Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1156 AM EDT Sun Jun 16 2024
Day 1
Valid 16Z Sun Jun 16 2024 - 12Z Mon Jun 17 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
NORTHERN PLAINS AND UPPER MISSISSIPPI VALLEY, ALONG WITH PARTS OF
THE CENTRAL GULF COAST...

...16Z Update...

The previous thoughts on the overall evolution within both SLGT
risk areas has not changed significantly, however there were some
small shifts that were notable within the latest HREF and
associated CAMs. The premise remains the same on a targeted area of
heaviest rainfall confined to the stationary frontal boundary that
will bisect areas of the northern plains and points east through
the Upper Mississippi Valley and Midwest. Recent HREF blended mean
QPF targets the quasi-stationary boundary with widespread 1-3"
totals with general maxima (>3") located over southeast SD and
southwest MN where the nose of the developing nocturnal LLJ aims
into that corridor providing the strongest deep moist convergence
signature along the front. This signal has been the most consistent
based on run-to-run continuity leading to this zone being one of
the focal points for flash flood opportunities during the period.
Considering the antecedent conditions being primed for flood
concerns thanks to the preceding rainfall overnight into this
morning, the setup is bordering on the upper threshold of SLGT risk
with some locally significant flash flood concerns a possibility
within the above corridor to just south of the MSP metroplex where
6+" fell overnight leading to a targeted, low FFG signature.

One of the notable shifts from the 12z deterministic suite was the
southern extent of the heavy rainfall being push a bit further to
the south with Minneapolis-St Paul now on the northern fringe of
the heaviest QPF footprint. This is also reflected in the
probability fields with the neighborhood 1/2/3" markers all now
lower compared to the previous HREF forecasts overnight. This is a
trend worthy of monitoring, but that does not mean the metro
corridor will escape all, if any flash flood potential as the
probability for at least 1" of rainfall is still over 80% within
the metroplex with the >2" probabilities holding around 25-30%.
Considering the setup and FFG intervals in place for Minneapolis
and surrounding suburbs, this is still within the SLGT risk
threshold and will maintain continuity as such. The threat will
extend into northern and west-central WI where the anticipated
convective cluster will exit MN and shift focus to those areas by
the back-end of the forecast period. A secondary maxima is being
depicted on guidance for higher totals once into northwestern WI
just north of La Crosse where another stronger convergence
signature is noted within the CAMs and relative probability fields,
likely within the confines of a developing SLP center focused
along the front. This is subject to some variability, but the
growing consensus was enough to warrant an extension of the SLGT
further to the northeast into WI.

Another area of interest for flash flood concerns this evening will
be across northern IL with the passage of an MCV as it ejects out
of the central plains and makes headway into the region by late
this afternoon. Increasing destabilization ahead of the mesoscale
circulation will aid in redevelopment of area convection under the
guide of the area vorticity. Hi-res CAMs are insistent on
redevelopment close to the IA/MO/IL border with a reflection to the
northeast as it moves into the Chicago suburbs and eventually the
urban corridor by early evening. A signature for localized
enhancement within the western confines of the circulation is noted
with the maximum QPF being depicted just west of the Chicago metro
into the adjacent suburbs between Rochelle and Naperville. This is
reflected well in the neighborhood probability fields with the >3"
probs around 20-25% with a significant drop off outside the
bullseye. There is even a small probability for upwards of 5" (10%)
in the fields as well signaling the upper bounds of the threat. A
general 0.75-1.5" is most likely within the impacted areas around
Chicago and northern IL leading to a MRGL risk with some prospects
of a targeted SLGT if the setup produces near the upper bounds of
the potential. General MCV vorticity will continue to the northeast
eventually exiting back into southwestern MI by the end of the
period. Some locally enhanced convective signatures are forecast
within a multitude of CAMs between South Bend, IN up through the
Grand Rapids area. The MRGL was extended into those locations to
account for the risk.

Lastly, the setup over the Tennessee Valley and adjacent southern
Appalachia through the Carolinas, as well as the SLGT risk across
the Gulf coast remain fairly steadfast with little wavering in the
overall setup and probability fields. Locally heavy rainfall with
rates topping at 2-3"/hr will create some isolated flash flood
concerns across southern LA with the urban areas the most
susceptible for impacts. Rates will top out between 1-2"/hr across
the MRGL area encompassing the Tennessee Valley and Carolinas.
Urban flooding and complex terrain will be the areas of
susceptibility through the afternoon and evening hours as general
initiation will occur after 18z and end prior to 05z with the loss
of diurnal heating.

Kleebauer


...Previous Discussion...

...Northern Plains to the U.P. of Michigan...

Elevated convection along/ahead of the surface warm front late
Saturday night had resulted in some prolific rainfall totals,
including portions of northeast NE and south-central to southeast
MN. Part of the reshaping of the Slight Risk was with this in mind,
i.e. with the notable increase in 0-40cm relative soil moisture/soil
moisture percentiles and subsequent lowering of the FFG over these
areas. The other reason was with the model trends, particularly the
high-res CAMs. The outlook areas here will see a break in the more
organized, heavier rainfall today as the warm front pushes northeast
and is followed by flat shortwave ridging aloft. Overnight, as an
upper level jet streak traverses the Dakotas and southern Manitoba-
Ontario, upper level divergence and low-level frontogenesis in its
right-entrance region will generate new convection which should have
no trouble growing upscale given the favorable dynamical,
thermodynamic, and kinematic environment. Strengthening S-SW low-
level inflow into the quasi-stationary boundary will likely equal if
not exceed the magnitude of the mean 850-300 mb flow, while becoming
aligned with the mean flow by late evening/toward midnight. This
will allow for an enhanced risk for cell training along/near the
surface stationary boundary across northeast NE, southeast SD,
southern MN, and parts of western WI -- areas where the guidance
shows TPWs peaking aoa 1.75" overnight. This along with elevated
MUCAPEs of at least 1000-2000 J/Kg will likely lead to 1.5-2.0/hr
rainfall rates where convection can train prior to the best deep-
layer forcing shifting east-northeast.

The Slight and surrounding Marginal Risk areas do have support from
the CSU ERO first-guess fields (UFVS-verified), though as with
yesterday's Day 2 ERO, will keep the Slight Risk out of the
Sandhills.


...Central Gulf Coast...

Broad, weak upper level difluence will affect much of the Gulf Coast
downstream of the weak upper level trough over eastern TX. Scattered
weak/subtle mid-level impulses along the western periphery of the
upper ridge will lead to a rather spotty, disorganized convective
footprint, though the latest CAM guidance/trends favor better
clustering along the central Gulf Coast (south-central to southeast
LA), along with western portions of the FL Peninsula where the deep-
layer easterly flow will push the Gulf-East Coast Sea Breeze
boundary a bit farther west than what would otherwise would be. PWs
between 2-2.25" along with ML CAPEs between 1500-2500 J/Kg during
the day will result in fairly prolific short-term rainfall rates,
likely 2+ inches within an hour or less, based on the latest HREF
neighborhood probabilities. 0-6km bulk shear values generally 20
kts or less will favor more pulse type convection and thus more
short- lived intense rainfall rates, however based on the
deterministic CAMs and HREF probabilities, isolated 3-5" totals
within a couple of hours can be expected, again especially (perhaps
a bit more scattered) along the Central Gulf Coast region where
the Slight Risk is noted.

...Tennessee Valley to the Southern Appalachians and Parts of the
Southeast...

Mainly pulse-variety convection (with 0-6km bulk shear aob 20 kts)
along the western periphery of the expanding upper ridge may lead to
localized instances of flash flooding across this area today. This
as MLCAPE values climb between at least 1000-2000 J/Kg by the
afternoon, while at the same time TPWs reach ~1.75". High-res CAMs
show spotty 1.5-2.0+ inch/hr rainfall rates during peak diurnal
heating this afternoon, then continuing (though becoming more
isolated) into the evening hours. Most of the CAMs show isolated QPF
totals of 3-5+ inches, which despite the relatively dry soils
initially (relatively high FFGs), could nevertheless lead to
localized runoff issues, especially over urban areas or more sloped
terrain.

Hurley