004  
FXUS02 KWBC 130620  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
119 AM EST MON NOV 13 2017  
 
VALID 12Z THU NOV 16 2017 - 12Z MON NOV 20 2017  
   
..PATTERN OVERVIEW
 
 
DOWNSTREAM FROM A HIGHLY AMPLIFIED PATTERN ACROSS THE  
NORTH/CENTRAL PACIFIC, REASONABLY PROGRESSIVE FLOW SHOULD REMAIN  
IN PLACE ACROSS THE CONUS FOR MUCH OF THE MEDIUM RANGE. BY NEXT  
WEEKEND THE PATTERN SHOULD SETTLE INTO A BROAD/AMPLIFIED WESTERN  
RIDGE-EASTERN TROUGH CONFIGURATION WITH AN ELONGATED MEAN TROUGH  
OVER THE EASTERN PACIFIC. THE REX BLOCK WHICH HAS BEEN PERSISTENT  
IN THE VICINITY OF THE DATE LINE SHOWS LITTLE SIGN OF BREAKING  
DOWN OVER THE NEXT WEEK, ALTHOUGH A SLIGHT WESTWARD PROPAGATION IS  
POSSIBLE. TELECONNECTIONS ASSOCIATED WITH THE PROMINENT PACIFIC  
HEIGHT ANOMALY CENTERS SUPPORT THE EXPECTED NORTH AMERICAN  
PATTERN, WET CONDITIONS ALONG THE WEST COAST, AND PERIODIC  
INTRUSIONS OF COLD CANADIAN AIR FROM THE NORTHERN TIER  
SOUTHEASTWARD.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
WHILE MODEL SPREAD REMAINS LOW INITIALLY ON THU, A SIGNIFICANT  
INCREASE IN SPREAD/UNCERTAINTY CONTINUES TO PLAGUE THE EVOLUTION  
OF PACIFIC SHORTWAVE ENERGY MOVING INTO THE WESTERN U.S. LATE THU  
AND FRI, AND THEN QUICKLY AMPLIFYING ACROSS THE PLAINS/GREAT LAKES  
FRI-SAT. THE DETERMINISTIC ECMWF HAS CONTINUED SLOWING THE  
PROGRESSION OF THIS SHORTWAVE OVER THE PAST FEW RUNS, WHILE THE  
GFS HAS BEEN RELATIVELY CONSISTENT FOR SEVERAL RUNS NOW ON THE  
FASTER SIDE OF THE SPREAD. REGARDLESS OF THE TIMING OF THE WAVE,  
MODELS ARE IN GENERAL AGREEMENT THAT A SURFACE CYCLONE WILL  
DEVELOP AND RAPIDLY DEEPEN ACROSS THE UPPER MIDWEST/GREAT LAKES  
FRI INTO SAT. THE AFOREMENTIONED UPPER-LEVEL TIMING DIFFERENCES  
ARE ALSO REFLECTED THROUGH THE SURFACE COLD FRONT SOUTH OF THE  
DEEPENING CYCLONE. AT THIS TIME A COMPROMISE SOLUTION IS PREFERRED  
- BUT THE CONTINUED SLOWING TREND OF THE ECMWF CANNOT BE IGNORED.  
THE 12Z CMC WAS GENERALLY WELL-CENTERED WITHIN THE GUIDANCE SPREAD  
ENVELOPE WITH RESPECT TO THIS FEATURE THROUGH DAY 5 (SAT). THUS, A  
BLEND OF THE 12Z ECMWF/CMC AND 18Z GFS COMPRISED A SIGNIFICANT  
PORTION OF THE FORECAST FOR DAYS 3-5.  
 
BEYOND SAT THERE IS A GENERAL CONSENSUS ON THE RESULTANT LONG WAVE  
PATTERN ACROSS THE CONUS, WITH RIDGING BUILDING INTO THE WEST AND  
A BROAD UPPER TROUGH OVER THE EAST. UNCERTAINTY IS MUCH GREATER  
SURROUNDING THE PATTERN ACROSS THE EASTERN PACIFIC DOWNSTREAM OF  
THE REX BLOCK. WHILE ENSEMBLES SHOW GOOD CONSENSUS ON AN ELONGATED  
AREA OF NEGATIVE HEIGHT ANOMALIES EXTENDING FROM THE GULF OF AK  
SOUTH TOWARD HAWAII, DETERMINISTIC SOLUTIONS VARY SIGNIFICANTLY AS  
TO THE EXACT EVOLUTION OF THE FLOW WITHIN THIS BROAD TROUGH. A  
NUMBER OF SOLUTIONS, INCLUDING THE LAST SEVERAL GFS RUNS DEVELOP A  
CUTOFF LOW OVER THE NORTHEAST PAC, WITH A RELATIVELY SLOW  
PROGRESSION INTO THE WESTERN CONUS RIDGE. THE 12Z ECMWF SHOWED A  
LARGE SHIFT FROM ITS PREVIOUS RUN, KEEPING THE FLOW OVER THE NORTH  
PAC MORE PROGRESSIVE AND BRINGING A COUPLE SIGNIFICANT SHORTWAVES  
ONSHORE IN THE PAC NW SAT-SUN, SERVING TO FLATTEN THE RIDGE  
SUBSTANTIALLY. AT THIS TIME, ECENS/GEFS/NAEFS ENSEMBLE MEANS  
SUPPORT A SOLUTION MORE SIMILAR TO THE GFS. GIVEN THE UNCERTAINTY  
AND LOW CONFIDENCE, ECENS/GEFS/NAEFS MEANS WERE WEIGHTED  
PROGRESSIVELY MORE IN THE FORECAST DURING DAYS 6-7.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
PERSISTENT ONSHORE FLOW AND THE ARRIVAL OF A COUPLE LOW PRESSURE  
SYSTEMS THROUGH THE PERIOD WILL KEEP CHANCES FOR PRECIPITATION IN  
PLAY ALONG THE WEST COAST AND INTO PORTIONS OF THE  
CENTRAL/NORTHERN ROCKIES. HEAVY RAIN IS POSSIBLE CLOSE TO THE  
COAST ALONG WITH MOUNTAIN SNOW. AS RIDGING BUILDS ACROSS THE WEST  
BY NEXT WEEKEND MUCH OF THE ROCKIES WILL DRY OUT, AND SNOW LEVELS  
WILL RISE, WITH PRECIPITATION CHANGING OVER TO RAIN EVEN ACROSS  
THE SIERRAS, AND SNOW REMAINING ONLY AT THE HIGHER ELEVATIONS OF  
THE CASCADES AND NORTHERN ROCKIES. EXPECT THE HIGHEST PRECIP  
TOTALS OVER FAVORED TERRAIN AREAS. FARTHER EAST, A WEAK AND  
FAST-MOVING FRONTAL BOUNDARY WILL BRING A MIX OF LIGHT RAIN AND  
SNOW SHOWERS TO PORTIONS OF THE GREAT LAKES/NORTHEAST THU INTO  
EARLY FRI. ANOTHER ROUND OF PRECIP IS POSSIBLE FROM THE MS VALLEY  
TO THE GREAT LAKES AND NORTHEAST SAT-SUN IN ASSOCIATION WITH THE  
DEEP GREAT LAKES CYCLONE. COLD AIR NORTH OF THE CYCLONE COULD  
PRODUCE AREAS OF SNOW ACROSS THE UPPER MIDWEST/GREAT  
LAKES/NORTHEAST, WITH GUSTY WINDS POSSIBLE AS WELL DUE TO THE  
RAPIDLY DEEPENING NATURE OF THE SYSTEM.  
 
TEMPERATURES WILL GENERALLY BE BELOW AVERAGE ACROSS THE  
MIDWEST/GREAT LAKES/EASTERN SEABOARD THROUGH MUCH OF THE PERIOD AS  
UPPER TROUGHING BECOMES MORE DOMINANT THROUGH TIME. ABOVE AVERAGE  
TEMPERATURES WILL GENERALLY PREVAIL FROM THE SOUTHWEST TO THE  
CENTRAL/SOUTHERN ROCKIES AND EAST INTO THE HIGH PLAINS. WARM  
ADVECTION AHEAD OF THE DEEPENING PLAINS LOW PRESSURE SYSTEM WILL  
INITIALLY BOOST TEMPERATURES THU-FRI, BUT THE INCREASED RIDGING BY  
LATE IN THE WEEK WILL FURTHER SUPPORT ABOVE NORMAL TEMPS.  
 
RYAN  
 

 
 
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