396  
FXUS02 KWBC 061508  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1007 AM EST WED DEC 06 2017  
 
VALID 12Z SAT DEC 09 2017 - 12Z WED DEC 13 2017  
 
15 UTC UPDATE...  
 
THE FORECAST WAS NOT MODIFIED SIGNIFICANTLY FROM OVERNIGHT.  
MODELS/ENSEMBLES SHOW EXCELLENT LARGE SCALE AGREEMENT WITH A  
HIGHLY STABLE AND AMPLIFIED FLOW PATTERN WITH A WESTERN NORTH  
AMERICA RIDGE AND EASTERN TROUGH PERSISTING THROUGH THE ENTIRE  
FORECAST PERIOD. WAVE DEVELOPING ALONG FRONTAL BOUNDARY OFF THE  
EAST COAST ON DAY 3 (SAT) STILL SHOWS SOME SPREAD AMONG THE  
GUIDANCE IN TERMS OF PLACEMENT, WITH THE 00Z UKMET ON THE WESTERN  
SIDE OF THE ENVELOPE (ALTHOUGH NOT AN OUTLIER), CLOSE ENOUGH TO  
SPREAD SOME PRECIPITATION WELL INLAND ACROSS THE MID-ATLANTIC  
STATES, AND THE 00Z ECMWF ON THE EASTERN SIDE OF THE SPREAD  
ENVELOPE, KEEPING ANY PRECIP CONFINED CLOSER TO THE MID-ATLANTIC  
COAST. A MULTI-MODEL BLEND APPROACH WAS TAKEN DURING DAYS 3-4,  
INCLUDING THE 00Z ECMWF/UKMET AND 00Z/06Z GFS, RESULTING IN A LOW  
POSITION OFF THE EAST COAST ON DAY 3 JUST A HAIR WEST OF THE 00Z  
ECMWF. NONETHELESS, THIS SYSTEM HAS THE POTENTIAL TO BRING THE  
FIRST SNOW OF THE SEASON TO PORTIONS OF THE MID-ATLANTIC FRIDAY  
NIGHT/SATURDAY, WITH RAIN LIKELY ALONG THE COAST.  
 
ELSEWHERE, A STREAM OF CLIPPER SYSTEMS WILL TRAVERSE THE  
NORTHWESTERLY FLOW BRINGING REINFORCING ROUNDS OF COLD AIR INTO  
THE CENTRAL/EASTERN U.S. INTO NEXT WEEK. MODELS CONTINUE TO SHOW  
THE POTENTIAL FOR AN AMPLIFYING SHORTWAVE ACROSS THE EAST BY  
TUE-WED TO RESULT IN A RATHER COMPLEX SURFACE CYCLONE EVOLUTION  
ALONG/OFF THE MID-ATLANTIC/NORTHEAST U.S. COAST. THIS TYPE OF  
EVOLUTION WOULD FAVOR POTENTIAL SNOWFALL FOR NEW ENGLAND MORE SO  
THAN THE MID-ATLANTIC, BUT THE SPECIFIC PRECIPITATION PATTERN WITH  
THE SYSTEM WILL DEPEND ON THE SYNOPTIC EVOLUTION WHICH HAS TOO  
MANY POTENTIAL VARIABLES TO SPECULATE MUCH ON A WEEK OUT. USED A  
GRADUAL TREND TOWARD 00Z ECENS/NAEFS MEANS FROM DAY 5 ONWARD, BUT  
KEPT ABOUT 30 PERCENT OF DETERMINISTIC SOLUTIONS (ECMWF/GFS) IN  
THE FORECAST THROUGH DAY 7 GIVEN THE ABOVE AVERAGE GUIDANCE  
CONSENSUS AT THE LARGE SCALE.  
 
RYAN  
 
PREVIOUS DISCUSSION (ISSUED AT 0455 UTC)...  
   
..OVERVIEW
 
 
RESILIENT RIDGE/TROUGH PATTERN SHOWS NO SIGN OF BREAKING DOWN INTO  
THE MIDDLE OF NEXT WEEK. THIS WILL CONTINUE THE STABLE SYNOPTIC  
PATTERN OF MILD/DRY IN THE WEST, A TRANSITION ZONE OF TEMPERATURES  
IN THE CENTRAL STATES (PLAINS TO MISSISSIPPI VALLEY), AND COLD IN  
THE EAST WITH PRECIPITATION FOCUSED IN THE GREAT LAKES.  
 
...GUIDANCE AND PREDICTABILITY ASSESSMENTS AND WEATHER  
HIGHLIGHTS...  
 
THE DETERMINISTIC MODELS HAVE WAVERED GENERALLY WITHIN THE  
MULTI-CENTER ENSEMBLE SPREAD FOR THE PAST SEVERAL DAYS WITH NO  
MODEL SINGLY THE BEST OVER ALL THE CONUS. PREFER TO MAINTAIN THE  
WESTERN RIDGE WITH NO INVASIONS OF PACIFIC SHORTWAVES (PER  
PREVIOUS GFS/ECMWF RUNS) WHILE FOLLOWING THE LEAD OF THE ENSEMBLE  
CONSENSUS IN THE EAST WITH RESPECT TO TIMING/TRACK OF EMBEDDED  
SHORTWAVES IN THE BROAD CYCLONIC FLOW. GIVEN THE RELATIVELY WEAK  
SURFACE REFLECTION, OPTED TO USE A DETERMINISTIC CONSENSUS TO  
START WITH A TRANSITION TOWARD THE ENSEMBLE MEANS FOR NEXT  
TUE/WED. SEVERAL CLIPPERS WILL SWING THROUGH THE EAST WITH A  
RENEWED COLD SHOT OF CANADIAN AIR BUT GENERALLY LIGHT PRECIP --  
THE EXCEPTION WILL BE AROUND THE GREAT LAKES. TEMPERATURES WILL BE  
ABOUT 5-15 DEGREES ABOVE AVERAGE IN THE WEST (OUTSIDE THE INTERIOR  
VALLEY AREAS OF WA/OR UNDER AN INVERSION) AND ABOUT 5-15 DEGREES  
BELOW AVERAGE IN THE EAST.  
 
THERE REMAINS THE CHANCE FOR A MORE ROBUST NORTHERN STREAM SYSTEM  
TO TAKE THE LONG WAY THROUGH THE EAST (INTO THE DEEP SOUTH) NEXT  
WEEK AND PERHAPS AMPLIFY OFF THE EAST COAST, BUT THIS HAS BEEN  
ADVERTISED FOR A FEW DAYS RUNNING AT THE DAY 6-8 LEAD TIME.  
HOWEVER, THE ENSEMBLES ARE IN GOOD AGREEMENT AND HAVE BEEN  
CONSISTENT CONSIDERING THE PATTERN BY NEXT WEDNESDAY WITH LOW  
PRESSURE NEAR THE GULF OF MAINE.  
 
FRACASSO  
 

 
 
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