065  
FXUS02 KWBC 041551  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1150 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 A BROADER CONSENSUS TIMING WHICH WAS  
SLOWER THAN THE 00Z CANADIAN AND MORE DEFINED THAN THE 00Z UKMET.  
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 REMAINED STRONGER THAN OTHER MODELS WITH A LEADING OHIO  
VALLEY TO EASTERN U.S. WAVE AT THE SURFACE AND ALOFT WED-THU  
UNLIKE THE OTHER MODELS (INCLUDING THE 06Z FV3-GFS), SO THAT WAVE  
WAS DEEMED OVERDONE.  
 
THE REST OF THE FORECAST INVOLVES MORE BROADLY DISPERSED  
MODELS/ENSEMBLE MEMBERS FOR SPECIFICS. 00Z GFS RUNS WAS AMONG THE  
MORE WESTERN OR SOUTHWESTERN SOLUTIONS FOR THE UPPER LOW THAT MAY  
CLOSE OFF WITHIN THE WESTERN U.S. TROUGH ALOFT BUT THE 00Z ECMWF  
AND ITS ENSEMBLE MEAN SHIFTED WESTWARD FROM ITS EARLIER RUN.  
PREFERRED THE 06Z GFS WITH THE 00Z ECMWF ALONG WITH CONTINUITY TO  
MITIGATE FUTURE SHIFTS IN THE EVOLUTION OF THE WESTERN TROUGH,  
GIVEN THE RECENT ENSEMBLE CHANGES.  
 
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. LATEST 00Z ENSEMBLES INDICATED A MUCH MORE ROBUST SYSTEM  
ENTERING THE HIGH PLAINS LATE FRI INTO SAT IN ADDITION TO A  
STRONGER AND PERHAPS MORE NORTHERN SFC LOW INTO THE EASTERN GREAT  
LAKES FRIDAY. FORMED A BEST CONSENSUS AMONG THE 06Z GFS/00Z ECMWF  
AND THE ENSEMBLE MEANS TO TAKE THAT FRONT/WAVE OFF THE NORTHEAST  
COAST EARLY NEXT SATURDAY, BUT WITH LOW CONFIDENCE IN ANYTHING  
OTHER THAN BROAD DETAILS.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
CONTINUE TO EXPECT A THREAT OF HEAVY RAINFALL OVER THE  
CENTRAL/SOUTHERN PLAINS AND MISSISSIPPI VALLEY THROUGH MUCH OF THE  
PERIOD (TUE-FRI). 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  
INCLUDING SOUTHERN UTAH.  
 
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 BUT HIGH TEMPERATURES ABOUT 5-10F ABOVE  
NORMAL. THE PACIFIC NORTHWEST SHOULD SEE MULTIPLE DAYS WITH HIGHS  
10-20F ABOVE NORMAL AND STAND THE BEST CHANCE OF TYING/BREAKING  
RECORD HIGHS.  
 
FRACASSO/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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