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.