PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
358 PM EST SUN FEB 12 2012
 
VALID 00Z MON FEB 13 2012 - 00Z THU FEB 16 2012
 

DAY 1...

...PACIFIC NORTHWEST/CA...

THE MODELS REMAIN IN AGREEMENT ON THE DEVELOPMENT OF A 700-300 MB
WAVE IN THE NORTHEAST PACIFIC AND FOR THIS WAVE TO MOVE SOUTHEAST
ACROSS THE COAST OF OR/CA AND INLAND. AS THE WAVE
APPROACHES...WARM AND MOISTURE ADVECTION DEVELOP AND PROGRESS DOWN
THE SPINE OF THE SIERRA RANGE.  DIFLUENT FLOW ALOFT IN ADVANCE OF
THE APPROACHING UPPER TROUGH FAVORS PERIODS OF UPPER DIVERGENCE TO
SUPPORT ASCENT ACROSS THE OR-CA
CASCADES/SISKIYOUS/SHASTAS/NORTHERN AND CENTRAL SIERRA. STEEPENING
LAPSE RATES IN THE POST FRONTAL AIR MASS SUPPORT SNOW SHOWERS
CONTINUING IN THE OR CASCADES. THUS WE CONTINUED THE HIGH RISKS OF
4 INCHES IN PARTS OF THE SIERRAS INTO THE
SHASTAS/SISKIYOUS/SOUTHERN OR CASCADES WHERE THERE IS GOOD OVERLAP
OF MULTI-MODEL/09-15Z SREF MEAN QPF MAXIMA.  THE RISKS FOR 8-12
INCHES SHOWN IN THE SIERRAS ARE DUE TO A LONG DURATION EVENT.

CENTRAL TO SOUTH CENTRAL PLAINS AND MS VALLEY...

A MID-UPPER TROUGH OVER CO THIS EVENING PRODUCES SNOW FOR SEVERAL
MORE HOURS 
BEFORE THE DEPARTURE OF THE TROUGH ON TO THE PLAINS. 
AS THE MID/UPPER LEVEL SYSTEM EMERGES OVER THE PLAINS...A CENTER
OF ARCTIC HIGH PRESSURE SHOULD FINALLY BE EASING EASTWARD INTO THE
OH/TN VALLEY.  AN INCREASING LOW-MID LEVEL SOUTHERLY COMPONENT OF
FLOW AHEAD OF THE WAVE INDUCES WARM/MOISTURE ADVECTION ACROSS THE
REGION.  THIS SHOULD COMBINE WITH BANDS OF 850-700 MB CONVERGENCE
TO RESULT IN PCPN BECOMING MORE WIDESPREAD OVER THE MS VALLEY. 
QPF AMOUNTS ARE GENERALLY EXPECTED TO BE LIGHT...WITH
CORRESPONDING SNOWFALL OVER THE CENTRAL PLAINS TO THE CENTRAL MS
VALLEY. 

FURTHER SOUTH...CONCERN IS POTENTIAL FOR ICING MAINLY IN OK/AR TO
EXTREME SOUTHERN MO AND MUCH OF NORTHWEST TX.  SINCE MULTIPLE
PRECIPITATION TYPES ARE EXPECTED...IT WAS DIFFICULT TO FIND AN
AREA WHERE HIGHER AMOUNTS WERE ONE PRECIPITATION TYPE FOR AN
EXTENDED PERIOD.  THE CONTINUING WARM ADVECTION SHOULD RESULT IN
SNOW CHANGING TO A MIXTURE OF SLEET/FREEZING RAIN AND THEN
EVENTUALLY RAIN.  THE NAM AND GFS CONTINUE TO SHOW POTENTIAL FOR
UP TO A QUARTER INCH OF FREEZING RAIN IN WEST CENTRAL TO NORTHWEST
AR...WITH NO OVERLAP OF THE THREAT AREAS.


DAY 2...

...NV/NORTHERN AZ/UT/CO...

THE MODELS SHOW THE NEXT UPPER TROF/CLOSED LOW MOVING SLOWLY
SOUTHEAST FROM NV TO UT/AZ/ADJACENT CO.  UPPER DIVERGENCE MAXIMA
IN THE LEFT EXIT REGION OF THE JET CROSSING UT/CO/NM SHOULD BE
COINCIDING WITH A BAND OF CONVERGENCE AHEAD OF AND ALONG THE 700
MB FRONT AS IT PROGRESSES EAST ACROSS THE REGION.  THIS RESULTS IN
THE PRIMARY AREA OF SNOW...ALONG WITH CONVERGENCE NEAR THE 700 MB
LOW IN CENTRAL NV.  SNOWFALL AMOUNTS ARE EXPECTED TO BE ENHANCED
WHERE PRE-FRONTAL SOUTHWEST FLOW TURNS UPSLOPE IN TERRAIN FACING
THE FLOW.  CONSEQUENTLY...HIGHER AMOUNTS AND PROBABILITIES ARE
FORECAST IN PARTS OF THE UT WASATCH IN THE SAN JUANS...WITH A
SECONDARY MAXIMA DEVELOPING IN THE AZ MOGOLLON RIM.  THE MODELS
WERE IN AVERAGE AGREEMENT WITH THE HIGHER PROBABILITIES WHERE
TERRAIN AIDS ASCENT.   

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.


DAY 3...
  
...CO/NORTHERN NM/CENTRAL HIGH PLAINS...

THE 700-500 MB WAVE OVER UT/AZ 00Z WED MOVES SLOWLY EAST ACROSS
CO/NORTHERN NM THROUGH THE PERIOD...EVENTUALLY ON TO THE CENTRAL
HIGH PLAINS.  
SNOW INITIALLY OCCURS WITH THE UPPER LOW ACROSS CO AND BANDS OF
SNOW WITH CONVERGENCE IN THE VICINITY OF THE 700 MB LOW AND
TRAILING FRONT INTO SOUTHWEST CO...WITH BRIEF POST FRONTAL SNOW
SHOWERS IN THE SAN JUANS. 

A LOW FORMS NEAR THE SOUTHEAST CO/KS BORDER AND DRIFTS SLOWLY
EAST.  NORTH OF THE CIRCULATION...LOW LEVEL
CONVERGENCE/DEFORMATION MAY DRIVE SNOW DEVELOPING IN THE PLAINS OF
NORTHEAST CO...EXTENDING INTO ADJACENT NORTHWEST KS/SOUTHWEST NE. 
THERE IS A CLUSTER OF SOLUTIONS AMONG THE NAM AND SREF MEMBERS/12Z
UKMET WHICH PREDICT A DEFINED SNOW BAND.  THE 12Z ECMWF HAS MADE A
LARGE CHANGE TO MOVE THE SNOW AREA NORTH INTO SD...AS IT SHOWS A
SECONDARY 850 MB CIRCULATION CENTER NEAR THE NE/SD BORDER.  
FURTHER SOUTH IN KS...850 MB TEMPS ARE ABV FREEZING MOST OF THE
FORECAST PERIOD...OMITTING POTENTIAL FOR SIGNIFICANT SNOWFALL.

...CA/NV/OR/ID/UT....

THE MODELS INDICATE A 700-500 MB WAVE MOVING ONSHORE BY 00Z WED
WITH THE WAVE PROGRESSING SOUTHEAST WITH TIME.  THE MODELS ARE
WIDELY DIVERGENT ON THE PATH AND TIMING OF THE WAVE...MAKING THIS
FORECAST MORE UNCERTAIN THAN USUAL.
THE 12Z ECMWF JUMPED FROM AN INLAND TRACK TO A FURTHER WEST TRACK
IN OR/CA LIKE THE 12Z GFS/00-12Z UKMET.  GIVEN THE LACK OF
CLUSTERING AND RUN TO RUN CHANGES...AREAS WHERE RISKS FOR
SIGNIFICANT SNOWFALL WILL BE MINIMAL.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

PETERSEN