Quantitative Precipitation Forecast
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FOUS30 KWBC 291457
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1056 AM EDT SAT APR 29 2017

...VALID 15Z SAT APR 29 2017 - 12Z SUN APR 30 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
BMQ 25 WSW MWL 30 W LTS 25 WNW JWG 10 E FOE MSN FKS 15 E APN
BAX PTK 20 NE FDY 4I3 20 WNW MGW 15 N EKN 25 ENE CRW 40 S EHR
10 SW UTA 30 SE ESF 25 NNE KCMB 20 ENE KXIH 20 E IAH DWH
25 E HYI BMQ.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
35 W EOK 15 NE FEP 20 SSW LDM 15 NNE MOP 20 N LAN 10 SSW BTL
10 SSE ASW 25 NNW DAY 20 ESE LHQ 30 E PKB 25 WNW 48I 10 E LOU
20 S HSB 20 ENE SGT 20 S BAD 30 WNW UTS 30 NNW LHB 10 NW TKI
25 SSE HBR 20 SSE WDG 20 S OJC 35 W EOK.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 NE DMO 25 ESE MQB 20 ENE BMI 10 NW 1H2 20 SW MDH 25 ENE ARG
10 NE LRF 10 SSE MWT 35 N PRX 15 SSE CHK 35 ENE SWO 50 N JLN
25 NE DMO.

HIGH RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 SSW PPQ 10 SW TAZ 15 SW SAR 35 N ARG 30 WSW BVX 20 N MWT
30 ESE MLC 15 SSW SNL TUL 45 NNW SGF 15 SSW PPQ.


...HIGH RISK AND TREMENDOUS RAIN/FLOOD EVENT EXPECTED FROM EASTERN
OKLAHOMA INTO THE WESTERN OH VALLEY...

EARLY 12Z MODEL GUIDANCE PICK UP ON WHAT APPEARS A SLIGHT
REALIGNMENT AND NORTHWESTWARD SHIFT TO THE RISK AREAS IN
ACCORDANCE WITH THE MAIN SYNOPTIC BOUNDARY DRAPED AND
STRENGTHENING FROM OKLAHOMA NORTHEASTWARD. OTHERWISE, THE FORECAST
APPEARS ON TRACK WITH A PREFERENCE TOWARD THE OVERNIGHT HI-RES
NSSL WRF AND HI-RES ARW RUNS FOR TRACKING RAIN RATES AND TIMING,
WHICH ARE DISPLACED QUITE A BIT SOUTH OF THE 12Z NAM. THE REST OF
THE OVERNIGHT DISCUSSION IS APPENDED BELOW.

PREVIOUS:

ALL THE INGREDIENTS ARE MAXIMIZED. A 3.5 STANDARD DEVIATION UPPER
LOW WITHIN A STRONGLY BUCKLING LARGE SCALE FLOW PATTERN / COUPLED
JET STRUCTURE...HEADING INTO A LENGTHY STALLED FRONTAL ZONE WITH
MOISTURE AND INSTABILITY CHARACTERISTIC OF MID SPRING. FLOW OFF
THE GULF...ORIGINATING IN THE CARIBBEAN...IS UNUSUALLY
STRONG...SQUEEZED BY A 1032 MB HIGH OVER THE WESTERN ATLANTIC. THE
ENSEMBLE SITUATION AWARENESS INFORMATION BASED ON THE NAEFS HAS AN
AWFUL LOT OF 99TH PERCENTILE AND MAX VALUES FOR INTEGRATED VAPOR
TRANSPORT WITHIN THE CONVEYOR BELT FROM EAST TEXAS UP THROUGH
EASTERN OK / WESTERN AR AND INTO MISSOURI. AMPLIFICATION OF THE
LARGE SCALE SYNOPTIC SYSTEM WILL ALSO RETARD EASTWARD
PROGRESSION...RESULTING IN VERY INTENSE AND PERSISTENT RAINFALL
THIS AFTERNOON AND EVENING...WHICH WE SUSPECT WILL BE CENTERED
AROUND NORTHWEST ARKANSAS BASED ON OBSERVATIONAL AND MODEL TRENDS.
THE LOCATION OF THE HEAVIEST RAIN VARIES BETWEEN MODELS...BUT
WITHIN THE WARM CONVEYOR BELT THEY ALL...GLOBAL AND HI-RES MODELS
ALIKE...HAVE A SIGNAL FOR WIDESPREAD 5-PLUS INCH RAINS WITH
EMBEDDED 7 TO 10 INCH AMOUNTS. A FURTHER CONCERN IS THE BREADTH OF
THE 5-PLUS INCH RAINFALL...WHICH MAY AFFECT PARTS OF THREE
STATES...COVERING A LOT OF TERRITORY...RESULTING IN A LARGE VOLUME
OF WATER INTO CERTAIN WATERSHEDS.

AS PRECIPITABLE WATER INCREASES TO NEAR 1.75 INCHES...AND 2.5
STANDARD DEVIATIONS ABOVE AVERAGE FOR LATE APRIL...EXPECT HOURLY
RAIN RATES TO PUSH ABOVE 3 INCHES GIVEN THE HIGH LIKELIHOOD OF
TRAINING. THE EVENT IS EXPECTED TO PEAK THIS AFTERNOON AND EVENING
AS THE LARGE SCALE SYSTEM REACHES MATURITY AND FLOW BECOMES VERY
MERIDIONAL DURING PEAK HEATING / PEAK INSTABILITY. THE HEAVIEST
RAIN IS FORECAST IN AREAS THAT ARE NOT ONLY PRONE TO FLASH
FLOODING DUE TO TERRAIN...BUT ARE ALSO WET FROM ANTECEDENT HEAVY
RAIN EARLIER IN THE WEEK AND RAIN ALREADY OCCURRING EARLY SATURDAY
MORNING. THUS...LIFE-THREATENING CONDITIONS MAY BECOME COMMON
LATER TODAY AND TONIGHT WITHIN THE HIGH RISK AREA.

MODERATE RISK EXTENDS A LITTLE DEEPER INTO EASTERN AND SOUTHERN
OKLAHOMA...AND UP ACROSS SAINT LOUIS TO ILLINOIS...WHERE EXTREME
RAIN TOTALS ARE LESS LIKELY...BUT AVERAGE RAINFALL OF AT LEAST 2
TO 5 INCHES IS FORECAST...INCLUDING VERY HEAVY SHORT TERM RATES.
THE GROUND IN SOME OF THESE AREAS ARE ALSO WET FROM PRIOR RAINFALL.

...MIDWEST AND OHIO VALLEY...

THE FORECAST CARRIES MORE UNCERTAINTY FROM NORTHERN MISSOURI
EASTWARD AND NORTHEASTWARD. THE INITIAL SURGE OF GULF MOISTURE HAD
RELEASED AN EXTENSIVE LINE OF CONVECTION STRETCHING EAST INTO THE
APPALACHIANS...WITH PLENTY OF TRAINING AND FLASH FLOOD CONCERNS.
THIS MAY PERSIST THROUGH LATE MORNING PER CONSISTENCY IN THE HRRR
RUNS. SYNOPTIC FORCING THEN REGROUPS BACK OVER THE MIDWEST AND
LOWER GREAT LAKES. THE MAIN INHIBITING FACTOR COULD BE EXTENSIVE
CONVECTION EXPECTED IN THE SOUTHERN PLAINS AND LOWER MISSOURI
VALLEY THIS MORNING. THIS MAY LIMIT INSTABILITY FARTHER
NORTH...SUCH AS DEPICTED IN THE NAM CONUS NEST AND NCEP
WRF-ARW...WHICH FORECAST MORE TOLERABLE RAINFALL TOTALS FROM SAINT
LOUIS NORTH AND EASTWARD. GIVEN SUCH A DYNAMIC UPPER
SYSTEM...HOWEVER...WITH STRONG WIND FIELDS...IT WOULD NOT TAKE
MUCH OF A BREAK IN THE ACTION TO THEN ALLOW A RAPID NORTHWARD
TRANSPORT OF MOISTURE AND INSTABILITY INTO AREAS WHERE THE HIGHLY
AMPLIFIED JET CIRCULATION WILL BEGIN TO PRODUCE DEEP LIFT FROM
EASTERN MISSOURI TO SOUTHEAST WISCONSIN AND WESTERN
MICHIGAN...ACROSS CHICAGOLAND. PLAYING THE
PROBABILITIES...MODERATE RISK OF EXCESSIVE RAINFALL EXTENDS
THROUGH SAINT LOUIS TO CENTRAL ILLINOIS...WITH SLIGHT RISK FARTHER
NORTH AND EAST.

...TEXAS AND LOUISIANA...

STRONG DEEP LAYER FORCING WILL BE REQUIRED TO OVERCOME THE CAPPING
INVERSION HERE...LIKELY RESULTING IN A NARROW AND MORE PROGRESSIVE
LINE OF CONVECTION. EXTREME TOTALS ARE UNLIKELY...BUT VERY HEAVY
SHORT TERM RAIN RATES MAY CAUSE FLASH FLOODING THIS AFTERNOON AND
OVERNIGHT.

BURKE/JAMES
$$




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