262  
FXUS02 KWBC 161558  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1057 AM EST SAT DEC 16 2017  
 
VALID 12Z TUE DEC 19 2017 - 12Z SAT DEC 23 2017  
   
..OVERVIEW
 
 
GUIDANCE CONTINUES TO SHOW A STRONG SIGNAL TOWARD AMPLIFICATION OF  
THE LARGE SCALE FLOW OVER NORTH AMERICA DURING THE NEXT WEEK.  
MEAN TROUGHING EXTENDING FROM THE BERING SEA TO HAWAI'I WILL HELP  
TO AMPLIFY A RIDGE JUST OFF THE WEST COAST AND EXTENDING WELL  
NORTHWARD THROUGH AND BEYOND ALASKA, WHILE POSITIVELY TILTED MEAN  
TROUGHING WILL BECOME ESTABLISHED FROM NEAR HUDSON BAY THROUGH THE  
CENTRAL U.S. AND LIKELY INTO NORTHERN MEXICO. THERE ARE SOME  
SIMILARITIES TO OTHER RECENT AMPLIFIED EPISODES OVER THE CONUS IN  
RECENT WEEKS, BUT A STARK DIFFERENCE THIS TIME SEEMS TO BE THE  
WESTERN ATLANTIC SUBTROPICAL RIDGE ATTEMPTING TO BUILD INTO THE  
SOUTHEASTERN STATES. THIS PATTERN WOULD EVENTUALLY FAVOR A  
PERSISTENT, SHARP, AND WAVY BAROCLINIC ZONE FROM NEAR THE SOUTHERN  
PLAINS NORTHEASTWARD AS ARCTIC AIR IS ABLE TO PLUNGE SOUTH INTO  
THE NORTH CENTRAL CONUS WHILE WARM AND MOIST AIR FROM THE GULF OF  
MEXICO SPREADS NORTH INTO THE LOWER MS VALLEY AND SOUTHEAST.  
 
   
..GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
CURRENT SATELLITE IMAGERY SEEMS TO BE SUPPORTING LATEST GUIDANCE  
TRENDS THROUGH THE 00Z/06Z CYCLES THAT UPPER TROUGH ENERGY NOW  
OVER THE WEST SHOULD FAIRLY SOON FORM A CLOSED LOW SUFFICIENTLY  
SEPARATED FROM THE WESTERLIES THAT IT WILL REACH NO FARTHER EAST  
THAN THE SOUTHERN ROCKIES/NORTHERN MEXICO BY THE START OF THE  
MEDIUM RANGE PERIOD. IMPROVED AGREEMENT IN THE SHORT RANGE  
EVOLUTION LEADS TO BETTER CLUSTERING THAN SEEN PREVIOUSLY AS THE  
REMAINING SHORTWAVE EJECTS ACROSS THE CENTRAL/EASTERN STATES.  
WIDE SPREAD/RUN TO RUN VARIABILITY IN PRIOR CYCLES STILL PROVIDE  
LINGERING UNCERTAINTY FOR THE FORECAST OF THIS FEATURE. THE  
CURRENT ARRAY OF GUIDANCE SUGGESTS THE 00Z-06Z GFS/GEFS MEAN RUNS  
MAY BE SOMEWHAT TOO SUPPRESSED WITH THE SURFACE REFLECTION  
CROSSING THE SOUTHERN TIER DURING THE FIRST HALF OF THE PERIOD.  
 
SIMILAR TO YESTERDAY, THROUGH ABOUT NEXT THU THE MAJORITY OF  
GUIDANCE APPEARS FAIRLY WELL CLUSTERED FOR THE UPPER TROUGH  
AMPLIFYING INTO THE WEST WITH AN EMBEDDED LOW CENTER DROPPING  
SOUTHEAST FROM NEAR HAIDA GWAII TO UTAH. THEN MODELS/ENSEMBLES  
DIVERGE AND VARY FROM RUN TO RUN REGARDING THE STRENGTH OF HEIGHT  
FALLS ACROSS THE NORTHERN TIER/GREAT LAKES AND HOW MUCH TROUGHING  
LINGERS OVER THE SOUTHWEST. DIFFERENCES IN TELECONNECTION FAVORED  
FLOW FOR THE SOUTHWEST U.S. RELATIVE TO NEGATIVE HEIGHT ANOMALIES  
NORTH OF HAWAI'I (HIGHER HEIGHTS) AND POSITIVE ANOMALIES NEAR THE  
ALASKA PANHANDLE (MORE TROUGHING) IN RECENT D+8 MEAN CHARTS  
SUPPORT THE IDEA THAT THE SOUTHWEST U.S. IS THE AREA OF GREATEST  
UNCERTAINTY. THIS FAVORS AN INTERMEDIATE SOLUTION FOR SOUTHWEST  
TROUGHING. LATE IN THE PERIOD THE 00Z/06Z GFS, 00Z CMC, AND 00Z  
ECMWF MEAN ARE CLOSEST TO A MIDDLE GROUND WHILE THE 00Z ECMWF  
STRAYS EASTWARD AND THE 06Z GEFS MEAN LAGS. FARTHER  
NORTHEASTWARD, ENSEMBLE CLUSTERING CONTINUES TO INDICATE THAT LOW  
PRESSURE EMERGING FROM THE WEST IS MOST LIKELY TO REACH THE GREAT  
LAKES BY EARLY DAY 6 FRI AND NORTHEAST OF MAINE BY SAT. OVERALL  
THE OPERATIONAL SOLUTIONS FROM THE PAST DAY HAVE TENDED TO  
GRAVITATE TOWARD THE WEAKER HALF OF THE SPREAD FOR LOW PRESSURE  
DEPTH. SIMILAR STRENGTH IN THE ENSEMBLE MEANS WOULD SUGGEST THAT  
THERE ARE STILL SOME MEMBERS THAT KEEP A DEEPER SCENARIO IN PLAY.  
 
FINALLY THERE HAS BEEN DECENT AGREEMENT/CONTINUITY FOR A WEAK  
CLIPPER SYSTEM THAT MAY REACH THE UPPER GREAT LAKES BY LATE IN THE  
PERIOD. SUCH FEATURES TEND TO BE IN THE LOWER HALF OF THE  
PREDICTABILITY SPECTRUM SO IT WILL NOT BE SURPRISING IF SOME  
ASPECTS OF THIS SYSTEM CHANGE IN SUBSEQUENT DAYS.  
 
FOR THE MOST PART THE UPDATED FORECAST REFLECTS ONLY MINOR  
REFINEMENTS TO CONTINUITY. DAYS 3-5 EMPHASIZED THE OPERATIONAL  
MODELS WITH GFS/ECMWF/UKMET/CMC WEIGHTS SET TO REFLECT CONSENSUS  
DETAILS. DAYS 6-7 STARTED WITH A MORE EVEN WEIGHT OF  
MODEL/ENSEMBLE GUIDANCE WITH THE ECMWF MEAN GIVEN HIGHER PRIORITY  
THAN THE 06Z GEFS MEAN GIVEN BETTER DEFINITION OF THE GREAT LAKES  
SYSTEM AND INTERMEDIATE UPPER TROUGH POSITION OVER THE SOUTHWEST.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE PACIFIC NORTHWEST AND NORTHERN/CENTRAL ROCKIES WILL BE  
UNSETTLED EARLY IN THE PERIOD AS A STRONG LOW PRESSURE SYSTEM  
MOVES INLAND TUE-WED. HEAVY RAINS ALONG THE COAST AND MOUNTAIN  
SNOWS WILL BE POSSIBLE. FARTHER EAST, CONFIDENCE IS INCREASING  
WITH RESPECT TO HEAVY RAIN POTENTIAL FROM THE SOUTHERN PLAINS TO  
THE SOUTHEAST/SOUTHERN APPALACHIANS. EXPECT ONE ROUND OF ENHANCED  
ACTIVITY TUE-WED IN ASSOCIATION WITH THE EJECTING SOUTHERN ROCKIES  
SHORTWAVE AND ASSOCIATED FRONTAL WAVE, FOLLOWED BY AN EPISODE OF  
SHOWERS AND THUNDERSTORMS FRI-SAT AS AN UPSTREAM COLD FRONT  
CROSSES THE REGION. THE FRONT MAY STALL IN THE GULF OF MEXICO  
WITH OVERRUNNING MOISTURE MAINTAINING PERIODS OF RAINFALL OVER THE  
SOUTHERN STATES. A COUPLE ROUNDS OF SNOW ARE POSSIBLE ACROSS THE  
NORTHERN PLAINS/UPPER MIDWEST THU-SAT, THE FIRST WITH A SURFACE  
LOW MOVING FROM THE CENTRAL PLAINS ON THU TO THE GREAT LAKES ON  
FRI WITH POTENTIAL FOR IT TO BE ACCOMPANIED BY FAIRLY BRISK WINDS  
DEPENDING ON SYSTEM STRENGTH. A CLIPPER SYSTEM QUICKLY CROSSING  
THE NORTHERN PLAINS/UPPER MIDWEST LATE FRI INTO SAT COULD BRING  
ANOTHER ROUND OF SNOW ALONG WITH A MUCH COLDER AIR MASS.  
 
TEMPERATURES WILL INITIALLY BE ABOVE AVERAGE ACROSS MUCH OF THE  
CENTRAL/EASTERN U.S. TUE-THU BEFORE AN AREA OF ARCTIC HIGH  
PRESSURE BEGINS TO NOSE SOUTHWARD EAST OF THE ROCKIES THU INTO  
FRI, BRINGING BELOW AVERAGE TEMPERATURES TO THE NORTH CENTRAL U.S.  
THE AFOREMENTIONED SUBTROPICAL RIDGE WILL KEEP ABOVE AVERAGE  
TEMPS IN PLACE ACROSS THE SOUTHEAST INTO NEXT WEEKEND.  
 
RAUSCH/RYAN  
 

 
 
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