592  
FXUS02 KWBC 081851  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
249 PM EDT SUN MAY 8 2022  
 
VALID 12Z WED MAY 11 2022 - 12Z SUN MAY 15 2022  
 
...BLOCKY PATTERN NEXT WEEK TO SUPPORT RECORD HEAT OVER THE  
SOUTHERN PLAINS/LOWER MISSISSIPPI VALLEY INTO THE MIDWEST ALONG  
WITH EPISODES OF CENTRAL U.S. CONVECTION AND AN EMERGING ONSHORE  
RAINFALL FOCUS TO THE CAROLINAS...  
 
19Z UPDATE: THE 12Z MODEL GUIDANCE SUITE IS IN EXCELLENT SYNOPTIC  
SCALE AGREEMENT THROUGH THE END OF THE WEEK, WITH THE MAIN  
EXCEPTION BEING THE UKMET THAT IS MORE AMPLIFIED WITH THE TROUGH  
EXITING THE DESERT SOUTHWEST AND EMERGING OVER THE WESTERN HIGH  
PLAINS BY FRIDAY, AND ALSO SLIGHTLY FASTER WITH THE INLAND  
PROGRESSION OF THE HYBRID/CLOSED LOW SYSTEM APPROACHING THE  
SOUTHEAST U.S. COAST. ALTHOUGH SOME TIMING AND MAGNITUDE  
DIFFERENCES ARE MORE APPARENT WITH THE NORTH-CENTRAL U.S. TROUGH  
AND THE STORM SYSTEM APPROACHING THE PACIFIC NORTHWEST FOR NEXT  
WEEKEND, THERE IS STILL ABOVE AVERAGE AGREEMENT ON THE OVERALL  
PATTERN WITH THE DETERMINISTIC GFS AND ECMWF CLOSE TO THEIR  
RESPECTIVE ENSEMBLE MEANS. IN TERMS OF QPF, THERE WAS ENOUGH OF A  
SIGNAL IN THE UKMET/ECMWF/CMC TO ADD A SMALL SLIGHT RISK AREA FOR  
EXCESSIVE RAINFALL ON DAY 4 ACROSS PORTIONS OF NORTH-CENTRAL  
MINNESOTA. TAKING THIS INTO ACCOUNT, THE WPC FORECAST WAS  
PRIMARILY DERIVED FROM A CMC/GFS/ECMWF BLEND WITH A LITTLE BIT OF  
THE UKMET, AND THEN ABOUT 60% OF THE DETERMINISTIC GUIDANCE AND  
40% OF THE MEANS BY NEXT SUNDAY. THE PREVIOUS FORECAST DISCUSSION  
FOLLOWS BELOW. /HAMRICK  
---------------  
   
..OVERVIEW
 
 
LATEST MODELS AND ENSEMBLES MAINTAIN THE IDEA OF A BLOCKY PATTERN  
THROUGH LATE WEEK. AN UPPER RIDGE WILL SUPPORT A PROLONGED PERIOD  
OF WELL ABOVE NORMAL TEMPERATURES (CHALLENGING DAILY RECORDS OVER  
SOME AREAS) OVER THE MISSISSIPPI VALLEY/CENTRAL-SOUTHERN PLAINS  
EXTENDING INTO NEW ENGLAND. AT THE SAME TIME, MEAN TROUGHING  
CONTAINING A COUPLE SEPARATE FEATURES WILL PRODUCE CHILLY AND  
UNSETTLED WEATHER OVER THE WEST WHILE WAVES/FRONTS AND A DRY LINE  
WILL PROMOTE EPISODES OF RAIN AND THUNDERSTORMS BETWEEN THESE TWO  
REGIMES. A RETROGRADING UPPER RIDGE OVER THE ATLANTIC WILL LIKELY  
PUSH AN INITIAL WESTERN ATLANTIC UPPER LOW BACK TOWARD THE  
SOUTHERN PART OF THE EAST COAST, INCREASING RAINFALL OVER CENTRAL  
AND SOUTHERN PORTIONS OF THE EAST. MOST GUIDANCE SUGGESTS THAT  
THE PATTERN WILL OPEN UP DURING NEXT WEEKEND, LEADING TO A RIDGE  
BUILDING OVER THE WEST AND A MEAN TROUGH AXIS REACHING THE  
PLAINS/MISSISSIPPI VALLEY. THIS WOULD FINALLY BRING A DRYING  
TREND OVER THE NORTHERN PLAINS AND VICINITY AND INCREASING  
RAINFALL COVERAGE OVER THE EASTERN HALF OF THE COUNTRY.  
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
GUIDANCE CONTINUES TO DISPLAY SOME MEANINGFUL DIFFERENCES WITH  
IMPORTANT FORECAST DETAILS. THE INTERACTION AMONG AN EJECTING  
WESTERN U.S. TROUGH/POSSIBLE EMBEDDED UPPER LOW AND INCOMING  
PACIFIC FLOW HAS BEEN A PERSISTENT UNCERTAINTY OVER RECENT DAYS,  
WITH THE CHARACTER OF THE PACIFIC ENERGY AS IT CONTINUES ACROSS  
THE LOWER 48 ALSO IN QUESTION. MEANWHILE THE MODELS APPEAR TO  
HAVE BECOME A LITTLE WORSE OVER THE PAST DAY IN TERMS OF SPREAD  
AND RUN-TO-RUN VARIABILITY FOR THE UPPER LOW EXPECTED TO  
RETROGRADE INTO THE SOUTHEAST.  
 
ON THE POSITIVE SIDE FOR THE WESTERN EVOLUTION THROUGH LATE WEEK,  
PREVIOUSLY FAST GFS/SLOW ECMWF RUNS HAVE NOW TRENDED CLOSER TO  
EACH OTHER (GFS STARTING ITS SLOWER TREND IN ITS 18Z RUN) IN  
BRINGING THE INITIAL CALIFORNIA UPPER LOW INTO THE NORTHERN PLAINS  
BY EARLY FRIDAY--LEADING TO SIMILARLY DEEP SURFACE LOW PRESSURE IN  
18Z/00Z GFS AND 12Z ECMWF RUNS. CMC RUNS HAVE TENDED TO BE AN  
EASTERN EXTREME WITH THIS SYSTEM. THE NEW 00Z RUN IS STILL WELL  
EAST OF THOSE GFS/ECMWF RUNS BUT AT LEAST NOT TO SUCH A PRONOUNCED  
DEGREE AS THE PRIOR 12Z VERSION. ON THE OTHER HAND THE LATEST  
UKMET RUNS HAVE BEEN TRACKING THE UPPER LOW EXTREMELY FAR SOUTH  
FROM LATE THURSDAY ONWARD DUE TO BEING FLATTER WITH THE ARRIVING  
PACIFIC FLOW. BASED ON GUIDANCE AVAILABLE THROUGH THE 18Z CYCLE,  
A BLEND EMPHASIZING THE 18Z GFS/12Z ECMWF (TEMPERED A BIT DUE TO  
UNCERTAINTY) OFFERED THE MOST REASONABLE STARTING POINT FOR THE  
FORECAST. THE NEW 00Z ECMWF IS JUST A BIT FASTER/WEAKER ALOFT,  
RESULTING IN WEAKER/EASTWARD ADJUSTMENT FOR THE NORTHERN PLAINS  
SURFACE LOW.  
 
BY NEXT WEEKEND THE OPERATIONAL MODELS HAVE BEEN VARYING WITH THE  
DETAILS OF THE OVERALL TROUGH REACHING THE CENTRAL U.S. AS AN  
UPPER RIDGE BUILDS INTO THE WEST. WHILE NOT PERFECTLY CONSISTENT  
OVER THE PAST COUPLE DAYS, THE ENSEMBLE MEANS HAVE BEEN MORE  
STABLE THAN INDIVIDUAL MODEL RUNS SO THE PREFERENCE WAS TO ADD  
STEADILY MORE ENSEMBLE MEAN WEIGHT LATER IN THE FORECAST.  
 
AS FOR THE ATLANTIC UPPER LOW TRACKING INTO THE SOUTHEAST, THE  
TRACK AND TIMING APPEAR TO HAVE BECOME MORE SENSITIVE TO EMBEDDED  
SHORTWAVE DETAILS. AS OF THE 12Z/18Z CYCLES, THE GFS/ECMWF AND  
THEIR RESPECTIVE MEANS PROVIDED THE BEST CLUSTERING FOR MOST OF  
THE PERIOD WHILE THE UKMET STRAYED FARTHER WEST AND THE CMC WAS TO  
THE EAST. IN THE NEW 00Z RUNS THE UKMET/CMC LOOK MORE REASONABLE  
INTO FRIDAY WHILE GFS IS FARTHER NORTHEAST, BUT THEN THE CMC  
STRAYS INCREASINGLY NORTHWARD. THE NEW 00Z ECMWF HAS ALSO MADE A  
PRONOUNCED NORTHWARD ADJUSTMENT FROM FRIDAY ONWARD.  
PROGRESSION/OPENING OF THE LOW WILL DEPEND ON UNRESOLVED DETAILS  
OF THE TROUGH REACHING THE PLAINS/MISSISSIPPI VALLEY.  
 
ABOVE GUIDANCE COMPARISONS/PREFERENCES LED TO STARTING THE UPDATED  
FORECAST WITH A BLEND OF THE 18Z GFS/12Z ECMWF EARLY IN THE PERIOD  
FOLLOWED BY GRADUALLY MORE INPUT FROM THEIR ENSEMBLE MEANS,  
REACHING A TOTAL OF 40-60 PERCENT BY NEXT WEEKEND.  
   
..WEATHER/THREAT HIGHLIGHTS
 
 
STRONG UPPER RIDGING WILL LEAD TO PERSISTENT MUCH ABOVE NORMAL  
TEMPERATURES OVER THE CENTRAL-SOUTHERN PLAINS AND MOST OF THE  
MISSISSIPPI VALLEY THROUGH THE WEEK, WITH SOME OF THIS WARMTH ALSO  
EXTENDING THROUGH THE GREAT LAKES AND NORTHEAST. EXPECT A BROAD  
AREA OF HIGHS/MORNING LOWS 10-20F ABOVE NORMAL WITH SOME POCKETS  
OF ANOMALIES REACHING HIGHER THAN THAT. NUMEROUS DAILY RECORDS  
ARE LIKELY THROUGH THE WEEK OVER THE SOUTHERN PLAINS AND LOWER  
MISSISSIPPI VALLEY, AND MAY ALSO EXTEND INTO THE CENTRAL  
PLAINS/MIDWEST ESPECIALLY AROUND MIDWEEK. TEMPERATURES COULD ALSO  
CHALLENGE RECORD HIGHS OVER A FEW PLACES IN THE NORTHEAST LATE IN  
THE WEEK. HIGHEST HEAT INDEX VALUES WILL TEND TO BE FROM THE  
LOWER MISSISSIPPI VALLEY INTO SOUTHERN TEXAS WITH MAXIMUM VALUES  
APPROACHING OR REACHING 100F AT SOME LOCATIONS. THE EXCESSIVE  
HEAT, GUSTY WINDS, AND DRY CONDITIONS LEADING TO AN ELEVATED FIRE  
WEATHER THREAT ACROSS PARTS OF THE SOUTHERN HIGH PLAINS INTO  
MIDWEEK SHOULD MODERATE THEREAFTER. WEAKENING OF THE UPPER RIDGE  
AND AN APPROACHING COLD FRONT SHOULD HELP TO START A COOLER TREND  
OVER THE PLAINS/MISSISSIPPI VALLEY BY THE WEEKEND.  
 
OVER THE WEST, THE INITIAL SYSTEM BEGINNING TO PUSH INLAND SHOULD  
BRING HIGHS OF 10-15F BELOW NORMAL TO PARTS OF NEVADA AND  
CALIFORNIA ON WEDNESDAY AND THEN INCOMING PACIFIC ENERGY WILL  
BRING A BRIEF EPISODE OF MINUS 5-15F ANOMALIES FOR HIGHS OVER THE  
NORTHWEST/NORTHERN ROCKIES THURSDAY-FRIDAY. OTHERWISE THE REGION  
WILL SEE A WARMING TREND AS AN UPPER RIDGE BUILDS IN BY NEXT  
WEEKEND, BRINGING HIGHS UP TO 10-15F OR SO ABOVE NORMAL OVER AND  
JUST EAST OF CALIFORNIA AT THAT TIME. HIGHS OVER THE SOUTHWEST  
COULD EVEN APPROACH DAILY RECORDS AT A FEW PLACES ON SUNDAY.  
PERIODS OF RAIN AND HIGHER ELEVATION SNOW WILL TEND TO BE LIGHT TO  
MODERATE, BUT A SYSTEM ARRIVING AROUND THURSDAY MAY ENHANCE TOTALS  
SOMEWHAT OVER FAVORED TERRAIN IN THE NORTHWEST.  
 
BETWEEN THE TWO CONTRASTING REGIMES THROUGH LATE WEEK, THE CENTRAL  
U.S. WILL SEE PERIODS OF RAIN AND THUNDERSTORMS. INITIAL FOCUS ON  
WEDNESDAY WILL BE ALONG A WARM FRONT LIFTING THROUGH THE NORTHERN  
PLAINS/UPPER MISSISSIPPI VALLEY, FOLLOWED BY A SYSTEM EMERGING  
FROM THE WEST AND TRACKING OVER THE NORTHERN PLAINS  
THURSDAY-FRIDAY. THIS SYSTEM WILL THEN PUSH A WAVY NORTH-SOUTH  
FRONT ACROSS THE CENTRAL U.S. WITH TIMING AND WAVE DETAILS (AND  
THUS SPECIFICS OF HIGHEST RAINFALL TOTALS) STILL DEPENDENT ON  
SPECIFICS OF TRAILING DYNAMICS FLOWING THROUGH THE WEST. THESE  
FRONTS AND THE EJECTING WESTERN SYSTEM MAY PRODUCE SOME AREAS OF  
SIGNIFICANT RAINFALL THAT WOULD BE A RUNOFF THREAT OVER NORTHERN  
TIER AREAS THAT ALREADY HAVE VERY WET SOIL. SOME AREAS OVER THE  
CENTRAL U.S. MAY SEE A THREAT FOR STRONG TO SEVERE CONVECTION,  
WITH DETAILS LIKELY TO BECOME MORE CLEAR OVER COMING DAYS.  
 
THE WESTERN ATLANTIC UPPER LOW FORECAST TO RETROGRADE INTO THE  
SOUTHEAST SHOULD START TO SPREAD MOISTURE FARTHER INLAND AROUND  
FRIDAY. EXPECT MOISTURE TO EXPAND IN COVERAGE DURING THE WEEKEND  
AS THE UPPER LOW LIFTS NORTHWARD AND OPENS UP IN RESPONSE TO THE  
UPPER TROUGH REACHING THE CENTRAL U.S. CURRENTLY EXPECT THE  
HIGHEST RAINFALL TOTALS TO BE OVER PORTIONS OF THE CAROLINAS INTO  
VIRGINIA.  
 
RAUSCH  
 
ADDITIONAL 3-7 DAY HAZARD INFORMATION CAN BE FOUND ON THE WPC  
MEDIUM RANGE HAZARDS OUTLOOK CHART AT:  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP  
 
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, EXPERIMENTAL EXCESSIVE RAINFALL  
OUTLOOK, 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/#PAGE=ERO  
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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