807  
FXUS02 KWBC 221533  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1133 AM EDT MON APR 22 2019  
 
VALID 12Z THU APR 25 2019 - 12Z MON APR 29 2019  
   
..16Z UPDATE
 
 
GUIDANCE SOMEWHAT TRENDED IN OPPOSITE DIRECTIONS OVER THE  
SOUTHEAST WITH RESPECT TO THE EXITING CLOSED LOW (WEAKENING TO AN  
OPEN WAVE) WITH THE 00Z ECMWF AND MANY OF ITS ENSEMBLE MEMBERS  
QUICKER THAN THE GFS/UKMET WHILE THE 00Z CANADIAN WAS MOSTLY IN  
BETWEEN. OPTED TO RELY ON THE PREVIOUS FORECAST AS IT SAT NICELY  
IN BETWEEN THE QUICKER/SLOWER CAMPS GIVEN NO CLEAR REASON TO  
CHANGE AS OF YET. NEXT SYSTEM OUT OF THE GULF OF ALASKA INTO  
SOUTHWESTERN CANADA HAS MOSTLY TRENDED WEAKER IN THE GUIDANCE BUT  
THE GFS REMAINED STRONGER THAN THE REST, WHICH RESULTED IN  
(LIKELY) TOO MUCH RIDGING INTO THE PLAINS NEXT SUN/MON COMPARED TO  
THE ENSEMBLE CONSENSUS. UPPER LOW APPROACHING CALIFORNIA AND  
WEAKENING ON SUNDAY REMAINS RATHER WELL-FORECAST IN THE PAST TWO  
MODEL/ENSEMBLE CYCLES THOUGH THE ECMWF HAS HAD BETTER CONTINUITY.  
AGAIN, PREVIOUS FORECAST WITH ONLY VERY MINOR MODIFICATIONS PROVED  
TO BE A BEST STARTING POINT, BUT WITH A FEW CHANGES TO LATER  
PERIODS IN THE TEMPERATURE FORECAST BASED ON THE LATEST ECMWF  
ENSEMBLES AND NATIONAL BLEND GUIDANCE.  
 
PREVIOUS DISCUSSION IS BELOW...  
 
FRACASSO  
 
   
..OVERVIEW AND GUIDANCE PREFERENCES
 
 
UPPER-LEVEL FLOW ACROSS THE CONUS IS NOT EXPECTED TO CHANGE  
SIGNIFICANTLY DURING THE EXTENDED FORECAST PERIOD. A PROGRESSIVE  
NORTHERN STREAM AND LESS PROGRESSIVE SOUTHERN STREAM, SIMILAR TO  
THE PATTERN SEEN IN RECENT WEEKS, IS EXPECTED TO PERSIST.  
MODELS/ENSEMBLES DO HINT AT POTENTIAL NORTHERN STREAM FLOW  
AMPLIFICATION FROM THE NORTH PACIFIC INTO CANADA BY NEXT WEEKEND,  
HOWEVER. DEVELOPMENT OF A BROAD REX BLOCK EXTENDING FROM HAWAII TO  
ALASKA IS EXPECTED, WHICH WILL REINFORCE UPPER TROUGHING CENTERED  
WEST OF HUDSON BAY. CONCEPTUALLY, THIS SETUP SEEMS TO FAVOR  
POTENTIAL SOUTHWARD TRANSPORT OF POLAR AIR INTO CENTRAL CANADA AND  
THE NORTH CENTRAL U.S., AND THIS BEARS OUT WHEN TELECONNECTIONS  
ASSOCIATED WITH THE PACIFIC REX BLOCK ARE EXAMINED.  
 
MODELS HANDLE A SOUTHERN STREAM TROUGH INITIALLY ACROSS THE  
SOUTHERN PLAINS ON DAY 3 (THU) SIMILARLY THROUGH FRIDAY, BEFORE  
SOLUTIONS BEGIN TO DIVERGE ON TIMING. TIMING HAS BEEN AN ISSUE  
WITH THIS SYSTEM IN THE GUIDANCE FOR SEVERAL DAYS, LARGELY HINGING  
ON THE DEGREE OF SEPARATION BETWEEN NORTHERN/SOUTHERN STREAMS AND  
AMPLIFICATION OF NORTHERN STREAM SHORTWAVE ENERGY CROSSING THE  
GREAT LAKES ON FRI. IN GENERAL, THE TREND HAS BEEN TOWARD MORE  
SEPARATION OF STREAMS AND A SLOWER PROGRESSION OF THE TROUGH/UPPER  
LOW ACROSS THE SOUTHEAST THROUGH SAT. FARTHER WEST, TIMING OF  
NORTHERN STREAM SHORTWAVE ENERGY DIGGING INTO THE NORTHWESTERN  
U.S. ON SAT IS A PROBLEM, WITH MODELS SHOWING A FAIR AMOUNT OF  
VARIABILITY. IN GENERAL, A MULTI-MODEL DETERMINISTIC BLEND  
INCLUDING THE 12Z ECMWF/UKMET AND 18Z GFS WAS USED AS A BASIS FOR  
THE FORECAST DURING DAYS 3-5 (THU-SAT).  
 
AFTER A FEW DAYS OF WAFFLING, MODELS/ENSEMBLES SEEM TO HAVE  
LARGELY SETTLED ON THE IDEA OF ANOTHER CUTOFF SOUTHERN STREAM  
SYSTEM APPROACHING CALIFORNIA SAT-SUN. THE ECMWF/GFS ARE QUITE  
SIMILAR ON TIMING OF THE FEATURE, AT LEAST THROUGH SUN, WHICH ALSO  
HAS SUPPORT FROM THE RESPECTIVE ENSEMBLE MEANS. BY MON,  
DIFFERENCES EMERGE, WITH CONFIDENCE QUICKLY DECREASING. THE  
CULPRIT SEEMS TO ONCE AGAIN BE SIGNIFICANT NORTHERN STREAM  
DIFFERENCES. THE GFS HAS BEEN PERSISTENT IN DIGGING AN AMPLIFIED  
UPPER TROUGH INTO THE PACIFIC NORTHWEST BY SUN-MON, WHILE THE  
ECMWF/CMC SHOW A MORE SUBDUED FEATURE WITH WEAKER HEIGHT FALLS.  
ENSEMBLES SHOW A FAIR AMOUNT OF DISBURSEMENT, SUGGESTING LOW  
CONFIDENCE IN THIS FEATURE. THE NORTHERN STREAM EVOLUTION BY MON  
ALSO AFFECTS THE SOUTHERN STREAM FEATURE, WITH A MORE AMPLIFIED  
TROUGH IN THE NORTHWEST KICKING THE SOUTHERN STREAM WAVE INLAND  
MORE QUICKLY, AND A LESS AMPLIFIED TROUGH KEEPING MORE STREAM  
SEPARATION AND AN UPPER LOW LINGERING ACROSS THE SOUTHWEST FOR  
LONGER. THE PATTERN IN RECENT WEEKS WOULD SUGGEST THAT THE IDEA OF  
MORE STREAM SEPARATION IS MORE LIKELY, BUT THE BROADER NORTHERN  
STREAM FLOW AMPLIFICATION DESCRIBED ABOVE SUGGESTS THAT THE ANSWER  
MAY BE LESS SIMPLE. THUS, LEANED HEAVILY TOWARD ENSEMBLE MEANS  
(12Z ECENS/18Z GEFS) BY DAYS 6-7 (SUN-MON), REPRESENTING A  
COMPROMISE SOLUTION ACROSS THE WESTERN U.S.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
WIDESPREAD SHOWERS AND THUNDERSTORMS ARE EXPECTED FROM THE LOWER  
MISSISSIPPI VALLEY TO THE SOUTHEAST THU-FRI IN ASSOCIATION WITH  
THE SOUTHERN STREAM SHORTWAVE/UPPER LOW. MODELS SUGGEST THAT AT  
LEAST LOCALLY HEAVY RAINFALL IS POSSIBLE. THE FRONTAL BOUNDARY  
ASSOCIATED WITH THIS SYSTEM IS EXPECTED TO STALL ACROSS THE  
FLORIDA PENINSULA OVER THE WEEKEND AND LINGER INTO EARLY NEXT  
WEEK, WHICH SHOULD KEEP SCATTERED TO NUMEROUS SHOWERS AND  
THUNDERSTORMS A POSSIBILITY, ALONG WITH LOCALLY HEAVY RAINFALL.  
FARTHER WEST, RAIN AND MOUNTAIN SNOW WILL BE POSSIBLE ACROSS THE  
ROCKIES THU NIGHT INTO FRI AS SHORTWAVE ENERGY PASSES OVERHEAD.  
PRECIPITATION SHOULD SPREAD INTO THE NORTHERN/CENTRAL PLAINS FRI  
NIGHT INTO SAT IN ASSOCIATION WHERE A COUPLE STATIONARY FRONTAL  
BOUNDARIES WILL ADD TO THE LIFT GENERATED BY THE UPPER-LEVEL  
SHORTWAVES. THIS PRECIPITATION SHOULD SPREAD EAST INTO THE  
MIDWEST/GREAT LAKES BY SUN, WITH COLDER AIR ON THE NORTHERN  
FRINGES POTENTIALLY LEADING TO SOME CHANCE FOR SNOWS FROM NORTHERN  
MINNESOTA EAST TO NORTHERN WISCONSIN AND THE UPPER PENINSULA OF  
MICHIGAN. THE ARRIVAL OF ADDITIONAL NORTHERN STREAM ENERGY SHOULD  
BRING ANOTHER ROUND OF PRECIPITATION TO THE ROCKIES ON SUN AND  
INTO THE CENTRAL/NORTHERN PLAINS MON.  
 
TEMPERATURES ARE EXPECTED TO BE WELL ABOVE AVERAGE FROM CALIFORNIA  
EAST TO THE CENTRAL ROCKIES THU-FRI, WITH HIGHS EXPECTED TO REACH  
10 TO 20 DEG F ABOVE AVERAGE FOR MANY AREAS. THESE TEMPERATURES  
ANOMALIES MAY SPREAD EAST INTO THE SOUTHERN/CENTRAL HIGH PLAINS  
FOR SAT. MEANWHILE, A COOLER POLAR AIR MASS SHOULD BEGIN TO SPREAD  
INTO AREAS FROM THE NORTHERN ROCKIES TO THE UPPER MIDWEST NORTH OF  
A SURFACE FRONT NEXT WEEKEND, WITH HIGH TEMPERATURES AT LEAST 5 TO  
10 DEG BELOW AVERAGE EXPECTED. THE FLOW ACROSS CENTRAL CANADA BY  
NEXT WEEKEND WILL PLAY A SIGNIFICANT ROLE IN TEMPERATURES ACROSS  
THE NORTH CENTRAL U.S., WITH SOME MODEL SOLUTIONS MUCH COLDER THAN  
CURRENTLY FORECAST.  
 
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  
 

 
 
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