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258 
FOUS11 KWBC 182113
QPFHSD
 
PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
412 PM EST FRI JAN 18 2008
 
VALID 00Z SAT JAN 19 2008 - 00Z TUE JAN 22 2008
 
DAY 1...

GREAT LAKES...

STRONG COLD ADVECTION OVER THE RELATIVELY WARM OPEN WATERS OF
LAKES SUPERIOR/MICHIGAN WILL SET UP LAKE EFFECT SNOWS OVER THE
UPPER PENINSULA OF MI AND NORTHWEST LOWER MI DUE TO STEEP LOW
LEVEL LAPSE RATES AND DECREASING LOW-MID LEVEL WIND SHEAR. 
HIGHEST AMOUNTS WERE FOCUSED ALONG LAKESHORE AREAS WHERE LONGER
CROSS-SUPERIOR TRAJECTORIES AND SHORELINE CONVERGENCE AND LOW
LEVEL VERTICAL VELOCITIES PERSIST IN THE EASTERN UP OF MI IN THE
GFS OUTPUT...SUPPORTING THE HIGHER NAM QPF TOTALS HERE. 

SRN APPALACHIANS/SRN MID-ATLANTIC...

THE 12Z GUIDANCE IS COMING INTO BETTER AGREEMENT ON THE EVENT
EVOLUTION...MINUS THE NAM.  THE NAM REMAINS AN OUTLIER IN SHOWING
A SLOWER MOVING LOW LEVEL WAVE AND LONGER DURATION PRECIPITATION
WHERE ITS FORECASTS SHOW IT TO BE COLD ENOUGH TO SNOW.  THIS
CARRIES OVER INTO EARLY DAY TWO WHERE THE GFS/UKMET/CANADIAN
GLOBAL/ECMWF/CANADIAN REGIONAL MODELS SHOW THE PRECIP ENDING
SOONER THAN THE NAM.  UNCERTAINTY CONTINUES AS THE MODELS SHOW A
TIGHT QPF GRADIENT AND A TRANSITION OF PRECIP TYPES FROM RAIN TO
SNOW AS POST-FRONTAL COLD ADVECTION COOLS THE AIRMASS WITH TIME. 
MANUAL PROGS INDICATE A LOW RISK OF 4 INCH AMOUNTS ACROSS PARTS OF
THE CAROLINAS DUE TO THE UNCERTAINTY OF SNOWFALL DURATION/AMOUNTS.
 MOST OF THE AREA RECEIVING SNOWFALL WILL BE ABV FREEZING AT THE
SURFACE...LIMITING ROADWAY ACCUMULATIONS WHERE TEMPS TODAY ARE ABV
FREEZING.  MANUAL PROGS BLENDED THE MODEL/SREFMEAN CONSENSUS QPF
AND THERMAL PROFILES.    

NORTHERN ID/NORTHWEST MT...

THE MODELS INDICATE MODEST WARM ADVECTION WITH VEERING PROFILES IN
THE SOUNDINGS WITH PERSISTENT MOIST CONDITIONS.   THIS SHOULD
RESULT IN PERIODS OF SNOW IN AREAS WHERE THE LOW LEVEL FLOW TURNS
UPSLOPE IN THE TERRAIN OF THE ID PANHANDLE AND NORTHWEST MT.  
MANUAL PROGS USED A BLEND OF THE OPERATIONAL MODELS AND SREFMEAN
FOR QPF AMOUNTS/THERMAL PROFILES.
 
THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.


DAY 2...

GREAT LAKES...

THE COLD FRONT CROSSES THE LOWER LAKES THIS PERIOD...WITH LOW
LEVEL WEST WINDS COMBINING WITH FRONTAL CONVERGENCE TO PRODUCE
BANDS OF SNOW OVER LK ONTARIO SHORELINE AREAS INTO 
THE TUG HILL.  THIS IS FOLLOWED BY VEERING WINDS MARKING STRONG
COLD ADVECTION INTO NEW YORK.  

FRIGID TEMPS ALOFT AND FAVORABLE WINDS FOR LAKE EFFECT SNOW WILL
BE IN PLACE EARLY IN THE PERIOD DOWNSTREAM FROM LK SUPERIOR IN THE
EASTERN UP OF MI ...WITH PERSISTENT SHORELINE CONVERGENCE AND LOW
LEVEL RISING MOTIONS.  SNOW TO LIQUID RATIOS CLIMB INTO 
THE MID 20S:1...PRODUCING HEAVY AMOUNTS ACRS THE REGION.  MANUAL
PROGS LEANED TOWARD HIGHER NAM AMOUNTS GIVEN THE GFS SOUNDINGS
SHOW LOW WIND SHEAR WITH PERSISTENT MOIST CONDITIONS AND STEEP
SFC-850 MB LAPSE RATES. 

NRN ROCKIES...

THE MODELS FCST A FRONT TO DRIFT SOUTH ACROSS THE NORTHERN ROCKIES
AND INDICATE A 700 MB LOW DEVELOPS WITHIN THE FRONTAL ZONE. 
CONVERGENCE IN THE FRONTAL ZONE NEAR THE FORMING CYCLONIC
CIRCULATION SUPPORTS ONE AREA OF SNOW ACRS THE RANGES OF CENTRAL
ID AND NORTHERN OR...INCLUDING THE CASCADES AND BLUES.   
OVER THE NORTHERN PLAINS...BUILDING HIGH PRESSURE TURNS THE
BOUNDARY LAYER FLOW NORTHEAST...WITH CONVERGENCE AND RISING MOTION
AS THE FLOW COMES INTO THE TERRAIN OF CENTRAL MT AND NORTHERN
WY/BLACK HILLS.  THE MODELS ARE DECENT AGREEMENT ON THE EVOLUTION
OF THESE EVENTS AND A MULTI-MODEL/SREFMEAN CONSENSUS WAS SUED TO
DETERMINE LIKELY AMOUNTS/PROBABILITIES.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.


DAY 3...

NRN ROCKIES/CENTRAL PLAINS/MID MS VALLEY...

THE AFOREMENTIONED CLOSED LOW AND FRONT WILL DRIFT SOUTH FROM
ID/WY INTO UT/CO...WITH THE MODEL CONSENSUS SHOWING ANOTHER 700 MB
CLOSED CENTER FORMING NEAR NORTHERN CA.  
THE RESULTING LONGER DURATION OW LEVEL CONVERGENCE WILL LAST
ACROSS NORTHERN UT INTO NORTHERN CO/SOUTHEAST WY...WITH A MAXIMUM
OVER THE LAST SEVERAL HOURS IN THE NORTHERN SIERRA.  THE FRONTAL
BOUNDARY WILL EXTEND FROM THE ROCKIES ACROSS THE NORTHERN PLAINS
AND MID MS VALLEY.  THE MODELS SHOW A WEAK WAVE DEVELOPING AND
PRODUCING CONVERGENCE AND SNOW ALONG THE BOUNDARY FROM NEBRASKA
ACROSS IA INTO ADJACENT NORTHERN IL/SOUTHERN WI. 
MANUAL PROGS BLENDING THE OPERATIONAL NAM/GFS/ECMWF/CANADIAN
GLOBAL MODELS/SREFMEAN.

GREAT LAKES...
THE MODELS SHOW THE LOW LEVEL AIR MASS STARTS TO MODERATE AND LOW
LEVEL WINDS START TO BACK.  IN THE UP OF MI THIS PRODUCES A
REVERSAL FROM ONSHORE TO OFFSHORE FLOW AND THUS SHORELINE
CONVERGENCE AND ASCENT DISSIPATES.   THE TIMING OF THE WINDS
PROFILE TRANSITION IS THE PRIMARY UNCERTAINTY.  MANUAL PROGS
BLENDED THE GFS/ECMWF/SREFMEAN/CANADIAN GLOBAL MODEL QPF.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

PETERSEN
$$