Quantitative Precipitation Forecast
Issued by NWS

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QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
523 AM EDT THU MAR 30 2017

VALID 12Z THU MAR 30 2017 - 12Z SUN APR 02 2017

DAYS 1 THROUGH 3...


...GREAT BASIN/ROCKIES...

A STRONG SHORTWAVE WILL MOVE INTO THE PACIFIC NORTHWEST THIS
MORNING...DIGGING SOUTHEAST INTO THE INTERMOUNTAIN WEST AND
SOUTHWEST INTO FRIDAY. HEAVY SNOWFALL IS LIKELY WITH THIS
SYSTEM...AS WILL HAVE PLENTIFUL MOISTURE TO WORK WITH...AS WELL AS
A PROLONGED PERIOD OF FAVORABLE LIFT AS THE WAVE DIGS AND CLOSES
OFF ACROSS THE SOUTHWEST. TO THE NORTH OF THE CLOSED LOW STRONG
EAST TO NORTHEASTERLY FLOW WILL HELP ENHANCE UPSLOPE FLOW INTO THE
EASTERN FACING SLOPES. THIS STRONG AND PERSISTENT UPSLOPE FLOW
COMBINED WITH ANOMALOUS PWAT VALUES AND DIFFLUENT FLOW ALOFT WILL
SET THE STAGE FOR SOME PRETTY IMPRESSIVE SNOWFALL TOTALS IN SPOTS.
ONE AREA OF INTEREST IS THE EASTERN SLOPES OF THE WIND RIVER RANGE
IN WY...WHERE 2 FEET PLUS IS PROBABLE. HEAVIER TOTALS ARE ALSO
LIKELY ALONG THE HIGHER TERRAIN OF CENTRAL UT INTO THE UNITA
MOUNTAINS...WITH LOCALLY 1-2 FEET.

THE FOCUS SHIFTS TO THE COLORADO FRONT RANGE BY FRIDAY AFTERNOON
INTO SATURDAY. STRONG UPSLOPE FLOW INTO THE FRONT RANGE WILL
RESULT IN 1-2 FEET PLUS OF SNOW ACROSS THE HIGHER TERRAIN. MORE
UNCERTAINTY INTO THE FOOTHILLS AND PLAINS. EXPECT THAT THE
DEEPENING CLOSED LOW WILL RESULT IN A STRONG ENOUGH NORTHEASTERLY
COLD PUSH TO CHANGE PRECIPITATION OVER TO SNOW...ESPECIALLY ACROSS
THE FOOTHILLS. QUESTION WILL BE HOW MUCH QPF FALLS AFTER THIS
TRANSITION. CURRENTLY THINK THE UPSLOPE FLOW WILL REMAIN STRONG
ENOUGH TO RESULT IN THE POTENTIAL FOR SIGNIFICANT SNOWFALL
ACCUMULATIONS INTO THE BOULDER TO DENVER CORRIDOR...ALTHOUGH
CONFIDENCE IS LOW AND WILL HAVE TO CONTINUE TO MONITOR TRENDS. IT
IS POSSIBLE THAT THE BETTER CONVERGENCE AND MOISTURE SHIFTS SOUTH
OF THE DENVER/BOULDER CORRIDOR BY THE TIME THE COLD AIR MOVES IN.
OUR IN HOUSE 5KM ENSEMBLE DOES SHOW AROUND  50% CHANCE OF 8"+
MAKING IT INTO THE FOOTHILLS..WITH LIGHTER AMOUNTS POSSIBLE INTO
THE PLAINS.

...UPPER GREAT LAKES...

LOW PRESSURE WILL CONTINUE TO MOVE TOWARDS THE SOUTHERN GREAT
LAKES TODAY...WITH A SHIELD OF PRECIPITATION TO THE NORTH OF THE
LOW IN AN AREA OF LOW LEVEL WAA AND UPPER LEVEL DIFFLUENCE. AS
THIS PRECIPITATION SPREADS NORTH IT WILL ENCOUNTER A COLDER
AIRMASS. DESPITE THIS BEING A SHORT TERM FORECAST...A DECENT
AMOUNT OF UNCERTAINTY UNFORTUNATELY EXISTS. THE HIGH RES 3KM NAM
AND WRF ARW SHOW THE PROFILE COOLING AND PRECIPITATION HEAVY
ENOUGH FOR DECENT SNOWFALL ACCUMULATIONS ACROSS NORTHERN MI TODAY
INTO TONIGHT. MEANWHILE THE GFS AND HRRR ARE WARMER AND LIGHTER
WITH QPF AMOUNTS. THE ECMWF AND GEM REGIONAL REPRESENT MORE MIDDLE
GROUND SOLUTIONS...WITH RAIN TRANSITIONING TO SNOW..MAINLY ACROSS
PORTIONS OF NORTHEAST MI. A LOW CONFIDENCE FORECAST...BUT THE 50TH
PERCENTILE SNOW FORECAST IS CURRENTLY AROUND 2-4 INCHES...WITH A
LOWER (10-20%) CHANCE OF 6".


...NORTHEAST...

AS THE COMPACT SHORTWAVE MOVES ACROSS THE OH VALLEY ON
FRIDAY...AND TOWARDS THE MID ATLANTIC COAST FRIDAY NIGHT...EXPECT
TO SEE SURFACE LOW REDEVELOPMENT OFF THE MID ATLANTIC COAST. THIS
LOW WILL MOVE SOUTH OF NEW ENGLAND ON SATURDAY. A SWATH OF HEAVY
SNOWFALL ON THE NORTHERN EDGE OF THE PRECIPITATION SHIELD IS
BECOMING INCREASINGLY LIKELY WITH THIS SYSTEM. SOME PRETTY LARGE
THERMAL FIELD DIFFERENCES WITHIN THE MODELS RESULT IN A LOW
CONFIDENCE FORECAST WITH REGARDS TO THE DETAILS. THE 12KM AND 3KM
NAM BOTH SHOW A MUCH MORE PRONOUNCED WARM NOSE THAN THE GFS/ECMWF
SOLUTIONS. THE NAM USUALLY HANDLES WARM NOSES ALOFT THE BEST OF
ALL OUR GUIDANCE. HOWEVER IN THIS CASE THE NAM MAY BE A TAD TOO
FAR NORTH WITH THE LOW TRACK...WHICH COULD BE CAUSING IT TO BE A
BIT TOO WARM ALOFT AS WELL. ON THE OTHER HAND THE GFS CONSISTENTLY
FAILS TO PROPERLY HANDLE WARM LAYERS ALOFT...AND THUS DOWNPLAYED
ITS MUCH COLDER SOLUTION FOR NOW. THOUGHT A GOOD MIDDLE GROUND
SOLUTION WAS A COMPROMISE OF THE NAM/ECMWF AND SREF. THIS RESULTS
IN A 6-10" SWATH OF SNOWFALL FROM NORTHEAST NY INTO MUCH OF VT AND
NH AND SOUTHWEST MAINE...WITH AMOUNTS DECREASING RAPIDLY AS YOU
HEAD SOUTH INTO MA WHERE SLEET AND RAIN SHOULD BE PREDOMINATE. DO
THINK WE WILL PROBABLY SEE A SWATH OF 12" PLUS WITH THIS
SYSTEM...HOWEVER THE EXACT LOCATION OF THESE HIGHER AMOUNTS IS
QUITE UNCERTAIN. SOUTHERN VT/NH HAS THE MOST UPSIDE FOR HIGHER
AMOUNTS (GIVEN THE HIGHER QPF AS YOU GO SOUTH) IF THE PROFILES CAN
STAY ISOTHERMAL. HOWEVER CURRENTLY ANTICIPATING AT LEAST SOME
SLEET MIXING IN...AND IF THE NAM COMES TO FRUITION IT VERY COULD
COULD BE ALMOST ALL SLEET ACROSS SOUTHERN VT/NH. NEEDLESS TO SAY A
LOW CONFIDENCE FORECAST WITH THE DETAILS...ALTHOUGH HIGH
CONFIDENCE IN A LATE REASON SIGNIFICANT WINTER STORM IMPACTING
PORTIONS OF THE AREA IN SOME ASPECT.

THE WARM NOSE ALOFT ALSO MEANS A FREEZING RAIN POTENTIAL. POCKETS
OF GREATER THAN A QUARTER INCH OF ICING ARE POSSIBLE FROM EASTERN
NY INTO CENTRAL/WESTERN MA. SURFACE TEMPERATURES WILL PROBABLY
ONLY BE RIGHT AROUND FREEZING WHICH MAY HELP LIMIT ICE ACCRETION
TO A DEGREE. HOWEVER SOME SIGNS THAT WE COULD SEE A SIGNIFICANT
AMOUNTS OF QPF THAT COULD FALL AS FREEZING RAIN...WITH PERSISTENT
LOW LEVEL EASTERLY FLOW KEEPING LOW LEVEL COLD AIR LOCKED IN. AS
ALWAYS WILL BE BORDERLINE BETWEEN SLEET AND FREEZING RAIN ACROSS
EASTERN NY INTO PORTIONS OF SOUTHERN NEW ENGLAND. BUT THE MIX
THREAT DOES APPEAR TO BE INCREASING AND WORTH KEEPING AN EYE ON.
WPC FREEZING RAIN AREAS FOLLOWED CLOSE TO THE 3KM NAM AND SREF
MEAN THROUGH THE PERIOD.

CHENARD



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