870  
FXUS02 KWBC 040659  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
259 AM EDT THU APR 04 2019  
 
VALID 12Z SUN APR 07 2019 - 12Z THU APR 11 2019  
   
..OVERVIEW
 
 
ONGOING PROGRESSION OF PACIFIC FEATURES INTO AND ACROSS THE LOWER  
48 WILL BRING ACTIVE WEATHER TO A DECENT PERCENTAGE OF THE COUNTRY  
FOR ONE OR MORE DAYS OF THE FORECAST PERIOD. THE SOUTHWESTERN  
U.S./SOUTHERN ROCKIES APPEAR TO BE THE ONE AREA THAT SHOULD HAVE  
FAIRLY DRY WEATHER. A LEADING UPPER TROUGH OVER THE CENTRAL U.S.  
ON SUN AND ITS ASSOCIATED SURFACE SYSTEM WILL SPREAD A BROAD AREA  
OF PRECIPITATION ACROSS THE EASTERN HALF OF THE COUNTRY SUN-TUE  
WITH SOME AREAS OF HEAVY RAINFALL POSSIBLE. DURING THAT SAME TIME  
FRAME EXPECT A STRONG JET AIMED AT THE NORTHWESTERN STATES TO  
BRING AREAS OF ENHANCED PRECIPITATION TO THE NORTHERN HALF OF THE  
WEST COAST INTO THE NORTHERN ROCKIES. AN UPPER TROUGH FORECAST TO  
REACH THE WEST COAST AROUND EARLY TUE AND CONTINUE INLAND SHOULD  
THEN PUSH THE BEST FOCUS FOR PRECIPITATION FARTHER EAST INTO THE  
NORTHERN-CENTRAL ROCKIES AND THEN PORTIONS OF THE  
PLAINS/MISSISSIPPI VALLEY AND EVENTUALLY OHIO VALLEY AND/OR GREAT  
LAKES. THIS TROUGH MAY SUPPORT A FAIRLY STRONG CENTRAL U.S. STORM  
SYSTEM BY WED-THU. TRAILING TROUGH ENERGY REACHING THE  
NORTHEASTERN PACIFIC LATE IN THE PERIOD MAY APPROACH THE WEST  
COAST BY NEXT THU.  
 
   
..GUIDANCE ASSESSMENT/PREFERENCES
 
 
NOT A LOT HAS BEEN RESOLVED YET FOR THE OVERALL UPPER TROUGH  
PROGRESSING EASTWARD FROM THE PLAINS. THERE IS STILL GOOD  
AGREEMENT ON ITS EXISTENCE AND GENERAL TIMING (ESPECIALLY IN THE  
MEANS) BUT INDIVIDUAL MODEL RUNS AND ENSEMBLE MEMBERS STILL VARY  
WIDELY WITH RESPECT TO HOW ENERGY IS DISTRIBUTED WITHIN THE  
TROUGH. OVER THE PAST DAY OR SO THERE IS BEEN A MODERATE SIGNAL  
TOWARD AT LEAST A PERIOD OF SEPARATION BETWEEN FLOW CROSSING THE  
SOUTHEAST QUADRANT OF THE COUNTRY AND THAT CROSSING THE NORTHEAST  
QUADRANT. THE SOUTHERN PART OF THE TROUGH WOULD SUPPORT A FRONTAL  
WAVE THAT COULD PROVIDE LOCALIZED FOCUS FOR HEAVY RAINFALL.  
DIFFERENT VARIATIONS OF STREAM INTERACTION PERSIST AS WELL.  
CONTINUE TO PREFER A MODEL/MEAN BLEND TO REFLECT THE MOST  
AGREEABLE ASPECTS OF THE FORECAST, IN PARTICULAR LOW PRESSURE  
CROSSING THE GREAT LAKES/NORTHEAST, WHILE OFFERING A MORE  
CONSERVATIVE APPROACH WHERE GREATER SPREAD EXISTS.  
 
FAST PACIFIC FLOW CROSSING THE NORTHWEST SUN-MON WILL LIKELY  
CONTAIN MULTIPLE EMBEDDED IMPULSES WHOSE SMALL SCALE AND FAST  
MOTION WILL MAKE THEM DIFFICULT TO RESOLVE UNTIL THE SHORT RANGE  
TIME FRAME. FOR THE UPPER TROUGH REACHING THE WEST COAST AROUND  
EARLY TUE, GFS/GEFS RUNS THROUGH THE 18Z CYCLE WERE STILL A BIT  
SLOWER THAN THAN THE ECMWF/ECMWF MEAN. FOR THE 12-18Z CYCLES THE  
FULL ARRAY OF GUIDANCE WAS NOT QUITE AS SHARPLY CLUSTERED AS IN  
THE PAST COUPLE DAYS WHEN THERE WAS A MORE PRONOUNCED MAJORITY IN  
FAVOR OF FASTER ECMWF TIMING. THIS LED TO A SOMEWHAT MORE EVEN  
BLEND FOR SOME ASPECTS OF EVOLUTION, BUT ULTIMATELY THE EJECTING  
TROUGH AND CORRESPONDING SURFACE LOW END UP CLOSER TO THE  
ECMWF/ECMWF MEAN LATE IN THE PERIOD. THE 18Z GFS WAS SIMILAR AS  
OF EARLY THU. NEW 00Z GUIDANCE THUS FAR, INCLUDING THE GFS, IS  
PROVIDING A BOOST IN CONFIDENCE TOWARD THE PROGRESSIVE SCENARIO.  
AT THE SAME TIME THE 00Z GFS/CMC HAVE TRENDED MUCH STRONGER WITH  
PLAINS/MISSISSIPPI VALLEY LOW PRESSURE WED-THU.  
 
FARTHER UPSTREAM MODELS/SPAGHETTI PLOTS STILL DEPICT A LOT OF  
SPREAD FOR NORTHERN INTO NORTHEASTERN PACIFIC DETAILS INTO WED.  
IN SPITE OF THIS SPREAD THROUGH WED THERE IS ACTUALLY SOME  
TRENDING TOWARD BETTER LARGE SCALE AGREEMENT BY THU WITH A TROUGH  
EVOLVING ALONG OR OFFSHORE THE NORTHERN HALF OF THE WEST COAST.  
 
COMBINING FORECAST CONSIDERATIONS ACROSS THE LOWER 48, THE UPDATED  
FORECAST STARTED WITH A BLEND PRIMARILY EMPHASIZING OPERATIONAL  
GUIDANCE EARLY BUT 50 PERCENT MEANS (12Z ECMWF MEAN AND LESSER 18Z  
GEFS/12Z NAEFS WEIGHT) ALREADY BY DAY 5 TUE. THE REST OF THE  
FORECAST TILTED A LITTLE MORE TOWARD THE MEANS BUT KEPT SOME  
ATTRIBUTES OF THE 18Z GFS/12Z ECMWF TO ENHANCE ENSEMBLE MEAN  
DETAIL.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
STRONG FLOW ALOFT ACROSS THE NORTHWEST WILL PROMOTE AREAS OF  
TERRAIN-ENHANCED RAIN AND HIGHER ELEVATION SNOW FROM NORTHERN  
CALIFORNIA AND THE PACIFIC NORTHWEST INTO NORTHERN ROCKIES FROM  
SUN INTO EARLY NEXT WEEK. CURRENT GUIDANCE SUGGESTS BEST  
POTENTIAL FOR HEAVIEST ACTIVITY WILL BE CENTERED ALONG THE WEST  
COAST NEAR THE CALIFORNIA/OREGON BORDER AND THE OREGON CASCADES.  
BY TUE THE UPPER TROUGHING MOVING INTO THE WEST WILL BEGIN TO  
SHIFT THE BEST MOISTURE FOCUS EASTWARD INTO THE GREAT BASIN AND  
NORTHERN-CENTRAL ROCKIES. CONTINUED PROGRESSION OF THIS SYSTEM  
SHOULD SPREAD PRECIPITATION ACROSS THE NORTH-CENTRAL PLAINS,  
MID-UPPER MISSISSIPPI VALLEY, AND OHIO VALLEY AND/OR GREAT LAKES  
BY WED-THU. SOME ACTIVITY MAY BE HEAVY AND AREAS WITH ONGOING  
FLOODING CONCERNS WILL NEED TO MONITOR THIS SYSTEM. A PERIOD OF  
LOW LEVEL UPSLOPE FLOW MAY ADD TO TOTALS OVER THE  
EASTWARD/NORTHWARD FACING SLOPES OF THE ROCKIES. IN ADDITION  
STRONGER TRENDS FOR WED-THU LOW PRESSURE IN LATEST GUIDANCE RAISE  
THE POTENTIAL FOR STRONG WINDS TO BE AN ISSUE OVER SOME AREAS.  
FINALLY, IF THE SYSTEM IS STRONG ENOUGH THERE MAY BE SOME SNOW IN  
THE NORTHERN PART OF THE PRECIP SHIELD TO THE EAST OF THE ROCKIES.  
 
THE EASTERN HALF OF THE COUNTRY WILL SEE A BROAD AREA OF  
PRECIPITATION WITH THE SYSTEM PROGRESSING EASTWARD FROM THE PLAINS  
SUN ONWARD. AMPLE FLOW OF MOISTURE FROM THE GULF WILL PROMOTE  
AREAS OF HEAVY RAINFALL. BEST POTENTIAL FOR HIGHEST TOTALS WILL  
BE OVER/NEAR THE LOWER MISSISSIPPI VALLEY AND CENTRAL GULF COAST  
REGION AND SOUTHERN APPALACHIANS. CONVECTION MAY BE STRONG TO  
SEVERE OVER SOUTHERN AREAS. CHECK SPC OUTLOOKS AS SPECIFICS  
BECOME BETTER RESOLVED OVER THE COMING DAYS. SIGNIFICANT RAINFALL  
IS ALSO POSSIBLE FROM THE OHIO VALLEY INTO NORTHERN  
MID-ATLANTIC/NORTHEAST. THERE ARE STILL SIGNIFICANT DETAIL  
UNCERTAINTIES WITH THIS SYSTEM SO CONFIDENCE IN LOCATION/MAGNITUDE  
OF HEAVIEST RAINFALL REMAINS SOMEWHAT LOWER THAN DESIRED. SNOW  
SHOULD BE CONFINED MOSTLY TO NEW ENGLAND.  
 
THE WARM SECTOR OF THE CENTRAL-EASTERN U.S. SYSTEM DURING THE  
FIRST HALF OF THE PERIOD WILL CONTAIN ABOVE TO MUCH ABOVE NORMAL  
TEMPERATURES. MORNING LOWS WILL BE PARTICULARLY ANOMALOUS WITH  
MANY LOCATIONS 15-25F ABOVE NORMAL SUN INTO TUE AND SOME DAILY  
RECORD WARM LOW VALUES MAY BE CHALLENGED. THE UPSTREAM SYSTEM  
WILL BRING A BRIEF PERIOD OF WARMTH ACROSS THE SOUTHWEST INTO THE  
SOUTHERN PLANS/SOUTHEAST OVER THE COURSE OF THE PERIOD, AND AT THE  
SAME TIME SPREAD BELOW NORMAL READINGS FROM THE NORTHWEST INTO THE  
PLAINS DURING TUE-THU.  
 
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  
 

 
 
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