989  
FXUS02 KWBC 091524  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1124 AM EDT THU MAY 09 2019  
 
VALID 12Z SUN MAY 12 2019 - 12Z THU MAY 16 2019  
 
15 UTC UPDATE...  
 
USED A BLEND OF THE LATEST DETERMINISTIC GUIDANCE (00Z ECMWF/CMC  
AND 06Z GFS) DURING DAYS 3-4 (SUN-MON) FOR THIS FORECAST UPDATE,  
WITH A GRADUAL SHIFT TO HEAVIER ENSEMBLE MEAN WEIGHTING (ECENS AND  
NAEFS) DURING DAYS 5-7 (TUE-THU), AND REMOVAL OF THE CMC FROM THE  
BLEND ENTIRELY AFTER DAY 5. ONE OF THE MORE SIGNIFICANT POINTS OF  
CONTENTION AMONG THE GUIDANCE REMAINS THE TRANSFER OF ENERGY FROM  
SYSTEM CROSSING THE GREAT LAKES TO A LOW OFF THE MID-ATLANTIC  
COASTLINE SUN-MON, AND THE EVENTUAL TRACK OF THE COASTAL SYSTEM.  
MODELS ARE RATHER POORLY CLUSTERED WITH THE COASTAL LOW FROM MON  
MORNING ONWARD, WITH THE ECMWF A GOOD BIT FARTHER NORTH AND CLOSER  
TO THE COAST THAN MUCH OF THE OTHER GUIDANCE. ENSEMBLE MEMBERS  
SEEM UNDER DISBURSED, WITH MOST ECENS MEMBERS CLUSTERED AROUND THE  
ECMWF AND SIMILARLY FOR THE GFS/GEFS AND CMC/CMCE. THUS, A  
COMPROMISE SOLUTIONS SEEMS BEST AT TIME. THE GFS/ECMWF DO AGREE  
THAT THE LOW PRESSURE SYSTEM WILL LINGER NEAR THE NEW ENGLAND  
COAST INTO WED (WHILE THE CMC QUICKLY MOVES THE SYSTEM AWAY FROM  
THE REGION), LEADING TO A PREFERRED ECMWF/GFS BLEND ALONG WITH THE  
AFOREMENTIONED ENSEMBLE MEANS BY THAT TIME FRAME. ELSEWHERE,  
MODELS HANDLE ADDITIONAL NORTHERN STREAM ENERGY DIVING INTO THE  
MIDWEST/GREAT LAKES WED/THU SIMILARLY, AND THE SAME HOLDS TRUE FOR  
SLOW-MOVING SOUTHERN STREAM ENERGY PROGRESSING ACROSS THE SOUTHERN  
PLAINS/LOWER MISSISSIPPI VALLEY THROUGH THE PERIOD. FINALLY, BROAD  
AGREEMENT WAS NOTED THAT AN AMPLIFIED UPPER TROUGH SHOULD REACH  
THE WEST COAST BY WED-THU. MODELS DIFFER ON THE EXACT TIMING AND  
CHARACTER OF THE FEATURE, BUT A HEAVILY ENSEMBLE MEAN-BASED BLEND  
BY THAT TIME SHOULD SMOOTH OUT ANY DIFFERENCES AND REPRESENT A  
GOOD CONSENSUS SOLUTION AT THIS TIME.  
 
RYAN  
 
PREVIOUS DISCUSSION (ISSUED AT 0659 UTC)...  
   
..OVERVIEW
 
 
MOST GUIDANCE SHOWS THE LARGE SCALE MEAN FLOW TRANSITIONING TO A  
TROUGH-RIDGE-TROUGH PATTERN ALOFT BY THE MID-LATE PART OF NEXT  
WEEK. ON THE WAY TO THIS CONFIGURATION, A STRONG UPPER TROUGH  
WILL CROSS THE EASTERN HALF OF THE COUNTRY SUN-WED WITH FINE-SCALE  
DETAILS LIKELY PLAYING A ROLE IN EXACT EVOLUTION OF SURFACE LOW  
PRESSURE AND COVERAGE/INTENSITY OF PRECIPITATION. UPSTREAM ENERGY  
FEEDING INTO THE MEAN TROUGH SHOULD PUSH A FRONT INTO THE NORTHERN  
PLAINS EARLY IN THE WEEK AND FARTHER SOUTHEAST THEREAFTER. OVER  
THE WEST THE PATTERN WILL TAKE ON A REX BLOCK APPEARANCE THROUGH  
THE FIRST PART OF NEXT WEEK AS AN UPPER LOW TRACKS ACROSS EXTREME  
SOUTHERN AREAS WHILE A RIDGE DRIFTS OVER THE NORTH-CENTRAL  
LATITUDES. IN PRINCIPLE THE UPPER TROUGHING THAT AMPLIFIES TOWARD  
THE WEST COAST HAS FAIRLY GOOD CONFIDENCE BUT THERE IS FAIR DEGREE  
OF DETAIL UNCERTAINTY.  
 
   
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
 
 
GUIDANCE IS STILL IN THE PROCESS OF TRYING TO RESOLVE SPECIFICS OF  
THE UPPER TROUGH CROSSING THE EAST SUN ONWARD, ALONG WITH THE  
CORRESPONDING SURFACE EVOLUTION. CONSENSUS HAS THE TROUGH  
CORE--MOST LIKELY A CLOSED LOW--PROGRESSING FROM THE UPPER MIDWEST  
THROUGH THE GREAT LAKES AND NORTHEAST. THE 18Z GFS/FV3 GFS  
STRAYED SOUTH OF CONSENSUS WITH THE UPPER LOW, ULTIMATELY LEADING  
TO AN OFFSHORE SURFACE LOW TO THE SOUTH OF MOST OTHER  
MODELS/ENSEMBLE MEMBERS BY TUE. THE NEW 00Z GFS MADE A PARTIAL  
NORTHWARD ADJUSTMENT WHILE THE 00Z UKMET HAS TRENDED SOUTH, AT  
LEAST MAINTAINING SUPPORT FOR A COMPROMISE RELATIVE TO THE FARTHER  
NORTH 12Z ECMWF/ECMWF MEAN. BASED ON GUIDANCE THROUGH THE 18Z  
CYCLE THE PREFERRED MULTI-MODEL/ENSEMBLE BLEND (USING THE 12Z RUN  
FOR THE GFS COMPONENT) YIELDED A SURFACE LOW TRACK BETWEEN THE 18Z  
GEFS/12Z ECMWF MEANS BY TUE. CURRENT CLUSTERING HAS THE UPSTREAM  
UPPER MIDWEST WAVE BECOMING FAIRLY SUPPRESSED AFTER EARLY SUN--AS  
PREVIOUSLY ADVERTISED BY SOME ECMWF/UKMET/CMC RUNS.  
 
THE NEXT FRONTAL SYSTEM DROPPING INTO THE NORTHERN PLAINS BY MON  
AND THEN CONTINUING SOUTHEAST HAS GOOD AGREEMENT ON EXISTENCE BUT  
DIFFERENCES ALOFT HAVE BEEN RESULTING IN MEANINGFUL TIMING SPREAD  
AND RUN-TO-RUN VARIABILITY. THIS FAVORS A MODEL/MEAN BLEND AS THE  
BEST DETERMINISTIC STARTING POINT. THE AVERAGE OF CURRENT  
GUIDANCE YIELDS A SLOWER TREND COMPARED TO THE CYCLE 12 HOURS AGO.  
NOTE THAT THE 00Z GFS HAS A LOW-CONFIDENCE NORTHWEST U.S.  
SHORTWAVE AROUND MON WHICH DISRUPTS DOWNSTREAM FLOW IN LATER DAYS,  
SO CONSENSUS OF OTHER MODELS/MEANS WOULD APPEAR BETTER.  
 
MULTI-DAY TRENDS FOR THE UPPER LOW FORECAST TO BE JUST SOUTHWEST  
OF ARIZONA AS OF EARLY SUN HAVE GENERALLY BEEN SLOWER ALONG WITH  
MAINTAINING SOMEWHAT BETTER DEFINITION AS THE FEATURE REACHES THE  
PLAINS. THIS FAVORS SOMEWHAT MORE ACCOUNT FOR OPERATIONAL MODEL  
SOLUTIONS RELATIVE TO THE MEANS LATE IN THE PERIOD VERSUS  
PREFERENCE FOR OTHER AREAS OF THE FORECAST DOMAIN.  
 
FOR THE AMPLIFYING UPPER TROUGH APPROACHING THE WEST COAST,  
MODEL/ENSEMBLE BEHAVIOR THUS FAR FAVORS SOLUTIONS IN THE MIDDLE TO  
SLOWER HALF OF THE SPREAD. IN CONTRAST TO GEFS MEAN RUNS THAT  
HAVE BEEN COMPARABLE TO THE ECMWF MEAN ASIDE FROM MODEST DETAILS,  
THE OPERATIONAL GFS HAS TENDED TO BE FASTER BRINGING IN HEIGHT  
FALLS OVER AT LEAST A PART OF THE WEST--NORTHERN AREAS IN SOME  
RUNS, FARTHER SOUTH IN OTHERS. THIS TILTS PREFERENCE TOWARD THE  
CMC/ECMWF AND GEFS/ECMWF MEANS.  
 
BASED ON ABOVE CONSIDERATIONS THE FORECAST STARTED WITH VARYING  
WEIGHT OF THE 12Z MODELS AND MINORITY 18Z GEFS/12Z ECMWF MEAN  
INPUT FOR ABOUT THE FIRST HALF OF THE PERIOD AND THEN RAPIDLY  
ADJUSTED TO 60-70 PERCENT WEIGHT OF THE ENSEMBLE MEANS WHILE  
KEEPING SOME IDEAS OF THE ECMWF/CMC.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
EXPECT A BROAD SHIELD OF PRECIPITATION OVER THE EAST AS FRONTAL  
WAVINESS LIFTS THROUGH THE REGION ON SUN FOLLOWED BY POSSIBLE  
CONSOLIDATION OF LOW PRESSURE OFF THE NORTHEASTERN COAST. BEST  
POTENTIAL FOR HIGHEST TOTALS WITH THIS SYSTEM SHOULD EXTEND FROM  
THE CENTRAL GULF COAST THROUGH THE MID-ATLANTIC BUT WITH LINGERING  
POTENTIAL FOR SIGNIFICANT AMOUNTS A BIT NORTHWEST OF THIS AXIS.  
CONFIDENCE IN SPECIFICS REMAINS LOWER THAN DESIRED IN LIGHT OF  
PERSISTENT MODEL SPREAD. AT LEAST SOME OF THE PRECIPITATION COULD  
FALL IN THE FORM OF SNOW OVER HIGHEST ELEVATIONS OF NEW ENGLAND AS  
THE CORE OF THE UPPER TROUGH PASSES THROUGH. BEHIND THIS SYSTEM A  
COLD FRONT DROPPING SOUTHEAST FROM THE NORTHERN PLAINS MAY SPREAD  
RAINFALL OF VARIOUS INTENSITY THROUGH THE NORTHERN PLAINS AND  
MIDWEST/GREAT LAKES/OHIO VALLEY.  
 
THE UPPER LOW TRACKING FROM THE WESTERN U.S.-MEXICO BORDER INTO  
THE PLAINS WILL BE RESPONSIBLE FOR LOCALLY ENHANCED AREAS OF  
PRECIPITATION, INCLUDING SOME CONVECTION, OVER SOUTHERN PARTS OF  
THE WEST AND THEN LIKELY EXTENDING INTO PARTS OF THE SOUTHERN  
PLAINS AND VICINITY. CONFIDENCE IS STILL MODEST REGARDING  
SPECIFICS OF SOUTHERN PLAINS ACTIVITY, WHICH WILL BE IMPORTANT TO  
MONITOR GIVEN RECENT AND UPCOMING SHORT-RANGE HEAVY RAINFALL OVER  
THE REGION.  
 
THE UPPER TROUGH NEARING THE WEST COAST SHOULD BRING INCREASED  
CLOUDS/PRECIPITATION TO THE REGION AFTER TUE. HOWEVER  
DISTRIBUTION AND INTENSITY OF PRECIP ARE UNCERTAIN AS GUIDANCE HAS  
YET TO AGREE ON THE FINER DETAILS OF THE TROUGH.  
 
SHORT-RANGE WARMTH OVER THE NORTHWEST WILL CONTINUE INTO THE FIRST  
HALF OF NEXT WEEK, THOUGH WITH A STEADY COOLING TREND OVER  
WASHINGTON/OREGON AS UPPER TROUGHING APPROACHES. EXPECT HIGHS OF  
10-20F ABOVE NORMAL OVER THE INTERIOR NORTHWEST AND NORTHERN  
ROCKIES SUN-TUE WITH COVERAGE OF PLUS 10F ANOMALIES LIKELY  
DECREASING SIGNIFICANTLY BY WED-THU. COOL HIGHS OF 10-15F BELOW  
NORMAL OVER THE SOUTHWEST INTO SOUTHERN HIGH PLAINS WILL MODERATE  
CLOSER TO NORMAL AS WELL. THE SYSTEM AFFECTING THE EAST DURING  
THE FIRST HALF OF THE PERIOD WILL KEEP HIGHS 10-15F OR SO BELOW  
NORMAL FOR ONE OR MORE DAYS FROM PARTS OF THE OHIO VALLEY/EASTERN  
GREAT LAKES INTO THE NORTHERN MID-ATLANTIC/NORTHEAST.  
 
RAUSCH  
 
ADDITIONAL 3-7 DAY HAZARDS INFORMATION CAN BE FOUND ON THE WPC  
MEDIUM RANGE HAZARDS CHART AT:  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP  
 
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDICES ARE AT:  
 
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page