U.S. Severe Weather Forecast (Convective Outlook)

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Day Three

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Storm Prediction Center ac 201945 


Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0245 PM CDT Mon Aug 20 2018 


Valid 202000z - 211200z 


..there is an enhanced risk of severe thunderstorms over parts of 
southern Illinois...southeast Missouri...northeast 
Arkansas...western Kentucky...western Tennessee...and northwest 
Mississippi... 


... 
Scattered severe storms are likely across the mid-south into early 
evening. Damaging winds should be the primary hazard, but a couple 
tornadoes and large hail are possible. 


..mid MS valley... 
Probabilities have been trimmed across portions of Arkansas/MO to account 
for the eastward progression of the primary band of thunderstorms. 
As this band spreads eastward through the afternoon, corridors of 
damaging wind remain possible, along with more isolated instances of 
large hail and perhaps a tornado or two. See the previous discussion 
below for more information. 


..eastern in/western Ohio... 
The 5% wind and 2% tornado probabilities have been expanded to the 
north and east to cover the rest of eastern in and more of western 
OH, to account for the cluster of thunderstorms spreading northeast 
along a surface warm front. Transient supercell structures and small 
bowing segments will remain possible with this activity through the 
afternoon. See mesoscale discussion 1326 for more discussion regarding this region. 


... 
No other changes have been made to the outlook. See the previous 
discussion below for more information. 


.Dean/dial.. 08/20/2018 


Previous discussion... /issued 1130 am CDT Mon Aug 20 2018/ 


..mid MS valley... 
Only minor changes have been made to the ongoing forecast. An 
unseasonably strong upper low is present today over MO, with a band 
of moderately strong mid-level winds rotating cyclonically around 
the base of the low into parts of MO/AR/IL/KY/TN. It appears that 
initial convection will develop soon over south-central MO and 
north-central AR, and track eastward across the enh risk area 
through the afternoon. Forecast soundings show favorable winds 
aloft and sufficient cape for a risk of bowing line segment capable 
of damaging wind gusts. Initial activity could also pose a threat 
of hail and perhaps a tornado or two. Have expanded the risk areas 
a little farther north into central/southern Illinois where 
clearing/destabilization appears likely. Further details will be 
available in an mesoscale discussion that will be issued soon. 


..northern Illinois and vicinity... 
Easterly surface winds over northern IL, will result in marginally 
favorable hodographs for a few rotating storms later this afternoon 
along and north of the warm front lifting across the region. 
Widespread clouds will limit heating/destabilization, but morning 
model guidance suggests the potential for a few isolated storms 
capable of brief spin ups or damaging wind gusts. 


... 
Scattered thunderstorms are expected once again today over parts of 
NC, where ample low level moisture remains. Winds in the lower 
troposphere are weak, but the strongest cells will be capable of 
locally gusty/damaging winds. 


..northern Great Basin... 
A strong shortwave trough is moving across eastern or and will 
affect parts of Idaho/Mt/UT/WY later today. Low level moisture is 
limited, but forecast soundings suggest steep low-level lapse rates 
and increasing risk of a few damaging wind gusts as the mid-level 
wind Max overspreads the region. Small hail will also be possible 
in the higher elevations. 


$$ 

Mesoscale Discussion


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Storm Prediction Center mesoscale discussion 202257 
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Mesoscale discussion 1331 
National Weather Service Storm Prediction Center Norman OK 
0557 PM CDT Mon Aug 20 2018 


Areas affected...far northeast la and southeast Arkansas...north-central 
and northern MS...southern part of western Tennessee 


Concerning...severe potential...watch possible 345... 


Valid 202257z - 210000z 


Probability of watch issuance...40 percent 


Summary...isolated 50-60 mph gusts, capable of wind damage, are 
possible with any supercell or stronger multicell early this 
evening. Although a Severe Thunderstorm Watch will probably not be 
needed across north-central MS to the south of the existing watches, 
this area will be monitored. 


Discussion...radar shows a broken cluster of a mixed Mode of 
supercells and multicells across the lower MS valley early this 
evening. This activity has developed on the southern periphery of 
stronger forcing for ascent that is largely displaced to the north 
of the middle MS valley. An extremely moist axis extends 
north-northeastward over the Delta Region with upper 70s dewpoints 
yielding lowest 100mb mean mixing ratios of 19 g/kg. Despite the 
relatively weak lapse rates per forecast soundings, around 3000 j/kg 
of MLCAPE exists from northeast la into northern MS. An outflow 
boundary from earlier-day convection over southeast MS and 
west-central Alabama is slowly progressing northwestward across the 
greater Jackson-Columbus corridor. The focus for severe 
thunderstorm potential will be north and west of this boundary 
during the evening as storms gradually move east. Wet microbursts 
appear to be the primary threat with the stronger storms. Tornado 
potential will likely be relatively limited across northern MS given 
a veer-back-veer signature evident on the area vads. 


.Smith/Edwards.. 08/20/2018 


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


Attention...WFO...bmx...hun...Meg...Jan...lzk...shv... 


Latitude...Lon 32779007 32349146 32689257 34359064 35299008 35228837 
34448798 33428872 32779007