U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 231954 
Storm Prediction Center ac 231952 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0252 PM CDT Sat Sep 23 2017 

Valid 232000z - 241200z 

..there is a marginal risk of severe thunderstorms this 
afternoon/evening from central Minnesota to the southern High Plains... 

Isolated severe wind and hail will be possible from the upper 
Midwest to the southern High Plains, mainly this afternoon and early 

..northeast nm to southeast Colorado... 
Portions of this region have been removed from the severe 
probabilities/marginal risk area, as the passage of thunderstorms 
has stabilized the environment. 

..elsewhere across the rest of the marginal risk area... 
No changes needed. 

.Peters.. 09/23/2017 

Previous discussion... /issued 1111 am CDT Sat Sep 23 2017/ 

A closed midlevel low over Nevada will drift eastward to Utah by tonight, 
as embedded speed maxima eject north-northeastward around the 
eastern periphery of the larger-scale trough, from Arizona/nm to the 
northern plains. At the surface, a stalled front extends from 
central Minnesota to southeastern Colorado. The warm sector is characterized by 
a broad swath of mid-upper 60s boundary-layer dewpoints, and the 
remaining narrow corridor of steeper midlevel lapse rates overlaps 
the surface warm sector from West Texas to Minnesota. The frontal zone will 
serve as the primary focus for thunderstorm development through the 

..MN to eastern nm through this evening... 
Ongoing clusters of storms may persist into the afternoon, 
especially across eastern Colorado/nm, and additional storm development is 
likely this afternoon. The majority of the convection will be 
slightly anafrontal, with the stronger ascent expected atop the 
frontal surface (on the immediate cool side of the surface front), 
within the deeper moisture plume. Thus, while deep-layer vertical 
shear and midlevel lapse rates will favor some low-end threat for 
multicell clusters/marginal supercells capable of producing some 
hail and strong wind gusts, the slightly elevated nature of most of 
the convection from NE to Minnesota will temper the overall threat. 

More surface-based convection is expected from eastern nm into 
southeastern Colorado. Hodograph length and curvature will become 
sufficient for supercells by this evening across the southern High 
Plains, though midlevel lapse rates will weaken gradually as the 
last of the remaining elevated mixed layer is overturned by deep 
convection. Thus, the threat for large hail will become more 
limited with time, while the moist profiles and a transition to more 
linear convective modes should likewise stunt any wind/tornado 


Mesoscale Discussion

acus11 kwns 222058 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 222057 

Mesoscale discussion 1667 
National Weather Service Storm Prediction Center Norman OK 
0357 PM CDT Fri Sep 22 2017 

Areas affected...portions of Nebraska northeastward to far southwest 

Concerning...severe potential...watch unlikely 

Valid 222057z - 222300z 

Probability of watch issuance...20 percent 

Summary...thunderstorms are expected to gradually develop from 
central Nebraska northward into South Dakota by early evening. 
Isolated storms may be capable of large hail and damaging winds. 
While a watch is not currently anticipated, organizational trends 
will be monitored for a potential uptick in the overall severe 

Discussion...visible satellite data illustrate developing cumulus 
along a cold front from central Nebraska northeastward into South 
Dakota this afternoon. Ahead of this front, temperatures have risen 
into the 90s, owing to strong surface heating. While deep mixing has 
lowered dew points into the 50s across parts of Nebraska, 
boundary-layer moisture remains greater farther north and east. In 
tandem with steep mid/level lapse rates, these boundary-layer 
conditions are yielding around 2000-3000 j/kg of MLCAPE. 

Forcing for ascent is not particularly strong across the region and 
relatively dry 850-700mb conditions may stunt nascent updrafts 
initially; however, broad/weak ascent and further heating will 
likely be sufficient for isolated/widely scattered convective 
initiation. Southerly surface winds around 15-20 kt, veering to 
southwesterly mid/upper flow around 40-50 kt, should offer adequate 
effective shear for organization/rotation in the most robust cores, 
with an attendant large hail and damaging wind threat. Additionally, 
while large surface T/TD spreads will likely limit the tornado 
threat, favorable low-level hodographs suggest a tornado may be 
possible, primarily across far eastern South Dakota and far 
southwestern Minnesota later this evening. 

Although watch issuance is not currently expected, one may be 
considered if greater storm organization/coverage is observed. 

.Picca/Hart.. 09/22/2017 

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


Latitude...Lon 41480022 42399925 43339801 44879765 45189707 45159624 
44509574 43309597 41709720 40959843 40759907 40729974 
40960020 41160031 41480022