815  
FXUS02 KWBC 040653  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
152 AM EST SUN FEB 04 2018  
 
VALID 12Z WED FEB 07 2018 - 12Z SUN FEB 11 2018  
   
..PATTERN OVERVIEW
 
 
MODELS AND ENSEMBLES GENERALLY AGREE IN MAINTAINING A STRONG MEAN  
EASTERN PACIFIC RIDGE WITH DOWNSTREAM TROUGHING COVERING A  
MAJORITY OF NORTH AMERICA. FINER DETAILS OF A MIDWEEK EASTERN  
U.S. SYSTEM ARE STILL IN THE PROCESS OF BEING RESOLVED BUT THE  
PRIMARY UNCERTAINTIES OF THE FORECAST CONTINUE TO INVOLVE ONE OR  
MORE BUNDLES OF SHORTWAVE ENERGY PROGRESSING AROUND/THROUGH THE  
EAST PACIFIC RIDGE AND CONTINUING INTO THE LOWER 48, WITH NORTHERN  
STREAM CANADIAN FLOW ALSO PLAYING A ROLE IN GUIDANCE SPREAD FOR  
INDIVIDUAL FEATURES.  
 
   
..MODEL GUIDANCE/UNCERTAINTY ASSESSMENT/PREFERENCES
 
 
GUIDANCE STILL SHOWS SOME SPREAD/TRENDING FOR SPECIFICS OF THE  
SYSTEM AFFECTING THE EAST WED INTO THU. COMPARING GUIDANCE  
THROUGH THE 12Z/18Z CYCLES, ENSEMBLES TEMPER CONFIDENCE IN  
SPECIFICS OF THE 12Z ECMWF OR 12Z/18Z GFS SOLUTIONS. SPECIFICALLY  
THE 12Z ECMWF IS A BIT ON THE FASTER SIDE OF THE SURFACE LOW  
SPREAD WITH ITS MEAN SLIGHTLY SLOWER. PREVIOUSLY FAST GFS RUNS  
HAVE SLOWED CONSIDERABLY BUT THE GFS HAS CONTINUED TO EXTEND  
LEADING HEIGHT FALLS ALOFT SOMEWHAT EAST OF CONSENSUS RESULTING IN  
A FARTHER EAST LOW TRACK. GEFS MEANS HAVE BEEN AT LEAST SOMEWHAT  
CLOSER TO OTHER GUIDANCE. THESE CONSIDERATIONS FAVOR A GENERAL  
MODEL/MEAN COMPROMISE. WITH THE ARRIVAL OF THE NEW 00Z GFS/GEFS  
MEAN AND UKMET/CMC, AGREEMENT IS IMPROVING WITH TIMING STILL  
SLOWER THAN THE 12Z ECMWF AND THE NEW GFS COMPARING BETTER TO  
CONSENSUS AT THE SURFACE AND ALOFT.  
 
DIFFERENCES FOR PACIFIC SHORTWAVE ENERGY HEADING INTO/AROUND THE  
RIDGE OFF THE WEST COAST BECOME NOTICEABLE ALREADY IN THE LATE  
SHORT RANGE TIME FRAME. THIS SEEMS TO REFLECT THE TENDENCY FOR  
MEDIUM-SMALLER SCALE SHORTWAVES AND DIFFICULTY IN FORECASTING  
ENERGY PASSING THROUGH/AROUND RIDGES TO KEEP PREDICTABILITY BELOW  
AVERAGE. AS THE ENERGY PROGRESSES INLAND, GUIDANCE DIVERGES IN  
TWO PRIMARY RESPECTS--ONE BEING THE TRACK AND TIMING OF LOW  
PRESSURE WHICH MOST GUIDANCE SUGGESTS WILL TRACK ACROSS THE  
NORTH-CENTRAL STATES EAST OF THE ROCKIES PLUS THE SOUTHWARD PUSH  
OF THE TRAILING FRONT, AND THE SECOND BEING THE AMPLITUDE OF  
POTENTIAL TROUGHING OVER THE WEST. FOR THE NORTHERN U.S. WAVE  
RECENT GFS RUNS HAVE BEEN ON THE SLOWER SIDE OF THE ENVELOPE WHILE  
THE ECMWF/CMC AND THEIR MEANS RESIDE ON THE FASTER BUT MAJORITY  
PART OF THE SPREAD. CONFIDENCE IS LOW IN ANY SPECIFIC SOLUTION  
GIVEN DEPENDENCE NOT ONLY ON THE PACIFIC ENERGY BUT ALSO SPECIFICS  
OF CANADIAN FLOW. MEANWHILE THERE IS STILL VERY WIDE  
MODEL/ENSEMBLE SPREAD ALOFT ACROSS THE WEST BY DAYS 5-7 FRI-SUN.  
CMC RUNS AND CORRESPONDING ENSEMBLES CONTINUE TO BE MOST  
AGGRESSIVE WITH DEEPENING A WESTERN TROUGH, WITH OPERATIONAL RUNS  
AND SOME MEMBERS CLOSING OFF AN UPPER LOW. AT THE SAME TIME A  
SOMEWHAT HIGHER PERCENTAGE OF 12Z ECMWF MEMBERS SUGGEST POTENTIAL  
FOR AN AMPLIFIED WESTERN TROUGH COMPARED TO GEFS MEMBERS. AS A  
RESULT BY LATE IN THE PERIOD, THE 12Z CMC MEAN HAS THE DEEPEST  
WESTERN TROUGH AMONG THE MEANS AND THE GEFS MEAN THE WEAKEST.  
ADDING TO THE COMPLEXITY OF THE FORECAST WILL LIKELY BE ANOTHER  
SHORTWAVE APPROACHING THE WEST COAST. THE 12Z ECMWF DOES NOT  
COMPARE WELL TO OTHER GUIDANCE FOR ITS FAST FRI-SAT FEATURE WHILE  
THERE IS A LITTLE MORE OF A SIGNAL FOR A SHORTWAVE ARRIVING AROUND  
DAY 7 SUN. ALBEIT WITH SOME SENSITIVITY TO THE EXACT POSITION OF  
THE POSITIVE HEIGHT ANOMALY CORE OVER THE EAST PACIFIC,  
TELECONNECTIONS RELATIVE TO THAT FEATURE ON AVERAGE STILL FAVOR  
BEST POTENTIAL FOR MODERATE TROUGHING SUCH AS REFLECTED BY A BLEND  
OF GEFS/ECMWF MEANS.  
 
BASED ON THE ABOVE CONSIDERATIONS, THE EARLY-MIDDLE PART OF THE  
FORECAST INCORPORATED EQUAL PARTS OF EACH SOLUTION AMONG THE 12Z  
ECMWF/GFS/UKMET AND 12Z GEFS/ECMWF MEANS TO PROVIDE THE CLOSEST  
REFLECTION OF CONSENSUS/INTERMEDIATE IDEAS. THEN THE FORECAST  
TRENDED TO THE ENSEMBLE MEANS EXCLUSIVELY BY DAY 7 SUN.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
THE MIDWEEK SYSTEM AFFECTING THE EAST WILL LIKELY BRING  
SIGNIFICANT SNOWFALL TO PORTIONS OF THE NORTHEAST, WITH BEST  
PROBABILITIES FOR HIGHEST TOTALS OVER INTERIOR LOCATIONS. TYPICAL  
GUIDANCE ERROR 3-4 DAYS OUT IN TIME ALLOWS FOR SOME FURTHER  
ADJUSTMENT IN EXACT SURFACE LOW TRACK/AXIS OF HEAVIEST SNOW. IN  
THE WARM SECTOR EXPECT SOME AREAS OF LOCALLY HEAVY RAINFALL TO  
CROSS THE SOUTHEAST/SOUTHERN MID ATLANTIC WITH MORE MODERATE  
ACTIVITY FARTHER NORTH. BEHIND THIS SYSTEM, SHORTWAVE ENERGY  
COMING INTO THE NORTHWEST SHOULD HELP TO GENERATE A WAVE AND  
ASSOCIATED PRECIP STREAMING EASTWARD FROM THE NORTHERN ROCKIES  
THROUGH THE GREAT LAKES/NORTHEAST. A MAJORITY OF THIS PRECIP  
SHOULD FALL IN THE FORM OF SNOW ASIDE FROM PERHAPS THE SOUTHERN  
PERIPHERY OF THE MOISTURE SHIELD. FARTHER SOUTH, FROM LATE WEEK  
INTO THE WEEKEND RAINFALL SHOULD DEVELOP OVER THE SOUTHEAST  
PLAINS/LOWER MS VALLEY AND THEN EXTEND NORTHEASTWARD. UNCERTAINTY  
WITH DETAILS OF FLOW ALOFT BY THAT TIME MAKE IT DIFFICULT TO  
PINPOINT TIMING AND INTENSITY BUT THERE IS POTENTIAL FOR SOME OF  
THIS ACTIVITY TO BE LOCALLY HEAVY. IT IS POSSIBLE THAT WINTRY  
PRECIP TYPES MAY EXIST IN THE NORTHWEST FRINGE OF THIS MOISTURE  
DEPENDING ON SOUTHEASTWARD EXTENT OF COLD AIR.  
 
THE WESTERN STATES SHOULD SEE WARMEST TEMPS IN THE WED-FRI TIME  
FRAME WITH DECENT COVERAGE OF PLUS 10-20F ANOMALIES. BEST  
POTENTIAL FOR ISOLATED RECORD VALUES SHOULD BE ON THU. BY NEXT  
WEEKEND THERE SHOULD BE A COOLING TREND BUT WITH TEMPS MOST LIKELY  
REMAINING SOMEWHAT ABOVE NORMAL, BUT THERE IS CONSIDERABLE  
SENSITIVITY TO THE DEGREE OF TROUGHING ALOFT. MEANWHILE EXPECT  
THE COLDEST TEMPS RELATIVE TO NORMAL TO BE OVER THE PLAINS WITH  
ONE OR MORE DAYS OF READINGS AT LEAST 10-20F BELOW NORMAL.  
 
RAUSCH  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page