603  
FXUS02 KWBC 221608  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1108 AM EST FRI DEC 22 2017  
 
VALID 12Z MON DEC 25 2017 - 12Z FRI DEC 29 2017  
 
...ARCTIC AIR TO GRIP MOST OF THE LOWER 48 NEXT WEEK ALONG WITH  
SOME SNOW THREAT...  
 
   
..PATTERN OVERVIEW
 
 
DEEP CYCLONIC FLOW WILL CONTINUE TO DOMINATE THE PATTERN FROM THE  
INTERMOUNTAIN WEST TO THE EASTERN SEABOARD. LITTLE CHANGE IN SUCH  
A SYNOPTIC REGIME IS EXPECTED AS ADDITIONAL SHORTWAVES HELP ASSERT  
REINFORCING SHOTS OF ARCTIC AIR TO THE REGION. WITHIN THIS  
CONVEYOR BELT OF EMBEDDED SHORTWAVES...THE INITIAL ONE WILL BE  
RESPONSIBLE FOR A COASTAL STORM IN THE PROCESS OF EXITING NEW  
ENGLAND CHRISTMAS MORNING. IN ITS WAKE...ARCTIC HIGH PRESSURE  
SHOULD EXERT ITS DOMINANCE FROM THE ROCKIES EASTWARD AS  
TEMPERATURES CRASH OVER MUCH OF THE COUNTRY. WITHIN SOME OF THESE  
SURGES...FORECAST 700-MB TEMPERATURES MAY DROP BELOW -30C ALONG  
THE NORTHERN TIER OF THE COUNTRY AS ANOMALIES DROP TO AROUND 2 TO  
3 STANDARD DEVIATIONS BELOW AVERAGE. OVER THE EASTERN PACIFIC AND  
INTO THE WEST COAST...AN UPPER RIDGE IS FORECAST TO GENERALLY  
REMAIN IN PLACE FOR MUCH OF THE PERIOD. THERE IS A BRIEF MOMENT ON  
TUESDAY WHERE A POTENTIAL SHORTWAVE WILL MOVE INTO THE PACIFIC  
NORTHWEST BEFORE SHEARING THEREAFTER. WHILE THE UPPER RIDGE  
RE-ESTABLISHES ITS PRESENCE...A PROGRESSIVE SYSTEM IS EXPECTED TO  
SLIDE THROUGH WESTERN BRITISH COLUMBIA EARLY WEDNESDAY WHILE  
ACCELERATING SOUTHEASTWARD ALONG THE EASTERN EXTENT OF THE MEAN  
RIDGE. IT SHOULD EVENTUALLY ADJOIN THE LONGWAVE TROUGH OVER THE  
CENTRAL/EASTERN U.S. WHICH WILL PROVIDE A NOTABLE SOUTHERN STREAM  
FEATURE TO WORK WITH. GIVEN TREMENDOUS COLD AIR IN PLACE...THIS  
WAVE IS DEFINITELY WORTH MONITORING TOWARD THE DAY 7/DECEMBER 29  
AND BEYOND REALM GIVEN POTENTIAL WINTER WEATHER IMPLICATIONS.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT/PREFERENCES
 
 
AT THE START OF THE PERIOD ON CHRISTMAS DAY...AN EXITING COASTAL  
STORM IS WELL AGREED UPON BUT WITH SOME SPATIAL/INTENSITY  
DIFFERENCES. MOST NOTABLY...THE 00Z UKMET SITS ON THE WESTERN SIDE  
OF THE SPREAD WHILE THE 06Z/00Z GFS AND 00Z ECMWF DISAGREE ON  
INTENSITY. RECENT RUNS OF THE GFS HAVE TRENDED DEEPER WHILE THE  
ECMWF HAS MOVED IN THE OPPOSITE DIRECTION. FOR INSTANCE...THE 00Z  
GFS HAS A 993-MB CYCLONE OFF THE COAST OF MAINE WHILE THE 00Z  
ECMWF IS APPROXIMATELY 10-MB WEAKER AT THE SAME TIME. REGARDING  
TRAILING FEATURES TRAVERSING THE DEEP CYCLONIC FLOW...THE 00Z  
UKMET IS AN OUTLIER HERE AS WELL AS IT IS MARKEDLY QUICKER WITH  
SHORTWAVE ENERGY SURGING TOWARD THE NORTH-CENTRAL U.S. ON TUESDAY.  
OVERALL...DETAIL DIFFERENCES ARE EVIDENT IN THE MID/UPPER-LEVEL  
FLOW...BUT THE GENERAL IDEA OF ARCTIC HIGH PRESSURE SWEEPING  
THROUGH THE CENTRAL U.S. IS AGREED UPON WHILE A BOUNDARY REMAINS  
LOCKED UP AGAINST THE HIGHER TERRAIN OUT WEST. ONE OF THE BIGGER  
AND MOST CONTENTIOUS AREAS OF THE FORECAST REMAINS WITH THE  
EASTERN PACIFIC RIDGE. MULTI-DAY COMPARISONS HAVE SHOWN PERIODS OF  
TIME WHERE OPERATIONAL MODELS MOVE A MODEST SHORTWAVE THROUGH  
WHICH ESTABLISHES A BRIEF PATTERN SHIFT. HOWEVER...ENSEMBLE  
SPAGHETTI PLOTS HAVE STRONGLY PUSHED TOWARD A MORE SEMI-PERMANENT  
UPPER RIDGE WHICH AGREES WITH THE PATTERN SETTING UP. THE 06Z/00Z  
GFS ARE STRONGER WITH THE TUESDAY SYSTEM APPROACHING THE PACIFIC  
NORTHWEST RELATIVE TO THE FLATTER CONSENSUS. EVENTUALLY THE  
MENTIONED SYSTEM SWINGING SOUTHEASTWARD OUT OF WESTERN BRITISH  
COLUMBIA ON WEDNESDAY IS HANDLED QUITE DIFFERENTLY. COMPARING THE  
GFS/ECMWF ON A MULTI-RUN BASIS...THE GFS HAS TENDED TO BE THE  
SLOWER SOLUTION WHICH FAVORS A SHORTWAVE MOVING THROUGH THE FOUR  
CORNERS ON THURSDAY BEFORE ACCELERATING TOWARD THE OZARKS BY THE  
FOLLOWING DAY. THE 00Z ECMWF IS NEARLY A DAY QUICKER WITH THIS  
SYSTEM WHICH IS EVEN FASTER THAN ITS ENSEMBLE MEAN. THERE IS  
PLENTY OF UNCERTAINTY HERE BUT THE IDEA OF ARCTIC AIR BEING IN  
PLACE OVER THE EASTERN U.S. COUPLED WITH STRONG JET ENERGY MOVED  
THROUGH SEEMS TO BE MORE AGREED UPON THAN PREVIOUSLY. ENSEMBLE LOW  
PLOTS DO SHOW WAVES OF LOW PRESSURE ANYWHERE FROM THE SOUTHERN  
U.S. INTO THE GULF STREAM ON FRIDAY.  
 
GIVEN SUFFICIENT MODEL CLUSTERING THROUGH DAY 4/TUESDAY...WAS ABLE  
TO UTILIZE A MULTI-OPERATIONAL MODEL CONSENSUS FAVORING THE 00Z  
GFS/ECMWF/CMC WITH A BIT MORE WEIGHTING ON THE GFS/ECMWF. BY DAY  
5/WEDNESDAY...GRADUALLY WENT TOWARD AN EQUAL BLEND OF OPERATIONAL  
MODELS RELATIVE TO THE ENSEMBLES. GIVEN GROWING SPREAD BEYOND THIS  
PERIOD...WENT SOLELY WITH ENSEMBLE MEANS BEING DOMINATED BY THE  
00Z GEFS/ECMWF ENSEMBLE MEANS. CONFIDENCE IN THE FORECAST BY DAY  
7/DECEMBER 29 BECOMES BELOW AVERAGE GIVEN MANY DETAILS WHICH NEED  
TO COME TOGETHER FIRST.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE TEMPERATURES DURING THE PERIOD WILL BE DOMINATED BY LARGE  
NEGATIVE ANOMALIES GIVEN THE INFLUENCE OF ARCTIC AIR MASSES ON THE  
FORECAST. THE LARGEST DEPARTURES FROM CLIMATOLOGY WILL BE LIKELY  
FROM THE UPPER INTERMOUNTAIN WEST EASTWARD INTO THE NORTHERN  
PLAINS/UPPER MIDWEST. SUCH READINGS ARE EXPECTED TO BE AROUND 25  
TO 30 DEGREES BELOW CLIMATOLOGY AS LOWS PLUNGE INTO THE -20S F  
WHILE HIGHS STRUGGLE TO GET OUT OF THE -10 TO 0F RANGE. ALL THIS  
COLD AIR WILL TRANSLATE SOUTH AND EAST ENSURING WELL BELOW NORMAL  
TEMPERATURES THROUGHOUT THE FORECAST PERIOD. MEANWHILE...THE WARM  
SPOT THROUGHOUT THE PERIOD WILL BE OVER THE SOUTHWESTERN U.S. INTO  
THE CENTRAL GREAT BASIN AND FOUR CORNERS REGION AS TEMPERATURES  
WILL GENERALLY BE 5 TO 15 DEGREES ABOVE AVERAGE.  
 
THE BIG STORY EARLY ON SHOULD BE THE HEAVY SNOW OVER NEW ENGLAND  
GIVEN THE COASTAL STORM IN PLAY CHRISTMAS MORNING. AS IT MOVES  
AWAY TOWARD THE CANADIAN MARITIMES...EXPECT GUSTY WINDS TO PREVAIL  
OVER MUCH OF THE GREAT LAKES/NORTHEASTERN U.S. DEEP CYCLONIC FLOW  
WILL FAVOR LAKE EFFECT SNOW OVER ALL OF THE MAJOR GREAT LAKES.  
SHIFTING FOCUS TO THE WEST...PERIODIC WET CONDITIONS ARE EXPECTED  
FROM THE PACIFIC NORTHWEST EASTWARD THROUGH THE UPPER  
INTERMOUNTAIN WEST AND INTO THE NORTHERN/CENTRAL ROCKIES. GIVEN  
THE 00Z ECMWF SHOWS A MORE ZONAL FLOW REGIME LATER IN THE  
FORECAST...IT WOULD SUPPORT HEAVY PRECIPITATION OVER THE PACIFIC  
NORTHWEST BY THURSDAY/FRIDAY. AS MODEL DIFFERENCES ARE HIGH AT  
THAT POINT...IT IS DIFFICULT TO PUT SIGNIFICANT STOCK IN ONE  
MODEL. LIKEWISE...WHILE THE POTENTIAL LOOMS FOR A COASTAL STORM BY  
DAY 7 OVER THE SOUTHEASTERN U.S...DETAILS ARE VERY UNCERTAIN AT  
THIS TIME. HOWEVER...THE THREAT FOR WINTRY PRECIPITATION WOULD  
EXIST OVER MUCH LOCATIONS OF THE TN/OH VALLEYS AND INTO THE  
MID-ATLANTIC GIVEN THE EXTENT OF MODIFIED ARCTIC AIR.  
 
RUBIN-OSTER  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page