Quantitative Precipitation Forecast
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FOUS11 KWBC 250903
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
402 AM EST WED FEB 25 2015

VALID 12Z WED FEB 25 2015 - 12Z SAT FEB 28 2015


...RED RIVER VALLEY TO LOWER MS VALLEY/SOUTHEAST/SOUTHERN
APPALACHIANS/MID-ATLANTIC...

AN UPPER LEVEL WAVE WILL PROGRESS STEADILY ACROSS THE SRN
PLAINS/LOWER MS VALLEY/SOUTHEAST ON WED AND ACROSS THE SRN
APPALACHIANS TOWARDS THE MID ATLANTIC THU MORNING. OUT AHEAD OF
THE WAVE...A COUPLED UPPER JET STRUCTURE PRODUCES AN UPPER
DIVERGENCE/LOWER CONVERGENCE MAXIMA.  A SURGE IN LOW LEVEL
MOISTURE FROM THE GULF OF MEXICO WITH MOISTURE FLUX ANOMALIES
EXTENDING ACROSS SRN MS/AL/GA/CAROLINAS.
THE MOISTURE GETS LIFTED IN TO THE MID LEVEL DEFORMATION
AXIS/FRONTAL ZONE TO PRODUCE SNOW IN NORTHERN LA/SRN AR EAST
ACROSS NORTHERN MS/AL/GA TO NORTHEAST NC AND SOUTHERN VA.  A
PRECIP TYPE TRANSITION ZONE SOUTH OF THIS AREA CUTS DOWN ON
AMOUNTS. ON THE OTHER END...HOW FAR NORTH PRECIP EXTENDS IS THE
PROBLEM FOR CENTRAL AR TO CENTRAL TN/ALONG THE WV/VA BORDER AND
THE DELMARVA.

THE MODELS/ENSEMBLES STILL DIFFER ON THE 850-700 MB WAVE
TIMING/AMPLITUDE AND PHASING WITH THE NORTHERN STREAM.  THE 00Z
NAM/21Z SREF MEAN REMAIN ON THE MORE AMPLIFIED END OF THE SOLUTION
SUITE...PARTICULARLY THE 00Z NAM AND ITS STRONGER 700 MB LOW IN LA
EARLY TODAY.  THE SREF MEAN AND 00Z NAM QPF IS HIGHER THAN THE
GLOBAL MODELS ALONG THE MAIN AXIS OF SNOW AND EXTENDS THE THREAT
AREA FURTHER NORTH AND IS THUS IS GIVEN LESS WEIGHT.  THE 00Z GFS
AND ECMWF HAVE TRENDED NORTH WITH THE PRECIP SHIELD IN THE
SOUTHEAST TO THE MID ATLANTIC SO GRADUAL CONSOLIDATION OF
SOLUTIONS IS DEVELOPING.  MORE WEIGHTING WAS GIVEN TO THE 00Z GEFS
MEAN AND 12Z ECMWF ENSEMBLE MEAN AS A CONSENSUS PAIRING IN BETWEEN
THE NAM/SREF AND GFS/ECMWF AND GOOD AGREEMENT/OVERLAP ON THE SNOW
AMOUNTS AND AXIS.

...MO VALLEY TO MID MS VALLEY DAY 1...

AN 850-700 MB WAVE PROGRESSES ACROSS UPPER MIDWEST AND MID MS
VALLEY WED-THURS.  THETA-E ADVECTION IN THIS LAYER COMBINES WITH
FRONTOGENESIS TO PRODUCE SNOW ACROSS
SOUTHEAST SD/SOUTHWEST MN DOWN ACROSS IA AND WRN IL/NRN MO.  THERE
ARE STILL RESIDUAL DIFFERENCE ON THE QPF WITH RECENT TRENDS
TOWARDS HIGHER AMOUNTS IN THE EASTERN CLUSTER IN IA/MO/IL.
CONSEQUENTLY...SLIGHTLY GREATER WEIGHTING WAS GIVEN TO THE RECENT
TRENDS AND GUIDANCE ACROSS IA.  A DECREASE IN SNOW IS EXPECTED THU
AS THE WAVE DEAMPLIFIES AND THE LOW LEVEL FRONTOGENESIS WEAKENS IN
RESPONSE.

...MAINE DAY 1...

A DEVELOPING LOW OFFSHORE OF SRN NEW ENGLAND WILL DEEPEN RAPIDLY
WHILE LIFTING TO THE NORTH.  THIS SHOULD RESULT IN A ZONE OF HEAVY
DEFORMATION SNOWFALL THROUGH THE ERN PORTION OF MAINE...WITH
RECENT RADAR AND SATELLITE TRENDS WITH DEVELOPING BANDS OF SNOW IN
SOUTHEAST NEW ENGLAND SHOWING THE SNOW EVENT IS UNDERWAY ON
SCHEDULE. MANUAL PROGS USED A MULTI-MODEL/SREF MEAN
QPF/TEMPERATURE PROFILE FOR THE DETERMINISTIC AND PROBABILISTIC
SNOW FORECASTS.

...CO/NM ROCKIES AND HIGH PLAINS DAYS 1-3...

ON WED AN UPPER JET EXTENDS SOUTH ACROSS THE MT AND WY ROCKIES
WITH THE FAVORED LEFT EXIT REGION OVER CO PRODUCING ASCENT IN A
COUPLED UPPER DIVERGENCE/LOWER CONVERGENCE BANDS.  THIS COMBINES
WITH LOW LEVEL FRONTOGENESIS AND UPSLOPE CONDITIONS TO PRODUCE
LOCALLY HEAVY SNOW IN THE FRONT RANGE AND FOOTHILLS OF CO INTO HE
SANGRE DE CRISTO RANGE OF NM.  ON THU...THE INITIAL WAVE DEPARTS
SO A BREAK IN ACTIVITY DEVELOPS.  A SECOND JET STRENGTHENS OVER
THE SOUTHWEST WITH AN UPPER DIVERGENCE MAXIMA FORMING IN TH E LEFT
EXIT JET REGION OVER THE FOUR CORNERS REGION AND MOVING INTO AND
ACROSS NM TO THE HIGH PLAINS OF THE TX PANHANDLE.

ON FRI TO FRI NIGHT...THE CONTINUING JET ACROSS SOUTHERN NV/CA
THROUGH AZ/NM LEADS TO AN UPPER DIVERGENCE MAXIMA CROSSING
SOUTHERN UT/NORTHERN AZ AND THEN INTO SOUTHERN CO TO NORTHERN NM.
THE LONGER DURATION COMBINATION OF UPSLOPE CONDITIONS AND UPPER
DIVERGENCE FAVORS HEAVY SNOW IN THE SAN JUANS OF CO. AGAIN...AS
THE UPPER DIVERGENCE MAXIMA STREAKS EAST FROM THE SOUTHERN ROCKIES
TO THE HIGH PLAINS...LIGHTER OVERRUNNING SNOW WILL BE POSSIBLE
INTO THE TX/OK PANHANDLES.  THE 00Z NAM HAD HIGHER AMOUNTS WITH A
PAIR OF 700 MB THETA-E ADVECTION AND CONVERGENCE MAXIMA ABOVE THE
GFS VALUES.  I WOULD WAIT UNTIL OTHER MODEL COME ON BOARD BEFORE
GIVING MORE THAN A LOW PROBABILITY OF HEAVY SNOW IN THE TX
PANHANDLE.

...PACIFIC NORTHWEST/CA/GREAT BASIN DAYS 2-3...

THE MODELS SHOW HEIGHT FALLS WITH THE DEVELOPING MID-UPPER LEVEL
TROUGH ACROSS THE NORTHWEST.  AS HEIGHT FALLS OCCUR...THE JET AXIS
SINKS SOUTH WITH TIME...SO ON DAY 2 THE OR CASCADES GET INTO THE
FAVORED LEFT EXIT REGION OF THE UPPER JET.  INCREASING LOW-MID
LEVEL MOISTURE COMBINES WITH LOW LEVEL ASCENT AND UPSLOPE LOW
LEVEL FLOW TO PRODUCE SNOW IN THE OR CASCADES.  MOIST CONDITIONS
UP TO 700 MB AND CONTINUING 850-700 MB ASCENT LEADS TO MORE SNOW
THE FIRST HALF OF DAY 3.  BUT AS THE JET SINKS FURTHER SOUTH INTO
CA AND THE GREAT BASIN...THE FAVORED UPPER DIVERGENCE DOES TOO.
ALSO...THE 700 MB CONVERGENCE RELOCATES SOUTH INTO CENTRAL NV AND
UT..SO SNOW WINDS DOWN IN THE OR CASCADES AND PICKS UP IN THE
RANGES OF NORTHERN CA AND THEN INTO CENTRAL NV AND
CENTRAL/SOUTHERN UT...WHERE SOUTHWEST FLOW ADVECTS INCREASING
MOISTURE INTO THE SOUTHERN WASATCH. MANUAL PROGS FAVORED A BLEND
OF THE ECMWF/GFS/21Z SREF MEAN QPF AND TEMPERATURE PROFILES.

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

PETERSEN



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