Quantitative Precipitation Forecast
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989
FOUS30 KWBC 210829
QPFERD

EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
429 AM EDT SAT OCT 21 2017

...VALID 12Z SAT OCT 21 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
35 SSE PSN 20 ENE 3T5 BAZ 40 NNW 6R9 25 WSW SPS 25 ESE CSM
30 NNW WDG 25 N EMP 30 ESE ICL 20 NW CNC 35 NNE IRK 30 ENE UNO
20 S LIT 35 SSE PSN.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 W ONP 40 W S47 35 WSW AST 25 WNW AST 25 SSW HQM 15 NNW HQM
20 E UIL CLM 20 SE CLM 25 NW PWT 10 SW PWT GRF 15 ENE TCM
10 E RNT 25 ENE RNT 20 ESE 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 35 NW RDM 25 W RDM 20 WSW RDM 35 SW RDM 55 ESE EUG
30 ENE EUG 15 SSE SLE 10 E SLE 10 SE MMV 10 W SLE 10 NNE CVO
10 WSW EUG 15 W RBG 25 WNW SXT 35 SSW SXT 40 ESE CEC 35 NE ACV
30 NNE ACV CEC 40 NNW CEC 50 NNW CEC 35 SSW OTH 35 SSW ONP
25 W ONP.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
OKC 25 SE PNC 25 E WLD 25 SW UKL 25 NE UKL OJC 10 SSW SZL
30 NNE ROG 30 SSE HRO 25 SSE RUE 30 SSW MWT 15 SW OSA 15 W GVT
35 W GLE 10 NNE SPS LAW OKC.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
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 SE TDO 15 NE TDO 30 E TCM SMP
35 SSE SMP 30 WSW YKM 35 NNW DLS.


...NORTHWEST INTO THE NORTHERN ROCKIES...

AN IMPRESSIVE AND LONG DURATION ATMOSPHERIC RIVER EVENT WILL
IMPACT THE PACIFIC NORTHWEST THROUGH THE DAY 1 PERIOD. RAINFALL
RATES THIS MORNING WILL INITIALLY BE LIGHTER IN NATURE IN THE
BROAD WARM AIR ADVECTION REGIME WITH LOW LEVEL FLOW NOT IDEALLY
SITUATED INTO THE TERRAIN. HOWEVER BY THIS AFTERNOON RAINFALL
RATES WILL INCREASE AS LOW LEVEL FLOW TURNS MORE WESTERLY AND
OROGRAPHIC ASCENT IS THUS MAXIMIZED. THE ATMOSPHERIC RIVER AXIS
WILL FLUCTUATE A BIT TO THE NORTH THIS AFTERNOON AND THEN BACK
SOUTH TONIGHT...WHICH WILL SHIFT THE HEAVIEST RAINFALL RATES FROM
OR EARLY TODAY...INTO WA THIS AFTERNOON/EVENING AND THEN BACK
SOUTH INTO OR BY LATER TONIGHT.

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 GUIDANCE ARE LEGITIMATE. ANTICIPATE PRETTY
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 THROUGH 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. THIS ATMOSPHERIC RIVER EVENT WILL
HAVE A PRETTY GOOD INLAND EXTENT...THUS ANTICIPATING 1-2" THROUGH
12Z SUNDAY INTO PORTIONS OF THE NORTHERN ROCKIES AS WELL...WITH A
GOOD PORTION OF THIS FALLING AS RAIN AS SNOW LEVELS RISE THROUGH
THE DAY.

WILL MAINTAIN THE SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS
OF THE WESTERN OR/WA MOUNTAINS...WHERE THE LONG DURATION HEAVY
RAINFALL WILL LEAD TO SOME DEVELOPING FLOODING CONCERNS. MINOR
URBAN FLOODING CONCERNS COULD ALSO ARISE IN AND AROUND PORTLAND.


...SOUTHERN PLAINS INTO THE MS VALLEY...

A MID/UPPER LEVEL TROUGH AXIS WILL PUSH INTO THE PLAINS THROUGH
THE DAY TODAY...WITH A COLD FRONT STEADILY PROGRESSING EAST AS
WELL. WILL HAVE PLENTY OF MOISTURE AND INSTABILITY ACROSS THE
PLAINS AND MS VALLEY OUT AHEAD OF THESE FEATURES...WITH PRETTY
GOOD MOISTURE TRANSPORT SEEN AS WELL. THUS THE COMBINATION OF
DIURNAL HEATING...FALLING HEIGHTS AND STRONG SURFACE CONVERGENCE
ALONG/NEAR THE FRONT...WILL LIKELY ALLOW FOR WIDESPREAD CONVECTION
ALONG THE FRONT EXTENDING FROM IA SOUTHWEST INTO TX. OVERALL THE
FRONT AND TROUGH WILL REMAIN PROGRESSIVE THROUGH THE DAY 1
PERIOD...WHICH SHOULD MEAN THE CONVECTION SHOULD ALSO BE STEADILY
PUSHED EAST. THIS FORWARD MOTION SHOULD CAP THE MAGNITUDE OF
RAINFALL AMOUNTS THAT CAN BE EXPECTED. HOWEVER THE ANOMALOUS PWATS
IN PLACE AND THE EXPECTED WIDESPREAD NATURE OF CONVECTION WILL
STILL SUPPORT SOME HEAVY RATES AND A BROAD AREA OF 1-2" EXTENDING
FROM PORTIONS OF KS/MO SOUTH INTO TX.

DO HOWEVER THINK THERE IS SOME OPPORTUNITY FOR BRIEF TRAINING OF
CONVECTION...ESPECIALLY THIS AFTERNOON AND EVENING DURING THE
FIRST SEVERAL HOURS OF THE EVENT. MEAN WINDS ARE PARALLEL TO THE
FRONT...SO COULD IMAGINE THAT WE SEE SOME REPEAT CONVECTION FOR A
PERIOD ACROSS NORTHEAST TX INTO EASTERN OK/KS...BEFORE THE
CONVECTIVE LINE ORGANIZES AND PROPAGATES MORE QUICKLY OFF TO THE
EAST. THUS DO THINK SOME ISOLATED 2-4" AMOUNTS ARE POSSIBLE IN
THESE AREAS...AND WILL MAINTAIN THE SLIGHT RISK OVER PORTIONS OF
EASTERN KS/OK AND WESTERN MO/AR TO ACCOUNT FOR THE MAINLY MINOR
FLASH FLOOD POTENTIAL. CURRENTLY NOT ANTICIPATING THIS EVENT TO
RESULT IN WIDESPREAD SIGNIFICANT FLASH FLOOD CONCERNS GIVEN THE
GENERALLY LINEAR AND PROGRESSIVE NATURE OF CONVECTION AND
RELATIVELY HIGH FFG OVER THE REGION...BUT WILL CONTINUE TO MONITOR.


CHENARD
$$





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