653  
FXUS02 KWBC 141558  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1058 AM EST TUE NOV 14 2017  
 
VALID 12Z FRI NOV 17 2017 - 12Z TUE NOV 21 2017  
   
..UNUSUALLY UNCERTAIN FORECAST FOR MUCH OF THE CONUS
 
 
   
..PATTERN OVERVIEW
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT AND PREFERENCES...
 
 
THE EVOLUTION OF THE UPPER FLOW OVER THE LOWER 48 WILL BE AT THE  
MERCY OF THE NORTH PACIFIC IN THE SHORT TERM, FIRST AFFECTING THE  
WEST AND THEN TRANSLATING EASTWARD. A LEAD TROUGH WILL PUSH  
THROUGH THE PLAINS AND EASTERN STATES FRI-SUN AND CARRY A SFC  
FRONT INTO THE ATLANTIC AND GULF. ANOTHER SYSTEM WILL PUSH INTO  
THE WEST, BUT THE DIFFERENCES IN TIMING/STRENGTH AMONG THE  
ENSEMBLES IS STRIKING. IN SHORT, THE ECMWF AND ITS ENSEMBLE  
MEMBERS DIFFER FROM THE REST OF THE GUIDANCE (GFS/GEFS,  
CANADIAN/GEPS, AND UKMET) SOUTH OF ALASKA AS A SHORTWAVE IN THE  
SHORT TERM (I.E. TOMORROW) DIGS SOUTH OF THE ALEUTIANS. THE ECMWF  
AND ITS ENSEMBLES WERE STRONGER AND FARTHER SOUTHWEST THAN THE  
REST OF THE PACK, WHICH ALLOW IT TO DIVE SOUTHWARD ON THE EAST  
SIDE OF STRONG UPPER RIDGING BUILDING INTO THE WESTERN ALEUTIANS  
AND CLOSE OFF AND SEPARATE FROM THE TROUGH. THE REST OF THE  
MODELS/ENSEMBLES ALLOW IT TO STAY FARTHER EAST TO ALLOW IT TO BE  
REINFORCED BY UPSTREAM VORTICITY DIGGING THROUGH THE GULF OF  
ALASKA WHICH RESULTS IN A COHERENT DEEPER/SLOWER TROUGH RATHER  
THAN THE SPLIT ORIENTATION OF THE ECMWF. THIS HAS THE RIPPLE  
EFFECT OF A WEAKER EASTERN TROUGH IN THE ECMWF COMPARED TO THE GFS  
(AND OTHERS) BY THE WEEKEND. THIS WAS ONE OF THOSE FEW DAYS A YEAR  
WHERE THE CLUSTERING BY ENSEMBLE SYSTEM WAS NEARLY MUTUALLY  
EXCLUSIVE TO EACH OTHER WHICH GIVES A FALSE SENSE OF IMPLIED  
CERTAINTY THROUGH "LOW" SPREAD WITHIN THE ENSEMBLE SYSTEM BUT HIGH  
SPREAD BETWEEN THE TWO (OR THREE, INCLUDING THE CANADIAN  
ENSEMBLES).  
 
THE NEAREST UPSTREAM SIMILARITY BETWEEN THE GEFS AND ECENS MEAN  
WAS OVER THE BERING SEA WITH THE STRONG UPPER RIDGE THIS WEEKEND.  
TELECONNECTIONS TO THIS POSITIVE ANOMALY (IMPRESSIVE AT +3 SIGMA  
EVEN IN THE MEAN) SUPPORT LESS AMPLIFIED FLOW OVER THE CONUS WITH  
WEAK RIDGING FROM OR/CA INTO THE PLAINS AND WEAK TROUGHING INTO  
THE GREAT LAKES. THIS SUPPORTS THE ECMWF IDEA. GIVEN A SIMILAR  
PREFERENCE IN THE PREVIOUS/OVERNIGHT MEDIUM RANGE SHIFT, OPTED TO  
CONTINUE THIS THEME UNTIL A MORE RELIABLE CONSENSUS APPEARS.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
WELL ABOVE AVERAGE TEMPERATURES WILL PRECEDE THE FRONT IN THE  
SOUTHERN PLAINS ON FRIDAY WHERE RECORD HIGHS MAY BE TIED OR BROKEN  
FROM NM INTO TX. WARMER THAN AVERAGE TEMPERATURES WILL SLIDE  
EASTWARD INTO THE SOUTHEAST SATURDAY BEFORE COOLER AIR FLOW IN  
BEHIND THE FRONT. DEEPER TROUGH FORECAST BY THE GFS/GEFS WOULD  
IMPLY MUCH COOLER AIR IN THE EAST SUN/MON THAT THE ECMWF AND THE  
WPC FORECAST WHERE ANOMALIES MAY BE 5-10F BELOW AVERAGE VS 10-25F  
BELOW AVERAGE. IN THE WEST, KEPT ANOMALIES CLOSER TO ZERO PER THE  
ECMWF GUIDANCE RATHER THAN DECIDEDLY ABOVE AVERAGE SUN-TUE GIVEN  
THE DIFFERENCES SEEN IN THE ENSEMBLES.  
 
PRECIPITATION WILL FOCUS IN THE GREAT LAKES AND NORTHEAST INTO THE  
SOUTHEAST AHEAD OF AND ALONG THE FRONT AS IT PUSHES OFFSHORE, WITH  
LAKE ENHANCED SNOW IN ITS WAKE. NEXT SYSTEM WILL BRING SOME  
RAIN/SNOW TO THE PAC NW AND CALIFORNIA SUN-TUE BUT AGAIN THE  
DIFFERENCES BETWEEN THE GFS AND ECMWF TRANSLATED TO MUCH HEAVIER  
VS LIGHTER PRECIPITATION, RESPECTIVELY. IN ADDITION, RETURN FLOW  
OUT OF THE GULF MAY SUPPORT INCREASED LIGHT RAIN THROUGH THE LOWER  
MS VALLEY NEXT WEEK.  
 
FRACASSO  
 

 
 
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