Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
410 PM EDT Thu Apr 18 2019

Day 1
Valid 16Z Thu Apr 18 2019 - 12Z Fri Apr 19 2019

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE LOWER MISSISSIPPI VALLEY...

16z Update: The Moderate Risk looks good into this afternoon
across portions of LA/MS. See MPD #138 for the latest details on
this threat. Ingredients are in place for continued upscale
convective development across this region into the afternoon
hours. And while the system as a whole remains progressive, the
orientation of the 850 mb moisture transport plume and the
developing low level boundary supports some backbuilding and
southwest to northeast training from central LA into central MS.
Thought the 14z HRRR was doing a decent job with this event, and
would suggest at least localized 3-5" amounts are likely within
the Moderate Risk area. This should result in growing flooding
concerns with time, with localized significant flash flooding a
possibility.

Opted to remove the northern Moderate Risk with this update. The
evolution of things this morning has put this region into the more
stable airmass. And while shower coverage and intensity should
increase through the day given the impressive dynamics in place,
instability should remain lacking. Thus appears like this activity
will be more of a broad shield of showers with just some embedded
heavier convective cells. Thus in general rainfall rates should
stay low enough to prevent a more significant flash flood risk
from developing. With that said, still looking at a widespread
1-2" (locally higher) of rain across this area, which will likely
result in growing areal flooding concerns and localized flash
flooding. Think a Slight Risk best characterizes this threat level
at this time.

Chenard


...Previous Discussion...

...Lower Mississippi Valley into the Lower Ohio Valley...
A widespread heavy to excessive rainfall event is likely day 1
from portions of the Lower Mississippi Valley...northward into the
Lower Ohio Valley.   There is a strong model signal for an axis of
heavy rains stretching along and ahead of the cold front pushing
southeast Thursday through the Lower Mississippi Valley into the
Lower Ohio Valley.  Active convection early this morning
stretching from eastern portions of the Southern
Plains...northeastward into the lower Arkansas and portions of the
Middle Mississippi Valley will be maintained Thursday in a very
favorable heavy rain set up ahead of the strong amplifying mid to
upper level trof stretching from the Lower Mississippi Valley into
the Lower Ohio Valley.  PW values 1.5-2+ standard deviations above
the mean in the vicinity of the front...strong frontal convergence
and well defined upper difluence will support widespread heavy
precip totals.  There is potential for a period of training of
cells parallel to the front...primarily in the 1200 utc Thu to
0000 utc Fri period from central to northeast AR...far southeast
MO...far western TN...southern IL...western KY into southwest IN. 
Much of this area has seen heavy rains over the past
week...upwards to 300 percent of normal resulting in much above to
high stream flow as per the National Water Model. There has been a
northwest trend in the NAM...NAM conest...GFS and EC qpf axes
across these area.  This has resulted in the risk areas on the 
Excessive Rainfall Outlook also shifting westward from NW
LA...central to eastern AR...into the Lower OH Valley.  A moderate
risk area was added across these areas given the model
trends...good agreement on this heavy axis and the above mentioned
high stream flows.

There is somewhat more uncertainty with the heavy to potentially
excessive precip potential farther to the southeast across
portions of northeast LA into south-central MS.  Much of the hi
res guidance is depicting a secondary precip max farther to the
south of the above mentioned qpf axis closer to and parallel to
the main frontal boundary.  This is in the area of greater
forecast instability...higher pw values and stronger low level
inflow off the western Gulf of Mexico.  Convection in this area
will likely be more progressive than areas closer to the above
mentioned front.  However...given the greater instability...hourly
rates of 1.5-2.5"+ are possible.  There is a larger model spread
with respect to this potential heavy axis.  However...given this
area has also seen much above average precip over the past 1 to 2
weeks...a moderate risk was also added after collaboration with
WFO JAN from northeast LA into southwest to central MS.

...Central Gulf Coast into the Southern Appalachians...
There were no significant changes made to the risk areas farther
to the east across portions of the Central Gulf Coast into the
Southern Appalachians.  Convection should be progressive to the
east across these areas and rainfall amounts over the past week
have not been as heavy as areas farther upstream.  A slight risk
area was maintained across these areas.

Oravec


Day 2
Valid 12Z Fri Apr 19 2019 - 12Z Sat Apr 20 2019

...THERE IS A SLIGHT RISK ALONG AND EAST OF THE APPALACHIANS FROM
THE CAROLINAS TO THE MID-ATLANTIC REGION AND INTO PORTIONS OF
NORTHERN NEW ENGLAND...

A line of organized convection will move eastward across the
Southeast and southern Appalachians Friday morning associated with
a potent low pressure system moving from the TN Valley into the
Ohio Valley. The cold front draped from the surface low will allow
anonymously high precipitable water values (nearly 4 standard
deviations above the mean) and instability (2000+ J/kg MUCAPE) to
surge east of the boundary.  This will help maintain and expand
the convective line north into the central Appalachians and
mid-Atlantic region as the front moves quickly eastward across the
south throughout the day. Rain rates associated with this line
will be upwards of 1-1.5"+/hour. In addition, some training of
convection is possible farther north into Pennsylvania as the
corfidi vectors become better aligned with the mean 300-850mb
flow. The Slight Risk was maintained with some minor refinements
made to account for the highest rainfall amounts/rates expected,
mainly along the mountains to the piedmont of the Carolinas north
into Pennsylvania.  Also, the urban coordinator from DC toward NY,
anticipate the threat for flash flooding to be elevated as FFG is
reduced.  It should be noted that model spread it quite large in
terms of QPF, likely due to the convective nature of this system.
A lot hinges on the moisture flux convergence and instability for
the maintenance of the convective line with the largest
variability in model solutions noted across the mid-Atlantic. 

Meanwhile, a couple rounds of moderate rain are expected along an
inverted trough/slow moving cold front from the eastern Great
Lakes up the interior northeast corridor across far northern
Maine. Increasingly difluent/divergent flow aloft juxtaposed with
deep moisture streaming northward ahead the aforementioned surface
low will promote brief periods of heavy rain. With snowpack across
the northern tier of New England resulting in snow liquid
equivalent values nearing 8 inches, expect snow melt to 
exacerbate the flood potential.  Therefore the Slight Risk was
expanded from northern Maine west across northern New York,
Vermont and New Hampshire.


Bann/Pagano


Day 3
Valid 12Z Sat Apr 20 2019 - 12Z Sun Apr 21 2019

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
MAINE AND NEW HAMPSHIRE...

A plume of deep moisture will still be oriented from the waters of
the Atlantic Ocean into parts of southern New England early in the
period on Saturday. The risk of excessive rainfall will linger
until the deep moisture, strong moisture transport in the
pre-frontal environment, and the moisture flux convergence along
the front all get shunted out to sea Saturday evening or even
during the early morning hours on Sunday. Additional rainfall of
around a half an inch to an inch is possible with locally higher
amounts along the upslope of the terrain.  While the hourly rates
may not be excessive, with this prolonged storm system and
expected snowmelt across the north, the region will likely become
sensitive to additional precipitation accumulations.  Therefore
the Slight Risk covers the more vulnerable regions of Maine and
southern New Hampshire. 

Bann/Pagano


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt