318  
FXUS02 KWBC 211601  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1200 PM EDT SUN APR 21 2019  
 
VALID 12Z WED APR 24 2019 - 12Z SUN APR 28 2019  
   
..OVERVIEW AND GUIDANCE PREFERENCES
 
 
MODELS AND ENSEMBLES CONTINUE TO SHOW A LOT OF SPREAD FOR POSSIBLE  
SOLUTIONS WITHIN THE PROGRESSIVE NORTHERN STREAM AND SLOWER  
SOUTHERN STREAM, AND DEGREE OF INTERACTION BETWEEN THE TWO  
STREAMS. THIS TYPE OF SCENARIO LENDS ITSELF TO LOW PREDICTABILITY  
AT EXTENDED TIME FRAMES. IN ADDITION LARGER SCALE PATTERN  
DIFFERENCES THAT DEVELOP OVER PARTS OF THE NORTHERN/CENTRAL  
PACIFIC AND BERING SEA CONTRIBUTE TO DIFFERENCES THAT ARISE  
DOWNSTREAM LATE IN THE PERIOD. THESE ISSUES KEEP FORECAST  
CONFIDENCE BELOW AVERAGE--ESPECIALLY BY THE FRI-SUN.  
 
ESPECIALLY WITH THE ARRIVAL OF THE 06Z GFS AND AGREEMENT FROM  
LATEST GEFS MEANS, THERE IS BETTER CLUSTERING FOR THE  
TROUGH/POSSIBLE EMBEDDED UPPER LOW TRACKING EASTWARD ACROSS THE  
SOUTHERN TIER STATES. THE OLD 12Z ECMWF HAD JUMPED SOMEWHAT  
TOWARD MORE PHASING THAN PRIOR RUNS BUT THE NEW 00Z RUN HAS  
REVERSED BACK TO MORE SEPARATION AND NOW THE 00Z CMC IS THE ONLY  
MODEL TO SHOW FASTER PROGRESSION. ENSEMBLE MEANS OFFER REASONABLE  
SUPPORT FOR THE 06Z GFS/00Z ECMWF IN PRINCIPLE FOR MOST OF THE  
PERIOD. NOTE THAT WHILE THE PROBABILITY FOR MAINTAINING  
SEPARATION PER LATEST MAJORITY CLUSTER IS GREATER, IT IS A CLOSE  
ENOUGH CALL WHETHER NORTHERN STREAM FLOW AFFECTS THE SOUTHERN  
FEATURE THAT SIGNIFICANT CHANGES IN FUTURE GUIDANCE ARE STILL  
POSSIBLE. MEANWHILE GUIDANCE FOR THE MIDWEEK SYSTEM CROSSING  
SOUTHEASTERN CANADA AND NEW ENGLAND HAS GRAVITATED TOWARD A  
CONCENTRATED UPPER LOW AND FAIRLY VIGOROUS SURFACE SYSTEM.  
 
THERE ARE TWO MAJOR QUESTION MARKS IN THE FORECAST OVER THE  
EASTERN PACIFIC INTO WESTERN NORTH AMERICA. FIRST IS THE DEGREE  
OF SEPARATION THAT DEVELOPS WELL OFFSHORE THE WEST COAST DURING  
THE LATTER HALF OF THE WEEK. ECMWF/ECMWF ENSEMBLE RUNS STILL SHOW  
CONSIDERABLE SEPARATION LEADING TO A SOUTHERN STREAM CLOSED LOW  
BUT ARE A BIT FASTER THAN 24 HOURS AGO. THE 00Z UKMET/CMC PROVIDE  
ADDED SUPPORT FOR THE ECMWF IDEA. THE 06Z GFS ADJUSTED TO A  
SOLUTION ABOUT HALFWAY BETWEEN THE ECMWF CLUSTER AND FASTER 00Z  
GFS. THESE CONSIDERATIONS PROVIDE GREATER SUPPORT FOR TILTING THE  
FORECAST MORE TOWARD THE ECMWF CLUSTER. THE SECOND ISSUE IS  
RAPIDLY INCREASING SPREAD THAT OCCURS OVER THE BERING SEA (RANGING  
BETWEEN DEEP GFS TROUGHING AND STRONG ECMWF RIDGE) AND FARTHER  
SOUTH OVER THE PACIFIC (DIFFERENCES IN CLOSED LOW POSITION).  
RESPECTIVE ENSEMBLE SYSTEMS HAVE TENDED TO REFLECT HINTS OF THEIR  
OPERATIONAL COUNTERPARTS BUT ACTUALLY AGREE REMARKABLY WELL ON  
BROAD CYCLONIC FLOW OVER THE NORTHWESTERN STATES--IN CONTRAST TO  
THE DEEP TROUGH/UPPER LOW WHICH 00-06Z GFS RUNS SHOW OVER THE WEST  
COAST BY NEXT SUN. THUS PREFER TO EMPHASIZE THE ENSEMBLE  
MEAN/ECMWF IDEAS FOR THIS PART OF THE FORECAST.  
 
BASED ON ABOVE CONSIDERATIONS THE UPDATED FORECAST STARTED WITH A  
BLEND OF OPERATIONAL GUIDANCE FOR DAYS 3-5 WED-FRI AND THEN  
TRANSITIONED TOWARD PRIMARY WEIGHT ON THE 00Z ECMWF/ECMWF MEAN AND  
06Z GEFS MEAN.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
WIDESPREAD SHOWERS AND THUNDERSTORMS ARE LIKELY WITH THE SYSTEM  
CROSSING THE SOUTHERN PLAINS AND LOWER MISSISSIPPI VALLEY MID-LATE  
WEEK AND MODELS/ENSEMBLES CONTINUE TO SHOW A STRONG SIGNAL FOR  
HEAVY RAINFALL FROM PORTIONS OF CENTRAL TEXAS EAST TOWARD THE  
MISSISSIPPI RIVER. SEVERE THUNDERSTORMS ARE ALSO POSSIBLE.  
PLEASE SEE CONVECTIVE OUTLOOKS ISSUED BY THE STORM PREDICTION  
CENTER FOR FURTHER DETAILS ON THE SEVERE WEATHER THREAT. EXPECT  
SCATTERED TO NUMEROUS SHOWERS AND THUNDERSTORMS TO PROGRESS  
EASTWARD INTO THE SOUTHEAST AS WELL AS THE TENNESSEE/OHIO VALLEYS  
(WHERE GULF MOISTURE MAY INTERACT WITH A FRONT) BY LATE THU INTO  
FRI. THE HEAVY RAINFALL SIGNAL IS LESS CLEAR OVER THESE AREAS--AS  
WELL AS THE SOUTHEAST COAST/FLORIDA INTO THE WEEKEND--BUT WOULD  
EXPECT CONTINUED POTENTIAL FOR AT LEAST LOCALLY HEAVY RAINFALL  
GIVEN THE RECENT GRAVITATION OF GUIDANCE TOWARD THE SLOWER HALF OF  
THE SPECTRUM. FARTHER WEST, THE ENERGETIC UPPER-LEVEL TROUGH  
ENTERING THE NORTHWESTERN U.S. LATE IN THE WEEK SHOULD PRODUCE  
RAIN AND MOUNTAIN SNOW FROM THE NORTHWEST TO THE NORTHERN ROCKIES.  
RAIN AND POSSIBLY EVEN SNOW (THOUGH MOSTLY ALONG/NORTH OF THE  
CANADIAN BORDER) MAY SPREAD INTO AREAS FROM THE NORTHERN PLAINS TO  
THE UPPER MIDWEST/GREAT LAKES AS ENERGY ALOFT AND ASSOCIATED LOW  
PRESSURE MOVE INTO THE CENTRAL U.S. BY NEXT WEEKEND. MULTIPLE  
UNCERTAINTIES OVER DETAILS OF FLOW ALOFT BY NEXT WEEKEND LOWER  
CONFIDENCE IN COVERAGE OF SCATTERED PRECIPITATION OVER PARTS OF  
THE WEST NEXT WEEKEND.  
 
ABOVE AVERAGE TEMPERATURES ARE LIKELY THROUGH MUCH OF THE PERIOD  
ACROSS CENTRAL AND SOUTHERN PORTIONS OF THE WESTERN U.S. EXPECT  
BROAD COVERAGE OF HIGH TEMPERATURES 10 TO 20 DEG F ABOVE AVERAGE  
FROM CALIFORNIA TO THE GREAT BASIN WED THROUGH FRI OR SAT. THE  
MOST LIKELY DAILY RECORDS WILL BE FOR WARM LOWS WED-FRI BUT A FEW  
RECORD HIGHS ARE POSSIBLE AS WELL. THE WARMTH SHOULD SPREAD INTO  
THE ROCKIES AND PORTIONS OF THE HIGH PLAINS BY FRI-SAT WHILE  
COOLER TEMPERATURES OVERSPREAD THE NORTHWEST/NORTHERN ROCKIES AND  
EVENTUALLY NORTHERN PLAINS IN ASSOCIATION WITH THE ARRIVING UPPER  
TROUGH/COLD FRONT.  
 
RAUSCH/RYAN  
 
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