U.S. Severe Weather Forecast (Convective Outlook)

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 
too messy. 

.Edwards/Peters.. 09/19/2018 


Mesoscale Discussion

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 
large hail. 

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. 

.Peters/Edwards.. 09/19/2018 

..please see www.Spc.NOAA.Gov for graphic product... 


Latitude...Lon 45180113 45590016 45589871 45569747 45559590 45039580 
44349596 44089629 44069756 44189889 44520031 45180113