382
FOUS30 KWBC 121952
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
352 PM EDT Sun May 12 2024

Day 1
Valid 16Z Sun May 12 2024 - 12Z Mon May 13 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF
EASTERN TEXAS INTO CENTRAL LOUISIANA...

...Summary...
Upper low evident on WV imagery this morning moving across eastern
CO will deepen as it traverses to the east, closing off over
Kansas by tonight, and producing height anomalies as ow as
-2 sigma between 500-700mb according to the NAEFS ensemble tables.
This evolution will result in a downstream increase in low-level
wind speeds emerging from the Gulf of Mexico, noted by 850mb winds
surging out of the S/SW as high as 20-40 kts by this evening. This
manifests as increasing moisture and instability advection, as more
favorable thermodynamics are drawn northward, reflected by a
ribbon of PW anomalies of +1 to +3 sigma via NAEFS (1.25" into KS,
as high as 2" in TX), collocated with MUCAPE reaching as high as
2000-3000 J/kg south of a surface warm front across TX/LA, and as
high as 1000 J/kg farther north into the Central Plains. These
thermodynamics become increasingly acted upon by strengthening
ascent as spokes of vorticity/shortwaves lift northeast around the
upper low, the upper jet streak positions favorably to produce
diffluence, and the warm front gradually pivots northeast. This
will result in two primary areas of heavy rain and areas of
potential flash flooding.

...Texas, Lower Mississippi Valley, and Central Gulf Coast...
The most significant rainfall and highest probability of flash
flooding will occur today from eastern TX into central LA. Here,
the most anomalous moisture (PWs to 2". above the 90th percentile
for mid- May according to the SPC sounding climatology) will
combine with MUCAPE approaching 3000 J/kg to provide robust
thermodynamics for heavy rainfall. This will occur in the vicinity
of the surface warm front which will drive low-level convergence to
serve as a focus for convective initiation in the presence of the
aforementioned impressive synoptic lift. There is high confidence
that multiple rounds of heavy rainfall will occur, with rainfall
rates of 1-3"/hr likely as progged by HREF neighborhood
probabilities and the HRRR 15-min rainfall rate fields. The
convection associated with these intense rates will likely expand
through the day, with training expected in the vicinity of the warm
front as reflected by mean cloud-layer winds generally parallel to
the warm front, so despite progressive Corfidi vectors this will
enhance the rainfall amounts today, and this will fall atop soils
that are saturated from recent heavy rainfall, especially in
eastern TX, noted by NASA SpoRT soil moisture reaching the 90-95th
percentile.

Instances of flash flooding today still appear likely, and the
inherited moderate risk remains. However, some adjustments were
made due to newer CAM guidance, and there is still uncertainty in
the placement of this heavy rain axis. The NSSL MPAS models appear
to be initializing quite well, and they have shifted a bit south,
which is pretty common in these setups, but they remain on the
southern edge of the unfortunately large ensemble envelope. There
has also been a bit of a pivot east in the max axis among the
various models. This necessitates a small expansion south and east
of the MDT risk, which is preferred over a shift due to the
likelihood of regeneration of cells to the west along the warm
front and in the area of greatest MUCAPE. Where the most pronounced
training occurs, rainfall of 3-5" is probable, with maximum
amounts exceeding 6" possible near the TX/LA border where moisture
convergence is maximized.

...Southern and Central Plains...
Farther to the north, a secondary axis of heavy rain is likely
across OK and into southern KS in the vicinity of the upper low.
Here, a narrow corridor of overlapped moisture and instability will
be drawn northward, resulting in an expansion of convection with
heavy rain rates exceeding 1"/hr ahead of a surface low and
occluded front. The CAMs indicate there could be multiple rounds of
thunderstorms this aftn/eve lifting northeast, and mean 0-6km
winds will likely be S/SW, parallel to the eastward advancing front
which could permit some training of echoes. There is some concern
that strong convection over TX could "steal" some of the
moisture/instability lifting north, capping the intensity of this
activity, resulting in a lowered flash flood risk, but the
latitudinal difference between the convection south and
regeneration north may be enough to reduce this impact. This area
of OK/KS has been generally dry in the past week except for
portions of north central OK into southeast KS, and after
coordination with OUN/ICT, added a small SLGT risk area where the
highest HREF and ECENS probabilities exist for 3"/24hrs, which
overlaps the lowest FFG due to recent rainfall. Still some
uncertainty in placement of heaviest rainfall, but any training of
cells across these more sensitive soils could result in scattered
instances of flash flooding, which is additionally suggested by
steadily increasing NWM RoFS probabilities and recent CSU first
guess fields.

Weiss


Day 2
Valid 12Z Mon May 13 2024 - 12Z Tue May 14 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL GULF COAST FROM EASTERN LOUISIANA TO THE WESTERN
FLORIDA PANHANDLE...

...Summary...
Mid-level low moving from the Central Plains to the Upper Midwest
Monday will remain amplified with 500-700mb height anomalies
reaching as low as minus-2 sigma according to NAEFS ensemble
tables. This will drive impressive height falls into an area of
enhanced divergence downstream, producing robust synoptic lift
aided additionally by a poleward arcing upper jet streak leaving
favorable diffluence within both its RRQ and LFQ. This evolution
will also result in increasing low-level flow emerging from the
Gulf of Mexico drawing moisture and instability northward, with the
attendant theta-e axis lifting as far north as the Great Lakes
before rotating into a deformation axis across the Upper Midwest to
cause a flash flood risk. Farther south, especially along and near
the central Gulf Coast, shortwaves and accompanying MCS moving
along a surface warm front will result in waves of heavy rainfall,
and a more widespread and significant risk for flash flooding.


...Lower Mississippi Valley through the Central Gulf Coast...
SW flow around the base of the mid-level trough will surge across
the lower MS VLY and central Gulf Coast, with shortwaves embedded
within the flow traversing west to east across the region. These
impulses will create enhanced mesoscale ascent atop the low-level
baroclinic boundary and band of frontogenesis associated with a
surface warm front lifting slowly northward from the Gulf Coast.
The guidance is in good agreement that two waves of heavy rainfall,
one overnight Sunday into early Monday, and a second, more intense
area Monday aftn/eve, will move along this front. However, there
remains considerable uncertainty as to exactly where they will
track due to the uncertainty in how the lead MCS will impact the
advection of the front. The NAM/GFS are on the north side of the
guidance envelope, and are likely too far north due to the typical
southern trend bias which occurs with these warmer season events,
and due to the general low-amplitude of the downstream ridge.

This suggests the heaviest rain will occur near and along the Gulf
Coast from eastern TX into the western FL Panhandle. With PWs
exceeding 2" and MUCAPE reaching 3000-4000 J/kg, this will support
rainfall rates of 2-3"/hr. The favorable orthogonal angle of the
moist low-level flow into the front/remnant cold pool combined with
boundary-parallel mean layer winds indicate an enhanced training
potential of these rain rates, which supports the model trends of
an axis of very heavy rainfall. In fact, recent HREF and RRFSE
probabilities suggest a 60-80% of 5 inches of rain from eastern LA
through the western FL panhandle, and there are even some lower
probabilities for 8 inches. The EAS probabilities, while extremely
impressive for 3"/24 hrs, are displaced just a bit north of the
neighborhood probabilities, and this, combined with generally
below normal soil moisture precludes an upgrade to the MDT ERO
risk area at this time. However, after collaboration with WFO MOB,
will continue to monitor the evolution of the guidance for a
possible upgrade with later issuances as rainfall of this magnitude
could result in widespread or significant flash flooding.

...Mid-Mississippi Valley and Upper Midwest...
Have added a narrow SLGT risk for portions of the region from
northern Missouri northeast towards Chicago, IL. There is good
model agreement here that a focus of slow moving convection will
develop within the impressive deformation axis. Prominent moist
advection into this axis will result in enhanced convergence to
drive ascent, occurring within a pronounced TROWAL reflected by
strong mid-level theta-e ridging surging northwest from the Gulf
Coast. This TROWAL will aid moisture and instability, with MUCAPE
progged to exceed 750 J/kg, and PWs likely climbing to around
1.25", slightly above the 90th percentile, additionally reflecting
the impressiveness of the thermodynamics. With strong ascent
pushing into this environment, convection is likely to become
widespread and heavy, with rainfall rates of more than 1"/hr
likely noted by HREF neighborhood probabilities reaching 40-50%.
These cells will move very slowly in this axis, and Corfidi vectors
become increasingly aligned anti-parallel to the mean winds
indicating backbuilding and training of these echoes. This could
result in localized rainfall exceeding 3 inches as noted by HREF
and RRFSE probabilities. This rain will occur atop soils with
compromised FFG as low as 1.5"/3hrs, so scattered instances of
flash flooding are possible.

Weiss


Day 3
Valid 12Z Tue May 14 2024 - 12Z Wed May 15 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE
SOUTHEAST AND GULF COAST...

20Z Update...
Changes to the previous forecast were minimal, and primarily
cosmetic based on latest GEFS/ECENS probabilities, new WPC QPF, and
updated NASA SPoRT soil moisture anomalies. The previous discussion
remains well aligned with current forecasts as well, so no
significant adjustments were needed to the ERO areas. The challenge
for D3 will generally be focused from the western FL Panhandle
through the coastal Southeast as heavy rain will move across
regions that have been saturated recently. Fortunately, features
should be more progressive by Tuesday so residence times of any
heavy rainfall should preclude any risks above the inherited SLGT.
The one caveat to this could be along the FL Panhandle where some
training along the progressive cold front could occur atop areas
that may get excessive rain on D2. If this comes to fruition, a
targeted higher risk area may be needed, but at this time
confidence in that evolution is too low for any upgrades.

...Tennessee Valley, Lower Ohio Valley, Mid Atlantic, and
Southeast...
While fairly transient, the models show a healthy dose of upper
divergence/deep-layer ascent ahead of a mid-level trough, aided by
the frontogenetic right-entrance region forcing from the upper jet
streak lifting through the northern Mid Atlantic region. Moisture
transport, IVT, and PW anomalies/percentiles remain rather robust
during Day 3 per the ensembles/ESAT. In fact, 850-700 mb moisture
flux sigmas climb to +4 to +5 eastern AL, southern 2/3 of GA, and
into the Carolinas, accompanied by IVT as high as 1000 kg/m/s.
Plenty of deep-layer instability will be present as well across
the Southeast, particularly behind the warm front, as MUCAPE surges
to nearly 2000 J/kg. While the ingredients are in play for heavy,
potentially excessive rainfall (both dynamical and
thermodynamical), the uptick in westerly MBE/Corfidi Vectors with
time Tuesday, coinciding with a more progressive frontal
progression over much of the region appears to be sufficient to
simply maintain the Slight Risk of excessive rainfall. This idea
matches the first-guess field from CSU which peak in the SLGT risk
category as well.

The one area that will need to be monitored for a potential upgrade
to a Moderate Risk in subsequent outlooks will be across southern
GA into northern FL, where the front and the 2.00+ TPW plume may
get hung up a bit Tuesday night as the southwesterly low-level flow
veers more westerly and aligns more parallel to the mean 850-300
mb flow later Tuesday afternoon into Tuesday night. At this point,

Hurley/Weiss

Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt