PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
318 AM EDT SAT MAR 16 2013

VALID 12Z SAT MAR 16 2013 - 12Z TUE MAR 19 2013

...DAY 1...SATURDAY...

...GREAT LAKES INTO MID-ATLANTIC...

A FAST-MOVING AA LOW RISK OF SNOWFALL EXCEEDING 4 INCHES IS ALSO
NOTED OVER CENTRAL AND WESTERN COLORADO.  THIS AREA OF SNOW IS NOT
DIRECTLY ASSOCIATED WITH THE DIVING TROUGH OVER THE NORTHWEST BUT
IS INSTEAD ASSOCIATED WITH A LESS DISTINCT BUT IMPORTANT TROUGH
MOVING EASTWARD ACROSS THE SOUTHWEST THAT LATER WILL ALLOW
SIGNIFICANT MOISTURE TO MOVE NORTHWARD FROM THE GULF OF MEXICO
ACROSS THE OHIO VALLEY INTO THE NORTHEAST ON DAYS 2 TO 3.
ND RELATIVELY WEAK SHORTWAVE WILL MOVE FROM THE GREAT LAKES ACROSS
THE NORTHEAST ON SATURDAY.  THE SURFACE LOW ASSOCIATED WITH THIS
SYSTEM WILL MOVE EASTWARD FROM THE OHIO VALLEY SATURDAY MORNING TO
OFF THE EAST COAST ON SATURDAY NIGHT.  AN AREA OF SNOW OVER LOWER
MICHIGAN ON FRIDAY NIGHT WILL CROSS PENNSYLVANIA AND REACH THE
MIDDLE ATLANTIC COAST ON SATURDAY.  TEMPERATURES WILL SUPPORT SOME
LIGHT SNOW ACROSS INTERIOR PENNSYLVANIA WITH LIGHT ACCUMULATIONS
AND SOME LIGHT FREEZING RAIN/SLEET ACROSS SW PA INTO WESTERN
MARYLAND BUT NO ACCUMULATIONS ARE EXPECTED NEAR THE COAST AS
PRECIPITATION WILL BE LIGHT AND TEMPERATURES MARGINAL AT BEST.

...DAYS 1 TO 3...SATURDAY TO TUESDAY MORNING...

...THE NORTHERN THIRD OF THE COUNTRY...

A HIGH AMPLITUDE PACIFIC STORM SYSTEM WILL PRODUCE A LARGE AREA OF
SIGNIFICANT WINTER WEATHER ACROSS THE NORTHERN THIRD OF THE NATION
OVER THE NEXT 3 DAYS...IN COMBINATION WITH A MODEST TROUGH MOVING
EASTWARD FROM THE SOUTHERN PLAINS ON SUNDAY TOWARD THE NORTHEAST
ON MONDAY.  THESE TWO SYSTEMS WILL COMBINE TO PRODUCE SIGNIFICANT
SNOW FROM THE HIGH ELEVATIONS OF THE PACIFIC NORTHWEST/NORTHERN
AND CENTRAL ROCKIES ON DAY 1...NORTHERN PLAINS AND UPPER
MISSISSIPPI VALLEY/A SMALL PORTION OF THE MIDDLE ATLANTIC
STATES/CENTRAL APPALACHIANS ON DAY 2/AND THE UPPER MIDWEST AND
INTERIOR NORTHEAST ON DAY 3.

...DAY 1...SATURDAY INTO SUNDAY...

...PACIFIC NORTHWEST AND NORTHERN AND CENTRAL ROCKIES...

AN AMPLIFYING UPPER TROUGH WILL DIVE SOUTHEASTWARD ACROSS THE
NORTHWEST COAST ON SATURDAY NIGHT AND BE CENTERED NEAR IDAHO BY
SUNDAY MORNING. A LARGE PLUME OF PACIFIC MOISTURE AND RAPIDLY
LOWERING SNOW LEVELS WILL PUSH SOUTH AND EASTWARD RESULTING IN
HEAVY SNOW FOR THE OLYMPIC AND CASCADE RANGES AS WELL AS THE BLUE
MOUNTAINS AND THEN MOVE ACROSS CENTRAL AND NORTHERN IDAHO/WESTERN
MONTANA AND NW WYOMING. HEAVIEST SNOW IS EXPECTED ACROSS THE
WASHINGTON CASCADES WHERE 6 TO 12 INCHES AND GREATER ARE EXPECTED
WIDELY ACROSS THE RANGE. SNOW LEVELS ARE EXPECTED TO DROP FROM
OVER 4000 FEET TONIGHT TO AROUND 1500 FEET BY SATURDAY NIGHT.
SIGNIFICANT SNOW IS ALSO EXPECTED ACROSS THE BITTEROOTS IN IDAHO
AND ACROSS NW MONTANA WITH A LOW RISK OF GREATER THAN 4 INCHES
ACROSS THE GRAND TETONS IN WYOMING BY SUNDAY MORNING.

A LOW RISK OF SNOWFALL EXCEEDING 4 INCHES IS ALSO NOTED OVER
CENTRAL AND WESTERN COLORADO.  THIS AREA OF SNOW IS NOT DIRECTLY
ASSOCIATED WITH THE DIVING TROUGH OVER THE NORTHWEST BUT IS
INSTEAD ASSOCIATED WITH A LESS DISTINCT BUT IMPORTANT TROUGH
MOVING EASTWARD ACROSS THE SOUTHWEST THAT LATER WILL ALLOW
SIGNIFICANT MOISTURE TO MOVE NORTHWARD FROM THE GULF OF MEXICO
ACROSS THE OHIO VALLEY INTO THE NORTHEAST ON DAYS 2 TO 3.


...DAY 2...SUNDAY INTO MONDAY MORNING...

...NORTHWEST/NORTHERN AND CENTRAL ROCKIES/NORTHERN PLAINS AND A
SMALL AREA OF THE MIDDLE ATLANTIC STATES/CENTRAL APPALACHIANS...

THE COMBINATION OF THE AMPLIFYING UPPER TROUGH OVER THE NORTHWEST
MOVING ACROSS THE NORTHERN PLAINS AND THE MORE DIFFUSE SYSTEM
CROSSING FROM THE SOUTHERN PLAINS TO THE OHIO VALLEY WILL PRODUCE
AN EXPANSIVE AREA OF PRECIPITATION AND LARGE AREA OF SNOWFALL WITH
SIGNIFICANT SNOW DEVELOPING ACROSS MONTANA INTO THE DAKOTAS INTO
MINNESOTA AND POSSIBLY IOWA.  THE SHARP TROUGH MOVING FROM THE
NORTHWEST TOWARD THE NORTHERN PLAINS IS RESPONSIBLE FOR THIS LARGE
AREA OF SNOW WHILE THE MOISTURE ASSOCIATED WITH THE LIFTING TROUGH
OVER THE OHIO VALLEY WILL LIKELY BE ASSOCIATED WITH SOME FREEZING
RAIN/SLEET AND SNOW OVER THE MID ATLANTIC STATES AS THE MOISTURE
MEETS UP WITH COLDER AIR ASSOCIATED WITH A LARGE CELL OF COLD HIGH
PRESSURE ANCHORED NORTH OF NEW YORK BY MONDAY MORNING.

WHILE SNOWFALL WILL NOT BE AS HEAVY ON DAY 2 ACROSS THE
CASCADES...THE FOCUS OF THE SNOW WILL BE MOVING EASTWARD ACROSS
MONTANA AND NORTHWEST WYOMING.  WHILE SNOWFALL ON DAY 1 OVER
WESTERN COLORADO WAS ASSOCIATED WITH THE TROUGH MOVING EASTWARD
ACROSS THE SOUTHWEST...SNOWFALL ON DAY 2 WILL BE ASSOCIATED WITH
THE EFFECTS OF PACIFIC NW TROUGH AND SHARP COLD FRONT CROSSING THE
STATE...RESULTING IN SIGNIFICANT SNOW EXCEEDING 4 INCHES ACROSS
MUCH OF THE ELEVATED WESTERN MOUNTAINS.

AS THE PACIFIC TROUGH CONTINUES EASTWARD TOWARD THE NORTHERN
PLAINS...A SURFACE LOW WILL CONSOLIDATE NEAR THE WYOMING MONTANA
BORDER EARLY SUNDAY AND THEN MOVE EAST NORTHEASTWARD TOWARD THE
MINNESOTA/NORTH DAKOTA BORDER BY MONDAY MORNING.  A LARGE AREA OF
SNOWFALL WILL SPREAD EASTWARD WITH THE HEAVIEST SNOW NEAR AND
NORTH OF THE SURFACE LOW WITH MOSTLY 4 TO 8 INCHES CURRENTLY
EXPECTED FROM NORTHEAST MONTANA ACROSS NORTHERN NORTH DAKOTA INTO
NORTHWEST MINNESOTA. EVEN SOUTHEAST OF THE LOW CENTER...MOSTLY
LIGHT SNOWS ARE EXPECTED TO FALL FROM IOWA NORTHWARD INTO
MINNESOTA.

FARTHER EAST...A NARROW BAND OF FREEZING RAIN/SLEET AND SNOW IS
EXPECTED ALONG THE NORTHERN FLANK OF THE EXPANDING AREA OF
PRECIPITATION ACROSS THE OHIO VALLEY.  WHILE SOME MODEL PROFILES
INDICATE COLDER TEMPERATURES THAN MIGHT BE EXPECTED FROM THE
SYNOPTIC SETUP...HAVE ELECTED TO MINIMIZE THREAT OF SNOW FOR NOW
IN THIS AREA SINCE I WOULD EXPECT WARMING ALOFT TO COUNTER LOW
LEVEL COLD AIR.  FARTHER EAST ACROSS THE APPALACHIANS...COLDER AIR
WILL BE DAMMED EAST OF THE MOUNTAINS AND GREATER LIKELIHOOD IS
GIVEN TO MORE SIGNIFICANT THREAT OF FREEZING RAIN/SNOW FROM
WESTERN VIRGINIA/EASTERN WEST VIRGINIA INTO MARYLAND PANHANDLE
WHERE THERE IS A LOW TO MODERATE THREAT OF FREEZING RAIN EXCEEDING
.25 INCHES AND SNOW EXCEEDING 4 INCHES.

...DAY 3...MONDAY INTO TUESDAY MORNING.

...UPPER MISSISSIPPI VALLEY/MIDWEST...INTERIOR NORTHEAST...

THE INTERACTION OF THE TWO WEATHER SYSTEMS WILL PRODUCE A
WIDESPREAD AREA OF SNOW AND FREEZING RAIN.  OVER THE UPPER
MISSISSIPPI VALLEY AND MIDWEST...A LOW TO MODERATE PROBABILITY OF
GREATER THAN 4 INCHES OF SNOW IS CONFINED TO CENTRAL AND NORTHERN
WISCONSIN AS WELL AS NORTHEAST MINNESOTA AND PORTIONS OF THE LOWER
AND UPPER PENINSULA OF MICHIGAN AS THE SURFACE LOW MAKES ITS WAY
TOWARD LAKE SUPERIOR. 

HOWEVER...A SURFACE WAVE OF LOW PRESSURE WILL DEVELOP ALONG A
DEVELOPING COASTAL FRONT AND BY TUESDAY MORNING WILL BE LOCATED
SOMEWHERE NEAR THE MIDDLE ATLANTIC COASTLINE.  THERE IS STILL
ENOUGH MODEL SPREAD THAT QUITE A BIT OF UNCERTAINTY REMAINS
ESPECIALLY WITH REGARD TO WHERE THE RAIN/SNOW/FREEZING RAIN
BOUNDARY SETS UP.  HEAVIEST SNOW IS EXPECTED ACROSS INTERIOR NEW
YORK INTO NEW ENGLAND AS THE SNOW IS SPREADING THROUGH NEW ENGLAND
AT THE END OF DAY 3...WHICH IS TUESDAY MORNING. 
THEREFORE...SNOWFALL AMOUNTS IN NEW ENGLAND ARE NOT TOTAL
ACCUMULATIONS SINCE IT WILL CONTINUE AFTER THE FORECAST PERIOD. 
WITH A LARGE CELL OF HIGH PRESSURE REMAINING NORTH OF NEW
ENGLAND...COLD AIR DAMMING LOOKS LIKE IT WILL BE PRETTY WIDESPREAD
FROM WESTERN VIRGINIA NORTHWARD ACROSS MUCH OF THE INTERIOR
NORTHEAST.  WITH THE SURFACE LOW HUGGING THE COASTLINE...ANY
SNOW/FREEZING RAIN NEAR THE COAST IS LIKELY TO CHANGE FAIRLY
QUICKLY TO RAIN.  HOWEVER...SOME SIGNIFICANT ICING IS POSSIBLE
ESPECIALLY OVERNIGHT ON MONDAY INTO EARLY TUESDAY FROM
PENNSYLVANIA INTO SOUTHEAST NEW YORK INTO SOUTHERN INTERIOR NEW
ENGLAND. FARTHER NORTH...BEST CHANCES FOR GREATER THAN 4 INCHES OF
SNOW IS MOST LIKELY FROM CENTRAL AND EASTERN NEW YORK INTO WESTERN
NEW ENGLAND.  

KOCIN