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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0153Z Apr 25, 2015)
 
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Abbreviations and acronyms used in this product
 
Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


EXCESSIVE RAINFALL DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
953 PM EDT FRI APR 24 2015

...VALID 03Z SAT APR 25 2015 - 00Z SUN APR 26 2015...
...REFERENCE AWIPS GRAPHIC UNDER...DAY 1 EXCESSIVE RAINFALL...


SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
NMM 30 NNW HEZ 20 WNW AEX 35 E UTS 30 N VCT 40 WSW VCT
40 NW ALI 25 WSW COT UVA 10 WSW BMQ 20 NE ACT 15 W OSA
30 ESE DEQ 20 S SRC 20 SE MKL 25 W CHA 30 SSW RHP 25 SW CEU
15 N 3J7 20 S LZU 20 ESE ANB NMM.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
15 SSW AFK 20 N CSQ 20 WSW IOW 25 ESE MLI 25 SSW LAF BMG
10 S AJG 20 NE BLV 35 NW SUS 20 NNW SZL 20 ENE UKL EWK 10 E RSL
20 WNW CNK 15 SSW AFK.


...SOUTHERN PLAINS EWD ACRS PORTIONS OF THE LOWER MS AND TN
VALLEYS...

A STRONG SHORTWAVE CONSISTING OF A CLOSED MID LEVEL CENTER AND
SOUTHWARD EXTENDING TROUGH AXIS WILL PRODUCE MODERATE HEIGHT FALLS
DOWN TO THE LATITUDE OF NORTHERN TX TO SOUTHERN TN. LOW LEVEL
MOISTURE TRANSPORT...INSTABILITY...AND DURATION OF DEEP ASCENT
WILL BE MAXIMIZED ALONG AND SOUTH OF THIS LINE...COINCIDENT WITH A
BROAD SLIGHT RISK OF EXCESSIVE RAINFALL THROUGH 26/00Z.

ASCENT WILL WORK WITH A VERY MOIST AIRMASS...PW VALUES GREATER
THAN 1.7 INCHES...LEADING TO WIDESPREAD MODERATE TO HEAVY
CONVECTIVE RAINS FROM TEXAS EASTWARD. EXPECTATIONS FOR HEAVY RAIN
ARE ESPECIALLY FOCUSED TONIGHT FROM NORTHEAST TEXAS INTO SOUTHERN
AR AND FAR NORTHERN LA...WHERE THE NOSE OF THE LOW LEVEL JET
CROSSES TO THE COOL SIDE OF AN OUTFLOW BOUNDARY AND WARM FRONT.
THE UPPER LOW WAS OPENING AND EJECTING...WHICH WILL ALLOW THE
COUPLET OF LOW LEVEL INFLOW / MID LEVEL ASCENT TO PROGRESS
STEADILY EASTWARD...BUT CONVECTION MAY BE SUSTAINED OVER A GIVEN
LOCATION FOR 2-3 HOURS PRIOR TO SEEING AN APPRECIABLE DECREASE OF
INTENSITY. THE HEAVY RAIN SIGNAL IN THE MESOSCALE MODELS IS
CONSISTENT IN THE SHORT TERM. THERE IS GREATER SPREAD LATER IN
TIME AND FARTHER DOWNSTREAM...BUT THE RAP FORECASTS SUFFICIENT
INSTABILITY AND BROADLY CONFLUENT 850-700 MB FLOW...TO MAINTAIN A
RISK OF HEAVY RAINFALL THROUGH EARLY MORNING...PROGRESSING TOWARD
MISSISSIPPI AND FAR SOUTHERN TENNESSEE. BEYOND EARLY
MORNING...BROAD TRANSPORT OF AN INCREASINGLY MOIST AND MARGINALLY
UNSTABLE AIRMASS WILL CONTINUE TOWARD THE EAST. ALTHOUGH LACKING A
LARGE SCALE FOCUS...THIS PATTERN SOMETIMES LEADS TO 2-3 HOUR
PERIODS OF TRAINING ON SMALLER SCALES IN THE SOUTHEAST STATES.

EARLY IN THE PERIOD...MODELS ALSO AGREE ON A MAXIMUM OF
PRECIPITATION IN SOUTH TEXAS. THIS WAS PLAYING OUT AT 0230Z...WITH
THE BULK OF CONVECTION SURVIVING TO THE SOUTH OF SAN ANTONIO.
STORMS THERE WILL CONTINUE A STEADY FORWARD PROGRESSION
PROGRESSIVE OWING TO A LACK OF FOCUSED INFLOW AND GREATER
DOWNDRAFT CAPE COMPARED TO AREAS FARTHER NORTH. BUT THE
ANOMALOUSLY MOIST ENVIRONMENT...PW VALUES AGAIN GREATER THAN 1.7
INCHES...WILL FAVOR LOCALLY INTENSE RAIN RATES IN AN AREA ALREADY
SATURATED BY A RECENT WET WEATHER PATTERN. THE MAXIMUM 3-HOUR
PRECIPITATION FROM THE SREF IS PARTICULARLY HIGH OVER SOUTH
TEXAS...AT JUST UNDER 3 INCHES IN THE MODEL.


...CENTRAL PLAINS / KS EASTWARD TO IL/IN...

NO CHANGE TO THIS AREA IN THE 03Z UPDATE. A HEAVY RAIN EVENT HAD
BEGUN TO UNFOLD EARLY FRI NIGHT. STRONGER FORCING AND STEEP LAPSE
RATES OVER KANSAS COMPENSATED FOR LESSER INSTABILITY COMPARED TO
AREAS FARTHER SOUTH. EVEN WITH THE THRUST OF GULF MOISTURE
SHIFTING SOUTH AND EAST OF THIS REGION...A RIBBON OF PW VALUES
APPROACHING 1.25 INCHES IS FORECAST TO BE INGESTED BY AND WRAP
AROUND THE TIGHT MID LEVEL CIRCULATION HEADING INTO THIS REGION
FROM WESTERN KS. CONVECTION WILL BE LESS APT TO FORWARD
PROPAGATE...GIVEN IT WILL BE SLIGHTLY ELEVATED...AND DRIVEN MORE
BY STRONG LOW LEVEL INFLOW AND MID LEVEL ASCENT RATHER THAN COLD
POOL FORCING. SOME SLOW MOVING CELLS PRODUCING HEAVY RAIN WERE
ALREADY SEEN...WITH ONE REPORT OF 2.25 INCHES OF RAIN NEAR LINCOLN
KS. THE HRRR HAD CAUGHT ONTO THIS ACTIVITY...AND THE 18Z NAM HAD
FORECAST THE EVENT FAIRLY WELL. THE BRIEF CLOSING OF A MID LEVEL
CENTER HERE...PERHAPS ENHANCED BY LATENT HEAT RELEASE / MCV
DEVELOPMENT...WILL HELP TO ORGANIZE THE EVENT AND INCREASE
DURATION OF CONVECTIVELY ENHANCED RAIN RATES. MODELS SHOW SOME
ELEVATED INSTABILITY WITHIN A DEVELOPING WARM CONVEYOR
BELT...INCLUDING WELL DOWNSTREAM OF THE SURFACE WARM FRONT INTO
PARTS OF MIDWEST/MID MS VALLEY THROUGH EARLY MORNING. THE
POTENTIAL FOR FOCUSED LOCALLY HEAVY RAIN SHOULD LINGER THROUGH
MIDDAY SATURDAY BEFORE THE MID LEVEL SYSTEM UNDERGOES RAPID
SHEARING UNDER DOWNSTREAM REX BLOCK SET UP.  WHILE WELL REMOVED
FROM DEEPER MSTR TO THE SOUTH...MODEST PWS AND SLOW MOVEMENT OF
THE MID LEVEL CIRCULATION ITSELF COULD LEAD TO A STRIPE OF 1 TO 2
INCH PLUS RAINS IN THE COMMA HEAD AND NEAR MID LEVEL COLD POOL.
ISOLD HEAVIER AMOUNTS WILL BE POSSIBLE...LEADING TO A SLIGHT RISK
OF EXCESSIVE RAIN.

BURKE/SULLIVAN