U.S. Severe Weather Forecast (Convective Outlook)

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Day Three

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Storm Prediction Center ac 210728 


Day 3 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0228 am CDT Thu Jun 21 2018 


Valid 231200z - 241200z 


..there is a slight risk of severe thunderstorms from central 
Oklahoma into southwest Missouri... 


... 
Scattered severe storms are expected from Oklahoma into southern 
Missouri on Saturday, with isolated activity extending from 
northwest Texas toward the lower Ohio Valley. A few strong storms 
are also possible across the coastal mid Atlantic. 


... 
A belt of moderate flow aloft will stretch from the Pacific 
northwest into the Central Plains, and from the mid Atlantic into 
the northeast. A leading shortwave trough will pivot quickly across 
the northeast, with low-amplitude ripples in otherwise zonal flow 
across the Central Plains into the Ohio Valley. 


A vast area of 60 f+ dewpoints will encompass the eastern half of 
the conus, with 70 f dewpoints generally south of 37 north resulting in 
areas of strong instability. Foci for severe storms include a 
stationary front/possibly outflow boundary across OK and MO, and a 
surface trough across the mid Atlantic. 


..northwest Texas into southwest MO... 
Models indicate a possible mesoscale convective system ongoing near the OK/KS/MO/AR border 
region, with some semblance of convection maintained throughout the 
day. This will likely result in a substantial outflow boundary, most 
likely across northeast OK into northern AR, and the primary severe 
risk will exist along and south of this boundary, as well as east of 
a synoptic wind shift from northwest Texas across central OK. However, 
if the mesoscale convective system fails to materialize or is meager, then more unstable air 
may proceed northward across MO, with a greater than forecast severe 
risk there. 


The air mass will become very unstable south/east of the boundaries, 
owing to heating, high dewpoints, and steep lapse rates aloft. 
Deep-layer shear will be favorable for severe storms, and low-level 
shear will support supercells and/or back-building storms. Large 
hail and damaging winds will be the most common severe Mode, but a 
few tornadoes cannot be ruled out on the mesoscale near boundaries. 
Isolated significant severe may occur, but predictability is 
currently low. Storms developing along the front into northwest Texas 
will likely be higher based, with hail and strong outflow winds 
likely. 


..MS and lower Ohio valleys... 
Storms that form over southern MO, OK and Arkansas are expected to move 
eastward with the mean flow toward the lower Ohio Valley overnight. 
Forecast soundings indicate the air mass may remain capped even with 
the loss of heating, thus supporting a damaging wind threat over 
southeast MO, northern AR, and into western Kentucky and Tennessee. 


..mid Atlantic... 
Substantial southwesterly 850 mb flow will allow 70s f dewpoints to 
surge northward across the Delmarva, with MLCAPE to around 2000 
j/kg. Lapse rates aloft are shown to be poor, however, 30-40 kt 
deep-layer mean wind speeds as well as lift in association with the 
upper trough should result in at least isolated strong storms, with 
damaging wind or marginal hail the main threats. 


.Jewell.. 06/21/2018 


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