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Excessive Rainfall Discussion
(Caution: Version displayed is not the latest version. - Issued 2022Z Aug 19, 2018)
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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
422 PM EDT Sun Aug 19 2018

Day 1
Valid 15Z Sun Aug 19 2018 - 12Z Mon Aug 20 2018


15z Update:
Only made minor changes to the risk areas based on a quick look at
the newly arriving 12z HREF members and recent HRRR runs. A broad
Slight Risk still seems warranted across portions of the Plains
into the MS Valley. Overall not a great overlap of instability and
slower storm motions today. The better instability and chance for
more intense convection and higher rates is across portions of
AR/MS (more progressive cells expected though)...but the longer
duration of rain will be further north within the developing comma
head over portions of KS/NE/MO/IA (but instability will be

Highest rainfall amounts through 12z Monday will likely be in the
comma head centered near the intersection of
KS/NE/IA/MO...although limited instability will limit hourly rates
here. 12z HREF members all show a swath of 4-7" within this comma
head given the long duration event. These totals may be a bit
high...but either way should see a swath of heavy rainfall. While
weaker rates may prevent a true flash flood...the long duration
rainfall should result in gradual inundation issues which may
eventually result in some flash flood concerns should any heavier
embedded convective elements develop. Areal averaged amounts will
be less over the southern end of the Slight Risk, but as mentioned
above, the better instability will support more intense rates and
localized 3"+ amounts where cell mergers occur.


Previous Discussion issued at 09z...

...Eastern portions of the Central to Northern Plains into the mid
to lower Missouri Valley...
A broad elongated slight risk area was maintained for the upcoming
day 1 period to the east of the amplifying upper trof across the
Plains into the lower Arkansas and middle to lower Missouri River
Valleys.  Overall there is good agreement this period with the
mass field evolution of this elongated trof---with the closing off
of the mid to upper level center over eastern portions of the
Central Plains.  With this closing off of the mid to upper level
center and the associated deepening of the surface
increasingly well defined comma head/deformation precipitation
band likely to form over eastern Kansas...eastern
Nebraska...northwest Missouri...western Iowa...far eastern South
Dakota into far southwest Minnesota.  While instability will be
modest across these values in the 500-1000
j/kg...the slow movement/pivoting of the comma head will support
widespread heavy precip totals.  The slight risk area was aligned
along the model consensus heavy precipitation axis in this comma
head/deformation region. 

...Eastern portions of the Southern Plains into the Lower Arkansas
River valley...
More progressive areas of convection expected to push eastward on
the southern side of the developing Central Plains closed low
across eastern portions of the Southern Plains into the Lower
Arkansas River Valley.  An initial area expected to push eastward
early day 1...a continuation of early morning organized convection
moving into the Southern Plains.  This will be followed by
potential for a second round of pre-frontal/frontal convection
late Sunday afternoon into Sunday night from eastern OK/northeast
TX into western AR.  With pw values remaining 1 to 2 standard
deviations above the mean along this front and greater
instability...1000+ j/kg mu-cape...heavy rains possible with each
convective episode.   The slight risk area was maintained from the
previous issuance to reflect the heavy rain potential with each
convective episode.

...Southern Mid-Atlantic...Central to Southern Appalachians into
the middle Gulf Coastal region...
An axis of above average pw values will persist this period along
and to the south of the frontal boundary stretching from the
Southern Mid-Atlantic...across the Southern Appalachians and into
the Tennessee Valley region.  Widespread scattered convection
likely in the moist unstable airmass along and south of this
front.   This will support locally heavy rainfall amounts and
potential for isolated runoff issues,  No significant changes made
to the previous broad marginal risk area across these regions.


Day 2
Valid 12Z Mon Aug 20 2018 - 12Z Tue Aug 21 2018

...Upper Midwest arcing to the Mid-South... 
The shortwave over the central Plains today will strengthen and
close at mid-levels by Monday morning. The low will then turn
northeast and reach Lake Michigan by Tuesday morning. Strong and
deep moisture transport ahead this system will draw 2.0 inch
Precipitable Water values from the Deep South through much of the
Midwest with the rest of the area ahead of the low above 1.75
inches. A focus on precipitation in the warm conveyor belt over
IL/IN with the 12Z guidance enhances the flash flood risk there
and north along the warm front Monday afternoon into the evening.
The most notable preconditioning was heavy rain Friday morning
over southern WI, but this did not have the spatial extent to
warrant a Moderate Risk and the area has been otherwise dry. A
Slight Risk extends from northern IA to Lake Michigan south along
the IL/IN border and to the MS Delta or from the deformation zone
north of the low to the warm conveyor ahead.  

...Central and Southern Appalachians...
A surface ridge will continue to push southwest across the
Mid-Atlantic Monday. An increase in low level moisture overriding
this surface feature (ahead of low pressure into the Upper
Midwest) will maintain/worsen the threat for flash flooding from
the central VA Blue Ridge south to the Smokies. Thus, a Marginal
Risk expansion from the risks west of the Appalachians is
...Northern Rockies... 
A positively tilted upper trough persists over the northwest CONUS
Monday and Monday night. PW around 0.75 in is one standardized
anomaly above normal and some instability could lead to localized
heavy rain across western MT which has low FFG. A Marginal Risk of
excessive rainfall was maintained. 


Day 3

The Day 3 outlook will be updated by 2030Z.

Day 1 threat area:
Day 2 threat area:
Day 3 threat area: