Quantitative Precipitation Forecast
Issued by NWS

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FOUS11 KWBC 220845
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
344 AM EST THU JAN 22 2015

VALID 12Z THU JAN 22 2015 - 12Z SUN JAN 25 2015


...DAYS 1 TO 3...

...SOUTHERN ROCKIES/SOUTHWEST/SOUTHERN HIGH PLAINS/TENNESSEE
VALLEY/APPALACHIANS/MID ATLANTIC/NEW ENGLAND...

A POTENTIALLY HIGH IMPACT STORM FIRST AFFECTING THE SOUTHERN
ROCKIES/SOUTHWEST AND SOUTHERN PLAINS ON FROM WEDNESDAY INTO
THURSDAY BECOMES A SNOW/ICE THREAT FOR THE TENNESSEE
VALLEY/APPALACHIANS/MID ATLANTIC FROM LATE THURSDAY INTO FRIDAY.

ON THURSDAY INTO FRIDAY/DAY 1...RESIDUAL SNOW WILL CONTINUE ACROSS
SOUTHEASTERN NEW MEXICO AND WESTERN TEXAS WITH A MOSTLY LOW TO
MODERATE PROBABILITY OF GREATER THAN 4 INCHES OF SNOW IN PARTS OF
THE AREA.  A COMBINATION OF PRIMARILY THE 12Z ECMWF/THE 18Z BIAS
CORRECTED SUPERENSEMBLE AND THE 12Z UKMET WITH SMALL CONTIBUTIONS
FROM THE 12Z ECMWF ENSEMBLE MEAN AND THE 21Z SREFMEAN WAS USED TO
GENERATE THE INITIAL BLEND FOR THE SNOWFALL.

ON DAY 2/FRIDAY INTO SATURDAY MORNING...THE UPPER TROUGH MOVES
EASTWARD AND THEN BEGINS TO EVOLVE INTO AN INCREASINGLY NEUTRAL
AND THEN NEGATIVELY TROUGH MOVING ALONG THE GULF COAST BEFORE
STARTING TO HEAD NORTHEASTWARD ALONG THE ATLANTIC COAST. A SURFACE
LOW WILL DEVELOP OVER THE WESTERN GULF OF MEXICO WHILE AN AREA OF
CONFLUENCE OVER THE EASTERN US IS ASSOCIATED WITH RISING PRESSURES
ALONG THE EASTERN US AHEAD OF THIS SYSTEM...ALLOWING COLDER AIR TO
PRECEDED THE STORM BEFORE MOVING UP ALONG THE EAST COAST...THE
CONFLUENCE SUBSIDES AND HIGH PRESSURE MOVES OFF THE EAST COAST.
MIXED PRECIPITATION CHANGING TO SNOW APPEARS TO BE THE QPF
TRANSITION ACROSS THE TENNESSEE VALLEY ON DAY 2 OVER
TENNESSEE/KENTUCKY AS WELL AS OVER THE APPALAHIANS FROM WESTERN
NORTH CAROLINA NORTHWARD ACROSS WESTERN VIRGINIA INTO WEST
VIRGINIA. IN THE HIGHER ELEVATIONS OF EASTERN WEST
VIRGINIA/WESTERN VIRGINIA...THERE IS A HIGHER CHANCE THAT MOST OF
THE PRECIPITATION COULD BE IN THE FORM OF SNOW. IN GENERAL...THERE
IS A LOW TO MODERATE PROBABILITY FOR 4 OR MORE INCHES ACROSS
PORTIONS OF THE APPALACHIANS AS WELL AS A LOW PROBABILITY FOR
GREATER THAN .25 INCHES OF FREEZING RAIN.

DAY 3/SATURDAY INTO SUNDAY MORNING IS A BIT MORE PROBLEMATIC WITH
A HIGHER DEGREE OF UNCERTAINTY ESPECIALLY WITH REGARD TO FROZEN
QPF AMOUNTS AND LOCATION. WITH THE EXCEPTION OF PORTIONS OF THE
CENTRAL APPALACHIANS AND PARTS OF CENTRAL TO NORTHERN NEW
ENGLAND...TRANSITIONS FROM RAIN/FREEZING RAIN/SNOW AND VICE VERSA
COULD CHARACTERIZE A FAIRLY LARGE AREA FROM THE MID ATLANTIC TO
SOUTHERN NEW ENGLAND.

A DEEPENING SURFACE LOW NEAR THE CAROLINA COAST ON SATURDAY
MORNING IS EXPECTED TO CONTINUE NORTHEASTWARD AND DEEPEN QUITE
RAPIDLY. WHILE PRECIPITATION IN PARTS OF THE MID ATLANTIC COULD
START AS SNOW/FREEZING RAIN...IT COULD THEN CHANGE TO RAIN AND
THEN POSSIBLY BACK TO SNOW AS THE CYCLONE CONTINUES TO DEEPEN AND
A SIGNIFICANT COMMA HEAD IS DEPICTED TO DEVELOP AS THE TROUGH
AMPLIFIES AND LIFTS NORTHEASTWARD.  THE CHANGEOVER BACK TO SNOW IS
DEPENDENT ON THE AMOUNT OF COOLING ASSOCIATED WITH STRONG LIFT
POSSIBLE IN THE COMMA HEAD AND THE CHANGEOVER BACK TO SNOW COULD
BE MORE PRONOUNCED THE FARTHER NORTH ALONG THE EAST COAST AS ONE
TRAVELS BETWEEN WASHINGTON AND BOSTON.  THERE ARE STILL MANY
VARIABLES IN HOW THIS SYSTEM ULTIMATELY DEVELOPS.

UNCERTAINTIES IN THE TRACK OF THE SYSTEM MAKE THE FORECAST QUITE
UNCERTAIN ON DAY 3 SINCE THE RECENT VERSIONS OF THE GFS/GEFS
MEAN/NAM/UKMET KEEP THE TRACK FARTHER EAST THAN OTHER MODELS AND
SUBSEQUENTLY...LOWER QPF AMOUNTS ALONG THE COAST FROM NEW JERSEY
TO NEW ENGLAND.  HOWEVER...A CLOSER TRACK AND HIGHER QPF AMOUNTS
ARE INDICATED BY THE 12Z ECMWF/ENSEMBLE MEANS.  A COMPROMISE
SOLUTION BETWEEN THE TWO CAMPS WAS USED TO BRIDGE THE DIFFERENCES.
THIS RESULTS IN SNOWFALL ACCUMULATIONS BETWEEN THE LOW AND
MODERATE ENDS WITH 4 TO 8 INCH AMOUNTS POSSIBLE FROM NEW YORK CITY
TO BOSTON BUT IS HIGHLY UNCERTAIN AT THIS STAGE OF THE FORECAST.

THE BIG QUESTION REVOLVES AROUND THE THERMAL FIELDS ASSOCIATED
WITH THE SYSTEM.  COLD AIR MAY BE IN PLACE AT THE BEGINNING OF THE
EVENT SOME SNOW/SLEET AND SOME FREEZING RAIN.  WITHOUT AN ARCTIC
HIGH IN PLACE...A TRANSITION TO RAIN ACROSS THE MID ATLANTIC
REGION AND NORTHEASTERN STATES WILL BE POSSIBLE.  LOOKING AT THE
AMOUNT OF QPF PER PTYPE SHOWS MOST PLACES HAVE SOME POTENTIAL FOR
RAIN OR SNOW WITH THE BEST FOR FREEZING PCPN POTENTIAL IN THE
HIGHER ELEVATION OF THE APPALACHIAN CHAIN.


THE PROBABILITY OF 0.25 INCHES OF ICE IS LESS THAN 10 PERCENT ON
DAY 1.

KOCIN

$$





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