292
FOUS30 KWBC 191555
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1155 AM 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...

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...

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