U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 210556 
Storm Prediction Center ac 210554 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1254 am CDT Sat Oct 21 2017 

Valid 211200z - 221200z 

..there is an enhanced risk of severe thunderstorms across 
Oklahoma...North Texas...southeast Kansas...southwest 
Missouri...extreme northwest Arkansas... 

..there is a slight risk of severe thunderstorms from the Southern 
Plains to the upper Midwest... 

..there is a marginal risk of severe thunderstorms from the 
Southern Plains to the upper Midwest surrounding the slight risk... 

Scattered severe thunderstorms are expected especially across 
portions of the south-Central Plains late this afternoon and 
evening, with very large hail, damaging winds, and a few tornadoes 

An upper-level trough centered over the Great Basin and northern 
intermountain region Friday evening will continue to dig/amplify 
from the northern/Central High plains this morning toward the 
central/Southern Plains and upper Midwest by early Sunday. An 
east/southeastward-moving cold front will be a primary focus for 
severe thunderstorm development later today, with a frontal wave and 
dryline also factors across the Southern Plains. 

..southern/Central Plains... 
An active second-season severe-weather day is expected across the 
region by late afternoon and evening. Ample moisture and moderate 
buoyancy, especially by mid/late October standards, will support 
intense thunderstorm development during the mid/late afternoon hours 
into evening as upper trough-related forcing for ascent overspreads 
the frontal zone. Low to middle 60s f surface dewpoints, as 
currently found across much of OK/TX, will become increasingly 
established by afternoon northward across eastern Kansas into far 
eastern NE, IA, and southern Minnesota. 

It seems likely that storms will initially develop and increase 
during the mid/late afternoon (roughly 21z-23z) near the 
east/southeastward-moving cold front spanning far eastern NE/western 
Iowa and northeast/south-central Kansas into northern/western OK. While a 
couple of initial supercells capable of hail could occur, a 
relatively quick transition to a linear Mode seems likely within 
this corridor in vicinity of the advancing cold front with damaging 
winds becoming the main concern. 

Farther south, a somewhat greater spatiotemporal window for 
semi-discrete supercells will likely exist across southern OK into 
western North Texas where deep-layer shear vectors will be somewhat 
more line-normal oriented. Current thinking is the most probable 
semi-discrete-supercell-related very large hail and potential for a 
couple of tornadoes should be focused across 
southwest/west-central/south-central OK in vicinity of the surface 
low/dryline-related triple point between 22z-02z. 

Quasi-linear bands of storms will be increasingly prevalent through 
the mid/late evening hours with damaging winds being the primary 
hazard, although a couple of tornadoes cannot ruled out across 
central into eastern OK with any line-embedded circulations amid 
adequate low-level shear/srh. These linearly organized storms should 
otherwise tend to gradually weaken during the overnight hours as 
they move into south-central MO/west-central Arkansas/southeast OK and The 
Ark-la-tex region. 

..upper Midwest... 
Although low-level moisture and buoyancy will be considerably more 
limited into the upper Midwest including parts of Iowa into Minnesota/western 
WI, strong south-southwesterly deep-layer winds (850mb winds around 
40 kt) will promote relatively shallow linear/bowing segments that 
rapidly spread north-northeast. The strongest convection during the 
evening hours may be capable of damaging gusts and perhaps a 

..west-central/southwest Texas... 
While the coverage/likelihood of storm development along the dryline 
is not certain, a few supercells capable of large hail cannot be 
ruled out during the late afternoon and early evening from the Low 
Rolling Plains south-southwestward toward the Edwards Plateau and 
Big Bend vicinity. A more likely scenario will be for increased 
storm coverage later this evening into the overnight as the cold 
front overtakes the dryline with some hail and wind being possible. 

.Guyer/Cook.. 10/21/2017 


Mesoscale Discussion

acus11 kwns 192021 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 192020 

Mesoscale discussion 1720 
National Weather Service Storm Prediction Center Norman OK 
0320 PM CDT Thu Oct 19 2017 

Areas affected...southwest New Mexico into a portion of far western 

Concerning...severe potential...watch unlikely 

Valid 192020z - 192245z 

Probability of watch issuance...5 percent 

Summary...stronger storms will remain capable of producing a few 
instances of downburst winds and hail through early evening. Overall 
threat is marginal, and coverage of severe events is not expected to 
become sufficient for a ww. 

Discussion...surface temperatures have climbed to near 80f with 
dewpoints in the mid to upper 40s in the presence of boundary-layer 
mixing, boosting MLCAPE to 400-800 j/kg. Storms over southern nm 
have increased in overall intensity during the last hour, and 
further increase in coverage may occur as forcing for ascent 
attendant to a progressive shortwave trough over Arizona spreads into 
western nm. No substantial increase in winds aloft is expected with 
the approach of the impulse, and vertical shear will remain weak and 
supportive of multicells. Nevertheless, the thermodynamic 
environment with steep lapse rates and inverted-v boundary layers 
will continue to promote a risk for a few instances of downburst 
winds and hail through early evening. 

.Dial.. 10/19/2017 

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


Latitude...Lon 31800602 31970707 32370817 33530845 33830682 33460584 
32500539 31800602