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FOUS30 KWBC 192005
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
405 PM EDT Sun May 19 2024

Day 1
Valid 16Z Sun May 19 2024 - 12Z Mon May 20 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN AND CENTRAL KANSAS...

...16Z Update...

An additional SLGT risk was added to the current update across
portions of KS. More information is provided in the "Central
Plains" sub-heading below...

No adjustments were made to the northern periphery of the
inherited MRGL risk across the Upper Mississippi Valley as models
have maintained relative continuity in a few convective bullseyes
across southern MN up to the MSP area. The other area with no
change is across coastal SC down to southeast GA where convergence
pattern is well-documented within the past several runs of guidance
where a small section of the coastal plain between Charleston to
just south of Savannah will see locally 2-4" of rainfall with some
CAMs approaching upwards of 5". Neighborhood probability of at
least 3" is running at 80-90% within the entire zone encompassed by
the MRGL risk, so the prospects that someone will see elevated
rainfall totals is high with the urban corridors in Charleston and
Savannah as the focal points in the current MRGL risk area.

...Central Plains...

Current satellite and UA analysis pins a shortwave trough ejecting
to the northeast across eastern NE this morning with some organized
convective clusters within the confines of the mid-level ascent
pattern. Another shortwave is currently moving out of northern NM
with sights on the central plains later this afternoon and evening.
Convective initiation along the dryline positioned over the KS/CO
border will occur after 19/20z with expected forward propagation
through KS as we move into the evening. The increased mid-level
ascent and Southwesterly  flow east of the Rockies in the
mid/upper levels will carry waves of vorticity across the central
Plains into the Corn Belt today/tonight, culminating in a more
coherent/robust vort max out of NE into Iowa overnight. Surface
frontal boundary through CO/KS will lift back northward today as a
warm front with increasing moisture in the boundary layer
(precipitable water values ~1.50" or +1.5 sigma), leading to a
severe weather threat over KS late this afternoon. Additionally,
warm front lifting through Iowa will help spur some rainfall
(though generally beneficial) atop relatively lower FFG values
(compared to KS). 00Z HREF probs for exceeding FFG values run about
10-50% from KS northeastward to southern MN, roughly in line with
the Marginal Risk outline and weighted toward late afternoon into
the overnight hours as the stronger vort max moves through.
Question will be the details of the mode of convection and how
things play out downstream.ar under the influence of the shortwave
moving out of NM, along with better instability fields downstream
over central OK/KS into southern NE will lead to expansive
convective growth and congealing cold pools to aid in large,
organized convective pattern that will move through KS and northern
portions of OK. Latest HREF probability fields for 1-2"/hr rates
are running much higher compared to the previous forecast iteration
with a solid area encompassing northern KS down to the KS/OK
border eclipsing 60-70% chances for those rates reaching towards
the base of the current hourly FFG`s in place over the region
(~1.5-2"/hr FFG indices). Taking it further, the 3-hr FFG markers
are generally around 2.5-3" as well with the probability fields now
signaling upwards of 70% chances to exceed that range with an
areal average between 40-60% over northern KS. Despite the forward
propagation of the complex, the environment is ripe with elevated
PWAT anomalies between 1.5-2 deviations above normal anticipated
for the afternoon and evening hours with the advection of warm,
moist air occurring during much of the forecast period thanks to a
modest LLJ.

With the signals increasing for heavy QPF in the means and with the
alignment of the probability fields showing an uptick in the
potential, felt it was necessary to upgrade portions of the central
and southern plains with a SLGT risk. The Slight Risk encompasses
much of north-central and central KS down to about Wichita where
the best probabilities align. There is some uncertainty on the
southern extension into OK as guidances is less adamant on the
potential as you move closer to the OK/KS border, but the cold pool
propagation from the developing complex will be the primary
component as a further south push on the southern flank of any
convection would likely enhance local rates south along I-35 to the
OK border. This will be a near term trend that will be monitored in
real time during the convective evolution.

Kleebauer


...Previous Discussion...

...Coastal South Carolina and Georgia...

Vort max over northern GA this morning will slip southeastward
across the coastal areas this afternoon, yielding another chance
for showers/storms as it passes overhead. Maintained the Marginal
Risk for this area of coastal GA/SC beneath its path where surface
convergence is greater. 00Z HREF probs of exceeding FFG values
(~3"/3h) climb as high as 40% in a very small area where CAPE will
be sufficient. Urban areas will be most susceptible.

Fracasso


Day 2
Valid 12Z Mon May 20 2024 - 12Z Tue May 21 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL PLAINS/CORN BELT, NORTHERN ILLINOIS, AND SOUTHEAST
WISCONSIN...

...20Z Update...

An additional MRGL risk was added to the western side of Lake
Michigan extending from Green Bay down to Chicago and surrounding
suburbs. More information on this setup included in the sub-heading
below...

As for the inherited MRGL, an expansion to the northeast was made
to include more of northwestern IA and the southwest portion of MN.
Strengthening upper level forcing and accompanying surface
reflection will provide ample support for a swath of heavy precip
to stream northeastward within a broad axis of diffluence. The
biggest change was a bit more progressive evolution of the surface
low with a strong vorticity maxima ejecting out of NE into the
northern Midwest towards the end of the forecast period. Widespread
convection is being depicted within many of the CAMs in the 12z
suite with some hints of the potential already given via multiple
ML algorithms over the past few days. A few of the models are
still holding back on the threat within the area added, but this is
becoming more of the outlier with the probabilistic data now
supporting the expanded threat into IA/MN. These areas will have
seen rainfall in the previous period as well, further priming the
soils with streamflows continuing to be elevated based on the
latest river data available. This area will ultimately be impacted
further on D3, so this is just a quicker time scale that guidance
is catching up on with the threat. Expect a general 1-2" with
locally up to 4" possible across the outlined MRGL risk area.

...Northern Illinois and Southeast Wisconsin...

Mid-level vorticity maxima over KS will continue to propagate to
the northeast from the D1 period into the front half of D2 with
some additional strengthening of the energy anticipated given the
latest hi-res deterministic and ensemble suite. The trend has
risen for more widespread convection to impact portions of the
Midwest with its approach due to the vigor and increasingly
favorable thermodynamics as a warm front lifts north of the region
and places the area directly within the warm sector. HREF blended
mean QPF has jumped from near 1" to above 1.5" now in spots with
some individual CAMs signaling the threat for 2-3" for places
within the urban corridor extending from Chicago to Milwaukee. 12z
HREF neighborhood probabilities have also spiked with at least 1"
total precip now over 90% for a majority of southeast WI into the
northeast corner of IL along I-94. 2" probabilities are closer to
40-50% which gives you a general bound of what is to be expected
through the course of the period with only some minor probabilities
(10-15%) for up to 3". This grants a non-zero threat for higher
impact, but generally looking at localized flooding within the
urban centers along Lake Michigan. Considering the latest trends,
have added a MRGL risk area along the western Lake Michigan area
encompassing the region from Green Bay down to Chicago and
surrounding locales with lower FFG indices.

Kleebauer


...Previous Discussion...

...Central Plains and Corn Belt...

Similar upper pattern over the central US on Monday but with
changes upstream over the West. A southern stream shortwave will
race through the southern Rockies just ahead of a digging northern
stream trough into the northern Great Basin. Likely convection late
Sunday/early Monday may leave the area rather stable for the
daytime Monday, though the lingering frontal boundary will act as a
focus for some rainfall into the afternoon. Any heavier rainfall
may hold until overnight (00-12Z Tue) as the shortwave exits CO and
the LLJ increases. Heavier rainfall may lie over the NE Sand Hills
where FFG values are much higher, so the Marginal Risk outline is
on the smaller side to the south and east of this region for any
overnight isolated heavier rainfall as surface cyclogenesis starts
in earnest.

Fracasso


Day 3
Valid 12Z Tue May 21 2024 - 12Z Wed May 22 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE UPPER MIDWEST...

...20Z Update...

Current synoptic evolution and expected heavy rainfall footprint
across the Upper Midwest is still on target with little run-to-run
deviation from previous forecast issuance. Extended the area SLGT a
touch further to the south over IL given the indications of a bit
more warm sector convection that would allow for localized
enhancement of QPF within any prominent convective cores that are
established over the region. There`s some instances where the
convective threat may extend further west into IA pending the
eventual evolution of the surface low and track of convective
activity shuffling out of the central plains. The highest threat
for flash flooding will reside within the strong axis of
deformation to the north and northwest side of the surface low, as
well as more defined convective elements within the established
warm sector extending from western and central WI down into IL. A
cold front will move eastward out of the plains with more
thunderstorms likely to form along the frontal boundary as we shift
towards the end of the period. This will translate into another
active setup for D4 and could be more prevalent for the period if
the convective enhancement begins a bit earlier than currently
forecast. For now, the MRGL was maintained for the southern edge of
the cold front with the SLGT risk firmly outlining the deformation
zone and warm sector.

Kleebauer


...Previous Discussion...

Robust southern stream shortwave over eastern CO Tuesday morning
will head northeastward into southwestern MN by early evening as
surface low pressure deepens below 990mb over northeastern MN at
the end of the period. Strong southerly flow in the BL (850mb winds
to 50kts) will help surge moisture northward into and around the
low, with precipitable water values climbing to over 1.50" which is
about +2 sigma and 850-700mb moisture flux anomalies > +5 sigma
from MO northward to MN/WI. QPF mode will be deformation-driven on
the NW side of the wrapped-up/occluded low where rates will be
modest but perhaps exceeding longer-time FFG values vs warm-sector
QPF (MO northward/northeastward) where instability will be present
(CAPE > 2000 J/kg as far north as northern IA). The entire system
will be rather progressive, but expansive. Focused the Slight Risk
on the warm sector rainfall and nosed back toward eastern SD where
FFG values are lower than points farther west (covered by the
Slight Risk). Models show various axes of QPF tied to different
forcings in the evolution of the system, which is covered by the
broad Marginal Risk outline eastward to Lake Michigan. AI guidance
was displaced a bit to the east of the dynamical parent models, but
this was also reflected in the ensemble systems as well
(GEFS/ECMWF/GEPS). QPF spread was largest well into the warm
sector (MO into the Ozarks) where the flash flood threat may be
more isolated depending on the convective evolution.


Fracasso


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