FOUS30 KWBC 182028
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 PM EDT Mon May 18 2026

Day 1
Valid 16Z Mon May 18 2026 - 12Z Tue May 19 2026

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
MISSOURI, IOWA, NEBRASKA, AND KANSAS...

16Z Update...

Coordinated a D1 upgrade to Moderate Risk for portions of
northeast Kansas through far southern Iowa as an impressive setup
for heavy rain develops this evening and tonight.

Large trough to the west will gradually advect eastward, while
shortwaves embedded within the flow track northeast into the Ozarks
and Central Plains. At the surface, a stationary front will waver
across the area, drifing east/west in response to convective
outflows and the surging 850mb LLJ. Activity this morning focused
across far southeast Kansas into southern Missouri should gradually
wane diurnally and shift east in response to the veering of the
LLJ, but this decay will likely be slower than "typical" thanks to
an improving synoptic evolution to offset the weakening/veering of
the LLJ. The moderate risk area for Missouri has remain unchanged,
and while there is likely to be a break in the activity late this
aftn/eve, a secondary round of heavy rainfall appears likely
tonight as development upstream (noted in the next paragraph below)
translates southeast back across this region.

The more significant change is to expand the moderate west-northwest
into the quad-state area from eastern Kansas through far southern
Iowa. As the upper trough to the west amplifies and translates
slowly eastward, it will shed a shortwave aloft atop the wavering
stationary front. At the same time, 850mb southerly flow is
expected to locally back and climb to 40-50 kts by this evening,
drawing impressive thermodynamics (PWs above 1.75 inches, a record,
and MUCAPE above 4000 J/kg) northward. The accompanying theta-e
ridge will lift north as well, placing an intense gradient along
this front, suggesting a setup that at least marginally matches the
conceptual model for Maddox Synoptic heavy rain. As convection re-
develops this evening in this area, rainfall rates should quickly
reach 1-3"/hr (HREF and REFS probabilities for 2"/hr above 40%),
and with Corfidi vectors aligned to the mean 0-6km winds,
significant training is likely which could produce (30-50% chance)
at least 5 inches of rain. Eventually, as the storms organize
upscale into an MCS tonight they should become outflow dominated
and progress more rapidly east/southeast, but until that occurs,
long- duration training is expected, and even then, additional
training is likely on the SW flank of this MCS due to backbuilding
convection into the inflow.

After coordination with the impacted WFOs, the moderate risk was
expanded, and includes the Kansas City metro area, which matches
more closely with the recent CSU First Guess field output.

Other changes for this D1 update are primarily cosmetic, and
include expanding the SLGT risk into Indiana a bit farther eastward
and the MRGL deeper into the Ohio Valley.

With so much going on D1, left the relevant portion of the previous
discussion below.

Weiss

...Previous Discussion...
In coordination with SGF/Springfield, MO; EAX/Pleasant Hill, MO;
and LSX/St Louis, MO forecast offices, a Moderate Risk upgrade was
introduced with this update. Much of the CAMs guidance has made a
rather drastic shift south of the current line of convection
tracking across Kansas and Missouri. There is reasonably good
agreement that the line of storms will move into west-central
Missouri over the next few hours, then stall out. Corfidi vectors
across this part of Missouri essentially drop to light and variable
over the next few hours. This combined with an influx of
atmospheric moisture, with PWATs increasing up to 1.75 inches, will
give the storms additional "juice" to work with in the form of
heavy rain. Once the storms stall, the 40 kt southwesterly LLJ will
allow for efficient backbuilding of storms along a narrow axis. An
ensemble of models highlight the new Moderate Risk area for the
most likely axis where the backbuilding will be most efficient. The
result is several models highlighting extreme rainfall amounts,
with the RRFS the highest at over 9 inches. On average, most peak
the heaviest rainfall amounts between 5 and 7 inches. Despite dry
soils in the area, this amount of rain over a 6 hour period,
combined with topographic effects, will lead to numerous instances
of flash flooding, resulting in the Moderate Risk upgrade.

The Moderate Risk is a double upgrade (from Marginal) in all areas
from the inherited forecast, with the southern sliver of the
Moderate perhaps a triple upgrade. This extreme shift in the
forecast unfortunately has much to do with the CAMs struggling to
resolve rapidly evolving convective patterns well. The heavy rain
is likely to begin in the Moderate Risk area over the next few
hours, with the early period (between 12-18Z/7am to 1pm CDT) being
the period with the most likely chances of seeing multiple inch per
hour rates, and the Moderate Risk getting the most rainfall of the
day. There should be a break in the rainfall during the evening and
most of the overnight, before a second much more progressive line
of storms possibly impacts this same area for a much shorter time
in the 09Z to 12Z time frame tonight. This second line of storms
will impact the hard-hit areas from northeast Kansas to southwest
Iowa during the late afternoon and evening before shifting
southeast across Missouri. Some training will also be possible here
before the storms organize, and given the more favorable hydrology,
the higher-end Slight remains in place from Kansas City north and
west through the southeastern corner of Nebraska.

Wegman

Day 2
Valid 12Z Tue May 19 2026 - 12Z Wed May 20 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
CENTRAL TEXAS HILL COUNTRY AND OHIO RIVER VALLEY...

The main changes to the overnight forecast include a broadening of
the Marginal Risk, and introduction of embedded Slight Risk areas
over the Texas Hill Country and Ohio Valley. A relatively stagnant
synoptic pattern will continue through Day 2 as a positively
tilted mean trough slowly evolves over the Western U.S. This
configuration should maintain modest southwest flow over the
Central/Eastern U.S. to support a corridor of heavy rainfall ahead
of a northeast to southwest oriented cold front.

Toward the beginning of Day 2, thunderstorms are expected to
develop along the cold front as it enters the Southern Plains and
interacts with a moist and unstable airmass characterized by
2000-3000 J/kg and 1.6-1.8" PW. Fairly weak and unidirectional
cloud layer wind profiles atop a lingering 20-30 kt nocturnal low-
level jet suggest this convection along the front will have a
tendency to train before propagating southward with the initiating
boundary. The HREF and RRFS suggest hourly rainfall rates will
eclipse 2"/hr at times in the main axis of convection, which should
initially support isolated flash flooding during the day. The
greatest concern for excessive rainfall, however, will materialize
late tomorrow afternoon as the cold front enters the Texas Hill
Country and bisects a well-defined dryline. The resultant east-
west axis of thunderstorms will eventually grow upscale and train
within the very unstable and modestly sheared airmass. From late
Tuesday into early Wednesday, the HREF and RRFS are in good
agreement for increasing probabilities of 2-3"/hr rainfall rates,
which could lead to rainfall totals upwards of 3-5" by Wednesday
morning. When combined with the sensitive nature of the Hill
Country and urban areas in the Texas Triangle, a Slight Risk was
introduced in this update.


Otherwise, a separate Slight Risk area was introduced over portions
of the Ohio Valley as thunderstorms develop along the slow moving
front tomorrow afternoon. While weak height rises forecast in the
vicinity of the front could limit thunderstorm coverage somewhat,
forecast soundings featuring moist vertical profiles (1.8-2.0" PW),
3000-4000 J/kg of MUCAPE, and unidirectional flow suggest the
storms that develop should be quite efficient and slow to propagate
in the new Slight Risk area. This scenario is supported by the
HREF and RRFS, which depict increasing chances of hourly rainfall
rates eclipsing 2-3"/hr by late tomorrow afternoon. As this
activity overlaps with ongoing heavy rainfall in the region,
scattered flash flooding could result.

Asherman


Day 3
Valid 12Z Wed May 20 2026 - 12Z Thu May 21 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTH-CENTRAL TEXAS INCLUDING THE HILL COUNTRY AND EDWARDS
PLATEAU...

The Day 3 excessive rainfall risk is really just a translation
slightly southward of the Day 2 risk area due to a slowly evolving
synoptic pattern.

A large positively tilted trough positioned across the
Intermountain West and back into the Great Basin will gradually
amplify but maintain its NE to SW orientation. Gradually pinching
mid-level flow will help funnel additional moisture northeast from
the Pacific across the Southern Plains and into the Ohio
Valley/Central Appalachians. Within this flow, subtle shortwave
impulses lifting through the flow will help enhance ascent, aiding
low-level convergence along a nearly stalled front draped from Ohio
through Texas. At the same time, locally backing low-level flow
from the south at 850mb will converge into the front, producing
impressive moisture convergence along the front in a region of PWs
that are progged to be 1.5 to 1.75 inches. Although inflow may be
somewhat modest (850mb winds around 20 kts), this will be
sufficient within the moist column to provide plentiful advection
to support widespread convection with rainfall rates above 1"/hr
nearly certain.

Although there continues to be uncertainty into the exact
placement of the heaviest rainfall due to prior-day convection
which will cause wavering of the surface boundary, 0-6km mean winds
becoming increasingly boundary-parallel will support training of
these rainfall rates, and the SREF/ECENS/GEFS all suggest at least
a low-end risk (5-10%) of 3"/24 hrs around the updated SLGT risk
region. While this in itself may not force widespread FFG
exceedance, much of this rainfall D3 will fall atop areas that are
expected to receive significant rainfall as well D2, so the SLGT
risk was drawn to account for compounding impacts potential over
already vulnerable terrain features.

Farther northeast, the MRGL risk was cosmetically adjusted for new
guidance as convection with heavy rainfall will likely repeat from
SW to NE along the front as it slowly sags southward through the
Tennessee and Ohio River Valleys.

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