U.S. Severe Weather Forecast (Convective Outlook)

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Storm Prediction Center ac 181630 


Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1130 am CDT sun Aug 18 2019 


Valid 181630z - 191200z 


..there is a slight risk of severe thunderstorms this afternoon 
from southern Illinois to lower Michigan... 


... 
The main severe-thunderstorm threat today will be damaging winds 
from Illinois into eastern lower Michigan. 


..Great Lakes to the Ozarks this afternoon... 
A northern stream shortwave trough will progress eastward from 
northern Minnesota across the upper Great Lakes, as midlevel heights begin 
to rise over the Southern Plains. In advance of the northern stream 
trough, multiple remnant mcvs now from MO to WI will move eastward 
to Illinois and lower Michigan during the afternoon. Ongoing convection in 
lower Michigan has weakened slowly the past couple of hours. Some 
strengthening is possible early this afternoon along the leading 
edge of the outflow into eastern lower Michigan as the low levels 
destabilize with surface heating, though confidence is not 
particularly high. Larger buoyancy will reside farther south into 
in/Illinois where boundary-layer dewpoints in the low 70s persist beneath 
the eastern extent of an elevated mixed layer. Convective 
inhibition will weaken as surface temperatures warm into the upper 
80s to lower 90s this afternoon, and new thunderstorm development is 
expected along the remnant outflow boundaries from MO into IL/in. 
Rather modest vertical shear with up to 40 kt midlevel flow will 
generally favor multicell clusters with damaging winds, though 
midlevel lapse rates will be sufficiently steep for some hail 
threat. 


Storm development is a little less certain from southern MO into 
eastern OK, given the ongoing midlevel convection and persistent 
clouds. There is some potential for development this afternoon in 
the differential heating zone near the southern influence of the mesoscale convective vortex 
over MO. Some warm advection will persist into this evening a bit 
farther to the northwest, though storm evolution/intensity is still 
in question. 


..NY/PA this afternoon... 
A weak midlevel trough will move east-northeastward from the lower 
Great Lakes across New York/PA this afternoon. A broken band of storms 
now forming in northwest PA is likely the precursor to a loose band 
of storms that will persist through the afternoon and spread into 
New York. Multicell clusters and/or low-end supercell structures will 
pose some threat for damaging gusts this afternoon. 


.Thompson/Elliott.. 08/18/2019 


$$ 

Mesoscale Discussion


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Storm Prediction Center mesoscale discussion 181735 
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Mesoscale discussion 1791 
National Weather Service Storm Prediction Center Norman OK 
1235 PM CDT sun Aug 18 2019 


Areas affected...portions of in...southern Illinois and southeast MO 


Concerning...severe potential...watch possible 


Valid 181735z - 181930z 


Probability of watch issuance...60 percent 


Summary...thunderstorms will increase in coverage and intensity 
through the afternoon. Damaging wind gusts are the main concern with 
storms heading into early evening, and a watch may be needed this 
afternoon. 


Discussion...latest visible satellite imagery shows increasing cumulus 
development along the composite outflow/front extending from 
northeast in southwestward toward southern portions of the greater 
St. Louis Metro area and the Missouri portion of the Ozarks. A few 
lightning strikes have been noted with this deepening convection 
near the Illinois/in border as well as across Madison/St. Clair counties 
near St. Louis. Strong heating of this region, where surface 
dewpoint are generally in the low to mid 70s, has resulted in 
1500-2000 j/kg MLCAPE with little inhibition remaining. While the 
main shortwave trough will continue to lift northeast across the 
Great Lakes, several shortwave impulses and remnant mcvs are 
embedded in westerly flow and shifting east across MO, as noted in 
midday WV loop. Storms should continue increasing along the boundary 
though the afternoon, with an attendant severe threat possible. 


The area will remain on the fringes of stronger deep layer flow, 
with around 25-30 kt effective shear forecast. However, moderate 
instability and steep low level lapse rates in the presence of high 
precipitable water (1.75-2.0 in) should support areas of strong downburst winds. If 
storm clusters become better organized through cell mergers or 
outflow interactions, the damaging wind threat could increase. Given 
modest shear/midlevel lapse rates and storm Mode, the hail threat 
should remain secondary to that of strong wind gusts, though a few 
marginally severe hail reports are possible. Trends will continue to 
be monitored and a watch may be needed this afternoon. 


.Leitman/Thompson.. 08/18/2019 


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


Attention...WFO...iln...lmk...iwx...ind...pah...ilx...lsx...sgf... 


Latitude...Lon 39708525 40128500 40618492 40928515 41048556 40528656 
39288876 37719145 37459157 37209162 36949147 36769095 
36839041 37308918 38448710 39708525