PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
319 PM EDT TUE MAY 01 2018

VALID 00Z WED MAY 02 2018 - 00Z SAT MAY 05 2018


DAYS 1 THROUGH 3...


...GREAT BASIN/CENTRAL AND SOUTHERN ROCKIES/SOUTHWEST...

A POSITIVELY TILTED LONG WAVE TROUGH EXTENDING FROM THE NORTHERN
ROCKIES INTO SOUTHERN CA ON DAY 1 BRIEFLY CLOSES OFF IN THE MID
LEVELS OVER THE GREAT BASIN LATE ON DAY 1 INTO DAY 2. THE LONG
WAVE TROUGH THEN EXITS INTO THE NORTHERN AND CENTRAL PLAINS DURING
DAY 3. UPSLOPE FLOW ASSOCIATED WITH DEVELOPING LOW PRESSURE OVER
SOUTHEAST CO...COMBINED WITH LIFT WITH THE MID LEVEL SYSTEM...IS
EXPECTED TO PRODUCE HEAVY SNOWFALL OVER PORTIONS OF THE WASATCH
RANGE IN UT...AS WELL AS THE CENTRAL AND SOUTHERN ROCKIES. THERE
WAS GOOD MODEL AGREEMENT WITH THE TIMING AND PLACEMENT OF THE
SYNOPTIC SCALE FEATURES...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...
AS A POSITIVELY TILTED LONG WAVE TROUGH CROSSES THE GREAT BASIN
AND CENTRAL AND SOUTHERN ROCKIES DURING DAY 1...LIFT ASSOCIATED
WITH THE MID LEVEL SYSTEM AIDS IN SPINNING UP SURFACE LOW PRESSURE
ON A LOW LEVEL BAROCLINIC ZONE EXTENDING ACROSS CENTRAL AND
SOUTHEAST CO. AS THE LOW LEVEL CIRCULATION BECOMES BETTER
ORGANIZED...THE UPSLOPE COMPONENT TO THE LOW LEVEL FLOW BECOMES
FOCUSED ON THE SAN JUAN RANGE IN SOUTHWEST CO. THERE WAS A MULTI
MODEL SIGNAL FOR 12+ INCHES OF SNOWFALL ACROSS THE HIGHER TERRAIN
(WHERE SNOW LEVELS EVENTUALLY DROP TO BELOW 10000 FEET)...WITH THE
12Z NAM AND 09Z SREF SHOWING 12+ INCH AMOUNTS.
INTERESTINGLY...NONE OF THE MEMBERS OF THE MOST RECENT ECMWF
ENSEMBLE OUTPUT SHOWED 12+ INCHES OF SNOWFALL HERE.

FURTHER WEST ACROSS THE WASATCH AND UINTA MOUNTAINS IN UT...SNOW
LEVELS DROP BELOW 8000 FEET. THERE SHOULD BE SUFFICIENT MOISTURE
IN THE UPSLOPE FLOW TO SUPPORT LOCAL 4 TO 8 INCHES SNOWFALL
AMOUNTS ABOVE THIS LEVEL. THESE AMOUNTS WERE SUPPORTED BY MEMBERS
OF BOTH THE 09Z SREF AND THE MOST RECENT ECMWF ENSEMBLE OUTPUT
SHOWING THE POTENTIAL FOR 8+ INCHES OF SNOW OVER THE HIGHER
TERRAIN OF EACH RANGE.

FINALLY...SNOW LEVELS ACROSS THE MOGOLLON RIM IN AZ FALL BELOW
8000 FEET WITH THE APPROACH OF THE MID LEVEL SYSTEM. THE FALLING
SNOW LEVELS SUPPORT LOCAL 3 TO 6 INCH SNOWFALL AMOUNTS ACROSS THE
NORTHERN PORTION OF THE RIM...WITH LESSER AMOUNTS FURTHER
SOUTHEAST.

DAY 2...
THE SURFACE LOW FORMING OVER SOUTHEAST CO DEEPENS AS IT TREKS
ACROSS KS DURING DAY 2...REACHING NORTHEAST KS BY THE END OF THE
PERIOD. AS THE SURFACE LOW BECOMES BETTER ORGANIZED...THE LOW
LEVEL UPSLOPE FLOW FOCUSES 0.50 INCH PRECIPITABLE WATER AIR ON THE
CENTRAL AND NORTHERN FRONT RANGE IN CO AND PORTIONS OF SOUTHEAST
WY. SNOW LEVELS ARE EXPECTED TO FALL TO NEAR 8000 FEET AS THE BEST
MID LEVEL LIFT CROSSES THE REGION (GENERALLY BETWEEN 03/00Z AND
03/06Z). THE COMBINATION OF MOISTURE IN THE UPSLOPE FLOW AND
MARGINAL INSTABILITY IN THE COLUMN (DUE AT LEAST IN PART TO
STEEPENING LAPSE RATES) IS EXPECTED TO PRODUCE AN AREA OF 12 TO 18
INCHES OF SNOWFALL...CENTERED ON THE NORTHERN CO FRONT RANGE.
THESE AMOUNTS ARE WELL SUPPORTED BY OPERATIONAL 12Z MODELS (SUCH
AS THE 12Z NAM/00Z ECMWF)...AS WELL AS MEMBERS OF THE MOST RECENT
ECMWF ENSEMBLE OUTPUT. IN FACT...BOTH THE 12Z NAM/00Z ECMWF
INDICATED THE POTENTIAL FOR AS MUCH AS 20 INCHES OF SNOWFALL...AND
THESE AMOUNTS COULD BE THE MAXIMUM AMOUNTS ABOVE 8000 FEET IN THIS
REGION.

RESIDUAL MOISTURE AND UPSLOPE FLOW ACROSS THE WASATCH AND UINTA
RANGES IN UT SHOULD BE ENOUGH TO PRODUCE LOCAL 3 TO 6 INCH
SNOWFALL AMOUNTS ABOVE 7000 FEET.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

HAYES