665  
FOUS30 KWBC 160106  
QPFERD  
 
EXCESSIVE RAINFALL DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
906 PM EDT MON OCT 15 2018  
 
DAY 1  
VALID 01Z TUE OCT 16 2018 - 12Z TUE OCT 16 2018  
 
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER CENTRAL AND  
NORTHEAST TEXAS...  
 
0100Z UPDATE: FOR THE FINAL ISSUANCE OF THE EXCESSIVE  
PRECIPITATION OUTLOOK...MODERATE RISK AREAS WERE ADDED OVER  
PORTIONS OF CENTRAL TO NORTHEAST TEXAS WHERE FFG VALUES HAVE  
LOWERED FROM RECENT RAINS. ADDITIONAL MODERATE TO HEAVY RAINS  
LIKELY THROUGH THE REMAINDER OF THIS FORECAST PERIOD. HOURLY  
RAINFALL RATES OF .50-1"+ POSSIBLE...WITH THESE VALUES POSSIBLY  
EXCEEDING THE LOWERED FFG VALUES. SEE WPC'S MESOSCALE  
PRECIPITATION DISCUSSION # 0982 VALID UNTIL 0500 UTC OCT 16 FOR  
ADDITIONAL INFORMATION IN THE VICINITY OF THE TX HILL COUNTRY.  
 
ORAVEC  
   
..EARLIER DISCUSSION
 
 
   
..CENTRAL TEXAS TO THE ARKLATEX
 
 
HERE WE CONFINED SLIGHT RISK TO AREAS WHERE THE GREATEST 3 TO  
6-HOUR RAIN RATES OVERLAP WITH LOWER FLASH FLOOD GUIDANCE VALUES.  
MOST RAINFALL WILL BE ELEVATED / ROOTED IN THE MID LEVELS AND  
LOCATED WELL NORTH OF A FRONTAL BOUNDARY. THIS WILL PLACE A LIMIT  
ON AVAILABLE INSTABILITY AND SHORT TERM RAIN RATES, WITH MOST OF  
THE HEAVIER CONVECTION OCCURRING IN A PROGRESSIVE FASHION CLOSER  
TO THE GULF COAST. STILL, GENERALLY SATURATED SOILS ALONG WITH  
CONTINUED RAINFALL AND OCCASIONALLY EMBEDDED, CONVECTIVELY  
ENHANCED RAIN RATES, WILL SUPPORT MARGINAL AND SLIGHT RISK AREAS  
FROM CENTRAL AND SOUTHWEST TEXAS UP THROUGH THE ARKLATEX. SOME OF  
THE HEAVIER RAIN RATES ARE EXPECTED LATER IN THE DAY 1 PERIOD,  
MAINLY MONDAY NIGHT.  
   
..TEXAS COAST
 
 
A COLD FRONT WILL ENCOUNTER INCREASINGLY UNSTABLE AIR AS IT DROPS  
TOWARD THE COAST THIS MORNING. COVERAGE OF THUNDERSTORMS IS  
EXPECTED TO INCREASE, ESPECIALLY BETWEEN 12 AND 15Z. MODERATE CAPE  
VALUES AND HIGH PW VALUES - ANOMALOUSLY HIGH EVEN FOR SOUTH TEXAS  
- WILL SUPPORT INTENSE SHORT TERM RAIN RATES. A RELATIVE ABSENCE  
OF MID AND UPPER LEVEL SUPPORT, HOWEVER, SHOULD GIVE THIS  
CONVECTION A RELATIVELY DISORGANIZED CHARACTER. HI-RES MODELS  
SUGGEST OUTFLOWS AND ADVANCEMENT OF THE FRONTAL ZONE WILL PUSH THE  
STORMS ALONG TO THE SOUTH AND EAST, OFFSHORE, BY LATE MORNING TO  
EARLY AFTERNOON. ISOLATED INSTANCES OF FLASH FLOODING ARE  
POSSIBLE, MAINLY IN URBAN, POORLY DRAINED AREAS, AND ANY  
PARTICULARLY SENSITIVE BASINS.  
   
..EAST COAST OF THE U.S
 
 
A STRONG SHORTWAVE TROUGH WILL PASS THROUGH THE NORTHEAST AND  
MID-ATLANTIC STATES THIS AFTERNOON/EVENING. THE TROUGH IS MARKED  
BY GREATER THAN 200 METER 12-HOUR HEIGHT FALLS OVER SOUTHERN  
QUEBEC, WITH MODERATELY STRONG HEIGHT FALLS EXTENDING TO THE  
LATITUDE OF MARYLAND / NEW JERSEY. MOISTURE RETURN WILL RUSH UP TO  
MEET THE ASSOCIATED COLD FRONT, AND HI-RES MODELS DEPICT A  
STRONGLY FORCED LINE OF CONVECTION FORMING OVER COASTAL SECTIONS  
OF SOUTHERN NEW ENGLAND THE THE MID ATLANTIC. RAPID MOVEMENT WILL  
KEEP EVENT RAINFALL TOTALS CONTAINED, BUT WITH PW VALUES OF 1.50  
INCHES IN A STRONGLY FORCED CONVECTIVE REGIME, WE WOULD EXPECT  
RAPID ACCUMULATION OF A HALF TO ONE INCH, WHICH SUPPORTS  
INTRODUCTION OF MARGINAL RISK FOR THIS HIGHLY POPULATED CORRIDOR.  
   
..SOUTHERN ARIZONA
 
 
THERE IS A SMALL ZONE IN WHICH SOME THUNDERSTORMS CARRYING HEAVY  
RAIN MAY OCCUR THIS AFTERNOON, FROM PHOENIX DOWN TO JUST WEST OF  
TUCSON. A SQUEEZE PLAY WILL TAKE PLACE AS A COLD FRONT DROPS DOWN  
FROM THE NORTH AND COOLER, DRIER AIR PUSHES WEST THROUGH THE  
MOUNTAIN GAPS IN NEW MEXICO. THERE IS POTENTIAL, HOWEVER, FOR A  
FEW ORGANIZED THUNDERSTORMS EAST OF AN UPPER LOW CENTER IN  
SOUTHERN ARIZONA, WITHIN A NARROW ZONE WHERE AFTERNOON DEWPOINTS  
ARE FORECAST TO HANG IN THE 50S, WITH A FEW HUNDRED J/KG OF CAPE.  
SHEAR IS SUFFICIENT FOR ORGANIZED UPDRAFTS AND SOME REDEVELOPMENT  
ALONG FLANKS WHICH COULD BOOST LOCAL RAIN TOTALS. THE PW  
ENVIRONMENT IS MARGINAL AT AROUND 0.65 INCHES, BUT WITH DECENT QPF  
SIGNAL FROM THE GFS AND THE UNIVERSITY OF ARIZONA WRF, WE  
INTRODUCED A VERY SMALL MARGINAL RISK AREA.  
 
BURKE  
 
DAY 2  
VALID 12Z TUE OCT 16 2018 - 12Z WED OCT 17 2018  
 
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF  
NORTH CENTRAL TEXAS...  
   
..SOUTHERN PLAINS INTO THE LOWER MISSISSIPPI VALLEY
 
 
LOCALLY HEAVY RAIN IS EXPECTED TO BE ONGOING AT THE START OF THE  
DAY 2 OUTLOOK PERIOD (12Z/16) WITH STRONG 850-700 MB FRONTOGENESIS  
IN PLACE FROM NEAR DEL RIO TO NORTHEASTERN TX. PRECIPITABLE WATER  
VALUES OF 1.25-1.75 INCHES SHOULD BE IN PLACE OVER CENTRAL AND  
NORTHERN TX WITH EVEN HIGHER VALUES NEAR THE COAST. REMNANT  
ISENTROPIC LIFT WILL HELP SQUEEZE OUT MOISTURE ACROSS  
CENTRAL/NORTHEAST TX BUT A LACK OF GREATER INSTABILITY SHOULD BE A  
SIGNIFICANT LIMITING FACTOR WITH REGARDS TO RAIN RATES.  
EVENTUALLY, LOW LEVEL CONVERGENCE AND FRONTOGENESIS WEAKEN AS  
MID-LEVEL HEIGHTS RISE ACROSS THE REGION WITH THE DEPARTING  
NORTHERN STREAM JET.  
 
THERE MAY STILL BE SOME EXCESSIVE RAINFALL CONCERNS ACROSS  
PORTIONS OF CENTRAL TO NORTHERN TX GIVEN ANTECEDENT HEAVY RAINFALL  
WITH AS MUCH AS 5-10 INCHES OVER THE PAST 7 DAYS, BUT THE THREAT  
IS EXPECTED TO SLOWLY DIMINISH THROUGH THE PERIOD AS FORCING  
WEAKENS. THE LATEST 12Z HI-RES MODELS SHOW AN ELONGATED WSW-ENE 24  
HR QPF MAX THROUGH 12Z/17 OF 2-4 INCHES OVER PORTIONS OF CENTRAL  
TO NORTHERN TX WITH ONLY MINOR LATITUDE DIFFERENCES. 3-HOURLY RAIN  
RATES ARE EXPECTED TO PEAK IN THE 1 TO 1.5 INCH RANGE, MAINLY  
PRIOR TO 00Z/17, WHICH WOULD OVERLAP WITH THE WET ANTECEDENT  
CONDITIONS IN PLACE. ONLY MINOR ADJUSTMENTS WERE MADE TO THE  
PREVIOUS DAY 2 OUTLOOK TO EXPAND THE MARGINAL AND SLIGHT AREAS A  
BIT WEST, EAST AND SOUTH BASED ON THE LATEST QPF GUIDANCE.  
   
..ARIZONA
 
 
A CLOSED LOW IS FORECAST TO REMAIN QUASI-STATIONARY OVER AZ FROM  
TUESDAY MORNING INTO WEDNESDAY MORNING WITH WEAK CAPE GIVEN THE  
500 MB TEMPERATURES OF -20C. PRECIPITABLE WATER VALUES ARE  
EXPECTED TO DECREASE DURING THE PERIOD, BUT MAY START OFF NEAR 1  
INCH AT 12Z/16. GIVEN THE LOCATION BENEATH THE CLOSED LOW, ANY  
CONVECTION THAT DEVELOPS MAY BE SLOW MOVING. BASED ON THE PIVOTING  
AXIS OF THE CLOSED LOW/VORTICITY ALOFT AND DIMINISHING MOISTURE  
FEED, FEEL THAT THE POTENTIAL FOR EXCESSIVE RAINS WILL RAPIDLY  
DIMINISH AFTER 00Z/17. WHILE A LACK OF HIGHER INSTABILITY/CAPE  
PRECLUDES THE ADDITION OF A MARGINAL RISK AT THIS TIME, IT IS AN  
AREA WE ARE WATCHING FOR A POSSIBLE UPGRADE ON THE NEXT CYCLE.  
 
OTTO/PAGANO  
 
DAY 3  
VALID 12Z WED OCT 17 2018 - 12Z THU OCT 18 2018  
 
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER SOUTH  
TEXAS...  
   
..SOUTH TEXAS
 
 
AS A CLOSED LOW JUST WEST OF THE CENTRAL ROCKIES AT 12Z/18 BEGINS  
TO OPEN AND LIFT TOWARD THE EAST-NORTHEAST, A 850-700 MB RIDGE IS  
EXPECTED TO EXPAND NORTHWESTWARD FROM THE EASTERN GULF COAST INTO  
OK AND KS. THIS ORIENTATION WILL ALLOW A STRENGTHENING OF  
EASTERLY/SOUTHEASTERLY 850 MB FLOW ACROSS THE GULF INTO THE SOUTH  
TEXAS COAST WITH WIND SPEEDS PEAKING NEAR 35 KT VIA A CONSENSUS OF  
THE 12Z MODELS, MANLY BEYOND 00Z/18.  
 
WHILE A SURFACE FRONT IS EXPECTED TO BE LOCATED FAR ENOUGH  
OFFSHORE WEDNESDAY MORNING TO LIMIT ANY APPRECIABLE CAPE OVER  
LAND, THE BOUNDARY IS FORECAST TO EDGE CLOSER TO THE COAST BY  
THURSDAY MORNING. THE INCREASE IN LOW LEVEL MOISTURE IS EXPECTED  
TO INCREASE PRECIPITABLE WATER VALUES TO PEAK OVER 2 INCHES BY  
THURSDAY MORNING FROM NEAR MATAGORDA BAY TO THE RIO GRANDE AND THE  
SHALLOW NATURE OF THE FRONT SHOULD ALLOW FOR AT LEAST WEAK  
ELEVATED INSTABILITY TO DEVELOP FOR COASTAL LOCATIONS.  
 
GIVEN THE HIGH MOISTURE AND WEAK INSTABILITY IN PLACE, ALONG WITH  
THE INCREASE IN LOW LEVEL FLOW, THERE MAY BE SOME SLOW MOVING OR  
TRAINING OF CONVECTIVE CLUSTERS LIMITED TO LOCATIONS WITHIN 25-75  
MILES OF THE COAST OVER SOUTH TEXAS WITH COULD PRODUCE LOCALIZED  
MAXIMA OF 2-4 INCHES.  
 
OTTO  
 
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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