715  
FOUS11 KWBC 021927  
QPFHSD  
 
PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
326 PM EDT WED MAY 02 2018  
 
VALID 00Z THU MAY 03 2018 - 00Z SUN MAY 06 2018  
 
 
DAYS 1 AND 2...  
 
   
..GREAT BASIN/CENTRAL AND SOUTHERN ROCKIES
 
 
A BROAD LONG WAVE TROUGH CROSSES THE CENTRAL AND SOUTHERN ROCKIES  
DURING DAY 1...BEFORE WEAKENING AND EXITING INTO THE PLAINS DURING  
DAY 2. SURFACE LOW PRESSURE FORMING IN RESPONSE TO THE MID LEVEL  
SYSTEM TRACKS FROM SOUTHEAST CO INTO THE MID MS VALLEY...AND THE  
UPSLOPE FLOW ASSOCIATED WITH THE SURFACE LOW TRANSPORTS MOISTURE  
FROM THE PLAINS FOR HEAVY SNOWFALL ACROSS THE NORTHERN FRONT RANGE  
IN CO INTO SOUTHEAST WY. THERE WAS GENERALLY GOOD MODEL AGREEMENT  
WITH THE SYNOPTIC SETUP...SO THE THERMAL PORTION OF THE FORECAST  
WAS BASED ON A MULTI MODEL BLEND. THE QPF PORTION OF THE FORECAST  
WAS BASED ON THE MOST RECENT WPC QPF.  
 
DAY 1...  
LIFT ASSOCIATED WITH A BROAD POSITIVELY TILTED LONG WAVE TROUGH  
MOVING ACROSS THE CENTRAL AND SOUTHERN ROCKIES DURING DAY 1 AIDS  
IN SPINNING UP SURFACE LOW PRESSURE OVER SOUTHEAST CO ON A LOW  
LEVEL BAROCLINIC ZONE BISECTING THE STATE. AS THE LOW BECOMES  
BETTER ORGANIZED AND CROSSES WESTERN KS DURING THE FIRST PART OF  
THE PERIOD... INCREASING UPSLOPE FLOW TRANSPORTS 0.50/0.75 INCH  
PRECIPITABLE WATER AIR ACROSS PORTIONS OF NORTHEAST CO AND  
SOUTHEAST WY...WHICH PEAKS BEFORE 03/12Z. SNOW LEVELS DROP WITH  
THE APPROACH OF THE MID LEVEL SYSTEM...BOTTOMING OUT BETWEEN  
8000/9000 FEET (BASED ON THE MOST RECENT NBM) OVER NORTHEAST CO  
AND SOUTHEAST WY DURING THE PERIOD OF BEST SYNOPTIC SCALE ASCENT.  
 
THE BEST LIFT ASSOCIATED WITH THE MID LEVEL SYSTEM ALSO PEAKS  
DURING THIS TIME...AND THE COMBINATION OF SYNOPTIC SCALE ASCENT  
AND UPSLOPE FLOW IS EXPECTED TO PRODUCE AN AREA OF 8 TO 12 INCHES  
OF SNOWFALL ALONG THE NORTHERN FRONT RANGE OF CO. THESE VALUES ARE  
SUPPORTED BY MEMBERS OF THE MOST RECENT ECMWF ENSEMBLE OUTPUT...AS  
WELL AS A FEW 12Z MODEL SOLUTIONS (SUCH AS THE 12Z NAM/12Z GFS AND  
12Z HREF MEAN). IN FACT...BOTH THE 12Z NAM/09Z SREF SUGGEST THAT  
LOCAL 20 INCH SNOWFALL AMOUNTS ARE POSSIBLE OVER THE NORTHERN CO  
FRONT RANGE...BUT CONSIDERING THE BEST MOISTURE AND INSTABILITY  
ARE EXPECTED OVER FAR WESTERN NE...THESE VALUES MAY BE OUT OF  
REACH.  
 
FURTHER SOUTH AND WEST...THE MOISTURE PLUME IS NOT AS  
IMPRESSIVE...BUT THERE SHOULD SUFFICIENT SYNOPTIC SCALE LIFT AND  
UPSLOPE FLOW TO PRODUCE LOCAL 3 TO 6 INCH SNOWFALL AMOUNTS OVER  
THE HIGHER TERRAIN OF THE UINTA MOUNTAINS IN UT...AS WELL AS THE  
SAN JUAN RANGE IN CO.  
 
DAY 2...  
THE SURFACE AND MID LEVEL SYSTEMS PULL OUT OF THE ROCKIES DURING  
DAY 2...AND LOW AND MID LEVEL WINDS SHIFT TO THE WEST NORTHWEST.  
THE MOISTURE PLUME FROM THE CENTRAL PLAINS EXITS AS WELL...BUT  
THERE SHOULD BE ENOUGH MOISTURE AND LIFT TO PRODUCE LOCAL 3 INCH  
SNOWFALL AMOUNTS OVER THE SAN JUAN MOUNTAINS IN CO AND NEARBY  
NORTHERN NM.  
 
THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.  
 
HAYES  
 
THIS IS THE LAST SCHEDULED DISCUSSION BETWEEN NOW AND SEPTEMBER  
15TH. UNSCHEDULED DISCUSSIONS ARE POSSIBLE AS EVENTS WARRANT.  
 

 
 
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