U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 241942 
Storm Prediction Center ac 241941 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0241 PM CDT sun Sep 24 2017 

Valid 242000z - 251200z 

..there is a marginal risk of severe thunderstorms late this 
afternoon and evening across parts of the upper Mississippi 

..there is a marginal risk of severe thunderstorms across parts of 
the southern High Plains... 

A few strong thunderstorms may impact parts of the upper Mississippi 
Valley region and portions of the southern High Plains late this 
afternoon and evening, accompanied by some risk for severe weather. 

No changes needed with this outlook issuance. 

.Peters.. 09/24/2017 

Previous discussion... /issued 1125 am CDT sun Sep 24 2017/ 

Much of the U.S. Will remain under the influence of an amplified 
southern branch of split westerlies emanating from the mid-latitude 
Pacific, with the axes of embedded large-scale ridging centered near 
the Atlantic Seaboard and west of the Pacific coast, and large-scale 
troughing near The Rockies. Models suggest that the trough will 
maintain a positive to neutral tilt with little eastward progression 
through this period, but an embedded closed low will weaken and 
become increasingly sheared while slowly accelerating northeast of 
the eastern Great Basin, into the Wyoming rockies by late tonight. 

A cold front associated with the upper troughing has advanced 
eastward through much of the northern plains, and southward through 
the Central High plains, with some further eastward/southward 
progression expected through 12z Monday. It appears that at least 
one impulse emerging from the upper trough may support some further 
development of a frontal wave across parts of western and northern 
Minnesota into northwest Ontario later today through tonight. 

Due to the meridional nature of the flow across the central U.S., 
Lower/mid tropospheric lapse rates are generally weak to the east of 
The Rockies. Additionally, high moisture content air (as 
characterized by surface dew points around 70f and precipitable 
water near 2 inches) is generally confined to portions of the Gulf 
Coast states and lower Rio Grande Valley. However, southeasterly 
low-level flow will continue to advect increasingly moist air into 
the southern High Plains, and a narrow tongue of mid 60s+ f surface 
dew points may advect northward ahead of the cold front, across the 
upper Mississippi Valley today through tonight. This may contribute 
to modest destabilization which could support some risk for vigorous 
thunderstorm development with some severe weather potential. 

..upper Mississippi Valley... 
A belt of strengthening southwesterly flow associated with the 
impulse emerging from the upper troughing appears likely to remain 
mostly to the cool side of the eastward advancing cold front front, 
particularly in the mid/upper levels. However, it still appears 
that a narrow corridor of stronger pre-frontal boundary layer 
heating and moistening could contribute to modest destabilization, 
and a window of opportunity for vigorous thunderstorm development in 
the presence of 30-40+ kt 850-500 mb flow. At the present time this 
seems most probable during the 23-03z time frame across parts of 
central/eastern Minnesota, where stronger storms may be accompanied 
by mainly a risk for localized potentially damaging wind gusts. 

..parts of the southern High Plains... 
At least lower/mid 60s surface dew points are expected to be 
maintained on southeasterly low-level flow into/across the Pecos 
Valley into the western slopes of the Davis/Sacramento Mountains of 
southwest Texas and southern New Mexico. Coupled with insolation, 
this may contribute to the development of modest boundary layer cape 
by late afternoon. Beneath moderate to strong cyclonic, 
southwesterly mid/upper flow, a window of opportunity appears to 
exist for the environment to become conducive to the formation of an 
isolated supercell or two early this evening. 


Mesoscale Discussion

acus11 kwns 242147 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 242146 

Mesoscale discussion 1670 
National Weather Service Storm Prediction Center Norman OK 
0446 PM CDT sun Sep 24 2017 

Areas affected...portions of Minnesota 

Concerning...severe potential...watch unlikely 

Valid 242146z - 242345z 

Probability of watch issuance...5 percent 

Summary...isolated severe hail and/or damaging wind reports are 
possible late this afternoon into this evening. However, watch 
issuance is not expected. 

Discussion...widely scattered thunderstorms have developed this 
afternoon near a cold front pushing east across Minnesota. Surface 
heating ahead of the front and sufficient boundary-layer moisture 
have promoted modest mixed-layer buoyancy (e.G., Values generally 
around 500-1000 j/kg), tempered by relatively poor mid-level lapse 
rates. However, mid/upper southerlies around 40-50 kt are offering 
adequate effective shear for occasional updraft organization, in 
turn yielding a threat of isolated severe hail. Moreover, dry 
mid-level air, well-mixed boundary-layer profiles, and 
small-scale/localized bowing segments may Foster a few stronger wind 
gusts prior to substantive nocturnal cooling this evening. Despite 
this wind/hail potential, a lack of more robust buoyancy is expected 
to keep most storms below severe limits, likely precluding watch 

.Picca/guyer.. 09/24/2017 

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


Latitude...Lon 44129530 45829511 48119357 48249265 47719222 46449245 
43969393 43609459 43709509 44129530