Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
405 PM EDT Tue Sep 29 2026
Day 2
Valid 12Z Wed Sep 30 2026 - 12Z Thu Oct 01 2026
...THERE ARE MODERATE RISKS OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL AND SOUTHERN PLAINS, AS WELL AS THE CORN BELT...
...Front Range to Southern Plains...
20Z Update: Maturing lee cyclone across the Central and Southern
High Plains will allow for a continued heavy rain threat across the
Sangre de Cristos into the San Juan Range as the flow shifts out
of the northeast along the northwest periphery of the surface low,
creating a textbook upslope flow regime to perpetuate the rainfall
over the terrain of southern CO into northern NM. The prospects of
flash flooding will be enhanced due to the antecedent moist soils
brought about by prior periods of rainfall, some of which will
likely cause flash flooding in their own right, leading to a
heightened probability for flooding given the overlapping nature of
heavy precip. 12z HREF continues to advocate for significant
totals, 2-4" with locally higher, of additional precip within the
mountains of the Southern Rockies into the Front Range as the
combination of upslope flow and enhanced axis of deformation will
lead to two primary periods of heavy precip; once in the morning
and then once again in the evening as we see the surface low
intensify and strong mid- level vorticity advection pivots around
the ULL center. HREF probabilities, both EAS and neighborhood
probabilities for >2" remain very high across the aforementioned
area leading to high confidence of the heavy rain threat with
multi-day totals likely breaching 3" over a large area over the
Front Range of southeastern CO into northern NM. The previous MDT
risk was maintained with little adjustment given the continued
signal.
Across the Southern Plains from Oklahoma down through Texas, the
setup is more classic with regards to increasing low-level
convergence along a propagating cold front that will materialize
from the surface cyclogenesis over the High Plains. The flow will
be fairly meridional ahead of the slow-moving mean trough/ULL
moving through the Southwest U.S. creating a broad area of
relatively backed flow across portions of west TX into the Red
River Valley down through the Central Rio Grande Valley (RGV) and
Edwards Plateau by Wednesday evening into early Thursday morning.
Model consensus is for the initiation of heavy convection along the
front as it migrates out of the High Plains of TX with the western
periphery of precip likely developing from the western Rolling
Plains down into the Lower Trans Pecos and adjacent Concho Valley
by late Wednesday afternoon. This will set the stage for a
broadening convective footprint over the Southern Plains as the
warm sector environment with deep tropical moisture presence will
interact with the frontal progression as it slowly moves eastward
through the area Wednesday night. Models indicate a formidable
35-45kt LLJ structure situated from the RGV up into the Central
Plains as the warm sector maturation will be well-defined with
continued moisture advection poleward until the front passes over
any given area. MLCAPE between 1500-2500 J/kg bisecting the eastern
two-thirds of the region will offer plenty of instability to
couple with 30-40kt Effective Bulk Shear, creating an environment
conducive for stronger mesocyclone cores and merging multi-cell
clusters that will provide heavy rain for a large area come
Wednesday evening.
HREF neighborhood probs for >3" are upwards of 60-90% over the
area from the Lower Trans Pecos to the central Red River Valley
between TX/OK with an average of 40-60% over the entire stretch
from Eagle Pass, TX up towards Norman, OK. This is a substantial
area of heightened probs for 3" with modest probs (25-50%) for >5"
locally with the best signal over the Lower Trans Pecos and
northern Hill Country between San Angelo and Fort Worth. This is
more than sufficient for a continuation of the MDT risk with some
adjustment to account for a bit slow forward propagation forecast
on the northern edge of the line that should keep the worst of the
rainfall just west of the DFW metroplex. This will be an area to
keep watching over the next 24 hrs as the threat for more
appreciable urban flooding in a scenario where the line makes
headway into the metro.
...Central Plains into the Corn Belt...
The maturing cyclone over the High Plains will lead to a
progressive warm front on the eastern flank of the low with a
general warm sector evolution occurring from the Southern Plains
into the southern reaches of the Midwest. Rainfall from isentropic
ascent will already be occurring on the northern periphery of the
low, but limited instability will maintain more "subdued" rates
that are coincident with more appreciable flash flood prospects.
By Wednesday evening, that will no longer be the case as the warm
front progression will open the door to a more favorable
environment with modest instability development along and south of
the front which is classic for these maturing lee cyclones. PWAT
anomalies of +2 to +3 deviations will have advected into the region
creating a formidable deep layer moisture profile to couple with
the tall, skinny CAPE signatures expected over the Central Plains
into northern MO and southern to central IA.
As of this time, there is a strengthening consensus for heavy
rainfall potential across northern KS into southeastern NE a strong
shortwave ejection is anticipated on Wednesday evening with a
budding LLJ occurring ahead of the trough located over the Rockies.
Convection will breakout by Wednesday afternoon and expand
northeastward into the Missouri River Valley and neighboring Corn
Belt by Wednesday night with training convection within proximity
of the warm front. Cells will develop across the warm sector and
move northeast, in tandem which will eventually feed into the
likely convective cluster migrating along the front from north-
central KS into southern NE, eventually making headway into western
and central IA after nightfall. The sufficiently buoyant
environment and deep moisture presence will lead to hourly rainfall
rates between 1-2"/hr in the strongest cell cores with some
repeating action along the warm front. This has led to some robust
signals in the deterministic CAMs and associated HREF blended mean
QPF output which shows a general 2-4", locally as high as 6" over
the course of Wednesday afternoon into early Thursday morning. The
most notable area was that corridor from the KS/NE border up
through Omaha over into Des Moines with a higher QPF footprint
situated along and south of I-80. There will likely be a sharp
cutoff in the greatest threat for flash flooding as north of the
warm front will allow for a more stable environment and less of the
threat of the higher rates coincident with flash flooding. 12z HREF
FFG exceedance probabilities over 3hrs notes a maximum prob of
30-50% from Hastings over into Omaha and surrounding areas from
00-09z, aligning well with a prolonged 1"/hr or great rate
forecast within that zone over the period of time in question.
In coordination with several WFO's over the Central Plains and
Midwest, a MDT risk was added for the new D2 update with highest
confidence in that area between I-70 to I-80 over KS/NE/IA.
Kleebauer
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
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