THE WEATHER PREDICTION CENTER

COLLEGE PARK, MD


PROBABILISTIC HEAVY SNOW AND ICE DISCUSSION




PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
318 AM EDT TUE APR 21 2015

VALID 12Z TUE APR 21 2015 - 12Z FRI APR 24 2015


DAYS 1 THROUGH 3...


...NORTHERN NEW ENGLAND...

A SHORT WAVE EJECTED FROM A STRONG CLOSED MID LEVEL LOW OVER
ONTARIO CROSSES LATE NORTHERN NEW ENGLAND LATE ON DAY 2 AND DAY 3.
THE SHORT WAVE INDUCES CYCLOGENESIS OVER THE WESTERN GULF OF ME
DURING THIS TIME. THE LOW THEN TRACKS OVER ME DURING DAY 3. THERE
IS SOME SPREAD CONCERNING THE TRACK OF THE SURFACE LOW OVER
ME...AS THE 00Z GFS APPEARS TO BE TOO FAR WEST. THE TRACK OF THE
LOW WILL DETERMINE BOUNDARY LAYER TEMPERATURES DURING DAY
3...WHICH AFFECTS SNOWFALL ACCUMULATIONS. AT THIS POINT...THE
THERMAL PORTION OF THE FORECAST WAS BASED ON A BLEND OF THE 00Z
NAM/12Z ECMWF...AND THE QPF PORTION OF THE FORECAST WAS BASED ON
THE MOST RECENT WPC QPF.

THE BEST LIFT AND MOISTURE ASSOCIATED WITH THE SHORT WAVE AND
SURFACE LOW OCCURS BETWEEN 24/06Z AND 24/18Z OVER NORTHERN NH AND
MUCH OF NORTHERN AND WESTERN ME. THE MAIN FORECAST PROBLEM WILL BE
BOUNDARY LAYER TEMPERATURES...AND GIVEN THE SPREAD IN THE
SOLUTIONS...THE SNOWFALL FORECAST FOR LATE DAY 2 INTO DAY 3 IS A
LOW CONFIDENCE FORECAST. AS THE SHORT WAVE TRACKS OVER NORTHERN NH
AND WESTERN ME BETWEEN 24/06Z AND 24/12Z...MOST MODELS SHOW SNOW
LEVELS DROPPING TO BETWEEN 2000 AND 3000 FEET. THE COMBINATION OF
MOISTURE AND LIFT SUPPORTS AN INCH OR TWO OF SNOWFALL HERE... WITH
BOUNDARY LAYERS TOO WARM FURTHER NORTH AND EAST THROUGH 24/12Z.

DURING DAY 3...THE SNOWFALL FORECAST BECOMES ELEVATION
DEPENDENT...AS MOST REGIONAL AND GLOBAL MODELS SHOWING BOUNDARY
LAYER TEMPERATURES BORDERLINE FOR SNOWFALL. MODEL SOUNDINGS ACROSS
WESTERN AND NORTHERN ME SUGGEST THAT SNOW LEVELS STABILIZE BETWEEN
2000 AND 3000 FEET (DROPPING A BIT LOWER ACROSS NORTHERN ME DURING
THE SECOND PART OF DAY 3). THERE IS A MULTI MODEL SIGNAL FOR 3 TO
6 INCHES OF SNOWFALL ACROSS THE HIGHER TERRAIN OF NORTHERNMOST NH
THROUGH THE MOUNTAINS OF WESTERN ME INTO FAR NORTHERN ME. IN
FACT...THESE AMOUNTS ARE SUPPORTED BY MEMBERS OF THE LATEST ECMWF
ENSEMBLE OUTPUT SHOWING 4+ INCHES OF SNOWFALL.

AT THIS POINT...GIVEN THE UNCERTAINTY OF THE BOUNDARY LAYER
TEMPERATURES ACROSS THE HIGHER TERRAIN OF NORTHERN NH AND WESTERN
ME...AS WELL AS FAR NORTHERN ME...SNOWFALL AMOUNTS WERE GENERALLY
CAPPED AT AROUND 4 INCHES ACROSS THE ROOFTOP OF ME. HOWEVER...THIS
WILL BE REVISITED WITH FUTURE FORECASTS...WHEN HOPEFULLY THERMAL
PROFILES WILL COME INTO BETTER FOCUS.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.


...UPPER MS VALLEY/UPPER GREAT LAKES...

A STRONG CLOSED MID LEVEL LOW MOVES OVER THE UPPER GREAT LAKES
DURING DAY 1...BEFORE IT CROSSES INTO ONTARIO AND SLOWS DURING DAY
2. MOISTURE AND LIFT ASSOCIATED WITH A SOUTHERN LOBE FROM THE
CLOSED LOW MOVES OVER THE UPPER MS VALLEY DURING THE FIRST PART OF
THE PERIOD...AND THE UPPER GREAT LAKES DURING THE SECOND HALF OF
THE DAY. THERE WAS GENERALLY GOOD MODEL AGREEMENT CONCERNING THE
TRACK OF THE CLOSED MID LEVEL LOW...SO THE THERMAL PORTION OF THE
FORECAST WAS BASED ON A MULTI MODEL BLEND. THE QPF PORTION OF THE
FORECAST WAS BASED ON THE MOST RECENT WPC QPF.

SNOWFALL WITH THIS SYSTEM SHOULD BE LIMITED TO NORTHEAST
MN...NORTHERN WI AND THE UP OF MI. MODEL CONSENSUS KEEPS BOUNDARY
LAYER TEMPERATURES BELOW THE FREEZING MARK IN THESE
LOCATIONS...BUT SNOWFALL ACCUMULATIONS MAY ULTIMATELY BE
DETERMINED BY SNOWFALL RATES. LIGHTER RATES COULD ALLOW A MIX OF
RAIN AND SNOW...ESPECIALLY ACROSS NORTHERN WI...WHILE HIGHER RATES
WOULD FAVOR MORE SNOW AND INCREASED SNOWFALL. THE BEST LIFT AND
MOISTURE ENDS AFTER 22/00Z. BOUNDARY LAYER TEMPERATURES SUPPORT 1
TO 3 INCHES OF SNOWFALL ACROSS THE REGION...WITH THE HIGHEST
AMOUNTS ACROSS THE UP OF MI. SNOWFALL AMOUNTS OF 4 INCHES OR
GREATER LOOKS LIKE A LOW PROBABILITY EVENT.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

HAYES





Last Updated: 318 AM EDT TUE APR 21 2015