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 232256 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 232255 

Mesoscale discussion 1669 
National Weather Service Storm Prediction Center Norman OK 
0555 PM CDT Sat Sep 23 2017 

Areas affected...portions of southeastern nm and far West Texas 

Concerning...severe potential...watch unlikely 

Valid 232255z - 240100z 

Probability of watch issuance...5 percent 

Summary...a couple instances of large hail and strong/damaging winds 
may occur with isolated supercells over the next several hours. The 
overall severe threat is expected to remain very isolated, and watch 
issuance is not anticipated. 

Discussion...a couple supercells have developed late this afternoon 
within a modest low-level upslope flow regime across parts of 
southeastern nm and far West Texas. Winds strengthen to around 35-40 kt 
at mid levels as a highly amplified large-scale trough remains 
centered over the Great Basin. A veering wind profile per recent 
vwps from kmaf is also contributing to around 35-45 kt of effective 
bulk shear, which will allow for continued supercell structures 
through the evening. A narrow corridor of weak to moderate 
instability has developed across this region where some clearing has 
occurred. Given ongoing supercell structures and recent mrms mesh 
data, isolated large hail will continue to be a threat. Strong to 
locally damaging winds may also occur where low-level lapse rates 
have steepened due to diurnal heating. Both instability and shear 
weaken with eastward extent, and with the loss of diurnal heating 
that will occur later this evening, storms should eventually 
decrease in intensity. The ongoing threat for large hail and 
strong/damaging winds is expected to remain too isolated to warrant 
watch issuance. 

.Gleason/Edwards.. 09/23/2017 

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


Latitude...Lon 31010570 31880547 32730498 33590412 33770282 32930238 
31820227 30880244 29910268 29440348 29500409 29720467 
30500499 31010570