535
ACUS01 KWNS 181956
SWODY1
SPC AC 181954

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0254 PM CDT Wed Sep 18 2024

Valid 182000Z - 191200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
NORTHEAST NEBRASKA INTO FAR NORTHWEST IOWA...EASTERN SOUTH
DAKOTA...AND WESTERN MINNESOTA...

...SUMMARY...
Thunderstorms capable of producing severe wind gusts and large hail
are possible today across parts of the eastern Dakotas and Minnesota
into the mid Missouri Valley. A marginal severe threat will also
extend southward into parts of the central and southern Plains.

...20Z Update...
No changes were made to the ongoing forecast. For mesoscale details
concerning eastern South Dakota and vicinity, see MCD 2083.
Additional details can be found in the previous discussion below.

..Wendt.. 09/18/2024

.PREV DISCUSSION... /ISSUED 1118 AM CDT Wed Sep 18 2024/

...Northern Plains/Upper Midwest to the Southern High Plains...
A closed upper low centered over eastern MT will continue
northeastward during the period, reaching southeast SK by early
Thursday morning. Upper-level ridging extending northward from the
northwest Gulf Coast into the upper MS Valley will slowly shift
eastward to the Upper Great Lakes. A belt of enhanced southwesterly
mid-level flow will extend from southern CA through the central High
Plains, while also wrapping cyclonically through the eastern Dakotas
and into southern Canada. At the surface, a trough/front extends
southward from central ND into SD and western NE, before continuing
southwestward over the central/southern High Plains. This boundary
may serve as a focus for renewed thunderstorm development later
today.

A band of elevated convection has generally moved northward from
northeast ND and northwest MN into southern MB. In the wake of this
activity, cloud cover remains prevalent across much of the warm
sector across the northern Plains/Upper Midwest. Still, filtered
daytime heating of an adequately moist boundary layer should aid in
the development of around 1500-2000 J/kg of MLCAPE by late
afternoon. Isolated to scattered thunderstorms may form along/east
of the surface trough/front within a favorable zone of 30-40 kt of
effective bulk shear. But, better large-scale forcing associated
with the upper low over the northern High Plains will tend to remain
displaced north and west (in southern Canada). Accordingly, 12Z
high-resolution guidance continues to vary regarding storm coverage
across the Slight Risk (isolated versus scattered) given the weaker
forcing aloft. Even with this continued uncertainty, various NAM/RAP
forecast soundings still suggest a somewhat conditional risk for
organized storms (mix of multicells and supercells). Severe
hail/wind may accompany the stronger storms from late afternoon
through early evening, before this activity eventually weakens.

Further south into the central/southern Plains, low-levels are
forecast to be somewhat drier near the surface trough, with surface
dewpoints expected to mostly range in the upper 50s to low 60s by
late afternoon with pronounced boundary-layer mixing. Warmer
mid-level temperatures and limited large-scale ascent across these
regions should temper the overall intensity of any updrafts that can
form and be sustained, and resultant intensity/coverage of this
convection. Still, some guidance shows locally greater thunderstorm
coverage and resulting isolated severe wind threat from the eastern
TX Panhandle into far northwest OK and southern KS during the late
afternoon/early evening, where low-level lapse rates should be
maximized with robust daytime heating.

$$