Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
353 AM EDT Mon Jun 24 2019
 
Day 2
Valid 12Z Tue Jun 25 2019 - 12Z Wed Jun 26 2019 

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS 
OF THE PLAINS AND LOWER MISSISSIPPI VALLEY INTO THE 
MID-MISSISSIPPI VALLEY...

Given a fairly weak flow pattern with an exiting shortwave trough 
moving through Ontario and a closed low off the Pacific Northwest, 
there is little in the way of any large scale forcing expected 
Tuesday into Tuesday night (Day 2).  As a result, small 
perturbations within the mid-level flow pattern combined with 
instability and surface convergence will be the main factors worth 
considering for excessive rainfall.  With such a pattern in place, 
it is quite difficult to discern the mid-level vorticity 
placement/trajectory and surface boundary orientation even one day 
in advance.  This is also evident by the large spread in model 
QPF.  However, when diving a bit deeper into the typical 
ingredients for flash flooding, small signals do arise for 
potential excessive heavy rainfall.  

Return flow west of the subtropical 850mb high will pump moisture 
and instability into the southern and central Plains riding along 
a weak boundary draped across northern MO into the OH Valley. 
While precipitable water values of 1.25 to 1.5 inches is not much 
above normal across this region, the 850mb moisture transport 
continues to feed moisture into TX/LA north into OK and toward the 
Great Lakes region. Strong instability will also be in place as 
noted by 4000+ J/kg MUCAPE, with weak capping in the low levels. 
In addition, this corridor may also house residual outflow 
boundaries which could act as a catalyst for convective 
development as mid-level impulses move overhead. So while there is 
not a strong signal for exactly where convective development will 
occur and the overall propagation, the ingredients are available 
for potential MCS development mainly across portions of the mid-MS 
Valley (southern IA into northern MO) and/or across OK. Since 
convection may initially start elevated and eventually be surface 
driven, feel cold pool propagation of convection, especially 
across OK, may move fairly quick.  This would limit the threat for 
flash flooding.  

Residual troughing across the TX/LA coast will promote convective 
development thanks to the pooling of moisture and instability.  
There does appears to be potential mesoscale impulses that ride 
north and east that may help provide additional support for 
convection just inland of the coast.  But organized convection 
with such weak flow seems unlikely at this time.  

Given the above logic, it appears that the best potential for 
flash flooding, albeit low, will be across portions of east TX 
into LA then northwest into OK and along a weak boundary into the 
Mid-MS Valley.  Again, quite a bit of uncertainty at this point, 
but with the above ingredients and some sensitive soils from prior 
heavy rainfall, feel this area has at least a small chance of 
observing excessive rainfall resulting in localized flooding.  
Therefore, placed this region in a Marginal Risk area.   


Pagano
 
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt