653  
FOUS30 KWBC 180859  
QPFERD  
 
EXCESSIVE RAINFALL DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
458 AM EDT MON JUN 18 2018  
 
DAY 1  
VALID 12Z MON JUN 18 2018 - 12Z TUE JUN 19 2018  
 
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER THE TEXAS  
GULF COAST...PORTIONS OF THE MONTANA...FROM THE CENTRAL PLAINS  
TO THE UPPER MIDWEST...AND ACROSS THE NORTHEAST...  
   
..TEXAS COAST AND SOUTH TEXAS
 
 
GUIDANCE CONTINUES TO SHOW AN INVERTED 700 MB TROUGH THAT DRIFTS  
WEST ACROSS SOUTH TEXAS, WITH AN OCCASIONAL SPOT LOW SFC-700 MB.  
TROPICAL MOISTURE STREAMS ONSHORE WITH PWAT OF 2.25-2.4 INCHES  
(AROUND RECORD VALUES FOR MID JUNE) SET TO MOVE INTO SOUTH AND  
EAST TX ALONG WITH BANDS OF 700 MB CONVERGENCE/300 MB DIVERGENCE,  
RESULTING IN HEAVY RAINS AND A POTENTIAL FOR A SIGNIFICANT  
RAINFALL EVENT. STILL UNCLEAR HOW ORGANIZED THE HEAVY RAINS WILL  
GET, BUT GIVEN THE TROPICAL MOISTURE AND INCREASED LOW LEVEL FLOW,  
A SLIGHT RISK WAS MAINTAINED ALONG THE TEXAS COAST INTO SOUTHWEST  
LA. RECENT DROUGHT CONDITIONS WILL MAKE IT HARDER TO FLASH FLOOD,  
ALTHOUGH SHORT TERM RATES COULD STILL GET HIGH ENOUGH TO CAUSE  
CONCERNS. MODEL GUIDANCE IS IN THE 3-6 INCH WITH THE 00Z NAM AND  
A FEW HIGH RES WINDOWS SHOWING ISOLATED HIGHER MAXIMA, SO THE  
SYSTEM WILL HAVE TO WATCHED CLOSELY.  
   
..CENTRAL PLAINS INTO THE MS VALLEY
 
 
A HEAVY RAIN THREAT WILL CONTINUE NEAR A SOUTHWARD MOVING FRONT  
STRETCHING FROM KS NORTHEAST ACROSS IA AND INTO WI.  
PWAT VALUES WILL REMAIN WELL ABOVE NORMAL ALONG THIS FRONT, WITH  
SUFFICIENT INSTABILITY LIKELY IN PLACE AS WELL.  
THE PRESENCE OF HIGH MOISTURE, A RELATIVELY SLOW MOVING FRONT AND  
CONTINUED LIFT OVER THE FRONT, SUPPORTS THE THREAT OF LOCALLY  
HEAVY RAINS AND THUS A SLIGHT RISK IS CONTINUED. THE 00Z NAM AND A  
FEW OF THE HIGH RES RUNS INDICATE POTENTIAL FOR BANDS OF 3-5  
INCHES IN NORTHERN IOWA TO SOUTHERN WI, SO THIS AREA WILL HAVE TO  
BE MONITORED CLOSELY.  
   
..NORTHEAST TO THE GREAT LAKES
 
 
A COLD FRONT WILL MOVE INTO THE NORTHEAST THIS AFTERNOON.  
ANTICIPATE AMPLE INSTABILITY AHEAD OF THE FRONT TO SUPPORT  
CONVECTIVE DEVELOPMENT. THE FRONT IS EXPECTED TO BE PROGRESSIVE  
AND THUS MOST STORMS SHOULD ALSO BE RATHER QUICK MOVING.  
HOWEVER THE VERY HIGH PWATS OF 2"+ FORECAST BY THE ECMWF INDICATE  
POTENTIAL FOR HEAVY DOWNPOURS WHERE THUNDERSTORMS DEVELOP.  
THESE VALUES COULD APPROACH RECORD VALUES FOR ANYTIME DURING THE  
SUMMER. THUS STORMS WILL LIKELY BE EFFICIENT RAINFALL PRODUCERS.  
STRONG WESTERLY 850 MB MOISTURE TRANSPORT AHEAD OF THE FRONT MAY  
SUPPORT SOME BACKBUILDING/REPEAT CELLS IN ADVANCE OF THE FRONT. THE  
PROGRESSIVE NATURE OF THE FRONT WILL LIMIT THE MAGNITUDE OF THE  
EVENT. A FEW OF THE HIGH RES RUNS SHOW POTENTIAL FOR 1-1.5 INCHES  
WITH CLUSTERS OF HEAVIER CELLS.  
   
..NORTHEASTERN OREGON TO CENTRAL MONTANA
 
 
A MID/UPPER LOW PERSISTS WITH THE 700 MB CIRCULATION NEAR THE  
OR/ID BORDER TODAY. MOISTURE CONTINUES TO ADVECT AROUND THE  
CIRCULATION WITH ASCENT OCCURRING NEARBY PLUS DOWNSTREAM OF THE  
UPPER LOW FORMING NEAR THE WY/MT BORDER, WITH PEAK 300 MB  
DIVERGENCE OCCURRING IN CENTRAL MT, WHERE GOOD MULTI-MODEL  
AGREEMENT IS IN PLACE FOR 1-2 INCHES OF RAIN. GIVEN THE HIGH  
PWATS AND DEEP LAYER RELATIVE HUMIDITY OF 90 PERCENT IN MT, WILL  
NEED TO MONITOR FOR LOCALLY HEAVY RATES. THE MODELS ALSO SHOW AN  
OROGRAPHIC COMPONENT WITH HIGHER AMOUNTS WHERE THE LOW-MID LEVEL  
SOUTHEAST FLOW TURNS UPSLOPE IN THE TERRAIN OF CENTRAL TO  
NORTHWEST MT, WHERE LOCALLY HIGHER AMOUNTS ARE POSSIBLE.  
   
..NORTHERN COLORADO TO WESTERN NEBRASKA AND SOUTH DAKOTA
 
 
THE MODELS INDICATE AN 850 MB WAVE EMERGES FROM NORTHEAST CO AND  
MOVES EAST NORTHEAST ACROSS NEBRASKA TODAY AND TONIGHT. LOW LEVEL  
DEFORMATION AND FRONTOGENESIS ALONG THE PATH AND NORTH OF THE WAVE  
PRODUCES INCREASING SHOWER ACTIVITY, INCLUDING NEAR A FORMING WARM  
FRONT CROSSING NORTHERN NEBRASKA TO THE SD BORDER. THE HIGH RES  
MODELS AND PARALLEL HIGH RES RAPID REFRESH INDICATE LOCALLY HEAVY  
DOWNPOURS, SO AREAS THAT HAVE GOTTEN WET THE LAST FEW DAYS COULD  
DEVELOP RUNOFF PROBLEMS. THE 00Z/06Z NAM CONUS NEST SHOWED HIGHER  
AMOUNTS IN SOUTHEAST SD THAN THE WRF ARW/NMMB/NSSL WRF, SO THE  
CONUS NEST LOOKS LIKE AN OUTLIER, AND THUS GIVEN LESS WEIGHT.  
 
PETERSEN  
 
DAY 2  
VALID 12Z TUE JUN 19 2018 - 12Z WED JUN 20 2018  
 
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF  
THE CENTRAL PLAINS AND ACROSS SOUTHEAST TEXAS...  
 
   
..CENTRAL PLAINS
 
 
A MODERATE RISK OF EXCESSIVE RAINFALL IS MAINTAINED FOR PORTIONS  
OF NORTHWEST KS AND SOUTHWEST NE, WITH VERY LITTLE CHANGE TO THE  
AREA ON THIS UPDATE. IN THE LOW LEVELS WILL HAVE A STATIONARY WEST  
TO EAST BOUNDARY, WITH PERSISTENT SOUTHERLY 850 MB MOISTURE  
TRANSPORT INTO AND OVER IT. IN THE MID AND UPPER LEVELS THE  
APPROACH OF THE CLOSED LOW WILL PROVIDE PROLONGED UPPER LEVEL  
DIVERGENCE OVER THIS AREA. MODELS SHOW AMPLE INSTABILITY IN THE  
VICINITY OF THE FRONT, WITH PWATS CONTINUING TO RUN WELL ABOVE  
AVERAGE. THUS THIS IS A GOOD SETUP FOR MULTIPLE CONVECTIVE  
CLUSTERS THROUGH THE DAY 2 PERIOD...WITH EACH ROUND CAPABLE OF  
PRODUCING HEAVY RAINFALL RATES. TOUGH TO REALLY PIN DOWN THE EXACT  
TIMING OF THE HEAVIEST RAIN, BUT TUESDAY NIGHT IS PROBABLY MOST  
LIKELY, AS THAT IS WHEN WE SEE THE NOCTURNAL INCREASE OF THE LOW  
LEVEL JET INTO THE FRONT. THE QPF SIGNAL IN MOST MODELS IS  
IMPRESSIVE, WITH THE VALUES DEPICTED USUALLY INDICATIVE OF A  
SIGNIFICANT FLASH FLOOD RISK. THUS GIVEN THE FAVORABLE ENVIRONMENT  
DESCRIBED ABOVE, AND THE MODEL QPFS, THINK A MODERATE RISK IS  
WARRANTED.  
 
GIVEN WE ARE STILL A FEW DAYS OUT THE EXACT LOCATION OF HEAVIEST  
RAINS MAY CHANGE, SO SOME LATITUDINAL ADJUSTMENTS TO THE MODERATE  
RISK AREA ARE CERTAINLY POSSIBLE. SOME CHANCE THE HIGHER END  
THREAT EXTENDS EAST NEAR THE KS/NE BORDER AS WELL...BUT ANTECEDENT  
CONDITIONS ARE NOTABLY DRIER THE FURTHER EAST YOU GO IN KS/NE,  
WHICH COMBINED WITH GROWING CROPS, SUGGESTS FLASH FLOODING WILL BE  
HARDER TO COME BY, AND THUS A SLIGHT RISK SHOULD COVER THE THREAT  
FOR NOW.  
   
..SOUTH TEXAS
 
 
GUIDANCE HAS COME INTO A BIT BETTER AGREEMENT WITH THE HANDLING OF  
THE ENERGY MOVING WEST ACROSS THE GULF TOWARDS SOUTH TX. TROPICAL  
MOISTURE (PWATS APPROACHING 2.5" INCHES IS AROUND RECORD VALUES  
FOR MID JUNE) WILL CONTINUE TO MOVE INTO SOUTH AND EAST TX, ALONG  
WITH THE APPROACH OF A MID LEVEL DISTURBANCE, RESULTING IN HEAVY  
RAINS AND A POTENTIAL FOR A SIGNIFICANT RAINFALL EVENT.  
 
STILL SOME UNCERTAINTY WITH HOW ORGANIZED THE HEAVY RAINS WILL  
GET, AND RECENT DROUGHT CONDITIONS WILL MAKE IT HARDER TO FLASH  
FLOOD. WITH THAT SAID, THIS SETUP CERTAINLY HAS THE POTENTIAL TO  
BE A SIGNIFICANT FLASH FLOOD EVENT FOR PORTIONS OF COASTAL TX. THE  
ENVIRONMENT IS CONDUCIVE WITH PWATS APPROACHING RECORD VALUES,  
SOUTHEASTERLY FLOW OFF THE GULF INCREASING TO 20-30 KTS, AND AT  
LEAST SOME MID LEVEL ENERGY MOVING WEST. THIS SEEMS TO FAVOR THE  
POTENTIAL FOR BACKBUILDING/TRAINING OF CELLS OFF THE GULF INTO THE  
COASTAL FRONT. MOST MODEL QPFS HAVE GENERALLY BEEN CONSISTENTLY  
IMPRESSIVE AS WELL (GFS APPEARS TOO WEAK/DISORGANIZED WITH THE  
ENERGY AND PROBABLY TOO LOW WITH QPF)...WITH THE 0Z HREF MEMBERS  
QUITE WET AS WELL. THIS COMBINATION OF FAVORABLE ENVIRONMENTAL  
PARAMETERS AND HIGH MODEL QPFS WOULD INDICATE AT LEAST THE  
POTENTIAL FOR SIGNIFICANT FLASH FLOODING TO DEVELOP.  
 
WITH THAT ALL SAID, SIGNIFICANT FLASH FLOODING WITH THIS SYSTEM IS  
CERTAINLY NOT A SLAM DUNK, AS MODELS ROUTINELY STRUGGLE WITH  
LOOSELY ORGANIZED TROPICAL FEATURES SUCH AS THIS, AND GIVEN HOW  
DRY ANTECEDENT CONDITIONS ARE. HOWEVER RAINFALL ON DAY 1 MAY HELP  
SATURATE GROUNDS FOR THIS DAY 2 ACTIVITY, AND AS MENTIONED ABOVE,  
THE SETUP DOES HAVE THE POTENTIAL TO DROP 3"-6"+ IN A SHORT  
PERIOD...WHICH WOULD FLASH FLOOD REGARDLESS OF ANTECEDENT  
CONDITIONS. GIVEN THE HIGH END CONDITIONAL THREAT, WE HAVE OPTED  
TO UPGRADE PORTIONS OF THE TX COAST TO A MODERATE RISK WITH THIS  
UPDATE.  
   
..MID MS VALLEY EAST INTO THE OHIO VALLEY AND MID ATLANTIC
 
 
A HEAVY RAIN THREAT WILL CONTINUE ALONG A SOUTHWARD MOVING FRONT  
STRETCHING ACROSS THE MS AND OH VALLEY INTO THE MID ATLANTIC. PWAT  
VALUES WILL REMAIN WELL ABOVE NORMAL ALONG THIS FRONT, WITH  
SUFFICIENT INSTABILITY LIKELY IN PLACE AS WELL. LARGE SCALE  
FORCING AND SHEAR ARE BOTH NOT ALL THAT GREAT, THUS NOT  
NECESSARILY LOOKING FOR ORGANIZED THUNDERSTORM COMPLEXES. BUT THIS  
PORTION OF THE FRONT DOES BECOME PRETTY MUCH STATIONARY BY THIS  
TIME, PROVIDING A PERSISTENT AREA OF LOW LEVEL CONVERGENCE FOR  
STORMS TO DEVELOP ALONG. GIVEN THE HIGH PWATS AND CONTINUED  
SCATTERED TO WIDESPREAD DEVELOPMENT OF STORMS NEAR THE STATIONARY  
BOUNDARY...WOULD ANTICIPATE SOME HEAVY RAINFALL AND AT LEAST SOME  
FLASH FLOOD RISK. THUS WILL HAVE A SLIGHT RISK EXTEND ACROSS THE  
AREA.  
 
OVERALL VERY LITTLE CHANGE WAS MADE TO THE SLIGHT RISK  
AREA...ALTHOUGH DID EXTEND IT A BIT FURTHER EAST INTO PORTIONS OF  
WESTERN MD AND NORTHERN VA. THE 0Z HREF MEMBERS ARE UNANIMOUS IN  
DEVELOPING SCATTERED CONVECTION ALONG THIS CORRIDOR TUESDAY  
AFTERNOON...AND GIVEN THE PWATS IN PLACE, SOME FLASH FLOOD THREAT  
LIKELY EXISTS.  
 
CHENARD  
 
DAY 3  
VALID 12Z WED JUN 20 2018 - 12Z THU JUN 21 2018  
 
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER THE CENTRAL  
TEXAS COAST...WITH A SLIGHT RISK EXTENDING FROM THE CENTRAL PLAINS  
EAST INTO THE MID ATLANTIC...  
 
   
..SOUTH TEXAS
 
 
A MODERATE RISK WAS CONTINUED ON DAY 3 ACROSS PORTIONS OF THE  
CENTRAL TX COAST. UNCERTAINTY IS A BIT HIGHER BY DAY 3 WITH  
REGARDS TO HOW PRONOUNCED THE MID LEVEL ENERGY REMAINS, AND  
EXACTLY WHERE IT IS CENTERED. IN GENERAL THE TREND HAS BEEN TO  
LIFT THE ENERGY A BIT MORE OFF TO THE NORTHEAST WITH EACH NEW  
FORECAST CYCLE. WPC THUS TRENDED IN THIS DIRECTION, BRINGING THE  
HEAVY RAIN THREAT MORE INTO EAST TX THAN PREVIOUSLY FORECAST.  
 
THE ENVIRONMENT WILL REMAIN CONDUCIVE FOR HEAVY RAIN, WITH PWATS  
APPROACHING RECORD VALUES, SOUTHEASTERLY FLOW OFF THE GULF  
INCREASING TO 20-30 KTS, AND AT LEAST SOME MID LEVEL ENERGY MOVING  
WEST. THIS SEEMS TO FAVOR THE POTENTIAL FOR BACKBUILDING/TRAINING  
OF CELLS OFF THE GULF INTO THE COASTAL FRONT. MOST MODEL QPFS HAVE  
GENERALLY BEEN CONSISTENTLY IMPRESSIVE AS WELL (GFS APPEARS TOO  
WEAK/DISORGANIZED WITH THE ENERGY AND PROBABLY TOO LOW WITH QPF).  
THIS COMBINATION OF FAVORABLE ENVIRONMENTAL PARAMETERS AND HIGH  
MODEL QPFS WOULD INDICATE AT LEAST THE POTENTIAL FOR SIGNIFICANT  
FLASH FLOODING.  
 
WITH THAT ALL SAID, SIGNIFICANT FLASH FLOODING WITH THIS SYSTEM IS  
CERTAINLY NOT A SLAM DUNK, AS MODELS ROUTINELY STRUGGLE WITH  
LOOSELY ORGANIZED TROPICAL FEATURES SUCH AS THIS, AND GIVEN HOW  
DRY ANTECEDENT CONDITIONS ARE. HOWEVER RAINFALL ON BOTH DAYS 1 AND  
2 SHOULD HELP SATURATE GROUNDS, THEORETICALLY MAKING IT EASIER FOR  
CONVECTION ON DAY 3 TO FLASH FLOOD, AND AS MENTIONED ABOVE, THE  
SETUP DOES HAVE THE POTENTIAL TO DROP 3"-6"+ IN A SHORT  
PERIOD...WHICH WOULD FLASH FLOOD REGARDLESS OF ANTECEDENT  
CONDITIONS. GIVEN THE HIGH END CONDITIONAL THREAT AND THE CURRENT  
WPC FORECAST OF 3 DAY RAINFALL TOTALS IN THE 6-12" RANGE OVER THE  
AREA, THINK THE BEST COURSE OF ACTION IS TO CONTINUE WITH A  
MODERATE RISK INTO THE DAY 3 PERIOD (ALTHOUGH CONFIDENCE IN THE  
DETAILS THIS FAR OUT ARE LOW).  
   
..CENTRAL PLAINS INTO THE MID MS VALLEY
 
 
AN IMPRESSIVE SETUP FOR HEAVY RAINFALL WILL CONTINUE INTO THE DAY  
3 PERIOD ACROSS PORTIONS OF THE CENTRAL PLAINS AND MID MS VALLEY.  
A WELL DEFINED SHORTWAVE IS EXPECTED TO EJECT INTO THE PLAINS ON  
WEDNESDAY. THIS FEATURE IS EXPECTED TO BE SLOW MOVING, AND LIKELY  
INTENSIFY, LIKELY BECOMING A CLOSED LOW. THIS EVOLUTION WILL  
SUPPORT A PROLONGED PERIOD OF IMPRESSIVE UPPER LEVEL DIVERGENCE.  
STRONG LOW LEVEL CONVERGENCE IS ALSO LIKELY, WITH PWATS REMAINING  
WELL ABOVE AVERAGE. THUS THE INGREDIENTS ARE CERTAINLY IN PLACE  
FOR PROLONGED HEAVY RAINFALL...WITH THIS SUPPORTED BY MODEL QPFS  
AS WELL.  
 
JUST LOOKING AT MODEL QPF VALUES WOULD SUGGEST THE NEED FOR A  
MODERATE RISK CENTERED OVER EASTERN NE. HOWEVER, A COUPLE OF  
QUESTIONS REMAIN THAT WARRANT KEEPING THE RISK AT THE SLIGHT LEVEL  
FOR NOW. THE HEAVIEST RAIN IN THE MODELS IS FORECAST NEAR AND JUST  
NORTH OF THE DEVELOPING CLOSED LOW, WHERE WE SEE VERY IMPRESSIVE  
850 MB MOISTURE TRANSPORT DEVELOP. THIS LOCATION CERTAINLY SEEMS  
FAVORABLE FOR PROLONGED HEAVY RAINFALL. HOWEVER, MODELS DO NOT  
REALLY BRING MUCH IN THE WAY OF INSTABILITY THAT FAR NORTH. THIS  
RAISES THE QUESTION AS TO WHETHER WE SEE RATES HIGH ENOUGH TO  
RESULT IN A SIGNIFICANT FLASH FLOOD THREAT, AND ALSO WHETHER SOME  
OF THE HIGH MODEL QPFS ARE SUFFERING FROM FEEDBACK ISSUES. FOR  
THESE REASONS, THINK KEEPING A SLIGHT FOR NOW IS THE WAY TO GO,  
WHILE WE CLOSELY MONITOR TRENDS.  
 
THE HEAVIEST RATES MAY ACTUALLY END UP FROM FAR SOUTHEAST NE INTO  
SOUTHERN IA AND NORTHERN MO...AREAS THAT ARE CURRENTLY FORECAST TO  
BE IN THE WARM SECTOR AND SHOULD HAVE PLENTIFUL INSTABILITY.  
HOWEVER WITH THE SYSTEM TAKING ON AN INCREASINGLY CUTOFF LOOK, WE  
MAY BEGIN TO LOSE THE BETTER SOUTHERLY 850 MB MOISTURE TRANSPORT  
ACROSS THE WARM SECTOR. THIS INCREASES UNCERTAINTY WITH THE  
MAGNITUDE OF THE FLASH FLOOD THREAT HERE AS WELL.  
 
THUS WHILE THIS SYSTEM SHOULD PRODUCE WIDESPREAD MODERATE TO HEAVY  
RAINFALL TOTALS...AND A SIGNIFICANT FLASH FLOOD THREAT MAY  
CERTAINLY EVOLVE ON WEDNESDAY...THERE ARE ENOUGH QUESTION MARKS TO  
WARRANT KEEPING JUST A SLIGHT RISK FOR NOW.  
   
..OH VALLEY AND MID ATLANTIC
 
 
A HEAVY RAIN THREAT WILL CONTINUE ALONG A NEARLY STATIONARY FRONT  
STRETCHING ACROSS THE MS AND OH VALLEY INTO THE MID ATLANTIC. PWAT  
VALUES WILL REMAIN WELL ABOVE NORMAL ALONG THIS FRONT, WITH  
SUFFICIENT INSTABILITY LIKELY IN PLACE AS WELL. LARGE SCALE  
FORCING AND SHEAR ARE BOTH NOT ALL THAT GREAT, THUS NOT  
NECESSARILY LOOKING FOR ORGANIZED THUNDERSTORM COMPLEXES. BUT THIS  
PORTION OF THE FRONT DOES BECOME PRETTY MUCH STATIONARY BY THIS  
TIME, PROVIDING A PERSISTENT AREA OF LOW LEVEL CONVERGENCE FOR  
STORMS TO DEVELOP ALONG. GIVEN THE HIGH PWATS AND CONTINUED  
SCATTERED TO WIDESPREAD DEVELOPMENT OF STORMS NEAR THE STATIONARY  
BOUNDARY...WOULD ANTICIPATE SOME HEAVY RAINFALL AND AT LEAST SOME  
FLASH FLOOD RISK. THUS WILL HAVE A SLIGHT RISK EXTEND ACROSS THE  
AREA. HAVE GENERALLY NOTED A NORTHWARD TREND AMONGST MOST OF THE  
GUIDANCE WITH THE POSITION OF THIS FRONT ON WEDNESDAY.  
 
CHENARD  
 
DAY 1 THREAT AREA: HTTP://WWW.WPC.NCEP.NOAA.GOV/QPF/94EPOINTS.TXT  
DAY 2 THREAT AREA: HTTP://WWW.WPC.NCEP.NOAA.GOV/QPF/98EPOINTS.TXT  
DAY 3 THREAT AREA: HTTP://WWW.WPC.NCEP.NOAA.GOV/QPF/99EPOINTS.TXT  
 

 
 
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