From
Mike Powell@618:250/6 to
All on Mon May 18 15:00:00 2026
FOUS30 KWBC 180800
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
400 AM EDT Mon May 18 2026
Day 1
Valid 12Z Mon May 18 2026 - 12Z Tue May 19 2026
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF MISSOURI...
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.
Elsewhere, the Slight Risk was also expanded well to the east into
western Indiana, where a separate area of training thunderstorms is
also coming into better agreement in the CAMs guidance. These
storms are also most likely to impact the area from about
Evansville/Terre Haute north and east to the Indianapolis metro.
While amounts won't be nearly as high as areas in the Moderate Risk
further west, the proclivity of training convection here should
result in widespread 1-3 inch rainfall amounts, requiring the
Slight Risk upgrade. Confidence is decreasing into southern
Minnesota for significant heavy rain, instead favoring either fast
moving storms or a more long-duration light rain. Thus, the Slight
Risk was trimmed south out of the Twin Cities metro area.
Wegman
Day 2
Valid 12Z Tue May 19 2026 - 12Z Wed May 20 2026
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE SOUTHERN
PLAINS TO THE LOWER OHIO VALLEY...
Relatively few changes were needed for the Day 2 outlook as
compared to the changes to the Day 1/Monday period. As the day 2
period enters the CAMs coverage period, there is an unsurprising
spread in the guidance as to how the convection will evolve,
especially moving just beyond the CAMs period Tuesday night. An
axis of heavier rainfall tracking along with the low level jet,
will persist from south Texas northeast into the Ohio Valley.
Depending on how the rainfall develops this evening in the lower
Ohio Valley, and where the areas of heaviest rainfall move in the
guidance, a few Slight Risk upgrades may be needed with future
updates. Should the storms D1 in Indiana outperform, then the
additional rainfall, while far from major, may be able to team up
with the newly favorable hydrology to result in more widely
scattered instances of flash flooding. The greater threat for a
Slight Risk upgrade will be in central Texas, where a maximum of
rainfall extends across the Texas Triangle (the area between the
Metroplex, San Antonio, and Houston). Given the urban concerns,
should there be better agreement in the CAMs as they move to
resolve the entire Day 2 period later today, a Slight Risk upgrade
may also be needed there.
Wegman
Day 3
Valid 12Z Wed May 20 2026 - 12Z Thu May 21 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR A PORTION OF
THE TEXAS HILL COUNTRY...
The heavy rain footprint not just across Texas but all across the
Marginal Risk area is a real mess by the Day 3/Wednesday period.
It's likely that the final flooding forecast will be determined by
the behaviors of prior convective regions both across Texas and
northeast to the mid-Appalachians. Thus, there remains a large
Marginal Risk from the southern Plains to the mid-Appalachians. The
inherited Slight Risk across west Texas was shrunk and shifted
south with this update. Higher rainfall totals are now expected in
the Texas Hill Country (but given the fickle nature of convection,
this is likely to change a lot over the coming days). This area is
particularly flood-prone should heavy rain occur over small and
topographically steep river channels common in the Texas Hill
Country. Thus, the Slight Risk highlights this higher flooding
risk, even though the heavy rainfall footprint in this area remains
highly uncertain.
Further east, an additional heavy rain area in the ensembles is
noted from east Texas, north of Houston east into northern
Louisiana. This area is relatively dry with no recent heavy
rainfall events, though that could change in some areas after the
Day 2/Tuesday period. The heaviest rainfall looks to occur over the
Piney Woods, and relatively sparsely populated areas of east Texas
and northern Louisiana, so for now, the Marginal Risk covers the
flooding threat, but a Slight may be needed if the heavy rainfall
shifts into urban areas such as Houston, or if the rainfall
forecast increases further in these lesser populated areas. The
moisture and instability plume will extend northeast from there,
where training storms are possible at times through the Tennessee
Valley and into the mountains of West Virginia. Given low FFGs and
the flood prone nature of the valleys of WV, the Marginal Risk was
expanded there to cover that potential flooding threat.
Wegman
$$
--- Scorpio BBS
* Origin: (618:250/6)