U.S. Severe Weather Forecast (Sääkatsaus)

Day Three

acus01 kwns 211617 
Storm Prediction Center ac 211615 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1115 am CDT Sat Oct 21 2017 

Valid 211630z - 221200z 

..there is an enhanced risk of severe thunderstorms this afternoon 
into tonight from southeastern Kansas and southwestern MO across 
northwestern Arkansas and OK to parts of the Red River valley... 

..there is a slight risk of severe thunderstorms from North Texas to 
Iowa and southeastern Minnesota... 

..there is a marginal risk of severe thunderstorms from central Texas 
to parts of the middle and upper MS valley... 

Scattered severe thunderstorms are expected especially across 
portions of the south-Central Plains late this afternoon into 
tonight. Very large hail, damaging winds, and a few tornadoes will 
all be possible. 

A pronounced midlevel trough over The Rockies will progress eastward 
to the plains by tonight. A surface cyclone in southern mb will 
move to the north-northeast, as a trailing cold front moves eastward 
into the upper MS valley, and southeastward across the 
central/Southern Plains and the Ozarks. Low-level moisture 
continues to spread northward in advance of the cold front and 
beneath a plume of steep midlevel lapse rates, which will combine 
with increasing deep-layer vertical shear and lift along the front 
to support a threat for severe storms this afternoon into tonight 
from Iowa to North Texas. 

..eastern Kansas to Iowa this afternoon through tonight... 
Initial severe thunderstorm development is expected by mid afternoon 
along the cold front in Kansas (possibly evolving from early midlevel 
convection), and storms will subsequently expand northeastward into 
Iowa through this evening. Remnant steep midlevel lapse rates and 
moderate buoyancy will support an initial large hail threat with the 
frontal convection, and deep-layer vertical shear will be sufficient 
for supercells. However, largely front-parallel, deep-layer shear 
vectors and flow aloft will tend to support rapid upscale growth 
into a squall line in the zone of linear ascent along the front by 
this evening. As such, damaging winds will become the main threat, 
though limited surface heating and moistening profiles aloft with 
time may tend to limit the downdraft/damaging-wind threat with 
northeastward extent. An isolated tornado or two cannot be ruled 
out with the more discrete initial storms, or with embedded 
circulations within the line. 

..OK to North Texas late this afternoon through tonight... 
More discrete supercell development will be possible across western 
OK and northwest Texas in the 21-23z time frame along the dryline, near 
and south of a weak triple-point low at the intersection of the cold 
front and dryline. MLCAPE of 2500-3000 j/kg, midlevel lapse rates 
greater than 8 c/km, and effective bulk shear in excess of 40 kt 
will favor supercells with the initial semi-discrete storm 
development. A couple of tornadoes and very large hail will be 
possible with the initial supercells, though low-level shear in the 
warm sector will not be particularly strong to the west of I-35. 
Upscale growth into an extensive squall line (likely merging with 
the convection farther north into ks) is expected this evening, with 
the severe-storm threat transitioning primarily to damaging winds by 
late evening. A couple of tornadoes will remain possible this 
evening into tonight with embedded mesovortices from southeastern 
Kansas/southwestern MO into eastern OK, especially with more 
southwest-northeast oriented bowing segments in the line. 

.Thompson/leitman.. 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