Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1147 AM 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 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt