Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 162016
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
315 PM EST TUE DEC 16 2014

VALID 00Z WED DEC 17 2014 - 00Z SAT DEC 20 2014


DAYS 1 THROUGH 3...


...PACIFIC NORTHWEST/NORTHERN ROCKIES...

A SERIES OF SHORT WAVES IN THE FAST MID LEVEL FLOW AFFECT THE
PACIFIC NORTHWEST AND NORTHERN ROCKIES DURING THE NEXT THREE DAYS.
THERE WAS FAIRLY GOOD MODEL AGREEMENT WITH RESPECT TO THE TIMING
OF THE SHORT WAVES...SO THE QPF PORTION OF THE FORECAST WAS BASED
ON THE MOST RECENT WPC QPF. THE THERMAL PORTION OF THE FORECAST
WAS BASED ON A BLEND OF THE 12Z GFS/09Z SREF MEAN/00Z ECMWF.

THE FIRST SHORT WAVE CROSSES THE PACIFIC NORTHWEST AND NORTHERN
ROCKIES ON DAY 1. SNOW LEVELS HOVER BETWEEN 5000 AND 6000 FEET
ACROSS THE GRAND TETONS OF NORTHWEST WY. THERE SHOULD BE ENOUGH
MOISTURE AND LIFT WITH THE SHORT WAVE TO SUPPORT 3 TO 6 INCHES OF
SNOWFALL. A SECOND...WEAKER SHORT WAVE CROSSES THE NORTHERN
ROCKIES DURING DAY 2. MOISTURE IS THE LIMITING FACTOR FOR
SNOWFALL...BUT UPSLOPE AND THE WEAK SYNOPTIC LIFT SHOULD BE ENOUGH
TO MAKE THE MOST OF THE MOISTURE IN THE COLUMN...AND LOCAL 3 INCH
SNOWFALL AMOUNTS ARE POSSIBLE IN THE BITTERROOTS OF ID AND THE
GRAND TETONS IN WY.

DEEPER MOISTURE AHEAD OF A STRONGER SHORT WAVE IN THE FAST MID
LEVEL FLOW CROSSES THE PACIFIC NORTHWEST LATE ON DAY 3. UNLIKE THE
PREVIOUS TWO DAYS...MOISTURE WILL NOT BE AN ISSUE FOR SNOWFALL
POTENTIAL. HOWEVER...DIFFERENTIAL WARM AIR ADVECTION AHEAD OF THE
SHORT WAVE HELPS BUMP UP SNOW LEVELS TO BETWEEN 6000 AND 7000 FEET
OVER OR AND WA. THE COMBINATION OF LIFT AND MOISTURE SUPPORTS
SWATHES OF 4 TO 8 INCHES OF SNOWFALL ACROSS THE WA AND OR
CASCADES. THESE VALUES ARE SUPPORTED WELL BY THE OUTPUT OF THE
LATEST ECMWF ENSEMBLE.


...CA/GREAT BASIN/SOUTHWEST/CENTRAL AND SOUTHERN ROCKIES...

A STRONG SHORT WAVE COMING ASHORE ON DAY 1 CROSSES THE GREAT BASIN
AND SOUTHWEST STATES LATE ON DAY 1 AND DAY 2...BEFORE IT EXITS THE
REGION ON DAY 3. THE NEXT SHORT WAVE AFFECTS NORTHERN CA DURING
DAY 3. THE ECMWF HAS BEEN HANDLING THE TIMING OF THE SHORT WAVES
WELL DURING THE PAST FEW CYCLES...SO THE THERMAL PORTION OF THE
FORECAST WAS BASED ON A BLEND OF THE 12Z GFS/00Z ECMWF. THE QPF
PORTION OF THE FORECAST WAS GLEANED FROM THE MOST RECENT WPC QPF.

A STRONG SHORT WAVE ROTATING ONSHORE ACROSS NORTHERN CA FUNNELS
DEEPER MOISTURE INTO NORTHERN AND CENTRAL CA DURING THE FIRST PART
OF DAY 1. THE LOW LEVEL FLOW FOCUSES THE MOISTURE ON THE HIGHER
TERRAIN...PRODUCING AN UPSLOPE FLOW ACROSS THE SIERRA NEVADA
RANGE. SNOW LEVELS ACROSS THIS RANGE SHOULD HOVER BETWEEN 5000 AND
6000 FEET THROUGH THE EVENT. THE COMBINATION OF SYNOPTIC SCALE
LIFT AND UPSLOPE FLOW SHOULD PRODUCE AN AXIS OF 6 TO 10 INCHES OF
SNOWFALL ACROSS THE SIERRA NEVADA RANGE.

AS THE BEST LIFT DROPS SOUTH ACROSS SOUTHERN CA...SNOWFALL IS
EXPECTED IN THE HIGHER TERRAIN THERE AS WELL. SNOW LEVELS DROP TO
AROUND 5500 FEET ACROSS THE SAN BERNADINO AND SAN JACINTO
MOUNTAINS. GIVEN THE MOISTURE AVAILABILITY AND SYNOPTIC SCALE
ASCENT...LOCAL SNOWFALL AMOUNTS IN EXCESS OF 4 INCHES ARE POSSIBLE
IN THE HIGHER TERRAIN.

THE BEST LIFT WITH THE SHORT WAVE ENERGY DROPS INTO THE THE GREAT
BASIN AND SOUTHWEST STATES DURING THE SECOND HALF OF DAY 1. SNOW
LEVELS HERE HOVER BETWEEN 5500 AND 6500 FEET...ESPECIALLY ACROSS
THE MOUNTAINS OF AZ AND NM. THE MOST ORGANIZED LIFT CROSSES THE
DRAGOON AND CHIRICHUA RANGES IN AZ...AND THE SAN JUAN MOUNTAINS IN
SOUTHWEST CO.  ABOVE 6000 FEET IN THESE RANGES...4 TO 6 INCHES OF
SNOWFALL IS EXPECTED. AS THE SHORT WAVE EXITS THE SOUTHERN ROCKIES
EARLY ON DAY 3...THERE SHOULD BE ENOUGH RESIDUAL MOISTURE AND LIFT
TO SUPPORT 3 TO 6 INCHES OF SNOWFALL ACROSS THE CO ROCKIES...AND
LESSER AMOUNTS ACROSS THE CHUSKA MOUNTAIN RANGE OF SOUTHWEST NM.



...CENTRAL PLAINS/MID MS VALLEY...

ISENTROPIC LIFT FOCUSES MOISTURE ON AN ELEVATED FRONT STRETCHING
ACROSS THE CENTRAL PLAINS INTO THE MID MS VALLEY...MAINLY FOR DAY
2. THIS SHOULD RESULT IN AN AREA OF ACCUMULATING SNOW IN THE
ABOVEMENTIONED LOCATIONS. THE ECMWF HAS BEEN THE MOST CONSISTENT
WITH THE MID LEVEL TROUGH AND THE LIFT AHEAD OF IT...SO THE
THERMAL FIELDS WERE BASED MAINLY ON A BLEND OF THE 00Z ECMWF/12Z
GFS. THE QPF PORTION OF THE FORECAST WAS DERIVED MAINLY FROM THE
MOST RECENT WPC QPF.

A SHORT WAVE TRACKING OUT OF THE SOUTHWEST STATES CROSSES THE
CENTRAL PLAINS AND MID MS VALLEY ON DAY 2. AHEAD OF THE SHORT
WAVE...THE LOW LEVEL FLOW TRANSPORTS MOISTURE NORTH ALONG THE I295
ISENTROPIC LEVEL...WHICH SPREADS OUT ALONG A LOW LEVEL BOUNDARY
EXTENDING ACROSS KS INTO MO AND SOUTHERN IL. THE COLUMN IS DRY TO
START...AS COLD AIR IS FED SOUTHWARD INTO THE REGION FROM SURFACE
HIGH PRESSURE OVER THE UPPER MIDWEST. AS THE PRECIPITATION
STARTS...IT COULD FALL IN THE FORM OF LIGHT RAIN ACROSS SOUTHEAST
KS/SOUTHERN MO. EVAPORATIVE COOLING SHOULD CHANGE THE RAIN TO
SNOW...AND THE BEST LIFT ALONG THE LOW LEVEL BOUNDARY OCCURS
BETWEEN 18/06Z AND 18/18Z.

AFTER THE CHANGE TO SNOW...THE COLUMN BECOMES ISOTHERMAL EXTENDING
FROM EASTERN CENTRAL KS THROUGH CENTRAL MO INTO SOUTHERNMOST IL.
THERE IS ENOUGH LIFT AND MOISTURE TO SUPPORT A SWATH OF 2 TO 4
INCHES OF SNOWFALL ACROSS THE ABOVEMENTIONED AREAS. AS THE BEST
WARM AIR ADVECTION ENDS AFTER 18/18Z...THE COLUMN DRIES OUT IN THE
MID LEVELS. THE COLUMN LOSES ITS ABILITY TO MAKE SNOWFLAKES AS THE
WARM AIR ADVECTION ENDS...SO LIGHT ICING IS POSSIBLE AFTER THE
SNOWFALL ENDS. THE LATEST ECMWF ENSEMBLE OUTPUT SHOWS THE
POTENTIAL FOR 4+ INCHES OF SNOW IN THIS AXIS...BUT AT THIS
TIME...THIS APPEARS TO BE A LOW PROBABILITY EVENT.


...MID ATLANTIC/NEW ENGLAND...

LIGHT ICING IS EXPECTED ACROSS PORTIONS OF THE INTERIOR MID
ATLANTIC AND NORTHERN NEW ENGLAND DURING THE FIRST PART OF DAY 1.
LOW PRESSURE FORMING OVER SOUTHEAST NEW ENGLAND WILL TRACK NORTH
ACROSS THE GULF OF ME ITO THE MARITIMES LATE ON DAY 2 AND EARLY ON
DAY 3. THERE IS SOME MODEL SPREAD WITH THE TRACK OF THE LOW...WITH
THE 12Z NAM PROBABLY TOO FAR WEST (AND WARMEST IN THE LOWER AND
MID LEVELS BECAUSE OF IT)...AND THE MOST RECENT UKMET (PROBABLY
TOO FAR EAST). THE QPF PORTION OF THE FORECAST WAS BASED MAINLY ON
THE MOST RECENT WPC QPF. IN AN ATTEMPT TO MITIGATE SOME OF THE
THERMAL DIFFERENCES...ESPECIALLY OVER NORTHERN ME...THE THERMAL
PORTION OF THE FORECAST WAS BASED ON A BLEND OF THE 12Z GFS/00Z
ECMWF.

AHEAD OF A STRONG SHORT WAVE RIDING THROUGH THE MID LEVEL FLOW
TOWARD THE MID ATLANTIC STATES DURING THE FIRST HALF OF DAY
1...LOW LEVEL MOISTURE POOLS ALONG THE TERRAIN ACROSS EASTERN WV
AND ACROSS NORTHERN NEW ENGLAND. MODEL SOUNDINGS ACROSS EASTERN WV
SHOWED MOISTURE DEEP ENOUGH TO SUPPORT A STRONG INVERSION...WITH
SURFACE TEMPERATURES CLOSE TO FREEZING IN THE SHELTERED LOCATIONS.
THIS COULD RESULT IN SPOTTY 0.01 INCH ICING IN PLACES. FURTHER
NORTH ACROSS NORTHERN NEW ENGLAND...MODEL SOUNDINGS SHOWED
MOISTURE THROUGH ABOUT 4000 FEET...THEN DRYING ABOVE. THIS
SUPPORTS LIGHT FREEZING RAIN/FREEZING DRIZZLE...AS THERE ARE NO
ICE NUCLEI IN THE CLOUD FOR SNOWFLAKE PRODUCTION. THE RESULTING
FREEZING RAIN/FREEZING DRIZZLE COULD SUPPORT LOCAL AREAS OF 0.10
INCHES OF ICE...ESPECIALLY ACROSS NORTHERN ME. THE COLUMN STARTS
MOISTENING AFTER 17/12Z...AND BECOMES SUFFICIENTLY MOIST TO CREATE
SNOWFLAKES IN THE COLUMN AFTER 17/18Z ACROSS THE WESTERN ME
MOUNTAINS...AS WELL AS NORTHERN ME.

AS SURFACE LOW PRESSURE FORMS ALONG THE SOUTHEAST NEW ENGLAND
COAST DURING DAY 2...THE LOW LEVEL FLOW AHEAD OF IT BECOMES
EASTERLY...AND THIS CREATES BOUNDARY LAYER ISSUES FOR ALL BUT THE
HIGHER TERRAIN OF NH AND NORTHERN AND WESTERN ME BY THE END OF DAY
2. MODEL SOUNDINGS ACROSS THE MOUNTAINS AND NORTHERN ME SHOULD
THAT PROFILES BECOME ISOTHERMAL JUST BELOW FREEZING...SUGGESTING
AN ALL SNOW EVENT HERE ONCE THE SURFACE SYSTEM DEEPENS AND THE
LOWER LEVELS START COOLING INTO EARLY DAY 3. THE BEST LIFT OCCURS
IN A FAIRLY NARROW FRONTOGENTICALLY FORCED RIBBON ACROSS NORTHERN
ME (CENTERED ON 18/00Z). THE BEST LIFT SITS NEARLY STATIONARY FOR
A TIME ACROSS THE ROOFTOP OF ME...BEFORE EXITING INTO QUEBEC AND
NEW BRUNSWICK AFTER 18/12Z. THIS SCENARIO SUPPORTS A SWATH OF 6 TO
12 INCHES OF SNOWFALL STRETCHING FROM THE WESTERN ME MOUNTAINS
ACROSS THE ROOFTOP OF ME...WITH THE HIGHEST AMOUNTS NEAR KCAR.
THERE ARE A NUMBER OF MEMBERS OF THE LATEST ECMWF ENSEMBLE OUTPUT
THAT SUPPORT 12+ INCHES OF SNOWFALL ACROSS NORTHERNMOST ME...BUT
THIS MAY BE OVERDONE GIVEN THE ECMWF QPF VALUES.

FURTHER SOUTH...THE LOW LEVEL EASTERLY FLOW MAKES THE BOUNDARY
LAYER TOO WARM FOR SNOWFALL OUTSIDE OF THE HIGHER TERRAIN LATE ON
DAY 2 AND EARLY DAY 3.  AS THE LOW LEVEL FLOW BACKS TO THE WEST
NORTHWEST BEHIND THE PASSING SURFACE LOW...THE BOUNDARY LAYER
COOLS ENOUGH FOR SNOWFALL ALMOST TO THE NORTHERN NEW ENGLAND
COAST. THE UPSLOPE FLOW COULD RESULT IN A FEW INCHES OF SNOW
ACROSS THE FAVORED UPSLOPE LOCATIONS IN VT/NH/WESTERN ME.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

HAYES

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