Quantitative Precipitation Forecast
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190
FOUS30 KWBC 220035
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
834 PM EDT SAT OCT 21 2017

...VALID 01Z SUN OCT 22 2017 - 12Z SUN OCT 22 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 WNW RBG 20 W SXT 35 SSW SXT 40 ESE CEC 35 NE ACV 25 SSE CEC
20 NNW CEC 55 NNW CEC 15 WSW OTH 40 NNW OTH 30 WNW ONP 40 W S47
35 WSW AST 25 WNW AST 25 SSW HQM 30 NNW HQM 20 E UIL CLM
25 NW PWT 15 NE SHN GRF 10 SSE SEA 20 E PAE 25 NNE AWO 30 S CWZA
40 SSE CYHE 25 WSW KS52 30 WNW EAT 20 ESE SMP 20 W ELN 25 SW YKM
20 NNE DLS 25 SSW DLS 25 NW RDM 25 SW RDM 40 SW RDM 50 ESE EUG
30 ENE EUG 15 SE SLE 10 NE SLE 10 NNW SLE SLE CVO 15 SW EUG
15 WNW RBG.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 S ABI 40 N CDS 15 SSW AVK 15 SSE MHK 15 SSE SDA 10 SSW DSM
20 WNW AWG 15 WSW MQB 45 NW SUS 30 NW UNO 25 NW SRC 10 N ELD OCH
25 E GTU 15 NNW 6R9 20 S ABI.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 ESE ACP 30 NNE ESF 25 N HEZ 15 SSE HEZ 15 SE BTR 20 NNE 7R3
10 E ARA 30 ESE ACP.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 ESE TCM 10 SSW SMP 35 WNW YKM 30 W YKM 35 NNW DLS 25 SW DLS
50 NNW RDM 35 WNW RDM 30 WSW RDM 50 ENE EUG 30 SE SLE 20 E SLE
UAO 10 NW UAO 10 NNW MMV 20 NNW CVO 20 WSW CVO 10 S ONP
20 NNW ONP 25 WNW S47 15 W AST 25 SSE HQM 20 WNW TDO 10 ESE TDO
20 SSE GRF 25 ESE TCM.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
10 NNE TXK 10 S TRL 15 WNW NFW 20 NNE FDR 15 N JWG 30 W WLD
20 N EMP 15 W STJ 25 NW CDJ 20 S IRK 15 N COU AIZ 20 NNW HRO
20 SSE RUE 10 NNE TXK.


...PACIFIC NORTHWEST...

SEE MESOSCALE DISCUSSION NUMBER 0896 FOR A DETAILED DESCRIPTION
THROUGH 06Z...AND LOOK FOR LATER MESOSCALE DISCUSSIONS WHICH MAY
BE ISSUED.

AN IMPRESSIVE AND LONG DURATION ATMOSPHERIC RIVER EVENT WILL
IMPACT THE PACIFIC NORTHWEST THROUGH THE DAY 1 PERIOD. AT 21Z A
BROAD RAIN SHIELD WAS ALREADY IN PLACE OVER MUCH OF WESTERN
WASHINGTON AND NORTHWEST OREGON...WITH OBSERVED HOURLY RAIN RATES
AT LEAST 0.30 INCHES IN FAVORED TERRAIN...WITH A FEW HIGHER
AMOUNTS. RAINFALL RATES WILL INCREASE FURTHER DURING THE OVERNIGHT
HOURS WITH THE APPROACH OF A LOW AMPLITUDE SHORTWAVE TROUGH AND
ASSOCIATED BOOST TO DEEP LAYER ASCENT.

THE ANOMALIES ASSOCIATED WITH THIS EVENT ARE QUITE IMPRESSIVE. LOW
LEVEL WESTERLY FLOW...INTEGRATED MOISTURE TRANSPORT...AND PWAT
VALUES...WILL ALL BE NEAR THEIR CLIMATOLOGICAL MAXIMUM VALUES FOR
AT LEAST A PORTION OF THIS EVENT. THUS THE IMPRESSIVE RAINFALL
TOTALS SEEN IN THE NUMERICAL GUIDANCE APPEAR LEGITIMATE.
ANTICIPATE WIDESPREAD 0.25" IN AN HOUR TOTALS ACROSS THE WESTERN
WA/OR TERRAIN...WITH RATES MAXIMIZING NEAR 0.75" IN AN HOUR IN THE
MOST PRONE UPSLOPE REGIONS. RAINFALL TOTALS FROM 12Z SATURDAY TO
12Z SUNDAY SHOULD BE IN THE 4-6" RANGE OVER MUCH OF THE WA/OR
COASTAL RANGES AND THE CASCADES...WITH ISOLATED AMOUNTS AS HIGH AS
6-8"...WITH 1-2" MORE COMMON OUTSIDE OF THE TERRAIN.

THE LONG DURATION HEAVY RAINFALL WILL LEAD TO SOME DEVELOPING
FLOOD CONCERNS. MINOR URBAN FLOODING COULD ALSO ARISE IN AND
AROUND PORTLAND OREGON.


...SOUTHERN PLAINS / LOWER MISSOURI VALLEY...

AT 00Z...A MATURING QLCS WAS IN PLACE FROM NORTHERN MISSOURI TO
NORTHWEST TEXAS...MOVING SLOWLY EASTWARD.

THE PARENT SYNOPTIC SYSTEM ITSELF IS PROGRESSIVE...BUT BRIEF
PERIODS OF TRAINING ARE POSSIBLE...AND CELL MERGERS WILL LOCALLY
BOOST RAINFALL TOTALS. ALTHOUGH THE ENVIRONMENT FAVORS MANY SEVERE
WEATHER HAZARDS...THE PARAMETERS FOR HEAVY RAIN ARE IN
PLACE...WITH PRE-CONVECTIVE PRECIPITABLE WATER VALUES APPROACHING
1.45 INCHES...MODERATE CAPE...AND STRONG SOUTHERLY LOW LEVEL
INFLOW. EXPECT A BROAD AREA OF 1 TO 2 INCH RAINFALL EXTENDING FROM
PORTIONS OF KS/MO SOUTH INTO TX...AND EAST INTO ARKANSAS. SOME
ISOLATED 2 TO 4 INCH AMOUNTS ARE POSSIBLE IN THESE AREAS...AND
WILL FALL QUICKLY ENOUGH TO EXCEED FLASH FLOOD GUIDANCE IN SOME
CASES...ESPECIALLY WERE FFG VALUES WERE SOMEWHAT LOWER OWING TO
ANTECEDENT RAINFALL OVER THE PAST SEVEN DAYS IN SOUTHEAST KANSAS
TO CENTRAL MISSOURI.


...LOUISIANA AND SOUTHWEST MISSISSIPPI...

ONGOING DEEP CONVECTION AT 00Z HAD SHOWN CONTINUED STRENGTHENING
AND A GRADUAL ABILITY TO CLUSTER INTO SOMETHING MORE ORGANIZED.
THE STORMS WERE SUPPORTED PRIMARILY BY RICH GULF MOISTURE
STREAMING INTO A REGION OF MODEST BUT PERSISTENT UPPER LEVEL
DIFLUENCE AHEAD OF THE LARGE SCALE AMPLIFYING TROUGH OVER THE
PLAINS STATES. STORMS WEST OF ALEXANDRIA...ALONG THE NORTHWEST
EDGE OF THE RICHEST MOISTURE PER TPW SATELLITE IMAGERY AND THE SPC
MESOANALYSIS...HAD BECOME ANCHORED WHERE SOUTHEASTERLY INFLOW WAS
BALANCED AGAINST SOUTHWESTERLY UPPER LEVEL FLOW. AT 0015Z DUAL POL
RADAR ESTIMATED SMALL SCALE RAIN RATES NEAR 3 INCHES IN AN HOUR.
FARTHER SOUTH FROM BATON ROUGE TO LAFAYETTE THE COMBINED EFFECTS
OF SCATTERED STORMS THROUGHOUT THE AFTERNOON HAD GRADUALLY BUILT
ENOUGH OF A COLD POOL TO ACT AS AN ANCHOR AGAINST THE DEEP LOW
LEVEL INFLOW...CAUSING ENHANCED PERSISTENT CONVERGENCE AND SLOWER
INDIVIDUAL CELL MOTION. CONTINUED MERGERS WITHIN THIS CLUSTER OF
DEEP THUNDERSTORMS IS LIKELY THROUGH AT LEAST 04Z...AND PERHAPS
LONGER...PER RAP MODEL FORECASTS OF INCREASINGLY DIFLUENT UPPER
FLOW. RECENT HRRR RUNS PRODUCE A SMALL BUT POTENTIALLY ACCURATE
AREA OF EXCEPTIONAL RAINFALL TOTALS THROUGH THE NIGHTTIME HOURS
OVER SOUTHERN LOUISIANA. THE MODEL PRODUCES A SIGNATURE OF MORE
THAN 10 INCHES OF RAIN...AND ALTHOUGH THIS WOULD REPRESENT A WORSE
CASE SCENARIO...ANYTHING APPROACHING THIS WOULD EASILY SURPASS
FLASH FLOOD GUIDANCE...AND WPC HAS UPGRADED A PORTION OF CENTRAL
AND SOUTHERN LOUISIANA AND SOUTHWEST MISSISSIPPI TO A SLIGHT RISK
OF EXCESSIVE RAINFALL.

BURKE/CHENARD
$$





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