U.S. Severe Weather Forecast (Convective Outlook)

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Storm Prediction Center ac 180548 


Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1148 PM CST Fri Nov 17 2017 


Valid 181200z - 191200z 


..there is a slight risk of severe thunderstorms across the lower 
Ohio Valley... 


..there is a marginal risk of severe thunderstorms across the 
greater Ohio Valley... 


... 
Thunderstorms capable of severe wind gusts, marginal hail, or a 
brief tornado will be possible Saturday mainly over the Ohio Valley 
region. 


... 


A vigorous mid-level trough will quickly move east from the plains 
toward the East Coast on Saturday. As this occurs, a surface cyclone 
near St. Louis will deepen and race to the northeast, reaching 
upstate New York by Sunday morning. Strong mid-level winds (nearing 
100 knots at 500 millibars) will overspread much of the Ohio Valley 
as a surface cold front pushes southeast. 


..lower Ohio Valley... 


Strong forcing for ascent along the southeastward-advancing cold 
front should allow for thunderstorms to develop in the 18-21z time 
frame. Despite widespread cloud cover, most-unstable cape values 
should still increase to between 500-1000 j/kg given the degree of 
Theta-E advection associated with the 50-60 knot low-level jet. The 
modest buoyancy and lower-tropospheric flow largely parallel to the 
front should result in thunderstorms organizing into one or more 
small, bowing segments. Damaging wind gusts will be possible with 
the strongest clusters/segments. Long hodographs with small loops in 
the lowest three kilometers suggest that a brief tornado cannot be 
ruled out within the larger thunderstorm clusters/segments. With 
time and eastward extent, thunderstorms should outrun the greatest 
buoyancy, diminishing the overall severe threat. 


..northeast Missouri eastward into Ohio... 


Strong, moist isentropic ascent on the nose of a 40-60 knot 
low-level jet should allow for elevated thunderstorms to occur along 
and north of the track of the surface low/warm front. Here, 
most-unstable cape values up to 1000 j/kg and mid-level-lapse rates 
between 7 and 8 c/km will support a marginal hail risk. 


.Marsh.. 11/18/2017 


$$ 

Mesoscale Discussion


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sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 181129 
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Mesoscale discussion 1768 
National Weather Service Storm Prediction Center Norman OK 
0529 am CST Sat Nov 18 2017 


Areas affected...parts of northeast MO into central Illinois and central 
in 


Concerning...severe potential...watch unlikely 


Valid 181129z - 181400z 


Probability of watch issuance...20 percent 


Summary...the potential for storms producing hail will increase some 
this morning across northeast Missouri through central Illinois into 
central and perhaps northern Indiana. Most of the hail should be 
sub-severe, though a marginal risk does exist for a couple to exceed 
1 inch in diameter. 


Discussion...trends in early morning radar imagery indicated an 
increase in thunderstorm development across west-central into 
central Illinois near and just north of a warm front that was analyzed at 
11z extending to the east-northeast from a surface low in central MO 
through central Illinois to central in. Forcing attendant to 
strengthening deep-layer ascent with the approach of a progressive 
trough moving east from the plains today should promote a further 
increase in thunderstorm development near the warm front, as the 
surface low tracks east-northeast along this boundary. Additional 
forcing for ascent will be attendant to the exit region of a strong 
(110-140 kt) 250-mb jet nosing into to the mid MS and lower Ohio 
valleys this morning to the afternoon. 10z rap soundings are in 
good agreement with surface temperatures/dewpoints near the warm 
front, and the forecast suggests the stable layer should persist 
such that storms remain rooted in the 900-850 mb layer. Steep 
midlevel lapse rates atop the shallow inversion combined with 
sufficient bulk shear suggest stronger storms could produce isolated 
hail, with a couple exceeding 1 inch in diameter. 


.Peters/Thompson.. 11/18/2017 


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


Attention...WFO...iwx...ind...lot...ilx...lsx...dvn...eax... 


Latitude...Lon 39719040 39379195 39559285 39819326 40239232 40419057 
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