494  
FXUS02 KWBC 150543  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1241 AM EST WED NOV 15 2017  
 
VALID 12Z SAT NOV 18 2017 - 12Z WED NOV 22 2017  
   
..PATTERN OVERVIEW
 
 
A HIGHLY AMPLIFIED PATTERN WILL REMAIN IN PLACE ACROSS THE  
CENTRAL/NORTHERN PACIFIC THROUGH THE MEDIUM RANGE. AN ANOMALOUSLY  
STRONG RIDGE, THE NORTHERN PORTION OF A REX BLOCK EXTENDING SOUTH  
GENERALLY ALONG THE DATE LINE, WILL CAUSE THE PRIMARY SOURCE OF  
SHORTWAVE ENERGY AFFECTING THE CONUS DURING THE MEDIUM RANGE TO BE  
ACROSS THE HIGH LATITUDES OF SIBERIA AND THE ARCTIC REGIONS. THIS  
SEEMS TO INTRODUCE SIGNIFICANT UNCERTAINTY INTO THE FORECAST,  
PERHAPS DUE TO DIFFICULTIES ASSIMILATING RELATIVELY SPARSE DATA AT  
THESE HIGH LATITUDES INTO THE MODEL GUIDANCE. LOOKING TO  
TELECONNECTIONS FOR SOME GUIDANCE, BOTH ANOMALY CENTERS ASSOCIATED  
WITH THE REX BLOCK SUPPORT SOME DEGREE OF PERSISTENT (ALBEIT NOT  
PARTICULARLY DEEP) EASTERN U.S. TROUGHING, AND FLAT TO MODESTLY  
ANTICYCLONIC UPPER-LEVEL FLOW ACROSS THE WESTERN U.S, WITH AN  
ELONGATED AREA OF TROUGHING ACROSS THE EAST PACIFIC DOWNSTREAM OF  
THE BLOCK. THIS IS GENERALLY CONSISTENT WITH THE FORECAST  
LARGE-SCALE PATTERN THROUGHOUT THE MEDIUM RANGE. GIVEN THE  
STRENGTH AND PERSISTENCE OF THIS FLOW REGIME ACROSS THE PACIFIC  
FOR WEEKS NOW, ANY MODEL SOLUTIONS WHICH DEVIATE SIGNIFICANTLY  
FROM THE CURRENT PATTERN (INCLUDING A SIGNIFICANT INCREASE IN FLOW  
AMPLIFICATION ACROSS THE CONUS) SHOULD BE VIEWED WITH SOME DEGREE  
OF SKEPTICISM, ESPECIALLY IF ACCOMPANIED BY A LARGE DEGREE OF  
RUN-TO-RUN VARIABILITY.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT AND PREFERENCES...
 
 
SIMILARLY TO PREVIOUS DAYS, THE GFS CONTINUES TO LAG THE ECMWF  
WITH RESPECT TO THE DETAILS OF THE SHORTWAVE/SFC CYCLONE CROSSING  
THE CENTRAL/EASTERN U.S. SAT-SUN. THE ECMWF HAS GENERALLY REMAINED  
STEADFAST ON A SLOWER TIMING, AND THE GFS HAS CONTINUED TO TREND  
TOWARD THE EC WITH EACH RUN. THE UKMET/CMC WERE ALSO GENERALLY ON  
BOARD WITH A SLOWER EC-LIKE PROGRESSION. FURTHER, AS THE CYCLONE  
MATURES ACROSS THE LOWER GREAT LAKES/ONTARIO/QUEBEC BY SUNDAY, THE  
ECMWF HAS SHOWN MUCH LESS AMPLIFICATION AT 500 HPA FOR SEVERAL  
CYCLES NOW. THE GFS CONTINUES TO SHOW A CUTOFF LOW AT 500 HPA  
ACROSS EASTERN CANADA ON SUN, BUT HAS TRENDED WEAKER (TOWARD THE  
ECMWF) WITH EACH SUCCESSIVE RUN. GIVEN THESE TRENDS, A SOLUTION  
HEAVILY WEIGHTED TOWARD THE ECMWF IS FAVORED THROUGHOUT THE  
EVOLUTION OF THIS SYSTEM ACROSS THE CENTRAL/EASTERN U.S.  
 
IN THE WAKE OF THE EASTERN U.S. CYCLONE, ATTENTION SHIFTS  
WESTWARD, WITH FAIRLY SIGNIFICANT UNCERTAINTY FROM MON (DAY 5)  
ONWARD. THE GFS HAS CONSISTENTLY TRIED TO AMPLIFY RIDGING ACROSS  
THE WEST FROM SUN ONWARD, WHILE THE ECMWF AND CMC HAVE GENERALLY  
WEAKER RIDGING/FLATTER FLOW. AS MENTIONED ABOVE TELECONNECTIONS  
FAVOR RELATIVELY WEAK RIDGING, MORE CONSISTENT WITH THE ECMWF/CMC  
IDEA, AND AS WITH THE PREVIOUS SYSTEM, THE GFS SEEMS TO BE  
TRENDING TOWARD A FLATTER SOLUTION WITH EACH RUN. SHORTWAVE ENERGY  
ORIGINATING ACROSS THE ARCTIC AND SIBERIA DURING THE SHORT RANGE  
IS FORECAST BY ALL GUIDANCE TO DIVE SOUTHWARD ALONG THE NORTH  
AMERICAN WEST COAST EARLY IN THE MEDIUM RANGE, REACHING THE PAC NW  
LATE SUN OR EARLY MON. THE DEGREE OF WESTERN U.S. RIDGING WILL  
PLAY A SIGNIFICANT ROLE IN THE EVOLUTION OF THIS WAVE AS IT  
REACHES THE U.S. WEAKER RIDGING AS SHOWN BY THE ECMWF WOULD FAVOR  
A FAIRLY AMPLIFIED SHORTWAVE MOVING INTO THE WEST MON-TUE (AS  
OPPOSED TO A WAVE WEAKENING AS IT MOVES INTO RIDGING AS HAS BEEN  
SHOWN BY THE GFS). ADDITIONAL ENERGY OF HIGH LATITUDE ORIGIN IS  
FORECAST TO DIVE ACROSS THE CANADIAN PRAIRIES MON-TUE BEFORE  
REACHING THE GREAT LAKES AS IT AMPLIFIES. AGAIN HERE THE ECMWF  
(SUPPORTED BY THE CMC) HAS BEEN FAIRLY CONSISTENT FOR A FEW RUNS,  
WHEREAS THE GFS SEEMS TO BE VARYING SIGNIFICANTLY FROM RUN TO RUN.  
RECENT ANECDOTAL EXPERIENCE HAS SUGGESTED THAT THE ECMWF (AND  
PERHAPS EVEN THE CMC) MIGHT HAVE SOME ADVANTAGE WITH RESPECT TO  
UPPER ENERGY ORIGINATING IN THE DATA SPARSE HIGH LATITUDES NORTH  
OF ALASKA/RUSSIA.  
 
GIVEN THESE CONSIDERATIONS, WILL CONTINUE TO LEAN SUBSTANTIALLY  
TOWARD THE ECMWF AND EC ENSEMBLE MEAN SOLUTIONS THROUGHOUT THE  
FORECAST. THE WPC FORECAST DURING DAYS 3-4 WAS BASED ON A MAJORITY  
DETERMINISTIC BLEND INCLUDING THE 12Z ECMWF/CMC AND 18Z GFS (MOST  
WEIGHT PLACED ON THE ECMWF). FROM DAY 5 ONWARD, WEIGHTING OF  
DETERMINISTIC SOLUTIONS REDUCED IN FAVOR OF HEAVIER ENSEMBLE  
WEIGHTING GIVEN THE UNCERTAINTY, BUT THE ECMWF/EC MEAN IN  
COMBINATION RETAINED A MAJORITY OF THE WEIGHTING THROUGH DAY 7.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
SHOWERS AND THUNDERSTORMS ARE EXPECTED FROM THE MS/OH VALLEYS TO  
THE EAST COAST IN ASSOCIATION WITH THE SURFACE FRONTAL SYSTEM  
SAT-SUN. LOCALLY HEAVY RAIN IS POSSIBLE, ALTHOUGH GIVEN A  
RELATIVELY QUICK MOVEMENT OF THE FRONT WIDESPREAD HEAVY RAIN IS  
NOT ANTICIPATED. ANOTHER COLD FRONT WILL QUICKLY CROSS THE  
CENTRAL/EASTERN U.S. MON-WED, ALTHOUGH MOISTURE AND PRECIPITATION  
ASSOCIATED WITH THIS SYSTEM LOOK RATHER SPARSE AT THIS TIME. THE  
ECMWF SUGGESTS SOME POTENTIAL FOR A WAVE TO DEVELOP ALONG THIS  
FRONT IN THE NORTHERN GULF OF MEXICO BY TUE-WED NEXT WEEK, WHICH  
WOULD POTENTIALLY SPREAD AN AREA OF OVERRUNNING PRECIPITATION  
ACROSS THE LOWER MS VALLEY/GULF COAST. GIVEN THE SIGNIFICANT  
UNCERTAINTY BY DAYS 6-7, CONFIDENCE IN SUCH SPECIFICS IS  
EXCEEDINGLY LOW AT THIS TIME. TEMPERATURES ACROSS THE EASTERN  
THIRD OF THE CONUS WILL GENERALLY REMAIN BELOW AVERAGE THROUGH THE  
MEDIUM RANGE DUE TO PERSISTENT UPPER TROUGHING AND INCURSIONS OF  
COLD CANADIAN AIR IN THE WAKE OF THE AFOREMENTIONED COLD FRONTS.  
 
ANOTHER FRONT WILL BRING HEAVY RAIN AND MOUNTAIN SNOW TO AREAS  
FROM NORTHERN CA NORTH INTO THE PAC NW OVER THE WEEKEND, ON SUNDAY  
IN PARTICULAR. ONSHORE FLOW WILL PERSIST ACROSS THE NW EVEN AS  
THIS FRONT DISSIPATES, WITH ANOTHER FRONT EXPECTED TO APPROACH BY  
NEXT TUE-WED. THIS WILL KEEP MUCH OF THE WEST COAST WET THROUGH  
THE PERIOD. FLAT TO WEAKLY ANTICYCLONIC UPPER FLOW ACROSS THE WEST  
SHOULD KEEP TEMPERATURES GENERALLY ABOVE AVERAGE FOR MOST AREAS,  
WITH THE EXCEPTION BEING THE WEST COAST WHERE CLOUDS AND PRECIP  
WILL HOLD TEMPERATURES DOWN.  
 
RYAN  
 

 
 
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