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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0826Z Oct 22, 2017)
 
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
426 AM EDT SUN OCT 22 2017

...VALID 12Z SUN OCT 22 2017 - 12Z MON OCT 23 2017...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 S ONP 15 SSW CVO 20 E CVO 25 SSE TTD 40 SSW DLS 25 W RDM
65 N LMT 35 NNW LMT 25 ESE RBG 10 SW RBG 25 SW SXT 15 NNE CEC
45 NNW CEC 15 WSW OTH 25 S ONP.

MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 E KVOA 15 E KMDJ 20 SSW SRN 15 NNW KVNP 25 S ACP 10 NNE ESF
BQP 25 NE LLQ 30 SSW UTA 45 ENE DYR 10 S EHR 25 N BWG
30 WSW K8A3 47A 10 SE LSF 15 WNW AAF 65 E KVOA 30 E KVOA.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 ENE MDQ 15 SSW RMG 20 SW LGC 40 N DHN 10 WSW PFN 35 WSW PAM
40 NNE KMIS 15 N 1B7 20 W KXPY 10 NNW SRN 10 W LFT 30 ESE ESF
15 WNW MCB MEI GTR 45 WNW MSL 50 SSW CKV 40 SSW MQY 30 ENE MDQ.


...NORTHWEST...

THE ONGOING ATMOSPHERIC RIVER DRIVEN HEAVY RAIN EVENT ACROSS THE
PACIFIC NORTHWEST WILL CONTINUE INTO THE MORNING HOURS AND SHIFT
SOUTHWARD WITH TIME. IN GENERAL THE MOISTURE AND WIND ANOMALIES
ASSOCIATED WITH THE ATMOSPHERIC RIVER EVENT WILL BE ON A DOWNWARD
TREND AFTER 12Z THIS MORNING...WITH RAINFALL RATES GRADUALLY
DECREASING WITH TIME AS WELL. AFTER 12Z ANTICIPATING AN ADDITIONAL
1-2" OVER THE TERRAIN OF WESTERN OR AND UPWARDS OF 1" OVER
PORTIONS OF THE NORTHERN ROCKIES. IN GENERAL MODELS WERE IN GOOD
AGREEMENT...WITH A MULTI MODEL BLEND WEIGHTED MORE TOWARDS THE
HIGH RES GUIDANCE USED FOR QPF. OVERALL THIS FORECAST REMAINS
PRETTY GOOD CONTINUITY FROM OUR PREVIOUS FORECAST...WITH JUST A
SLIGHT QUICKER AND SOUTHWARD SHIFT OVER OR. WILL MAINTAIN A
MARGINAL RISK OVER PORTIONS OF WESTERN OR.

...LOWER MS VALLEY/SOUTHEAST/TN VALLEY...

A COMPACT MID/UPPER LEVEL TROUGH WILL MOVE FROM THE PLAINS AND
INTO THE MS VALLEY THROUGH THE DAY. BY LATER TODAY SHORTWAVE
ENERGY WITHIN THE NORTHERN PORTION OF THE TROUGH WILL BEGIN TO
PEEL OFF TO THE NORTHEAST INTO CANADA...WITH ADDITIONAL ENERGY
FOCUSING IN THE SOUTHERN PORTION OF THE TROUGH AND CONSOLIDATING
OVER THE LOWER MS VALLEY. THUS FROM A LARGE SCALE FORCING
STANDPOINT...SHOULD GENERALLY SEE WEAKENING ASCENT ACROSS THE
NORTHERN HALF OF THE TROUGH. ON THE OTHER HAND SYNOPTIC ASCENT
SHOULD INCREASE FURTHER SOUTH AS THE TROUGH TRIES TO BECOME
NEGATIVELY TILTED AND/OR CLOSE OFF...WITH THE UPPER LEVEL FLOW
ALSO BECOMING QUITE DIFFLUENT AS WELL. ANOMALOUS PWATS ABOVE THE
CLIMATOLOGICAL 90TH PERCENTILE WILL EXTEND ALONG THE EXTENT OF
TROUGH...WITH THE HIGHEST ABSOLUTE VALUES (~2") FOCUSED CLOSER TO
THE GULF COAST. INSTABILITY BY LATER TODAY WILL GENERALLY BE
FOCUSED FROM THE TN VALLEY AND POINTS SOUTH AS WELL. THUS GIVEN
THAT THE BEST FORCING/MOISTURE/INSTABILITY ARE ALL MAXIMIZED AS
YOU GO SOUTH..WOULD ALSO ANTICIPATE THE HEAVIEST RAINFALL AMOUNTS
HERE AS WELL.


THE SYSTEM AS A WHOLE WILL GENERALLY REMAIN PROGRESSIVE...WHICH
SHOULD CAP THE UPPER LIMIT OF RAINFALL TOTALS. HOWEVER AS
SHORTWAVE ENERGY CONSOLIDATES WITHIN THE SOUTHERN EXTENT OF THE
TROUGH AND THE TROUGH BEGINS TO TAKE ON MORE OF A NEGATIVE
TILT...LOW LEVEL FLOW WILL BEGIN TO BACK...WHICH SHOULD SLOW THE
FORWARD PROGRESSION OF THE CONVECTION TO A DEGREE. ALSO ANTICIPATE
THAT THE CONVECTIVE LINE WILL BE MORE LOOSELY ORGANIZED THAN WHAT
WE HAVE SEEN OVERNIGHT...AND WE SHOULD BE ABLE TO GET SOME
DISCRETE CELLS OR CLUSTERS OF CELLS THAT FORM OUT AHEAD OF THE
LINE AND EVENTUALLY MERGE INTO IT. THE COMBINATION OF THESE
MERGERS AND THE GRADUAL SLOWING OF CONVECTION SHOULD RESULT IN
SOME AREAS OF LOCALLY ENHANCED RAINFALL TOTALS AND SOME FLASH
FLOOD RISK. THUS WHILE AREAL AVERAGED RAIN OF 1-2" WILL BE MOST
COMMON...ISOLATED POCKETS OF 3-5" ARE PROBABLE...WITH THE GREATEST
THREAT FROM SOUTHERN LA...SOUTHERN/EASTERN MS INTO AL AND THE
WESTERN FL PANHANDLE. GIVEN THE POTENTIAL FOR THESE HEAVIER
AMOUNTS...WILL MAINTAIN THE SLIGHT RISK. THE SLIGHT RISK AREA WAS
SHIFTED A BIT SOUTHEAST TO BETTER MATCH THE LATEST HREF
PROBABILITIES. ALSO CUT BACK SOME ON THE NORTHERN EXTENT OF THE
MARGINAL AND SLIGHT GIVEN THE LIMITED INSTABILITY AND EXPECTED
FOCUS OF CONVECTION A BIT FURTHER SOUTH AND EAST THAN WHAT IS
DEPICTED BY THE GLOBAL MODELS.

CHENARD