- Day Three
acus01 kwns 191256
Storm Prediction Center ac 191255
Day 1 convective outlook
National Weather Service Storm Prediction Center Norman OK
0755 am CDT Wed Sep 19 2018
Valid 191300z - 201200z
..there is a marginal risk of severe thunderstorms across parts of
the north-Central Plains to upper Mississippi Valley...
A few severe storms, mainly with a threat for hail and damaging
winds, are possible across parts of the north-Central Plains to
upper Mississippi Valley.
In mid/upper levels, ridging is forecast to shift eastward from the
Southern Plains and Ozarks across the Mississippi Valley to the
Tennessee and Ohio valleys. A synoptic-scale trough -- now evident
in moisture-channel imagery over the Pacific coast states -- is
forecast to shift eastward to western Mt, UT, and the Arizona/nm border
region by 12z. To its southeast, several minor shortwave troughs
and vorticity lobes will eject northeastward across the central/
southern rockies and central/northern plains.
At the surface, 11z analysis showed a low over northeastern co, with
warm front roughly eastward across southern NE and northern MO. A
weaker, quasistationary frontal zone was drawn across southern Minnesota,
southeastern SD, and northwestern NE. The low is expected to
move/redevelop to central/southern NE by the end of the period as
the mid/upper trough approaches, with the frontal zone consolidating
toward the current position of the northern boundary, but with
slight effective displacement southward by convective processes
possible across the southern Minnesota/northern Iowa and southern WI areas.
..north-Central Plains to upper Mississippi Valley...
An extensive area of clouds, rain and embedded clusters of
thunderstorms is present across portions of northern IA, southern
Minnesota, northern and eastern SD, and northern NE. Being near the
southern rim of the stronger mid/upper flow, but north of the
surface front, a combination of elevated frontal lift and warm
advection will support isolated strong-severe convection in roughly
the southeastern half of this precip shield for a few more hours,
with pulse hail and strong gusts possible.
The morning activity will reinforce antecedent baroclinicity
accompanying the front, and another round of convection should
develop later today, perhaps beginning late this afternoon but more
likely evening, just poleward of the surface boundary. Modified 12z
raobs, model forecast soundings and planar progs show the surface
front and warm sector may remain diurnally capped, though an
isolated surface-based storm with supercell characteristics, large
hail and severe gusts cannot be ruled out over parts of central/
northern NE. That potential is very conditional, too uncertain and
low in coverage to justify affixing a 15%/slight area to the broader
marginal outlook at this time.
The evening/overnight elevated convection will occur as large-scale
ascent strengthens ahead of the synoptic trough -- primarily in the
form of warm advection but also mesoscale enhancements to midlevel
DCVA immediately preceding ejecting southern-stream perturbations.
Coverage may become scattered to numerous north of the boundary this
evening over the corridor from southeastern South Dakota to southern WI near
the nose of a strengthening llj, with related intensification of
moisture transport and isentropic lift. Large hail will be the main
concern, though a few strong-severe gusts are possible, especially
with any convection that can access the warm-sector boundary layer
before too much nocturnal stabilization occurs. 40-50 kt effective-
shear magnitudes and 2000-2500 j/kg MLCAPE will support some hail
risk wherever relatively discrete storms can mature before modes get
acus11 kwns 190848
sels mesoscale discussion
Storm Prediction Center mesoscale discussion 190847
Mesoscale discussion 1479
National Weather Service Storm Prediction Center Norman OK
0347 am CDT Wed Sep 19 2018
Areas affected...north-central to eastern South Dakota and adjacent southwest
Concerning...severe potential...watch unlikely
Valid 190847z - 191145z
Probability of watch issuance...5 percent
Summary...isolated large hail should be the primary risk with the
ongoing storm moving to the east-southeast through north-central
South Dakota, and with any additional sustained storms that can
develop and move into northeast and east-central South Dakota to
southwest Minnesota through the early morning.
Discussion...trends in mosaic radar imagery indicated mainly
discrete storms and one that has evolved into a linear Mode across
north-central South Dakota (dewey County into Potter county). These storms
were elevated, given their presence well north of the southern NE to
northern MO synoptic boundary, with easterly surface winds across
South Dakota/Minnesota and temperatures and dew points in the 50s to lower 60s f.
Forcing for ascent appears to be primarily associated with a
transient midlevel impulse tracking east across South Dakota overnight and
into Minnesota after 12z, while accompanying stronger westerly 500-mb
winds/shear and steep lapse rates are supporting the threat for
The track of the Dewey/Potter counties storm indicated that it began
to accelerate around 0730z while moving through northern Ziebach
County, and has a current movement at 0840z toward the
east-southeast at 45 kt. Although this storm and others that
develop will be elevated, the potential may exist for a locally
stronger wind gust to reach the surface. Similar to early yesterday
morning (prior to sunrise), there likely exists dry air in the
sub-cloud layer above the stable boundary layer per modified 00z unr
sounding. Downdrafts penetrating this dry air layer could be
enhanced such that higher-momentum air could reach the surface.
Short-term guidance does not provide very much confidence in the
forecast of the ongoing storms, though consensus of href members
would suggest overall coverage of sustained and potentially severe
storms should be isolated into this morning as convection spreads
toward eastern South Dakota and adjacent southwest Minnesota.
..please see www.Spc.NOAA.Gov for graphic product...
Latitude...Lon 45180113 45590016 45589871 45569747 45559590 45039580
44349596 44089629 44069756 44189889 44520031 45180113