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Winter Weather Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 2106Z Feb 19, 2018)
 
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
406 PM EST MON FEB 19 2018

VALID 00Z TUE FEB 20 2018 - 00Z FRI FEB 23 2018


DAYS 1-3...

...PLAINS AND MISSISSIPPI VALLEY INTO GREAT LAKES AND OHIO
VALLEY...

WHILE MOST OF THE FOCUS IN THE MIDDLE OF THE COUNTRY WILL BE ON
THE EXCESSIVE RAINFALL AND LIKELY FLOODING TUES THROUGH THURS, THE
TOUGHEST PART OF THE FORECAST WILL BE ALONG THE WESTERN FRINGE OF
THE ANOMALOUS MOISTURE PLUME WITH THE THREAT OF ICING. THE LONG
WAVE TROUGH OVER THE WEST AND ANOMALOUS UPPER RIDGE OVER THE EAST
WILL SET UP A VOLATILE ZONE OF DEEP MOISTURE CONTENT AND COLLIDING
AIR MASSES. MULTIPLE SHORT WAVES WILL EJECT OUT OF THE AMPLIFIED
WESTERN TROUGH AND RIDE UP A POLAR/BORDERING ARCTIC FRONTAL ZONE.

THE FIRST WEAK WAVE IS ALREADY PRESENT OVER EASTERN IA/NORTHERN IL
AND SHOULD TRACK NORTH AND EAST BEFORE FILLING RAPIDLY. THIS IS IN
RESPONSE TO THE MAIN BATCH OF UPPER DYNAMICS IN THE BASE OF THE
UPPER TROUGH EMERGING OUT INTO THE PLAINS TO INDUCE ANOTHER WAVE
NEAR LAKE MI. THE RESULTANT MON EVENING THROUGH TUES WILL BE A
STREAK OF MODERATE TO HEAVY SNOW FROM WESTERN NE NORTH AND EAST
THROUGH SD/SERN ND INTO NORTHERN/CENTRAL MN. MEANWHILE, THE
SURFACE COLD FRONT WILL PRESS EAST THROUGH THE MID MS VALLEY INTO
THE OH VALLEY. THIS COLD ADVECTION AND DENSE SHALLOW AIR MASS WILL
UNDERCUT THE BROAD MOISTURE RETURN AND ENHANCED LARGE SCALE LIFT
FOR A POTENTIAL ZONE OF ICING. TYPICALLY THE ECMWF AND NAM
TRADITIONALLY HANDLE THIS COLD DENSE AIR MASS BETTER THAN THE GFS
BUT THERE WAS ENOUGH SPREAD WITH THE TRANSITION OF EACH WAVE AND
THE TRACK OF THE SURFACE COLD FRONT DOWNSTREAM. THUS WPC FOLLOWED
MORE OF A 80% GLOBAL MODEL BLEND (20% EACH OF THE ECMWF/GFS/EC
MEAN AND GEFS MEAN) WITH THE REMAINING 20% GOING TO THE NAM NEST.
THE LATTER MODEL AND ECMWF SOLUTIONS WERE MUCH BOLDER WITH THE
COLD FRONT ADVANCING DOWNSTREAM AND RE-SURGING FRIGID AIR MASS
MOVING INTO THE MID TO UPPER MS VALLEY THAN THE GFS. THE ZONE TO
WORRY ABOUT FOR POTENTIAL ICING IS FROM EASTERN KS/NWRN MO THROUGH
MUCH OF IA INTO SOUTH CENTRAL WI. NOW GIVEN THE SPREAD AND
UNCERTAINTY, THIS FORECAST TUES AND REMAINING DAYS COULD GO EITHER
WAY TO NOT MUCH OR MARGINAL THREAT TO A MORE EXTENSIVE, BORDERING
HI-END THREAT.

THE SURFACE COLD FRONT WILL TRANSITION FROM THE GREAT LAKES/OH
VALLEY TO THE NORTHEAST/MID-ATL TUES EVENING THROUGH WED. A BLEND
OF THERMALS, 50% GFS/GEFS MEAN AND 50% ECMWF/ECMWF MEAN, FOR A
THREAT ZONE OF POTENTIAL ICING FROM NWRN AR THROUGH SERN
MO/CENTRAL IL INTO NORTHERN IN AND PARTS OF LOWER MI, POSSIBLY
EXTENDING ALSO AS FAR SOUTH AS OK AND TX. FINALLY ON WED EVENING
THROUGH THURS, THE SERN UPPER RIDGE STAYS STRONG KEEPING THE
FRONTAL ZONE INTACT FROM THE LOWER MS VALLEY TO THE OH
VALLEY/NORTHERN APPALACHIANS. A COLD 1040 MB SURFACE RIDGE WILL
BUILD OVER EASTERN CANADA AND A POTENTIAL BACK DOOR COLD FRONT
WITH WEDGING MAY YIELD SOME LIGHT SNOW AND ICING FROM OH THROUGH
PA INTO NY STATE. MEANWHILE UPSTREAM, A DYNAMIC SHORT WAVE TROUGH
WILL TRANSITION FROM THE DESERT SW TO THE PLAINS AND THIS MAY
ALLOW A SURGE OF MOISTURE CONTENT TO OVERRUN INTO THE COLD SECTOR
FROM SOME SNOW FROM NORTHERN KS TO EASTERN SD/SWRN MN AND ICING
FROM THE TX PANHANDLE NEWRD TO NWRN MO AND WESTERN IA.

...WEST...

THE FOCUS OVER THE WEST WILL BE THE ANOMALOUS LONG WAVE TROUGH AND
LIKELY WELL BELOW AVG TEMPS BUT A DYNAMIC SPLIT SHORT WAVE TROUGH
WILL DIVE DUE SOUTH ALONG THE PAC NW COASTLINE LATE TUES INTO WED.
A PLUME OF MOISTURE WILL FLOURISH OVER SWRN WA AND EASTERN OR WITH
ENHANCED VERTICAL LIFT, ADD IN THE VERY BELOW AVG HEIGHT FALLS
WITH CRITICAL THICKNESS VALUES AND SNOW MAY REACH COASTAL SECTIONS
OF EACH STATE ALONG WITH THE THREAT FOR SNOW POTENTIAL AROUND PDX
METRO AREA. THIS SYSTEM WILL THEN PROCEED THROUGH THE MAIN UPPER
TROUGH AXIS OVER THE WEST BEFORE EJECTING OUT INTO THE PLAINS WED
EVENING INTO THURS, WHILE A MORE ROBUST UPPER JET AND STREAM OF
UPPER DYNAMICS DIG INTO THE WEST AGAIN TO PRODUCE MORE LIGHT SNOW
ACROSS THE REGION INTO THE CENTRAL ROCKIES.


MUSHER