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Excessive Rainfall Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1519Z Feb 24, 2018)
 
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Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1019 AM EST Sat Feb 24 2018

VALID 15Z Sat Feb 24 2018 - 12Z Sun Feb 25 2018


MARGINAL RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
20 SW OQT 20 SSE KCMB 20 SW KOPM 10 ESE 2R8 10 S LVJ 10 W DRI
20 NW POE 30 E OCH 10 NNW JSO 15 SW TYR 30 ESE LNC 10 NW CRS
10 SW GDJ 20 WSW MWL 35 WNW RPH 20 ENE HBR 20 NW GOK 15 ENE PNC
CFV 25 NE JLN 15 SSE VIH 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 NW LWB
30 WSW LNP 20 SW OQT.

SLIGHT RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
AFW 35 NNW 1F9 10 SE DUC 10 WNW PVJ SNL 30 E SWO 25 N GCM
15 N GMJ 25 ESE JLN 35 SSW TBN 25 NNE FAM 20 N SLO 30 WNW EYE
30 ENE FDY 20 N MFD 20 SE CGF 25 SW BFD 30 N UNV 35 SSE UNV
15 W HGR 25 W OKV 15 NE EKN 30 WNW W22 3I2 10 NNE LOZ 30 N MDQ
45 NW MEI 20 SE TVR 15 E MLU 15 WSW SHV 10 WNW TYR 10 S TRL RBD
AFW.

MODERATE RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
25 NNW HTS 45 NNW JKL 20 SSW BWG MKL 25 SE KM04 35 N GLH
20 ENE LLQ 20 WSW ELD 25 ENE 4F4 10 S SLR 10 NW TKI 20 NE ADM
25 ESE SNL 10 W OKM TUL 20 ENE GCM GMJ 25 NE ROG 25 NNW BPK
10 S FAM 20 NNW BMG 30 ESE MIE 25 SSW AOH 15 ESE MFD 20 NNE BTP
FIG AOO 20 N CBE 2G4 20 SE MGW CKB 30 E PKB 20 WSW PKB
25 NNW HTS.

HIGH RISK OF RAINFALL EXCEEDING FFG TO THE RIGHT OF A LINE FROM
30 SE FSM 40 E UNO 20 SE MVN 25 NNE KHLB 25 SSW LCK 45 SSE LUK
20 SE LOU 30 NE 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***

15z Update:

A well defined shortwave is evident on water vapor imagery over
northern TX and central OK this morning. Convection has rapidly
developed this morning ahead of this wave. This system will only
continue to become more dynamic as we head through the morning
into the afternoon hours. The combination of height falls moving
into the area and the development of very impressive upper level
divergence with a dual upper jet structure...will provide a very
favorable synoptic environment for convection. Ahead of this wave
in an area of increasing 850 mb moisture transport... will likely
see south to north moving convection expand in coverage over AR.
Back to the west anticipate convection will develop into a squall
line as the forcing and system as a whole intensifies. This squall
line and lead convection should merge...resulting in a period of
training and repeat convection across portions of central AR. also
likely to see some training northeast of here as this 850 mb
moisture transport axis moves along the old warm front around the
OH river. This thus continues to favor an axis of enhanced
rainfall totals of 2-4" (locally higher) across portions of AR
northeast along/near the OH river. Took a look at the new 12z high
res guidance and recent HRRR runs and think the high risk axis
still looks good. Still some uncertainty on the exact axis of
highest amounts, but think what we have going is still a good
consensus position. Given the newer arriving guidance and recent
radar trends...would say confidence is increasing in a period of
training and higher end QPF amounts in/near the high risk area.
Given the saturated conditions over this area and the expected
magnitude and rates of rainfall...significant and potentially life
threatening flash flooding appears probable in/near the high risk
area.

We currently extend the moderate risk northeast across much of
central and southern OH and southwestern PA. Think the axis of
this risk looks okay and remains in the middle of the 12z high res
model spread, however some question as to whether rates and totals
this far northeast will be high enough to produce a significant
flash flood risk. Latest guidance suggests amounts generally in
the 1-2" range. This may be enough to produce areal flooding and
river flooding concerns...however true flash flooding may be less
common. Given some uncertainty with the exact axis and magnitude
of totals...along with exactly how sensitive the area is to
additional rain...will maintain the moderate risk here for now.

Chenard


13z Update:

Made an adjustment west to the risk areas to include more of
eastern OK in the slight and moderate risks. Convective initiation
is occurring further west than most models indicated...and with
this activity moving over saturated grounds...an expansion of the
slight and moderate risks was warranted. Please see MPD #38 for
more details. -Chenard


...Previous 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