Flash Flood Warning 38, NWS Memphis, TN

Area Forecast Discussion
National Weather Service Memphis TN
747 PM CDT Fri Apr 4 2025

...New UPDATE...

.KEY MESSAGES...
Updated at 748 PM CDT Fri Apr 4 2025

-Severe storms and excessive rainfall are expected to continue
 through Saturday night.

-A Moderate Risk of severe weather is in effect through tonight
 for portions of northeast Arkansas and the Missouri Bootheel.

-An Enhanced Risk of severe weather is in effect for much of the
 Mid-South through Saturday night.

-A High Risk of excessive rainfall is in effect through Saturday
 night. Total rainfall amounts of 10 to 15 inches along and north
 of I-40 are likely. This is not your average flood risk.
 Generational flooding with devastating impacts are possible.

&&

.UPDATE...
Issued at 748 PM CDT Fri Apr 4 2025

A few storms were able to develop across the CWA this afternoon
and have lingered into the evening. This activity has largely
waned with the loss of diurnal heating and lack of large scale
forcing for ascent. Convection is much more organized just to our
north and west from central AR into the OH Valley. This activity
will propagate eastward into the CWA over the next several hours,
gradually overspreading northeast AR and the Bootheel through
approximately midnight. Some of this convection will be severe as
the near storm environment remains favorable for strong, rotating
updrafts. MLCAPE is forecast to remain near 2000 J/kg with ample
deep-layer shear and 0-1 km SRH on the order of 300-400 J/kg. Some
of the WoFS ML products and UH probabilistic data highlight this
greatest potential for supercells across Clay, Greene, Dunklin,
and Pemiscot Counties, but the area will expand a bit south and
east with time. The current watch really doesn't cover this time
frame so another watch cannot be ruled out after midnight.

Also of significant concern is the potential for a significant
hydrologic event overnight. The initiating boundary extends
southwest to northeast, roughly parallel 850-300 mb steering flow,
suggesting the likelihood of training cells. Right-moving
supercells could potentially propagate east off the boundary
given the Bunkers RM vectors. Storms could congeal into more of a
messy QLCS mode overnight with a potential for mesovortices and
tornadoes, in addition to damaging wind and large hail. Rainfall
amounts over the next 24-36 hours could be similar to what we've
already seen in many areas over the past 2 days given the training
convection and ridiculous rainfall rates. Additional rainfall
amounts of 3-6" are expected, but a band of higher amounts is
likely somewhere along or north of I-40 appears likely. CAM
guidance suggests higher end amounts of 8-10" in some areas (based
on HREF LPMM products). This flash flood threat has a high
ceiling and will need to be watched closely.

MJ

&&

.DISCUSSION...
(This evening through next Thursday)
Issued at 143 PM CDT Fri Apr 4 2025

Another day, and another round of severe weather as we continue
through an extremely active weather episode. A large upper trough
continues east today and is gradually picking up speed. To its
east and closer to the surface, a cold front extends south through
central Texas with a warm front lifting north from the Arklatex
through the Midsouth into the Ohio River Valley. A warm, tropical
airmass will then move into the region and persist through the
next 36-48 hours, producing heavy rain, severe thunderstorms, and
significant flooding.

The severe weather threat this afternoon will be characterized by
two rounds: warm sector cells and a QLCS overnight. Strong
southerly flow and upper height falls have allowed the retreat of
yesterdays cold air mass. Rapid destabilization is already under
way across our northern counties where upwards of 4000 J/kg of
SBCAPE are possible by midafternoon. The upper low to the west
will bring increasing upper winds and allow for 0-6 km bulk shear
in excess of 50 knots, favoring supercells and strong bowing line
segments within the QLCS. ESRH is currently lackluster (100-200
0-3 km) but is expected to increase into the evening and along the
warm frontal zone. Therefore, any storms that are able to form in
the open warm sector that reside through the evening or interact
with the warm front will pose a threat for all severe hazards,
especially across West Tennessee, East Arkansas, and the Missouri
Bootheel. However, there is still uncertainty in the coverage,
longevity, and location of warm sector storms going into this
afternoon and evening.

More severe weather is likely tomorrow as a cold front and
western upper low pass over through the region. A forced line and
the potential for prefrontal cells exist once again. However,
convection today will likely shove instability further south
towards the TN/MS border with 2000+ J/kg MUCAPE to the south. High
ESRH, especially in the lowest 1 km, will be high and streamwise.
Therefore, storms will rotate and organize quickly. Storm mode is
expected to be a line with embedded mesovortices/mesocyclones
that will have the potential to produce tornadoes and damaging
winds. Prefrontal cells are still possible tomorrow but extensive
cloud cover might preclude sufficient surface instability to break
through any capping. Storms are expected to leave the area by
Sunday afternoon with a few showers behind the front that are not
expected to add much rainfall.

Alongside the severe weather, significant and possibly
destructive flash flooding is likely starting today that will last
into Sunday across much of the Midsouth. PWATs currently sit above
the 99th percentile (1.8"-2") here and upstream towards the Gulf.
Strong integrated moisture transport and training convection will
aid in very heavy, long-lasting rain rates starting this
afternoon. Mean storm motion is not expected to change much
between tonight and Saturday evening. Expect north/south bands of
significant rainfall. Furthermore, much of the region has already
experienced 4-7 inches of rain. These saturated soils will fail
to contain new rainfall accumulations which are expected to be as
high as 6-10 inches through Sunday over western Tennessee,
Arkansas, and extreme northern Mississippi. Any areas that have
already experienced this rainfall will flood quickly. So heed
warnings immediately!

The upper pattern will then slow down after the upper trough exits
to our east. Northwest flow is expected to prevail over the region
for several days, with high pressure as the surface. This regime
will keep cooler than average temperatures around through Tuesday
with temperatures expected to warm into Wednesday and Thursday
ahead of a new upper surface low to our northwest. Moisture is
expected to be scoured off the Gulf as far south as the Yucatan
Peninsula. Therefore, significant moisture return is unlikely as
continental air is recycled into the CONUS. Some showers could
occur Thursday afternoon as the upper system responsible for the
surface low passes through, but no significant threat for severe
weather is anticipated at this time.

&&

.AVIATION...
(00Z TAFS)
Issued at 613 PM CDT Fri Apr 4 2025

Short term upper level ridging has limited TSRA coverage this
afternoon. TSRA corridor will enter the JBR vicinity by
03Z, remaining primarily north of MEM through the overnight.
Latest short range guidance indicates a potential for isolated
TSRA lifting up the MS River, in an open warm sector ahead of the
TSRA to the north. This isolated TSRA may affect MEM between 03Z
and 09Z.

For the non-JBR TAF sites, TSRA potential will ramp up
significantly after 15Z, as an upper level low over TX ejects to
the northeast.

PWB

&&

.MEG WATCHES/WARNINGS/ADVISORIES...
AR...Flood Watch through Sunday morning for ARZ009-018-026>028-035-
     036-048-049-058.

MO...Flood Watch through Sunday morning for MOZ113-115.

MS...Flood Watch through Sunday morning for MSZ001>005-007-008-
     010>014-020.

     Flood Watch from midnight CDT tonight through Sunday afternoon
     for MSZ006-009-015>017-021>024.

TN...Flood Watch through Sunday morning for TNZ001>004-019>022-
     048>055-088>092.

&&

UPDATE...MJ
PUBLIC FORECAST...JAB
AVIATION...PWB