Quantitative Precipitation Forecast
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630
FOUS30 KWBC 300033
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
832 PM EDT SAT APR 29 2017

...VALID 01Z SUN APR 30 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
20 WSW 3T5 15 ENE HYI 10 E TPL 15 E INJ DAL 15 NW GYI
15 ESE ADM 55 S OJC 35 N SZL 30 SE IRK 20 ESE GBG 10 NNE JVL
10 NNE MWC 10 SE CAD 25 N MBS 20 NE FNT PTK 20 NE FDY 4I3
20 WNW MGW 15 N EKN 25 ENE CRW 25 SSW FTK 35 WSW HOP 45 SSE MKL
20 NNE GWO 25 WNW HKS 25 WNW BTR 15 N KVNP 20 N KVBS 30 WSW BPT
20 E IAH DWH 20 WSW 3T5.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 ENE JAS 25 N JAS UTS LHB 30 N LHB 10 E CRS 10 E GVT
15 SW MKO 20 NNE GMJ 10 NE AIZ 20 ESE PPQ 15 NE PNT IGQ
25 SSW VPZ 10 WNW GUS 10 ENE MIE HAO 35 NE HNB 10 ENE EHR
25 E M30 30 ESE PAH 35 SSE PAH 15 ESE OLV 40 NNW GWO 25 NW GWO
20 SE GLH 20 NW HEZ 20 SE ACP 20 ENE CWF 20 SW DRI 20 ENE JAS.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
TBN CPS 10 N OLY 10 E HSB 30 S CIR 15 N NQA UTA 20 WNW GLH
35 E BAD 25 E OSA 15 S LBR 40 N LBR 15 SSW SLG 10 ENE SGF
20 WSW TBN TBN.

HIGH RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
45 NNE UNO ENL CGI 20 SE M19 10 N SGT 20 SSW LIT 15 SW HOT
30 WNW MWT 20 NE FSM 25 NNW HRO 35 NW UNO 45 NNE UNO.


...HIGH RISK AND TREMENDOUS RAIN/FLOOD EVENT SHIFTING ACROSS
ARKANSAS INTO THE WESTERN OH VALLEY OVERNIGHT...

01Z UPDATE...
CHANGES TO PREVIOUS OUTLOOK MAINLY REFLECT EWD PROGRESSION OF
EXPECTED STRONGER FORCING OVERNIGHT WITH FLANKING LINE FROM MCV
MOVING ACRS IL HANGING UP ACRS NORTHERN ARKANSAS WHILE ADDITIONAL
UPSTREAM CONVECTION ALONG COLD FRONT ACRS EASTERN TX WILL
PROPAGATE EWD OVERNIGHT AS A NEARLY N/S STGR LINE OF CONVECTION
WITH EMBDD BOWING SEGMENTS.  OF COURSE THE BIGGEST CONCERN WILL BE
WHERE WITH 30 TO 40KT SLY FLOW FEEDING VERY ANAMOLOUS PWS OVER
1.75 INCHES INTERECT ANY OF MULTIPLE E/W BOWING LINE SEGMENTS.
12Z HIGH RES MODELS AND RECENT HRRR RUNS ALL SUGGEST POTNL FOR
OVERNIGHT RAINS OF 4 TO 7" IN VICNITY OF THESE BANDS.  ALSO
EXTENDED SLIGHT AND MDT RISK AREAS SWD A BIT ACRS ARKANSAS INTO LA
WITH AREA ACRS CNTL/NRN LA HAVING SEEN LOCAL 2 TO 3 INCH RAINS
FROM EARLIER MCV THAT MOVED NWD DURING THE MID/LATE AFTERNOON.
ADDITIONAL CNVTV ACTIVITY WAS DVLPG SOUTH OF LCH AHEAD OF A WEAKER
VORT LIFTING NEWD FROM OFF THE MID TX COAST AREA WHICH NONE OF THE
HIGH RES MODELS SEEM TO HAVE A CLUE ON.   EVEN IF THIS ACTIVITY
DOES NOT ADD APPRECIABLE RAIN..SEVERAL RUNS OF THE HRRR SUGGEST
NEAR SOLID N/S LINE OF CONVECTION WITH THE COLD FRONT MOVING
THROUGH OVERNIGHT COULD END UP SUPPORTING ADDITIONAL 1 TO 3 INCH
RAINS OVER SOME OF THE SAME AREAS THAT SAW 2 TO 3 INCH RAINS
EARLIER THIS AFTN.  SULLIVAN

PREVIOUS DISCUSSION:

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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