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Winter Weather Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0856Z Feb 24, 2015)
 
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Abbreviations and acronyms used in this product
 
Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
356 AM EST TUE FEB 24 2015

VALID 12Z TUE FEB 24 2015 - 12Z FRI FEB 27 2015


...SOUTHERN ROCKIES ACROSS SRN PLAINS/LOWER MS VALLEY TO THE
SOUTHERN APPALACHIANS/SOUTHEAST/CAROLINAS/VA...

ON DAY 1...
AN AREA OF 700 MB CONVERGENCE OUT AHEAD OF THE CLOSED LOW PROVIDES
IMPETUS FOR CONTINUING SNOWS IN THE RANGES OF CENTRAL TO WESTERN
NM TO ADJACENT AZ. WITH THE LOW MOVING EAST ON TO THE HIGH PLAINS
TONIGHT...LOW LEVEL CONVERGENCE/UPPER DIVERGENCE WEAKENS AND THE
SOUTHERN ROCKIES SNOW COVERAGE AND INTENSITY WINDS DOWN. 
CONVERSELY...COOLING WITH HEIGHT FALLS TONIGHT ALLOWS A TRANSITION
FROM RAIN TO SNOW IN PARTS OF NORTHERN TX...WHERE A FEW INCHES OF
SNOW ARE POSSIBLE. 

AN AREA OF LIGHT SNOW DEVELOPING OVER THE SOUTHERN APPALACHIANS
TONIGHT MOVES EAST ACROSS INTERIOR NC TODAY WITH LIGHT SNOW
EXPECTED TO FALL IN INTERIOR EASTERN NC. LOW LEVEL COLD AIR HAS
ADVECTED INTO EASTERN NC AND GFS/NAM FORECAST SOUNDINGS SHOW A
WARM NOSE OF ABV FREEZING TEMPERATURES NEAR 850 MB THAT RESULTS IN
FALLING SNOW MELTING AND RE-FREEZING AT THE SURFACE. THIS FREEZING
RAIN OCCUR OVER MUCH OF EASTERN NC TO ADJACENT SC.  THE MODELS
HAVE WARMED FURTHER SOUTH IN SC SO THE ICE THREAT HAS EASED IN
SOUTHERN SC.

ON DAY 2...
THE 12-00Z UKMET AND ECMWF AND 12Z ECMWF ENSEMBLE MEAN SHOW A WELL
DEFINED 700 MB WAVE CROSSING THE LOWER MS VALLEY AND THE
SOUTHEAST/SOUTHERN APPALACHIANS AND TOWARDS THE MID ATLANTIC
COAST.  SNOW IS EXPECTED TO DEVELOP ACROSS SOUTHEAST AR/NORTHERN
MS AS UPPER DIVERGENCE/LOWER CONVERGENCE MAXIMA DEVELOP IN A
COUPLED JET REGION AT 300 MB.  THIS COUPLET MOVES ACROSS THE
SOUTHERN APPALACHIANS/NC/SOUTHERN VA.
THIS SYSTEM SHOULD BEGIN TO TAP A SURGE IN LL MOISTURE. THIS
MOISTURE WILL GET LIFTED OVER A LOW-MID LEVEL FRONT TO PRODUCE A
BAND OF SNOW AND MIXED PRECIPITATION TYPES.

A DECENT AMOUNT OF SPREAD EXISTS WITH THE GUIDANCE. THE 00Z NAM IS
A CLEAR OVER-AMPLIFIED SOLUTION AT 700 MB AND IS THE ONLY MODEL
WITH A CLOSED LOW IN THE ARKLATEX.  THIS RESULTED IN GREATER SNOW
AMTS AND THE AXIS EXTENDING FURTHER NORTH THAN THE OTHER MODELS. 
THE 00Z GFS FORECAST A FLATTER 700 MB WAVE AND THE RESULTANT SNOW
AXIS WAS FURTHER SOUTH IN THE SOUTHEAST/SRN APPALACHIANS/NC.  LESS
WEIGHTING WAS GIVEN TO THE GFS/NAM ...WITH THE 21Z SREF MEAN
SUPPORTED BY THE ECMWF ENSEMBLE MEAN AS WELL AS THE OPERATIONAL
ECMWF/UKMET RUNS. THE 03Z SREF MEAN AMPLIFIED THE 700 MB WAVE EVEN
MORE AND MOVED THE QPF AXIS FURTHER NORTH IN THE MID ATLANTIC..SO
WE WILL FOLLOW LATER GUIDANCE TO SEE IF THIS TRANSLATES INTO A
TREND IN THE MODELS.


...NORTHERN ROCKIES/NORTHERN PLAINS/MO VALLEY/MID MS VALLEY...

LIGHT SNOW IS EXPECTED IN THE NORTHERN ROCKIES ON DAY 1 AND EARLY
DAY 2 AS A COUPLED UPPER JET CROSSES THE REGION BETWEEN 2 SEPARATE
JET MAXIMA STRADDLING THE REGION...RESULTING IN UPPER DIVERGENCE
ACROSS THE AREA. LIGHT SNOW DEVELOPS NEAR A 700 MB JET MAX
CROSSING NORTHWEST TO SOUTHEAST ACROSS NORTHWEST ND TO SOUTHEAST
SD ON DAY 2.  AS THE LOW LEVEL WAVE AMPLIFIES AND A BETTER DEFINED
700 MB CONVERGENCE MAXIMA DEVELOPS...SNOW SHOULD INCREASE IN
COVERAGE/INTENSITY FROM SOUTHEAST SD ACROSS WESTERN IA TO NORTHERN
MO.  THE MODELS WEAKEN THE 700 MB WAVE SO LIGHTER SNOW AMOUNTS ARE
EXPECTED ON DAY 3.  THERE ARE STILL UNRESOLVED WEST-EAST
DIFFERENCES ON THE TRACK OF THE SFC LOW AND 850-700 BM WAVE WHICH
GIVEN  WEST-EAST VARIABILITY ON THE DAY 2 SNOW AXIS IN EASTERN NE
TO IA.  THE QPF AMOUNTS HAVE INCREASED SINCE YESTERDAY AND THUS
THE SNOW AMOUNTS HAVE TOO. MANUAL PROGS SUED A CONSENSUS APPROACH
AND MAINTAINED CONTINUITY AS WELL. 

...CO/NM ROCKIES DAYS 2-3...

ON DAY 2...IN CONFLUENT NORTHWEST FLOW ALOFT...A 300 MB JET MAXIMA
MOVES SOUTH OUT OF WY INTO WESTERN CO.  UPPER DIVERGENCE IN THE
LEFT EXIT REGION OF THE JET AND 700 MB CONVERGENCE LEADS TO SNOW
DEVELOPING IN THE RANGES OF NORTHERN CO. ON DAY 3 A SEPARATE JET
MAXIMA EXTENDS ACROSS AX INTO NM.  UPPER DIVERGENCE MAXIMA
DEVELOPS IN NM... WHERE A 700 MB WAVE INDUCES LOW LEVEL
CONVERGENCE MAXIMA AND WARM/MOISTURE ADVECTION. SNOW IS EXPECTED
AT HIGHER ELEVATIONS MOVING OUT OF CO AND ACROSS THE RANGES OF
NORTHERN NM...INCLUDING THE SANGRE DE CRISTOS. IN SOUTHERN NM
INITIAL TEMPS ARE TOO WARM FOR SNOW WITH LATER HEIGHTS AND COOLING
ALLOWING A TRANSITION TO SNOW TO OCCUR. MANUAL PROGS GAVE MORE
WEIGHTING TO THE THE GFS/ECMWF/SREF MEAN QPF AND TEMPERATURE
PROFILES.

...NEW ENGLAND...

A NRN STREAM SHORT WAVE TROUGH MOVING ACROSS THE GREAT LAKES TUES
SUPPORTS UPPER DIVERGENCE MAXIMA OVER SOUTHEAST NEW ENGLAND WITH
SNOW DEVELOPING ONCE THE UPPER JET MAXIMA INDUCES LOW LEVEL
CYCLOGENESIS.  THIS LOW FORMING WED MORNING WILL DEEPEN RAPIDLY
WHILE LIFTING TO THE NORTH. THIS SHOULD RESULT IN DEFORMATION WITH
A BAND OF SNOW IN THE MID LEVEL DEFORMATION BAND WEST OF THE
CIRCULATION ACROSS EASTERN MAINE.
THERE IS DECENT AGREEMENT AMONG THE UKMET/ECMWF/NAM /GFS WITH
SLIGHT TIMING DIFFERENCES ON THE LOW PROGRESSION AND SNOW TIMING. 
MANUAL PROGS TOOK A CONSENSUS APPROACH WITH NO SOLUTION WEIGHTED
MORE THAN THE OTHER FOR THE DETERMINISTIC/PROBABILISTIC FORECASTS.

THE PROBABILITY OF 0.25 OR GREATER INCHES OF ICING IS LESS THAN 10
PERCENT DAYS 2-3.

PETERSEN