U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 210545 
Storm Prediction Center ac 210544 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1144 PM CST Tue Feb 20 2018 

Valid 211200z - 221200z 

..there is a marginal risk of severe thunderstorms from eastern 
Texas to western Mississippi... 

Thunderstorms are forecast from Texas to the Ohio Valley and 
Appalachians today into tonight, and a few of these storms may be 
strong to severe from the Sabine valley to western Mississippi. 
Locally damaging winds and perhaps a tornado will be the main 

The mid/upper pattern across the country will witness little 
large-scale change today. An impressive ridge will persist 
along/offshore the East Coast, while a western-US trough is 
reinforced by a jet maximum advancing southward along the California 
coast. Between the two, a broad corridor of strong southwesterly 
flow will persist from the Southern Plains to the Great Lakes 
region. Meanwhile, surface high pressure will build over the 
northern plains and upper Midwest, helping push a cold front 
southeast across southern/eastern Texas, the lower Mississippi 
Valley, and the Ohio Valley. 

..eastern Texas to Mississippi... 
Aided by widespread precipitation across the cool sector, a cold 
front is forecast to continue progressing towards the Texas coast 
and Sabine valley through the early morning hours. However, as one 
or more weak shortwave impulses lifts northeast within the 
sub-tropical channel, a slowly amplifying southerly low-level jet 
should slow the southeastern advance of the combined front/outflow 
boundary over northern Louisiana. Furthermore, broad forcing for 
ascent may induce a weak surface low along this baroclinic zone 
during the afternoon and evening hours. Within the warm sector 
along/ahead of this frontal wave, surface dew points in the upper 
60s are expected to Foster upwards of 500-1000 j/kg of MLCAPE, 
despite modest 700-500mb lapse rates. 

Thunderstorms should organize along the front, with some enhancement 
in eastward propagation supported by any mergers from development 
within moist/confluent flow in the open warm sector. A few small, 
northeastward-advancing bowing segments may organize through the 
afternoon and evening, owing to primarily Uni-directional southerly 
flow within the cloud-bearing layer. These cells will chiefly pose a 
localized damaging wind threat. However, subtle backing of surface 
flow near the front, combined with any northwest-southeast oriented 
bands, may support a tornado or two as well. 

.Picca/leitman.. 02/21/2018 


Mesoscale Discussion

acus11 kwns 210318 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 210318 

Mesoscale discussion 0078 
National Weather Service Storm Prediction Center Norman OK 
0918 PM CST Tue Feb 20 2018 

Areas affected...portions of northern and central Illinois into central MO 

Concerning...winter mixed precipitation 

Valid 210318z - 210915z 

Summary...rain will transition to a wintry mix of freezing rain, 
sleet and snow across parts of northern and central Illinois into central 
MO over the next several hours. This wintry mix is expected to 
continue through the overnight hours, though precipitation rate 
should remain light. 

Discussion...a slow, eastward-advancing cold front was draped from 
roughly northwestern in into northwest Arkansas this evening. Cooler 
temperatures are filtering southward across the mid-MS valley, with 
readings generally in the 32-37 degree f range from near Chicago 
toward St. Louis and Springfield MO. Light to moderate precipitation 
was occurring over this area with mainly rain noted on the eastern 
edge of the mesoscale discussion area with a mix of precip types further west. This 
transition from rain to a wintry mix will slowly progress eastward 
through 09z. Precipitation should generally remain light as heavier 
precipitation translates eastward with the front. Freezing rain 
rates up to 0.05 inches per 3 hours are expected through the night. 
Sleet and occasionally light snow may mix with freezing rain. 

.Leitman.. 02/21/2018 

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


Latitude...Lon 41808743 40958789 40198839 39228910 38389018 37909104 
37389207 37279259 37309306 37609318 37959312 38549267 
39049218 40099101 40859017 41418940 41998851 42128797 
42038771 41808743