PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
423 AM EDT WED APR 18 2018

VALID 12Z WED APR 18 2018 - 12Z SAT APR 21 2018


DAYS 1-3...

...GREAT BASIN/FOUR CORNERS AND CENTRAL TO SOUTHERN ROCKIES/HIGH
PLAINS...

SNOW WILL RETURN AS AN EASTERN PACIFIC TROUGH AND JET APPROACHES
LATE WED WITH MOISTURE FLUXES AND LIFT PRODUCING LIGHT SNOWS IN
THE OR CASCADES.
ONCE THE UPPER TROUGH MOVES ONSHORE AN INLAND...EXPECT LIGHT
ELEVATED SNOWFALL FOR THE CENTRAL TO SOUTHERN CA SIERRA NEVADA
ACROSS THE RANGES OF CENTRAL NV. THEN ON THURS, A LARGE CLOSED 500
MB CIRCULATION WILL DRIFT DOWNSTREAM THROUGH THE FOUR CORNERS
REGION TO YIELD SOME DYNAMICALLY DRIVEN AND UPSLOPE SNOWFALL FROM
THE SOUTHERN WASATCH INTO THE CO ROCKIES, ESPECIALLY FOR THE SAN
JUANS INTO THE SANGRE DE CRISTO RANGE. SEVERAL MODELS/ENSEMBLE
MEANS INDICATE LOCALLY HEAVY SNOW IN THE SAN JUANS. 

AS THE CIRCULATION DRIFTS EAST INTO THE HIGH PLAINS...LOW-MID
LEVEL CONVERGENCE MAXIMA AND PERSISTENT DEEP LAYERED HIGH RELATIVE
HUMIDITY FAVOR SNOW DEVELOPING FRI-FRI NIGHT IN THE FRONT
RANGE/PALMER RIDGE AND FOOTHILLS OF CENTRAL CO DOWN INTO THE
SANGRE DE CRISTO MOUNTAINS OF NM.

THE MODELS HAVE SLIGHT TEMPERATURE DIFFERENCES IN CO EAST OF THE
MOUNTAINS AND RESULTANT IMPACTS ON SNOW SAT.  THE GFS HAS HIGHER
QPF TO ADJACENT WY AND THE NAM OVER IN NORTHEAST UT...WITH THE
RESPECTIVE MEANS GIVEN MORE WEIGHT UNTIL BETTER SUPPORT FOR THE
OPERATIONAL GFS AND NAM DEVELOPS WHERE THEY ARE OUTLIERS.  THE
NAM/UKMET/212Z SREF MEAN ARE IN BETTER THAN AVERAGE AGREEMENT IN
PRODUCING SEVERAL INCHES OF SNOW ON FRI NIGHT-SAT IN THE RANGE OF
CO. 


...UPPER/MID MS VALLEY ACROSS THE GREAT LAKES AND NEW YORK/NEW
ENGLAND...

A DYNAMIC SHORT WAVE TROUGH WILL PROCEED ACROSS THE UPPER/MID MS
VALLEY TO LAKE MICHIGAN ON WED. A CLOSED MID LEVEL LOW
ACCOMPANYING THIS COMPACT SYSTEM WILL YIELD A SWATH OF HEAVY
SNOWFALL ALONG A LOW LEVEL FRONTOGENESIS AND 700 MB CONVERGENCE
AXIS FROM NERN NE THROUGH NORTHERN IA INTO SOUTHERN WI.  BANDS OF
1-2 INCHES PER HOURS ARE POSSIBLE TODAY IN NORTHERN IOWA. THE
GUIDANCE HAS NUDGED THE QPF AXIS A BIT SOUTH AGAIN TONIGHT...
ALTHOUGH AGREEMENT IS NOT AS MUCH AS WE USUALLY SEE ON A DAY 1
FORECAST.  TIMING DIFFERENCES PERSIST AS THE CANADIAN GEM AND 12Z
ECMWF END SNOW SOONER IN EASTERN SD/EASTERN NE WHILE THE NAM/HIGH
RES WINDOWS ARE SLOWER AND HEAVIER. 

ON THURS, THE MAIN SHORT WAVE TROUGH WILL BEGIN TO INTERACT WITH A
DIGGING NORTHERN STREAM JET. THE PARENT LOW STREAKING ACROSS THE
OHIO VALLEY/PA WILL WEAKEN, WHILE A TRIPLE POINT JUST OFFSHORE OF
SOUTHERN NEW ENGLAND DEEPENS AND LIFTS INTO THE CANADIAN
MARITIMES. THIS SHOULD YIELD OVERALL LIGHTER SNOWFALL AMOUNTS
ACROSS INTERIOR NEW YORK AND NEW ENGLAND.  THE 00Z ECMWF TRENDED
FASTER IN MOVING THE LOW OFFSHORE, CUTTING BACK N QPF AND
POTENTIAL SNOW.

THE PROBABILITY OF FREEZING RAIN GREATER THAN 0.25 INCHES IS LESS
THAN 10 PERCENT DAYS 1-3.

PETERSEN