384  
FXUS02 KWBC 040701  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
300 AM EDT SAT MAY 04 2019  
 
VALID 12Z TUE MAY 07 2019 - 12Z SAT MAY 11 2019  
   
..OVERVIEW
 
 
VARIOUS DETAILS REMAIN UNCERTAIN BUT GUIDANCE IS CONTINUING TO  
SUGGEST THAT A STRONG EASTERN PACIFIC RIDGE WILL PROMOTE WESTERN  
U.S. TROUGHING, WHILE A BROAD/FLAT RIDGE WILL LIKELY BE ORIENTED  
NEAR OR JUST OFFSHORE THE EAST COAST. AN EJECTING SOUTHWEST U.S.  
UPPER LOW TUE ONWARD AND ASSOCIATED SURFACE WAVE/FRONTAL SYSTEM,  
FOLLOWED BY SLOW MOVEMENT/POSSIBLE STALLING OF THE TRAILING FRONT  
AS IT BECOMES ALIGNED WITH SOUTHWESTERLY FLOW ALOFT, WILL PRODUCE  
A HEAVY RAINFALL THREAT OVER PARTS OF THE PLAINS AND MISSISSIPPI  
VALLEY. SOME AFFECTED AREAS WILL LIKELY ALREADY HAVE WET GROUND  
THUS ELEVATING FLASH FLOOD CONCERN. A LESS EXTREME PORTION OF  
THIS MOISTURE SHOULD EXTEND INTO THE EAST, WHILE THE EJECTING  
SOUTHWEST UPPER LOW AND SUBSEQUENT LARGER SCALE TROUGHING SHOULD  
GENERATE TERRAIN ENHANCED PRECIPITATION OVER PARTS OF THE WEST.  
 
   
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
 
 
IN THE TUE-WED TIME FRAME THERE HAD BEEN CONSIDERABLE UNCERTAINTY  
OVER DETAILS ACROSS THE NORTHEASTERN QUADRANT OF THE LOWER 48 INTO  
CANADA. STARTING WITH YESTERDAY'S 06Z GFS THERE HAS BEEN BETTER  
AGREEMENT IN PRINCIPLE ON THE MORE SUPPRESSED SCENARIO WHICH THE  
MAJORITY CLUSTER HAD SHOWN 24 HOURS AGO. THERE HAS STILL BEEN  
SOMEWHAT OF A SOUTHWARD TREND WITH THE NORTHEASTERN FRONT DROPPING  
INTO THE MID-ATLANTIC.  
 
AS THE EJECTING SOUTHWEST U.S. UPPER LOW REACHES THE ROCKIES AND  
BEYOND, GUIDANCE DISPLAYS TYPICAL DIVERGENCE FOR TIMING--DUE IN  
PART TO DIFFERENCES THAT DEVELOP WITHIN THE AMPLIFYING WESTERN  
TROUGH. OVERALL PREFER AN INTERMEDIATE TIMING AT THE MOMENT, AT  
LEAST LEANING AWAY FROM THE SLOW SIDE OF THE SPREAD (LATEST GFS  
RUNS) AS THE FEATURE IS CARRIED ALONG BY PROGRESSIVE SOUTHWESTERLY  
FLOW ALOFT. ON THE OTHER HAND SLOWER TREND OF THE 00Z CMC LEAVES  
THE UKMET AS THE FASTEST SOLUTION. OVER THE PAST DAY GUIDANCE HAS  
TRENDED STRONGER FOR THE SURFACE WAVE REACHING THE MIDWEST BY  
EARLY THU, REFLECTING BETTER DEFINITION OF THE UPPER SYSTEM. ALSO  
WORTH NOTING, RECENT GFS RUNS ARE STRONGER THAN OTHER MODELS WITH  
A LEADING EASTERN U.S. WAVE AT THE SURFACE AND ALOFT WED-THU.  
 
THE REST OF THE FORECAST INVOLVES MORE BROADLY DISPERSED  
MODELS/ENSEMBLE MEMBERS FOR SPECIFICS. GFS/FV3 GFS RUNS ARE AMONG  
THE MORE WESTERN OR SOUTHWESTERN SOLUTIONS FOR THE UPPER LOW THAT  
MAY CLOSE OFF WITHIN THE WESTERN U.S. TROUGH ALOFT. ON THE OTHER  
HAND THE 12Z ECMWF MEAN IS ONE OF THE MOST AGGRESSIVE TO KNOCK  
DOWN THE UPSTREAM RIDGE TOWARD THE END OF THE PERIOD, LEADING TO  
FASTER TROUGH PROGRESSION. THE 12Z ECMWF IS SIMILAR WITH THE  
TROUGH IN SPITE OF HOLDING ONTO A LITTLE MORE RIDGE UPSTREAM.  
THERE IS A GENERAL SIGNAL FOR SOME EROSION OF THE RIDGE THOUGH.  
TOWARD THE END OF THE PERIOD A COMPROMISE AMONG THE FASTER (12Z  
ECMWF/ECMWF MEAN) AND SLOWER (18Z GEFS MEAN/OLD 00Z ECMWF)  
SOLUTIONS WOULD PROVIDE A REASONABLE STARTING POINT--AND IS CLOSE  
TO THE 12Z CMC MEAN.  
 
SEPARATE TROUGHING (WITH POSSIBLE EMBEDDED LOW) OVER SOUTHERN  
CANADA AND EXTENDING INTO THE NORTHERN TIER OF THE LOWER 48 WILL  
PROVIDE ADDED FORECAST CHALLENGES, ESPECIALLY DURING THE LATTER  
HALF OF THE PERIOD. THUS FAR GUIDANCE HAS NOT BEEN VERY AGREEABLE  
OR CONSISTENT WITH THE ASSOCIATED WAVE(S)/FRONT(S) REACHING THE  
NORTHERN U.S.--MAINTAINING LOW CONFIDENCE IN ANY SPECIFIC SOLUTION.  
 
FOR ABOUT THE FIRST HALF OF THE PERIOD A MULTI-MODEL BLEND  
EMPHASIZES THE MOST COMMON ELEMENTS OF GUIDANCE. THEN THE  
FORECAST SHIFTS TOWARD A BLEND OF THE 18Z GEFS/12Z ECMWF MEANS  
(MORE OF THE FORMER DUE TO AFOREMENTIONED ISSUE WITH THE EASTERN  
PACIFIC RIDGE) AND PAST TWO ECMWF RUNS.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
CONTINUE TO EXPECT A THREAT OF HEAVY RAINFALL OVER THE  
CENTRAL/SOUTHERN PLAINS AND MISSISSIPPI VALLEY. LESS EXTREME BUT  
STILL SIGNIFICANT RAINFALL MAY EXTEND INTO THE THE OHIO/TENNESSEE  
VALLEYS AND NORTHEAST/MID-ATLANTIC BUT WITH LESSER CONFIDENCE ON  
SPECIFICS. THE CENTRAL/NORTH-CENTRAL PLAINS ACTIVITY WILL LIKELY  
TAKE PLACE DURING THE FIRST HALF OF THE PERIOD. A LONGER DURATION  
OF HEAVY RAIN/THUNDERSTORMS WILL BE POSSIBLE FARTHER SOUTH--IN  
ADVANCE OF THE SYSTEM EMERGING FROM THE WEST INTO THE PLAINS AND  
THEN LATE WEEK INTO THE WEEKEND AS PERSISTENT LOW LEVEL FLOW FROM  
THE GULF INTERACTS WITH A FRONT THAT MAY STALL FOR A TIME OVER THE  
LOWER MISSISSIPPI VALLEY/WESTERN GULF COAST. THE WAVE REACHING  
THE EAST LATE IN THE WEEK WILL PROVIDE THE PRIMARY FOCUS FOR  
ENHANCED RAINFALL OVER THAT PART OF THE COUNTRY.  
 
ACROSS THE WEST, ANTICIPATE VARYING INTENSITY OF PRECIPITATION  
FROM CALIFORNIA THROUGH THE GREAT BASIN/NORTHERN ARIZONA AND INTO  
MOST OF THE ROCKIES AS THE UPPER LOW TRACKS THROUGH SOUTHERN AREAS  
EARLY IN THE WEEK AND THE UPPER TROUGH/POSSIBLE EMBEDDED UPPER LOW  
DEVELOP OVER THE WEST LATER IN THE WEEK. SOME OF THE  
PRECIPITATION SHOULD BE IN THE FORM OF SNOW OVER HIGH ELEVATIONS  
OF THE NORTHERN-CENTRAL ROCKIES. DEPENDING ON EXACTLY HOW  
FEATURES EVOLVE AND TIME OF DAY, SOME SNOW COULD EXTEND INTO THE  
NORTH-CENTRAL HIGH PLAINS. HIGHEST PRECIP TOTALS ARE MOST LIKELY  
OVER FAVORED TERRAIN WITHIN THE ROCKIES FROM WYOMING TO NORTHERN  
NEW MEXICO. SOME LOCATIONS FARTHER WEST MAY SEE SIGNIFICANT  
AMOUNTS AS WELL.  
 
CHILLY AIR PUSHING SOUTHWARD OVER THE PLAINS MAY BRING HIGHS DOWN  
TO 15-25F BELOW NORMAL OVER NORTH-CENTRAL/CENTRAL HIGH PLAINS ON  
WED-THU, WITH A BROADER AREA OF BELOW NORMAL READINGS OVER THE  
CENTRAL U.S./ROCKIES AND SOUTHWEST DURING MOST OF THE PERIOD. THE  
SYSTEM TRACKING OUT OF THE SOUTHWEST WILL BRING WARMTH INTO THE  
EAST LATER IN THE WEEK WITH LOW TEMPERATURES IN PARTICULAR 10F OR  
GREATER ABOVE NORMAL. THE PACIFIC NORTHWEST SHOULD SEE MULTIPLE  
DAYS WITH HIGHS 10-20F ABOVE NORMAL.  
 
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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