FXUS63 KIWX 091148
AFDIWX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NORTHERN INDIANA
648 AM EST WED DEC 9 2009

.AVIATION...
12Z TAFS.

DRY SLOT WORKING ACROSS THE AREA AT THIS TIME WITH CIGS AND VSBY
ALL OVER THE PLACE. EXPECTING WORSENING CONDITIONS AND RAPID
INCREASE IN WINDS BY MID MORNING AS COLD AIR RUSHES IN AND ALLOWS
FOR A QUICK BURST OF SNOW FOR MOST AREAS. STRONGER WINDS HAVE
BEGUN TO MIX DOWN OVER THE PAST COUPLE OF HOURS SO HAVE LEFT THE
WIND SHEAR MENTION OUT. FLIGHT CONDITIONS WILL BE WORSE AT KSBN
WITH VSBYS POSSIBLY LESS THAN 1/4 SM AT TIMES AND PERIODS OF SNOW
INTO THIS EVENING. TAF WILL LIKELY NEED AMENDING AS DEFORMATION
ZONE SETS UP. WINDS WILL REMAIN HIGH BUT LOSE SOME OF THEIR HIGHER
SPEEDS LATER TONIGHT AS GRADIENT TRIES TO RELAX SOMEWHAT.

&&

.PREV DISCUSSION... /ISSUED 409 AM EST WED DEC 9 2009/ 

SHORT TERM...

..NEAR RECORD SFC LOW PRESSURE TO BRING VERY STRONG WINDS AND 
ANOTHER ROUND OF WINTER WEATHER TO THE REGION...

SFC LOW NOW CENTERED OVER NORTHERN IL AT 983 MB /0830Z/...CONTINUES 
TO TRACK NE BEING STEERED BY A POTENT MID LATITUDE UPPER TROUGH 
AND ASSOCIATED POWERFUL CYCLONICALLY CURVED UPPER FLOW. 00Z RAOB 
DATA CONFIRMS 130+ KNOT FLOW OVER THE CENTRAL PORTION OF THE 
COUNTRY. IMPLIED FORECAST ASCENT FROM ROBUST SECONDARY CIRCULATION 
ON THE LEADING EDGE OF THE JET STREAK...HAS LIKELY SUPPORTED 
SIGNIFICANT ASCENT. PEGGED ASCENT FIELDS LIKELY LEAD TO STAUNCH 
ADIABATIC COOLING RETARDING THE SN/PL CHANGEOVER TO RAIN YESTERDAY 
EVENING. COLD FRONT ALOFT AND THE LEADING EDGE OF A HIGH DESERT EML 
CONTINUES TO ADV INTO IL/IN...WITH WIDESPREAD SHOWERS AND ISOLATED 
THUNDERSTORMS PRESENT ALONG A SFC TROUGH...AKIN TO A SPRING SYSTEM 
DRY LINE. THIS SYSTEM AND ITS TRACK ACTUALLY BEARS SOME RESEMBLANCE 
TO THE APR 11 1965 CYCLONE WHICH PRODUCED WIDESPREAD SEVERE WEATHER 
TO THE REGION. THE MAIN DIFFERENCE IS OFF COURSE THE MUCH COOLER AND 
DRIER THERMAL AND MOISTURE FIELDS. SFC COLD FRONT IS LOCATED IN 
EASTERN MO AND WILL CONTINUE TO RAPIDLY PUSH NE TOWARD THE 
FA...ENTERING THE WESTERN FA AROUND DAWN. MAIN FORECAST CONCERN 
IN THE PERIOD WILL BE THE STRONG GUSTY WINDS TODAY...AND WINTER 
WEATHER CONCERNS FROM A BRIEF PERIOD OF DEFORMATION SNOW/LAKE EFFECT 
SNOW/AND FREEZING OF ROADWAYS LATER THIS MORNING. HAVE FOLLOWED THE 
NAM12 HEAVILY WITH AID FROM THE SREF GIVEN GOOD TRACK RECORD OF LATE 
AND PREFERRED THERMAL AND MOMENTUM FIELDS.

COLD FRONT ALOFT WILL PUSH THROUGH THE REGION THIS MORNING 
SUPPORTING ANOTHER ROUND OF RAIN...POSSIBLY MIXED WITH SLEET GIVEN 
ELEVATED INSTABILITY AND STRONG CONVECTIVE COMPONENT. SFC COLD FRONT 
IS EXPECTED INTO THE FA AROUND 12Z...QUICKLY PUSHING EAST THROUGH 
THE FA IN 6 HOURS OR LESS. NEAR RECORD/HISTORIC SURFACE LOW PRESSURE 
CENTER VALUES EXPECTED AROUND 975 MB WILL CONTINUE TO SUPPORT VERY 
STRONG LL WIND FIELDS. NAM 12 GEOSTROPHIC ANALYSIS AT APPROX 1 KFT 
INDICATES 95 KNOTS OF GEOSTROPHIC FLOW COINCIDENT WITH STRONG CAA/ A 
VERY STRONG SFC PRESSURE GRADIENT/AND LL DRY ADIABATIC LAPSE RATES 
WITH THE SFC WIND PARALLEL TO THE ISALLOBARIC WIND FIELD IN THE 
PRESENCE OF A DEEPENING SFC LOW. THIS WILL SUPPORT VERY STRONG GUSTY 
WINDS BY LATE MORNING. SUSTAINED WINDS OF 35 TO 40 MPH WILL BE 
COMMON WITH EXPECTED GUSTS OF 60 TO 70 MPH. ANALOGS TO PAST 
EVENTS...12-15-87 AND 4-11-65...CONFIRM WIND GUSTS IN EXCESS OF 60 
MPH. THIS IS EXPECTED TO CAUSE SOME TREE DAMAGE WITH STRUCTURAL 
DAMAGE POSSIBLE. WINDS WILL PERSIST THROUGH THE AFTERNOON...WANING 
SOME BY EVENING. ISSUED A HIGH WIND WARNING EARLIER FOR THIS. 

NEXT IS THE WINTER WEATHER CONCERNS. TEMPS WILL QUICKLY FALL BELOW 
FREEZING LATE MORNING WEST-EAST WITH THERMAL PROFILES QUICKLY 
SUPPORTING SNOW. TAIL END OF THE ASSOCIATED DEFORMATION ZONE IS 
EXPECTED TO CLIP THE NW FA LATE MORNING AND EARLY AFTERNOON WITH 1-2 
INCHES POSSIBLE. FLOW WILL TURN STRONGLY CONVERGENT OVER LAKE MI 
THIS AFTERNOON SUPPORTING A STRONG LAKE ENHANCED SNOW BAND OVER SW 
MI. THIS BAND LOOKS TO STAY MOSTLY NORTH OF THE FA. HOWEVER...ANOTHER 
1-2 INCHES IS POSSIBLE IN NORTHERN BERRIEN AND CASS COUNTIES. VERY 
STRONG GUSTY WINDS WINDS WILL SUPPORT WIDESPREAD BLOWING AND 
DRIFTING SNOW WITH VIS RESTRICTIONS EXPECTED WHERE SNOW ACCUMS 
OCCUR. IT WILL NOT TAKE MUCH SNOWFALL WITH THESE KIND OF WINDS...AS 
ONLY AN INCH WOULD CAUSE SIG VIS REDUCTION. IN ADDITION...CAA OVER 
LAKE MI WILL SUPPORT DELTA T/S OF 20-23 C BY LATE TONIGHT WITH A 
VIGOROUS LAKE RESPONSE EXPECTED AS LAKE INDUCED CAPES REACH 600-700 
J/KG. HOWEVER...STRONG LL WIND WILL LEAD TO MODERATE BAND SHEARING 
WITH HEAVIEST ACCUMS LIKELY REMAINING NORTH OF THE FA. 
REGARDLESS...CONTINUED STRONG GUSTY WINDS WILL SUPPORT VERY DEGRADED 
TRAVEL CONDITIONS WITH ANY SNOWFALL. GIVEN THE ABOVE CONCERNS...HAVE 
ISSUED A WW ADV FOR IMPACTS TO TRAVEL.

LAKE EFFECT SNOW CONTINUES INTO THUR AS SFC HIGH PRESSURE BUILDS 
SOUTH OF THE REGION...WITH LAKE INDUCED CAPES REMAINING IN THE 
600-800 J/KG RANGE. FLOW WILL REMAIN WESTERLY SUPPORTING HEAVIEST 
SNOW NORTH OF THE FA. THERE IS TILL SOME QUESTION AS TO EXACT FLOW 
ORIENTATION AT THIS POINT BUT FEEL ADDITIONAL ACCUMS WITH BREEZY 
CONDITIONS WILL IMPACT TRAVEL IN SW MI. IF HEAVIER BANDS DO SHIFT 
FARTHER SOUTH THAN CURRENT EXPECTATIONS...HEAVIER ACCUMS MAY SUPPORT 
NEAR WARNING CRITERIA. HOWEVER...IT LOOKS TO BE A LONG DURATION 
LIGHT TO MODERATE SNOWFALL EVENT BELOW WARNING CRITERIA AT THIS 
POINT. WILL LET THE DAYSHIFT ASSESS. 

ARCTIC AIR MASS WILL PUSH INTO THE REGION TONIGHT AND LINGER 
THROUGH THUR. GIVEN UPSTREAM OBS AND MOSS TRENDS...HAVE ADJUSTED 
HIGHS/LOWS DOWN. TEMPS ARE EXPECTED TO HOVER IN THE TEENS 
THUR...WITH WIND CHILLS OF 5 TO 10 BELOW ZERO EXPECTED THU MORNING 
AND NIGHT.

LONG TERM /FRIDAY THROUGH TUESDAY/...

COLDEST AIR WILL BE DEPARTING AT THE START OF THE PERIOD. SW FLOW 
WILL STILL BE IN PLACE TO CONTINUE THE CHC OF LAKE EFFECT SNOW 
SHOWERS MAINLY ACROSS THE MICH COUNTIES BUT ACCUMS SHOULD NOT BE AN 
ISSUE WITH DIMINISHING MSTR AND INVERSION HGTS. H85 TEMPS WILL START 
OUT AROUND -14 TO -16 C AT 12Z FRI WARMING TO -1 TO -3 C BY 12Z 
SUNDAY. THE RESULT SHOULD BE MODERATING SFC TEMPS BACK CLOSER TO BUT 
STILL BELOW NORMAL LEVELS. FRIDAY HIGHS SHOULD MAKE IT INTO THE MID 
20S IN MOST LOCATIONS WITH THOSE AREAS THAT HAVE SNOW PACK ACROSS 
THE FAR NORTH LIKELY AT LEAST A FEW DEGREES COOLER. BY SATURDAY AND 
SUNDAY...THAW SHOULD BE UNDERWAY WITH TEMPS PUSHING BACK TOWARDS 
FREEZING IN MANY SPOTS. MODELS NOW POINT TOWARDS DRY FORECAST FOR 
SATURDAY AND AFTER COLLABORATION WITH SURROUNDING OFFICES HAVE 
REMOVED CHC POPS. 

TOWARDS THE END OF THE PERIOD...MUCH UNCERTAINTY REMAINS IN NEXT 
UPPER LOW TO DROP OUT OF CANADA. MED RANGE MODELS ALL TRY TO DIG THE 
LOW GENERALLY JUST SOUTH OF HUDSON BAY WITH TROUGH EXTENDING SOUTH 
INTO THE GREAT LAKES. TIMING ISSUES...STRENGTH OF TROUGH AND AMOUNT 
OF COLD AIR ALL MAKE THE FORECAST RATHER TRICKY. GFS IS MOST 
AGGRESSIVE ON COLD AIR WITH 850 MB TEMPS DROPPING BACK TOWARDS -15 C 
BY 16/00Z. ECMWF AND GFS ENSEMBLE MORE IN THE -6 TO -8 C RANGE. 
SINCE MAIN PUSH OF COLDEST AIR IS WELL INTO DAY 6 AND MORE DAY 7 AND 
BEYOND...WILL HOLD OFF ON INTRODUCING ANY NEW POPS. MEX GUIDANCE IS 
RIGHT AT CLIMO VALUES AND WOULD LIKE TO SEE IT A BIT HIGHER THAN 
CLIMO TO GIVE ADDED SUPPORT. 

&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...HIGH WIND WARNING FROM 10 AM EST /9 AM CST/ THIS MORNING TO 7 PM 
     EST /6 PM CST/ THIS EVENING FOR INZ003>009-012>018-020-
     022>027-032>034.

     WINTER WEATHER ADVISORY FROM NOON EST /11 AM CST/ TODAY TO 9 PM 
     EST /8 PM CST/ THIS EVENING FOR INZ003>005.

MI...HIGH WIND WARNING FROM 10 AM THIS MORNING TO 7 PM EST THIS 
     EVENING FOR MIZ077>081.

     WINTER WEATHER ADVISORY FROM NOON TODAY TO 9 PM EST THIS EVENING 
     FOR MIZ079.

     WINTER WEATHER ADVISORY FROM NOON TODAY TO 7 PM EST THURSDAY FOR 
     MIZ077-078.

OH...HIGH WIND WARNING FROM 10 AM THIS MORNING TO 7 PM EST THIS 
     EVENING FOR OHZ001-002-004-005-015-016-024-025.

LM...STORM WARNING UNTIL 7 PM EST THIS EVENING FOR LMZ043-046.

     GALE WARNING UNTIL 9 AM EST THIS MORNING FOR LMZ043-046.

&&

$$

SHORT TERM...JC
LONG TERM...FISHER
AVIATION...FISHER