZ005-ILZ006 UNTIL 9 PM
     THURSDAY.

     WINTER WEATHER ADVISORY...ILZ008-ILZ011-ILZ012-ILZ013-ILZ014
     UNTIL 9 PM THURSDAY.

IN...NONE.
LM...NONE.
&&

$$