Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1150 AM EDT Mon Jun 24 2019
 
Day 1
Valid 12Z Mon Jun 24 2019 - 12Z Tue Jun 25 2019 

1600 UTC update

Changes to the previous Excessive Rainfall Outlook based heavily 
on the evolution of the latest radar.   The marginal risk area was 
removed from the Upper Texas coast...eastward into southern 
Louisiana in the wake of the squall line moving offshore.

Over the Ohio Valley...the previous marginal risk area was trimmed 
on the southwest side over western to central Tennessee into the 
Lower Ohio Valley in the wake of the shortwave and well defined 
area of associated precipitation pushing northeastward out of TN 
into KY.

Oravec



...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER A PORTION OF 
SOUTH TEXAS...

...Middle-Upper Texas Coast to Southern Louisiana...
For much of this area, any excessive rainfall threat will be early 
in the period (mainly through 18Z), owing largely to the 
evolution/lingering effects of the ongoing MCS early this morning. 
0-6km bulk shear values are comparatively weak toward the Gulf 
Coast (aob 20 kts), likely to result in more fragmented convection 
as the morning progresses. Nevertheless, given the orientation of 
the QLCS (largely w-e), along with the alignment of the 950-850 mb 
inflow and mean 850-300 mb flow, the resultant weak Corfidi 
vectors would support some upwind propagation with an enhanced 
risk of backbuilding and cell training. The moisture profile 
remains favorable after 12Z Mon -- with PWs between 1.75-2.00" -- 
however deep-layer instability will be on the downward trend 
outside of south TX. This will significantly curtail the rainfall 
rates, especially by 15Z when MUCAPEs fall aob 1000 j/kg behind 
the QLCS.

...South TX...
Airmass will have a much better chance to destabilize during the 
peak daytime heating hours south of the stalled frontal boundary, 
as mixed-layer CAPES average between 3000-4000 j/kg while PWs 
continue to hover between 1.75-2.00". Meanwhile, shortwave energy 
traversing northern Mexico Monday will become 
convectively-enhanced upon reaching south-central TX toward 00Z, 
sparking renewed convection by late afternoon and into the 
evening. The upper perturbation will also allow for a wave along 
the effective surface boundary -- slowing the southward 
progression of this boundary somewhat as it reaches south-central 
TX.

Given the thermodynamic setup, as per the some of the high-res 
CAMs (especially the WRF-ARW and ARW2), scattered strong cells 
will be capable of hourly rainfall rates of 2-3 inches by the time 
the activity reaches South TX and the Rio Grande Valley (RGV). 
This as the cell motions later this afternoon and evening will 
average much slower compared to the convection earlier on Monday. 
The 00Z HREF shows highest 3 hourly probabilities of exceeding 3" 
over South TX between 2100-0300Z -- with 6hr probs of >3" over 
50%, and 12hr probs of >5" between 30-40%. WPC included a Slight 
Risk over this region as a result.
 

...Upper Midwest...
Vigorous upper level forcing early in the period ahead of a 
pivoting upper shortwave and associated surface cold front will 
combine with modest deep-layer instability (MUCAPEs ~1000 j/kg) in 
supporting a marginal excessive rainfall threat -- despite the 
negative PW advection with time today (near 1.5" at 12Z Mon, 
falling to near 1.00" by 00Z Tue). Individual cells are expected 
to exhibit more downwind propagation than otherwise; however, some 
renewed convection within the broad comma head/along the mid level 
deformation axis may cause isolated short-term runoff issues, 
especially where the 3 hourly FFG values are 2" or less.

...Ohio-Tennessee Valleys into the Mid Atlantic and portions of 
the Northeast...
Upper level forcing will be weaker across these areas, as the 
aforementioned shortwave lifts northeastward from Lower MI into 
eastern Ontario. Nevertheless, subtle divergence aloft just behind 
the departing upper ridge axis (aided by right entrance region 
forcing via an upper jet streak draped over eastern New England) 
will aid in convective development/organization. Deep-layer warm 
and moist advection will precede the surface warm front, which is 
expected to extend across the eastern Great Lakes to the mid 
Atlantic region by 00Z Tue. MUCAPEs averaging between 1000-2000 
j/kg will provide modest instability, while PWs climb between 
1.75-2.00". 0-6km bulk shear increasing to 30-40 kts will aid in 
the organization/coverage of convection, while west-southwesterly 
upslope effects aid in the development upon reaching the western 
slopes of the mountains. For now, based on the low HREF exceedance 
probabilities and the fact that the convection is expected to 
forward propagate, the excessive rainfall risk over this area is 
expected to be marginal/isolated.

Hurley
 
Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt