xcessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1157 AM EDT Sat Mar 27 2021
Day 1
Valid 16Z Sat Mar 27 2021 - 12Z Sun Mar 28 2021
...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS PARTS OF
THE MID-SOUTH AND TENNESSEE VALLEY...
16z Update: No major changes needed to the ERO at this time. 12z
HREF members and recent HRRR runs still support a Moderate risk
from eastern AR, into northern MS/AL and much of TN. Portions of
this area have already seen 1-3" of rain earlier today...and going
into the event soil saturation values were already well above
average. Thus antecedent conditions remain favorable for
additional flash flooding. Currently we are generally in between
forcing mechanisms, and a slight decrease in low level moisture
transport should result in some weakening/broadening of the low
level convergence axis. However, by mid to late afternoon we
should begin to see an uptick in mid/upper forcing from the west,
along with a renewed increase in low level moisture transport,
helping re-intensify the convergence axis. Thus the flash flood
risk will likely begin ramping up by mid/late afternoon into the
evening hours. This uptick by mid/late afternoon into the evening
is of high confidence. It is just late morning into early
afternoon where the risk may temporarily become more
isolated/scattered...but even during this period some flash flood
risk is still expected to persist.
The multiple convective rounds, combined with favorable antecedent
conditions, supports a scattered to widespread flash flood event
as we head through the evening hours. Embedded within this
Moderate risk there may very well be a narrower axis that receives
6-9" of rainfall. If this does occur then a focused corridor of
more significant and dangerous flash flooding is possible...with
the greatest chance of this from far northern MS/AL into
southwest/south central TN. Given amounts of this magnitude and
already saturated conditions, High risk level impacts are
certainly a possibility over a narrow axis. However, no upgrade
with this update, as the localized nature and uncertainty supports
maintaining a broader Moderate risk at this time.
Chenard
