PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
411 AM EST FRI JAN 16 2009
 
VALID 12Z FRI JAN 16 2009 - 12Z MON JAN 19 2009
 

DAY 1...

GREAT LAKES...
SURGE OF ARCTIC AIR AND A BAND OF PERSISTENT LOW LEVEL CONVERGENCE
TO THE LEE OF THE GREAT LAKES WILL CONTINUE TO PROVIDE A FAVORABLE
SETUP FOR HVY LAKE EFFECT SNOW BANDS.  850 MB TEMPS IN THE MINUS
23-27C RANGE OVER MUCH OF THE REGION WILL PROVIDE FOR STEEP LAPSE
RATES FROM THE SFC-775 MB.  THE COLD TEMPS LOFT PROVIDE FOR HIGH
SNOW TO LIQUID RATIOS ROUGHLY IN THE MID 20S:1 AND MINIMAL
AGGREGATION AND COMPACTION.  GREATER WEIGHTING WAS GIVEN TO THE
HIGHER RESOLUTION NAM. 

MN ARROWHEAD...

THE NEXT IN A SERIES OF CLIPPER SYSTEMS WILL DROP SOUTH INTO THE
MN ARROWHEAD AND ADJACENT PARTS OF THE WESTERN GREAT LAKES.  THERE
SHOULD BE SUFFICIENT SUPPORT ALOFT FROM A MID AND UPPER LEVEL JET
NOSING INTO THE REGION.  WE INDICATED A SLIGHT RISK OF 4 INCH
AMOUNTS IN THE REGION OF BEST UPPER LEVEL DIVERGENCE ASSOCIATED
WITH THE JET AND A MID LEVEL VORT CENTER. 

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

DAYS 2 AND 3...

GTLKS TO NORTHEAST AND MID ATLANTIC
...
THE SAME CLIPPER WHICH MOVES INTO THE MN ARROWHEAD ON DAY ONE WILL
CONTINUE TO DROP SOUTHEAST INTO THE GREAT LAKES REGION...BEFORE
TURNING EAST AND THEN NORTHEAST IN THE LONGWAVE TROF.  THE UPPER
LEVEL JET STRUCTURE SHOWS THE LEFT EXIT JET CROSSING THE
REGION...WITH EMBEDDED DIVERGENCE MAXIMA CROSSING THE REGION. 
GIVEN THIS SCENARIO...WE FOCUSED THE BEST SNOW ACROSS THE GREAT
LAKES AND THEN INTO THE NORTHEAST.  WE CONTINUED TO INDICATE THE
POTENTIAL OF 4 INCH SNOWFALL IN PARTS OF THE UPSLOPE REGION IN
WEST VIRGINIA AS WEST TO NORTHWEST WINDS DRAW A COLD AIRMASS WITH
HIGH RELATIVE HUMIDITY UPSLOPE.  HOWEVER...THE LACK OF ABUNDANT
DEEP LAYER MOISTURE SUGGESTS THAT NO MORE THAN A SLIGHT RISK WAS
WARRANTED.  WE OPTED FOR A 15/12Z ECMWF...16/00Z GFS/UKMET SOLN
FOR THE TIME BEING AND WILL WAIT TO SEE IF THE 16/00Z ECMWF GETS
ANY SUPPORT FOR ITS IDEA OF A SFC LOW FORMING OVER THE SOUTHEAST
THAT ROTATES UP TO THE CAPE HATTERAS REGION LATE ON DAY 3 AND
BEYOND. AT THIS POINT...IT SEEMS TOO AMPLIFIED COMPARED WITH THE
OTHER GUIDANCE AND WAS TOO BIG OF A JUMP IN CONTINUITY FOR US TO
MAKE ANY LARGE CHANGES IN THE FORECAST.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

BANN