812  
FXUS02 KWBC 131600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1059 AM EST WED DEC 13 2017  
 
VALID 12Z SAT DEC 16 2017 - 12Z WED DEC 20 2017  
   
..OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
MOST GUIDANCE IS ADHERING TO SIMILAR IDEAS FOR THE LARGE SCALE  
PATTERN EVOLUTION DURING THE PERIOD, WITH INCREASED INFLUENCE FROM  
NORTH PACIFIC FLOW BUT A TENDENCY TOWARD EASTERN U.S. MEAN  
TROUGHING BY NEXT TUE-WED IN RESPONSE TO A BROAD RIDGE NEAR THE  
WEST COAST. NORTHEAST PACIFIC TROUGHING SHOULD MAKE GRADUAL  
PROGRESS TOWARD THE NORTHWEST TOWARD MIDWEEK. IN CONTRAST TO THE  
EXPECTED MEAN FLOW THERE ARE STILL SIGNIFICANT DETAIL AND TIMING  
UNCERTAINTIES THAT REDUCE CONFIDENCE IN ANY SPECIFIC DETERMINISTIC  
FORECAST.  
 
ONE KEY UNCERTAINTY CONTINUES TO INVOLVE THE SHORTWAVE OVER THE  
WEST AS OF THE START OF THE PERIOD DAY 3 SAT. EVEN AT THIS TIME  
THERE ARE SIGNIFICANT DIFFERENCES IN WHAT HAPPENS TO ENERGY IN THE  
SOUTHWEST PART OF THE OVERALL FEATURE, SUCH AS THE OFFSHORE CLOSED  
LOW IN RECENT ECMWF RUNS (WITH EVENTUAL MIGRATION INLAND) OR VERY  
SHEARED DEPICTION IN GFS RUNS. THE UKMET/CMC ARE BETWEEN THE TWO  
EXTREMES BUT STILL WITH SUCH A SMALL FEATURE THAT CONFIDENCE IS  
MINIMAL. EVEN WITH THESE DIFFERENCES, THERE ARE POSITIVE TRENDS  
TOWARD BETTER CLUSTERING BY DAY 4 SUN AS ENSEMBLE MEANS ARE  
FURTHERING THEIR TRENDS TOWARD MORE ENERGY COLLECTING OVER THE  
FOUR CORNERS REGION AT THAT TIME WHILE THERE IS DECENT AGREEMENT  
FROM THE OPERATIONAL MODELS ALBEIT WITH TYPICAL DEPTH/AMPLITUDE  
DIFFERENCES. THEN MODELS/ENSEMBLES RAPIDLY DIVERGE FOR EJECTION  
OF THIS SHORTWAVE WITH THE 00Z ECMWF/UKMET/CMC ALL BECOMING FASTER  
THAN 00Z/06Z GFS RUNS. THIS TIMING DIFFERENCE APPEARS TO BE  
CONNECTED TO TIMING/AMPLITUDE OF APPROACHING NORTH PACIFIC FLOW  
THAT ULTIMATELY CARVES OUT THE ANTICIPATED EASTERN U.S. TROUGH BY  
MIDWEEK, AS THE GFS IS BY FAR THE SLOWEST WITH THE  
ARRIVAL/AMPLIFICATION OF THAT UPSTREAM FLOW. SO WHILE ENSEMBLE  
TRENDS/GFS CONTINUITY DO SUPPORT A SLOWER SCENARIO INTO SUN-MON,  
IT BECOMES MORE QUESTIONABLE TO WHAT EXTENT THE SLOW SIDE OF THE  
SPREAD WILL PREVAIL THEREAFTER. REGARDING FLOW AHEAD OF THE  
INITIAL WESTERN SHORTWAVE, THERE IS NOW MUCH BETTER AGREEMENT ON  
AN UPPER TROUGH/LOW EJECTING OUT OF NORTHERN MEXICO.  
 
MEANWHILE GUIDANCE HAS BEEN SHOWING A LOT OF DIFFICULTY IN  
RESOLVING THE EXTENT/TIMING OF HEIGHT FALLS REACHING THE NORTHWEST  
U.S. LATE IN THE PERIOD. JUST OVER THE PAST DAY GFS RUNS HAVE  
VARIED DRAMATICALLY BETWEEN DEEP TROUGHING AND FLAT RIDGING AS OF  
12Z NEXT WED, AND WHILE ECMWF RUNS HAVE TENDED TO ADVOCATE FOR THE  
TROUGHING SIDE OF THE SPECTRUM ITS PAST COUPLE RUNS ARE NOT AS  
AMPLIFIED AS YESTERDAY'S 12Z RUN. 00Z ENSEMBLE MEMBERS SHOW  
CONSIDERABLE SPREAD AS WELL, RANGING BETWEEN RIDGE AND TROUGH  
SOLUTIONS BY NEXT WED. WITH D+8 MEANS OVER THE PAST COUPLE DAYS  
FAIRLY CONSISTENT IN SHOWING A CORE OF POSITIVE HEIGHT ANOMALIES  
JUST SOUTH OF MAINLAND ALASKA, TELECONNECTIONS SUGGEST THAT  
POSITIVELY TILTED MEAN TROUGHING SHOULD DEVELOP OVER THE NORTHWEST  
BUT EXACTLY HOW MUCH TROUGHING OCCURS IS FAIRLY SENSITIVE TO THE  
EXACT STRENGTH/LOCATION OF THE POSITIVE ANOMALY CENTER. THUS IT  
WILL LIKELY TAKE ADDITIONAL TIME TO ACHIEVE BETTER  
AGREEMENT/STABILITY FOR THE PATTERN OVER THE EASTERN  
PACIFIC/WESTERN U.S.. RECENT GEFS/ECMWF MEAN TRENDS TO DELAY  
HEIGHT FALLS SOMEWHAT LEAVE THE 00Z CMC MEAN AS HAVING THE DEEPEST  
TROUGHING AMONG THE MEANS BY MIDWEEK.  
 
BASED ON ABOVE CONSIDERATIONS, THE UPDATED FORECAST DURING THE  
FIRST HALF OF THE PERIOD INCORPORATES SOMEWHAT MORE 06Z GFS/GEFS  
MEAN GUIDANCE IN TOTAL VERSUS OTHER SOLUTIONS THOUGH IT INCLUDES A  
SMALL WEIGHT OF THE 00Z ECMWF/CMC/UKMET TO ACCOUNT FOR THEIR  
COMMON THOUGH PERHAPS SLIGHTLY LESS LIKELY IDEAS. THEN THE BLEND  
TRENDS TOWARD A 06Z GEFS/00Z ECMWF MEAN SOLUTION GIVEN OPERATIONAL  
SPREAD, WHILE MAINTAINING A SLIGHT TILT TO THE GEFS MEAN.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
OVER THE PAST DAY MODEL/ENSEMBLE GUIDANCE HAS COME INTO BETTER  
RELATIVE AGREEMENT REGARDING THE POTENTIAL FOR AREAS OF LOCALLY  
HEAVY RAINFALL FROM THE ARKLATEX REGION EASTWARD ACROSS THE  
SOUTHEAST/GULF COAST, IN RESPONSE TO THE SHORTWAVE/UPPER LOW  
EJECTING OUT OF MEXICO AND THE SHORTWAVE EXPECTED TO EMERGE FROM  
THE FOUR CORNERS REGION AFTER SUN. TIMING OF SUPPORTING DYNAMICS  
IS STILL A QUESTION MARK BUT THERE IS IMPROVING CONFIDENCE FOR THE  
OVERALL HEAVY RAIN THREAT. FARTHER NORTH THERE WILL BE PERIODS OF  
PRECIP WITH SNOW IN NORTHERNMOST AREAS. SOME LAKE EFFECT ACTIVITY  
AND/OR LAKE ENHANCEMENT WITH SYNOPTIC SYSTEMS IS POSSIBLE.  
FAVORED TERRAIN FROM THE PACIFIC NORTHWEST SOUTHEAST ACROSS THE  
ROCKIES WILL BE ANOTHER AREA OF PRECIP FOCUS. EXACT TOTALS OF  
RAIN/HIGHER ELEVATION SNOW OVER THE NORTH WILL BE HIGHLY DEPENDENT  
ON EXTENT OF LATE PERIOD HEIGHT FALLS ALOFT-- A FAIRLY LOW  
CONFIDENCE ASPECT OF THE FORECAST AT THIS TIME. MOST ACTIVITY  
OVER THE SOUTHERN HALF OF THE ROCKIES SHOULD BE WITH THE LATE  
WEEKEND/EARLY WEEK SHORTWV CROSSING THE REGION.  
 
EXPECT THE MOST PERSISTENT WARMTH DURING SAT-WED TO BE OVER THE  
NORTHERN HALF OF THE PLAINS WITH SOME LOCATIONS IN THE NORTHERN  
TIER POSSIBLY SEEING MINS AVERAGING 15-20F ABOVE NORMAL FOR THE  
FULL 5-DAY PERIOD. HIGHEST MAX TEMP ANOMALIES MAY END UP BEING  
CLOSER TO THE CENTRAL PLAINS. CALIFORNIA WILL TEND TO SEE ABOVE  
NORMAL TEMPS WITH BROADER COVERAGE OF POSITIVE ANOMALIES THERE AND  
OVER THE GREAT BASIN NEXT WEEK, AFTER THE GREAT BASIN AND FOUR  
CORNERS SEE A BRIEFLY COOLER PERIOD WITH PASSAGE OF THE SHORTWAVE  
ALOFT. THE EASTERN STATES WILL SEE SOME BELOW NORMAL READINGS  
DURING THE WEEKEND BEFORE A TREND TO NEAR OR ABOVE NORMAL LEVELS.  
 
RAUSCH  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page