Quantitative Precipitation Forecast
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FOUS11 KWBC 160826
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
324 AM EST SAT DEC 16 2017

VALID 12Z SAT DEC 16 2017 - 12Z TUE DEC 19 2017


DAYS 1 THROUGH 3...


CASCADES AND NORTHERN ROCKIES

THERE WILL BE TWO SEPARATE EPISODES OF WINTER WEATHER ACROSS THE
NORTHWESTERN QUADRANT OF THE NATION.  THE FIRST ROUND WILL BE ON
SATURDAY ACROSS THE NORTHERN ROCKIES AND EXTENDING SOUTHWESTWARD
TO EASTERN NEVADA.  WITH SHORT WAVE ENERGY CROSSING THE PACIFIC
NORTHWEST AND NORTHERN ROCKIES, IT PROVIDES ENOUGH LIFT AND
OROGRAPHIC FORCING ACROSS THESE AREAS TO SUPPORT A GENERAL 2 TO 4
INCHES OF SNOWFALL ACROSS MUCH OF WYOMING AND SOUTH-CENTRAL
MONTANA, WITH MAXIMA ON THE ORDER OF 6 TO 10 INCHES POSSIBLE FOR
THE WIND RIVER AND BIG HORN MOUNTAINS.  ECMWF ENSEMBLE SPAGHETTI
PLOTS INDICATE THE HIGHEST POTENTIAL FOR 4+ INCH AMOUNTS OVER
THOSE MOUNTAIN RANGES.

THE SECOND EPISODE WILL BRING HEAVY SNOW BACK TO THE WASHINGTON
CASCADES FOR THE SUNDAY AND MONDAY TIME PERIOD.  A VERY
PROGRESSIVE MID LEVEL FLOW ADVECTS A STEADY STREAM OF PACIFIC
MOISTURE INLAND ACROSS THE CASCADES AND THEN OVER THE NORTHERN
ROCKIES.  THIS FLOW WILL ENHANCE 850-500MB LIFT, AND SNOW LEVELS
ARE EXPECTED TO FALL FROM ABOUT 6500 FEET TO 4000 FEET BASED ON
MODEL FORECAST SOUNDINGS FOR THE CASCADES AND REACHING THE VALLEY
FLOORS OVER THE NORTHERN ROCKIES.


ARIZONA AND NEW MEXICO

A LONG WAVE TROUGH DEEPENING OVER THE SOUTHWEST STATES ON SUNDAY
RESULTS IN LOWERING HEIGHTS ALOFT AND THUS SNOW LEVELS TO DROP,
PRODUCING ACCUMULATING SNOWFALL OVER THE HIGHER TERRAIN OF EASTERN
ARIZONA AND WESTERN NEW MEXICO.  STEEPENING MID LEVEL LAPSE RATES
COULD PROVIDE A CONVECTIVE CHARACTER TO THE PRECIPITATION, BUT
THERE SHOULD BE ENOUGH QPF TO SUPPORT LOCAL 2 TO 4 INCH SNOWFALL
AMOUNTS, GENERALLY ABOVE 7000 FEET OVER THE EASTERN MOGOLLON RIM
IN ARIZONA TO THE MOUNTAINS OF WEST CENTRAL NEW MEXICO.  RECENT
ENSEMBLE SPAGHETTI PLOTS INDICATED MULTIPLE MEMBERS WITH 4+ INCHES
OF SNOWFALL IN THESE LOCATIONS.  THERE IS NOW LESS MODEL SPREAD
CONCERNING HOW QUICKLY THE CLOSED MID LEVEL SYSTEM EXITS THE
REGION, COMPARED TO EARLIER RUNS.


GREAT LAKES REGION

THE ONGOING LAKE EFFECT SNOW EVENT FROM LAKES ERIE AND ONTARIO IS
FORECAST TO ABATE BY SATURDAY AFTERNOON AS DRIER AIR AND LOWER
INVERSION LEVELS BECOME ESTABLISHED OVER THE REGION, AND THE LOW
LEVEL WINDS VEER TO A NORTHERLY DIRECTION ACROSS LAKE ONTARIO AND
NORTHEASTERLY OVER LAKE ERIE.  THE MAJORITY OF THE DAY 1 SNOWFALL
IS EXPECTED FROM 12Z TO 18Z SATURDAY, WITH THE MOST FAVORABLE
PARAMETERS EXPECTED DOWNWIND OF LAKE ERIE FROM EXTREME NORTHEAST
OHIO TO SOUTHWEST NEW YORK.  THE LAKE EFFECT BANDS ARE EXPECTED TO
PROGRESS SOUTHWARD ACROSS THE SNOW BELTS BEFORE DISSIPATING LATER
IN THE DAY.


THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

D. HAMRICK

$$





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