Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 AM EDT Thu Jun 22 2023
 
Day 1
Valid 12Z Thu Jun 22 2023 - 12Z Fri Jun 23 2023 

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF 
THE SOUTHEAST AND HIGH PLAINS...

...Southeast...

The vertically stacked low over the Tennessee Valley this morning 
will slowly drift northward into the Ohio Valley by Friday 
morning. As it does so, it will continue pumping plentiful Gulf 
moisture northward up the Eastern Seaboard and into New England. 
Meanwhile, shortwave disturbances wrapping around the low will 
remain prolific at supporting additional rounds of showers and 
thunderstorms across the area. Both their slow movement and 
abundance of available moisture will support heavy rains that 
could cause additional flash flooding. The Slight Risk area is 
considered a high-end one in three separate portions of the Slight 
risk area. Starting in the south, the eastern Florida Panhandle 
through the Big Bend area will likely see yet another round of 
slow moving and backbuilding storms that will train over the same 
areas being hard-hit now and have been for the past several days. 
With no relief in sight in this area, and despite the high 
thresholds for flash flooding which include the very favorable 
antecedent conditions, the expected storms are likely to result in 
flash flooding yet again. The ongoing MCS over the area will 
gradually weaken through the afternoon, with many areas of north 
FL getting a brief break in the storms, but by afternoon the 
storms will redevelop over the Gulf and track over the Big Bend 
area again, taking well into the evening to finally drift far 
enough south to allow for a longer break in the rainfall. The area 
from east of Tallahassee to Gainesville has seen anywhere from 
200-350% of their normal rainfall over the last 2 weeks according 
to AHPS data, so soils in the area cannot handle any more rainfall.

Further north across south GA, an advancing front trying to bring 
a bit drier air to the region behind the low will force another 
round of storms to develop ahead of it across this region. Just 
like down in FL, this area has seen 200-400% of normal rainfall 
over the past 2 weeks, and over the last week over 600% of normal 
towards Albany. Most of the guidance is hinting that the area of 
rain will be just south and east of Albany, but rainfall amounts 
there are still 200-400% of normal over the past week, so soils in 
this region are also very saturated. The front will help to move 
the storms along towards the northeast, so fortunately no one area 
will see stationary convection get stuck, but since the front will 
be stalling out, it will act as a corridor for storms to train 
over many of the same areas over and over again. General consensus 
is for the storms to develop in the late afternoon, around 20Z, 
and persist through around midnight. 

Finally up towards the central Carolinas and southern VA, 
scattered showers and thunderstorms will be ongoing at the start 
of the period and continue well into the overnight tonight. The 
most organized convection will be in the morning associated with 
an upper level shortwave rotating around the low, becoming more 
widely scattered through the afternoon and overnight as the storms 
pop up and quickly move northward. While training convection is 
likely, the timing between individual storms will vary widely 
across this region. The Slight risk was maintained across the 
Carolinas, which have seen upwards of 200% of normal rainfall over 
the past week, and are therefore more susceptible than up into 
southern VA. The Slight risk area was expanded northward into 
southern VA to include Richmond and the Hampton Roads area due to 
locally lower FFGs due to urbanization and recent rainfall in this 
area. As with areas further south, moisture will remain plentiful 
with most areas staying at 1.75 inches PWATs, or up to 2 sigma 
above normal. 

...Plains...

In CO, the ongoing convection at the time of this writing will 
continue moving slowly eastward across the High Plains in the 
eastern part of the state. These storms have been responsible for 
numerous Flash Flood Warnings, several of which have are 
highlighting considerable flash flooding. Once this MCS finally 
moves off and begins turning southward into KS, there may be a 
break in CO for some of the morning hours. With daytime heating 
however, new storms are likely to develop along the Front Range, 
but fortunately should by this point be moving eastward fast 
enough to make the flash flooding threat considerably lower than 
last night. Nonetheless, the storms will continue to have ample 
moisture and the local dry line will provide additional forcing, 
so the storms are likely to be strong, even if they're more 
isolated than previous days. Given the ample rainfall the area has 
seen with parts of CO between Denver and Colorado Springs seeing 
over 600% of normal rainfall the past 2 weeks, any storms even if 
more isolated and faster moving have potential to result in 
localized flash flooding.

Northeast of there into South Dakota, a cool front will be 
advancing southeastward out of MT and ND. Ample Gulf moisture from 
a strong 40 kt LLJ will supply the storms with plenty of energy. 
The storms will develop in the late afternoon and continue into 
the evening as they organize into an MCS over eastern SD. While 
this area hasn't had nearly as much rain as eastern CO has, there 
is good potential for training and repeating storms as the MCS 
grows upscale during the evening, which should overcome the drier 
conditions in eastern SD with rates as high as 3 inches per hour 
possible.

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