367  
FXUS02 KWBC 281600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1159 AM EDT MON MAY 28 2018  
 
VALID 12Z THU MAY 31 2018 - 12Z MON JUN 04 2018  
 
...RECORD HEAT EXPECTED ACROSS THE SOUTHERN PLAINS & WESTERN GULF  
COAST...  
...POST-TROPICAL CYCLONE ALBERTO TRACKS FROM THE GREAT LAKES INTO  
CANADA...  
 
   
..OVERVIEW
 
 
THE EXPECTED TROUGH-RIDGE-TROUGH MEAN PATTERN COMPARES FAVORABLY  
TO THAT EXPECTED FROM TELECONNECTIONS RELATIVE TO THE CORE OF  
POSITIVE HEIGHT ANOMALIES WHICH MULTI-DAY MEANS SHOW OVER THE  
NORTHERN PACIFIC NEAR 150-160W LONGITUDE. THIS PROVIDES SOME  
DEGREE OF CONFIDENCE IN THE LARGE SCALE FORECAST BUT  
SPREAD/VARIABILITY FOR INDIVIDUAL FEATURES WITHIN THE MEAN FLOW  
SUGGEST A FAIR AMOUNT OF UNCERTAINTY IN THE SPECIFICS. MEANWHILE  
RELATIVELY STRONG ABOVE NORMAL HEIGHT ANOMALIES PERSISTING ACROSS  
THE NORTH ATLANTIC AND SOUTHEAST CANADA THROUGH THE PERIOD WILL  
CORRESPOND TO BLOCKY FLOW WITH THE PATTERN POSSIBLY TAKING ON THE  
SHAPE OF A REX BLOCK FOR A TIME ACROSS SOUTHEAST CANADA AND THE  
EAST.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
ALREADY IN THE FIRST HALF OF THE PERIOD THERE ARE SOME CONTINUITY  
ADJUSTMENTS/SPREAD WITH THE COMBINATION OF EJECTING WEST COAST  
TROUGH ENERGY AND INCOMING FLOW THAT AMPLIFIES TOWARD THE WEST  
COAST. THERE IS A STRONG SIGNAL FROM THE 00Z/06Z MODELS/MEANS  
THAT THE LEADING FEATURE WILL TRACK INLAND FASTER THAN PREVIOUSLY  
FORECAST. A COMPROMISE AMONG THE LATEST AVAILABLE OPERATIONAL  
RUNS (06Z GFS AND 00Z ECMWF/UKMET/CMC) REFLECTS THIS SCENARIO  
WITHOUT GETTING TOO FAR ON THE FASTER SIDE OF THE ENVELOPE THAT IS  
REPRESENTED BY THE 00Z GFS/ECMWF. GIVEN THE LACK OF CONSISTENCY  
IN THE GUIDANCE OVER THE PAST COUPLE DAYS, A SIMILAR APPROACH  
PROVIDES THE BEST RISK-MANAGED FORECAST FOR THE INCOMING EASTERN  
PACIFIC TROUGH--BETWEEN THE DEEPER 00Z GFS/CMC AND FLATTER 06Z  
GFS/00Z ECMWF.  
 
DURING THE LATTER HALF OF THE FORECAST EXPECT UPPER TROUGHING TO  
DEEPEN SOMEWHAT AS IT SLOWLY SETTLES OVER OR JUST INLAND FROM THE  
WEST COAST, WITH A MODEL/ENSEMBLE MEAN BLEND REPRESENTING  
CONSENSUS WELL. BY DAY 7 MON THE 06Z GFS BECOMES THE FAST EXTREME  
WITH ITS EMBEDDED UPPER LOW. FARTHER EAST THE MAIN QUESTION MARKS  
WILL BE TO WHAT EXTENT AND WHERE EASTERN U.S. TROUGH ENERGY FORMS  
A CLOSED LOW AND THEN HOW THE EJECTING WESTERN SHORTWAVE MAY  
INTERACT WITH THE INITIAL ENERGY OVER THE EAST. THUS FAR THE  
GFS/CMC ARE MORE AGGRESSIVE WITH A CLOSED/WESTWARD FEATURE BY THE  
WEEKEND AND ALSO ALLOW THE UPSTREAM ENERGY TO HOLD THE OVERALL  
FEATURE FAIRLY FAR WESTWARD INTO EARLY NEXT WEEK.  
TELECONNECTION-FAVORED FLOW SUGGESTS THE GFS CLUSTER COULD BE A  
LITTLE TOO FAR WEST WITH THE OVERALL MEAN TROUGH BUT THE BLOCKY  
PATTERN COULD LEAD TO A SLOWER/LESS OPEN EVOLUTION THAN ADVERTISED  
BY THE ECMWF/ECMWF MEAN. THUS AN EVEN BLEND AMONG THE TWO  
CLUSTERS WOULD APPEAR BEST AT THIS TIME.  
 
...WEATHER HIGHLIGHTS AND THREATS  
 
SOME COMBINATION OF POST-TROPICAL CYCLONE ALBERTO'S ENERGY ALOFT  
AND AN APPROACHING SHORTWAVE WILL LIKELY DROP SOUTHEAST INTO THE  
MID-ATLANTIC STATES, SETTING THE STAGE FOR POTENTIAL HEAVY  
RAINFALL OVER PARTS OF THE MID-ATLANTIC REGION AND VICINITY.  
UNCERTAINTY OVER WHETHER AND WHERE UPPER FLOW MAY CLOSE OFF A LOW  
TEMPERS CONFIDENCE IN LOCATION/EXTENT OF ANY HEAVY RAIN. ABUNDANT  
MOISTURE ALONG/AHEAD OF A FRONTAL BOUNDARY WILL ALSO PROVIDE A  
FOCUS FOR SOME EPISODES OF LOCALLY HEAVY RAINFALL OVER  
CENTRAL/SOUTHERN LATITUDES OF THE SOUTH AND EAST. A SHORTWAVE  
EJECTING FROM THE WEST COAST SHOULD GENERATE SOME RAINFALL OVER  
NORTHERN PARTS OF THE WEST LATE THIS WEEK AND LOCALLY HEAVY  
CONVECTION OVER THE PLAINS/MS VALLEY FRI INTO THE WEEKEND. THIS  
ENERGY ALOFT MAY PLAY A ROLE IN RAINFALL DISTRIBUTION/INTENSITY  
OVER THE EAST TOWARD SUN-MON. THE UPSTREAM TROUGH ARRIVING AT THE  
WEST COAST SHOULD BRING SOME AREAS OF PRECIP TO THE NORTHWEST.  
 
THE DOMINANT FOCUS OF THE TEMPERATURE FORECAST WILL BE ON A  
MULTI-DAY SOUTHERN PLAINS HEAT EVENT WHOSE INFLUENCE MAY EXTEND AT  
TIMES WESTWARD INTO THE ROCKIES AND EASTWARD INTO THE LOWER MS  
VALLEY. 100-110F READINGS SHOULD BE MOST COMMON IN THE SOUTHERN  
HIGH PLAINS/LOWER RIO GRANDE. IT IS LIKELY THAT RECORD VALUES FOR  
MAX/WARM MIN WILL BE REACHED/EXCEEDED ON MULTIPLE DAYS, WITH SOME  
OF THE RECORD-BREAKING HEAT ALSO EXTENDING FOR A TIME FARTHER  
NORTH/NORTHEAST. ON THE OTHER HAND MEAN TROUGHING ALOFT WILL TEND  
TO KEEP HIGHS NEAR TO BELOW NORMAL NEAR THE WEST COAST AND BY THE  
WEEKEND/EARLY NEXT WEEK OVER THE EAST.  
 
RAUSCH/ROTH  
 
WPC MEDIUM RANGE FORECASTS OF 500 MB HEIGHTS, SURFACE SYSTEMS,  
WEATHER GRIDS, QUANTITATIVE PRECIPITATION, AND WINTER WEATHER  
OUTLOOK PROBABILITIES CAN BE FOUND AT:  
 
HTTP://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTP://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTP://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTP://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTP://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
 

 
 
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