EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1129 AM EST WED FEB 26 2014 VALID 12Z SAT MAR 01 2014 - 12Z WED MAR 05 2014 ...FAR BELOW NORMAL TEMPERATURES ACROSS THE NORTHERN TIER... ...SNOW/ICE ACROSS THE PACIFIC NORTHWEST INCLUDING THE INTERIOR... ...SNOW/ICE FROM THE CENTRAL PLAINS TO THE OH/TN VALLEYS/MID-ATLANTIC... OVERVIEW... THE HIGH-AMPLITUDE PATTERN WITH A LONGWAVE RIDGE ACROSS THE WEST AND LONGWAVE TROUGH ACROSS THE GREAT LAKES IS EXPECTED TO MAINTAIN SOME SEMBLANCE THROUGH THE MEDIUM RANGE PERIOD/DAYS 3-7...PARTICULARLY ACROSS WESTERN CANADA PROVIDING SOME BLOCKING TO UPSTREAM SYSTEMS...BUT OTHERWISE IS EXPECTED TO FLATTEN AND BECOME MORE PROGRESSIVE. MODEL AGREEMENT ON THIS EVOLUTION IS QUITE GOOD. HOWEVER...HOW THE MODELS ARRIVE TO THIS POINT VARIES CONSIDERABLY AMONG THE GUIDANCE...ESPECIALLY WITH THE EVOLUTION OF THE UPPER TROUGH ACROSS THE GREAT LAKES. THUS...AS A WHOLE FORECAST CONFIDENCE IS ABOVE AVERAGE TROUGH DAY 3 OR 3 1/2...BEFORE TRANSITIONING THROUGH A SHORT PERIOD OF NEAR AVERAGE PREDICTABILITY DAY 4...THEN FALLING BELOW AVERAGE FROM DAY 5 ONWARD. THE SOLUTION SPREAD IS CONSISTENT WITH THESE LEVELS OF PREDICTABILITY. MODEL PREFERENCES... FOR DAY 3/SAT...MODEL AGREEMENT IS STRONG SUGGESTING NEARLY EQUAL USE OF ANY OR ALL SOLUTIONS. THE ONLY EXCEPTION WITH THE 00Z UKMET WHICH LIES NEAR THE SLOW EDGE OF THE GUIDANCE WITH AN UPPER LOW MOVING INTO SOUTHERN CALIFORNIA AND NEAR THE FAST EDGE OF THE GUIDANCE WITH THE LEADING EDGE OF A SMALLER UPPER LOW NORTH OF MONTANA. THE TRANSITIONAL PERIOD OF PREDICTABILITY IS DAY 4/SUN...WITH THE 00Z GFS MOVING WITHIN THE FAST HALF OF THE SOLUTION ENVELOPE WITH THE LOW NEAR MONTANA...WHILE THE 06Z GFS/00Z ECMWF ARE BEST SUPPORTED BY THE MOST RECENT ENSEMBLE MEANS AND THUS ARE GIVEN GREATER WEIGHTING AT THIS TIME. BY DAY 5/MON...THE 00Z ECMWF JUMPS AHEAD OF THE 00Z GFS ACROSS THE UPPER MIDWEST WITH THE LEADING EDGE OF THE WEAKENING LOW...GENERALLY REPRESENTING ONE OF THE FASTEST OVERALL SOLUTIONS. ALONG WITH THE FASTER NORTHERN STREAM SOLUTION...ITS SOUTHERN STREAM SOLUTION IS ALSO NEAR THE FAST EDGE OF THE GUIDANCE. WHILE PLAUSIBLE AND CONTAINED WITHIN THE SOLUTION ENVELOPE...UNCERTAINTY AND THE INCREASING SPREAD WITH BOTH STREAMS BY THIS PERIOD SUGGEST LIMITED CONFIDENCE WITH ITS TIMING...AND THUS ITS SOLUTION THROUGH DAYS 6 AND 7 IS ALSO ASSOCIATED WITH LIMITED CONFIDENCE. FOR LARGER-SCALE PREDICTABILITY AND TO ADDRESS THE OVERALL ABOVE AVERAGE UNCERTAINTY FROM DAY 5 ONWARD...THE MODEL PREFERENCE IS FOR AN EVEN BLEND OF THE GENERALLY AGREEING 00Z ECMWF ENSEMBLE MEAN AND 06Z GEFS MEAN. THE 06Z DETERMINISTIC GFS IS AN ACCEPTABLE MODEL TO DERIVE SOME SYSTEM DETAILS ALBEIT SOME CAUTION UPON HEAVY USE OF IT IS RECOMMENDED. SENSIBLE WEATHER HIGHLIGHTS... TEMPERATURES ARE EXPECTED TO REMAIN FAR BELOW NORMAL FOR THE NORTHERN TIER... PARTICULARLY FROM THE PACIFIC NORTHWEST ACROSS THE NORTHERN/CENTRAL PLAINS TOWARD THE GREAT LAKES...ALTHOUGH THE AIR WILL MODIFY APPRECIABLY FARTHER EAST. THUS...THE LARGEST NEGATIVE TEMPERATURES TEMPERATURE ANOMALIES OF 30 TO 40 DEGREES ARE EXPECTED ACROSS THE NORTHERN HIGH PLAINS DAYS 3/4...WITH SMALL NEGATIVE ANOMALIES DEVELOPING FARTHER EAST. THE EXTREME NATURE OF THE ANOMALIES ALONG WITH AT TIMES THE SHALLOW NATURE OF THE MODIFYING ARCTIC AIR WILL PROMOTE 2 DISTINCT AREAS OF POTENTIALLY SIGNIFICANT SNOW AND/OR ICE. ONE AREA IS ACROSS THE PACIFIC NORTHWEST...INCLUDING THE INTERIOR...WHERE FORECAST SOUNDINGS FOR SOME COASTAL AREAS AND THE INTERIOR ESPECIALLY FOR WASHINGTON FAVOR ACCUMULATING SNOWS AND/OR ICE ESPECIALLY FROM MONDAY ONWARD. SNOW/ICE AMOUNTS COULD BE SIGNIFICANT WITH SEVERAL OR MORE INCHES OF SNOW AND ACCUMULATING ICE...ALTHOUGH SNOW SEEMS MORE PROBABLE THAN ICE AT THIS TIME. A LARGER/BROADER SWATH OF SNOW/ICE IS EXPECTED TO DEVELOP FROM THE CENTRAL PLAINS EASTWARD BEGINNING SUNDAY...BEFORE REACHING PORTIONS OF THE OH/TN VALLEYS/MID-ATLANTIC SUNDAY NIGHT INTO MONDAY. THE PREFERRED MODEL SOLUTIONS ARE GENERALLY CONFLUENT IN NATURE...SUGGESTING THAT ICE AND/OR MIXED PRECIPITATION IS MOST LIKELY BUT STILL WITH A POTENTIALLY SIGNIFICANT BAND OF SNOW ALONG ITS NORTHERN PERIPHERY. THE 06Z GFS AND THE GLOBAL ENSEMBLE MEANS SUPPORT THIS. THE 00Z ECMWF PROVIDES ONE OF THE MORE EXTREME SOLUTIONS...PARTICULARLY FROM THE APPALACHIANS EASTWARD...WITH POSSIBLY HEAVY SNOW A GREATER LIKELIHOOD THAN ICE. THE ECMWF ENSEMBLE MEANS FAVOR THE SNOW CONCEPT ALSO BUT WITH A LARGE RANGE OF POSSIBILITIES...WITH MANY MEMBERS SUGGESTING A FEW INCHES OF SNOW ACROSS PORTIONS OF THE EAST COAST....TO AS MUCH AS 7 OR 9 INCHES...ALTHOUGH VERY FEW MEMBERS SUPPORT THIS. FOR NOW...WILL FAVOR EMPHASIS OF ICE OR MIXED PRECIPITATION OVER ALL SNOW GIVEN THE EXPECTED CONFLUENT NATURE OF THE FLOW. JAMES