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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1926Z Feb 24, 2018)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
226 PM EST Sat Feb 24 2018

VALID 1926Z Sat Feb 24 2018 - 12Z Sun Feb 25 2018


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 S EKN 25 N BKW 20 E 1A6 20 NE 4A9 20 NW NMM 40 NNE MCB
20 SSE KCMB 20 SW KOPM 10 ESE 2R8 10 S LVJ 30 NW BPT 25 WSW JAS
35 S PSN 10 WSW CRS 15 ENE TKI 25 SW JSV 10 SSW JLN 30 NNW SGF
15 ENE AIZ 10 NW SET 10 SSW DFI 10 SW CXPT 15 NNW ERI ELZ
30 S BGM 10 NW 12N 10 SE TTN 10 N APG JYO 25 N SHD 30 S EKN.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 N MDQ 40 SW TUP 30 SSW GLH 15 N IER 20 ENE LFK 20 WSW OCH
PSN LNC 15 NNE TRL 35 N PRX RKR 10 SSE SLG 25 SE JLN 25 WSW SGF
20 NE SGF 35 NW FAM 15 ENE BLV 20 WNW SLO HUF 10 WSW AID AOH
20 SW LPR 15 SSE CGF 10 NNW YNG 20 NE FKL 35 NW DUJ 30 SE BFD
25 NE UNV 20 ESE UNV 20 SSE AOO 20 SSW CBE 25 NE EKN 15 WSW CKB
30 S PKB 15 NE HTS 10 NNE LOZ 30 N MDQ.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 SSW AOH MNN 10 SE 4I3 15 NW UNI 35 NW HTS 45 NNW JKL
20 SSW BWG 30 SW BWG 55 SSW CKV 30 E OLV 35 N GLH 30 NNW GLH
10 SSW ELD 10 WNW SHV 15 WSW GGG 15 SE 3T1 20 NW OSA 10 NW LBR
40 WNW DEQ 15 E FSM 25 E ASG 30 NNE HRO 30 NNW BPK 10 S FAM
20 NNW BMG 30 ESE MIE 25 SSW AOH 20 SSW AOH.

HIGH RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 SE FSM 40 E UNO 30 N POF MWA 15 NNE CUL 15 SSW BMG
10 ENE BAK WMO 15 SSW ILN 45 SSE LUK 15 N LEX 30 SW FFT 25 S FTK
15 NNE HOP 25 ENE DYR 15 NNW SGT 10 SSE LIT 25 SSW LIT M89
25 SSW DEQ 15 NNE LBR 35 WNW DEQ 30 NW DEQ 25 SE RKR 30 SE FSM.



***A major flash flooding event likely Saturday afternoon into
Saturday night/early Sunday morning from the lower AR river valley
into the OH valley region***

19z Update:

The system continues to become better defined over the MS valley
with a significant flood and flash flood threat continuing into
tonight. The combination of height falls moving into the area and
the development of very impressive upper level divergence with a
dual jet structure, will provide a very favorable synoptic
environment for convection. Convection continues to stream
northeastward within the WAA regime ahead of this wave. Strong
850mb moisture transport within this band will continue to support
developing convection moving northeast across AR and along the OH
river. Meanwhile a squall line along the developing cold front has
become better defined as well over recent hours. So far there has
been enough separation of these two areas to prevent any real
robust training. As such QPF amounts from the 12Z arw/arw2 and
some older hrrr runs may have been overdone over AR, with these
solutions depicting a training scenario that has not yet unfolded.
With that said, still looking at two rounds of heavy rainfall
rates over much of AR (one with the WAA and another the squall
line), and given saturated conditions even the the 1-2" (localized
3") totals depicted by the 16z hrrr will still cause flash flood
concerns into this evening. Thus while the magnitude of flash
flooding may not be as significant as was possible...will maintain
the high risk for now until the squall line passes through.

The most concentrated area of training and repeat cells appears to
be setting up near the OH river from far northeast AR into far
southeast MO, western TN/KY, far southern IL and southern IN. The
area of strong 850 MB moisture transport ahead of the wave will
repeat across this area and run into the warm front in place. The
expectation is that the WAA convection will tend to repeat across
these areas into this evening, with the squall line from the west
eventually overtaking the lead convection. This thus sets up about
a 6 hour window OR so where locally heavy rates may be moving over
the same locations. In general expecting areal averaged amounts of
2-3" within this corridor, with the potential certainly there for
a narrow swath of 3-5". Given this will be falling over saturated
grounds, significant flash flooding will be a concern...and thus
the high risk will be maintained. Some uncertainty with the exact
axis of higher amounts remains. Thought the 12Z nam nest and nmmb
were probably too far north. Thought the 16z hrrr showed a pretty
good match to current radar and its evolution seemed reasonable,
thus the WPC QPF follows this hrrr solution the closest through
06Z tonight. The greatest flash flood threat appears to be this
afternoon through about 06Z-07z, as the convection looks to weaken
some after that and also become more progressive in nature. Given
this...opted to cut back some on the northeastward extent of the
moderate risk over portions of eastern OH and western PA. Think
rainfall magnitudes and rates will be lower here...and thus while
some areal and river flooding will be possible...significant flash
flooding appears less likely.

Chenard


...Original Discussion...

No significant changes made to the previous excessive rainfall
potential outlook from northeast TX/southeast OK northeastward
into the OH valley.  There continues to be a strong model signal
for widespread heavy and excessive rainfall over the broad area
listed above---with a very large moderate and high risk areas
maintained for the upcoming day 1 period. Organized heavy precip
expected in two distinct waves day 1.  The lead wave---currently
producing heavy rains from eastern OK---northwest
Arkansas---southern MO into south central IL will be pressing east
northeastward Saturday morning just to the north of the OH river
and into the northern mid Atlantic by early afternoon.  Rainfall
rates in this area expected to diminish by the beginning of the
day 1 period and will likely not be a flash flood threat as it
moves north of the OH river into the northern Mid Atlantic but
will further saturate the soil ahead of the more significant
precip area later day 1.  A smaller southern component of this
first wave associated with convection currently moving northeast
out of central TN will also expand northeastward into the upper OH
valley and central Appalachians Saturday morning.  This area
should be progressive but still produce locally heavy rains that
will move over lower ffg values across the central Appalachians.

This will be followed by a second and more significant wave of
heavy rains developing Saturday morning across central OK/north TX
ahead of the strong mid to upper level height falls ejecting
northeastward from the central/southern plains into the mid to
upper MS valley region.  The overall strong dynamics ahead of the
ejecting trof will support strengthening of the low level flow to
50-65 kts+ out of the south southwest across eastern portions of
the southern plains-- northeastward into the lower MS
valley---lower TN valley and through the OH valley.  Expect a well
organized convective squall line to push east northeastward across
these areas.   With pw values remaining 2-3+ standard deviations
above the mean and the very favorable dynamics ahead of the
ejecting mid to upper level trof---widespread additional heavy to
excessive rainfall amounts are likely.  While this squall line
will likely be progressive---hourly rain rates of 1.0-1.5"+ likely
along the length of the squall line into at least the 0600-0900
utc time frame early Sunday morning---with model consensus for
weakening of the squall line and rainfall rates toward the end of
the day 1 time period.  While most of the squall line will be
progressive this period---there may be a period of training along
the west to east oriented front over the lower OH valley region
from northeast AR---southeast MO---southern IL---southern
IN--northwest KY and southwest OH in the 1800 to 0600 utc time
frame. This is the axis where much of the hi res guidance is
showing a concentrated precip max this period.

Oravec