U.S. Severe Weather Forecast (Coup d'œil sur la convection)

Day Three

acus01 kwns 281947 
Storm Prediction Center ac 281945 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0245 PM CDT Wednesday Sep 28 2016 

Valid 282000z - 291200z 

..there is a slight risk of severe thunderstorms late this afternoon into 
tonight along and east of the Blue Ridge mountains through the 
Piedmont of the Carolinas and Virginia...and perhaps adjacent 
portions of the coastal plain... 

..there is a marginal risk of severe thunderstorms across surrounding areas of the 
Middle Atlantic States/central Appalachians region... 

Thunderstorms impacting the Piedmont of the Carolinas and Virginia 
late this afternoon into tonight will pose a risk for severe 
weather. This includes the possibility for a tornado or two...but 
locally strong surface gusts and hail appear the primary potential 

..20z outlook update... 
Some tweaks to probabilistic and categorical lines have been made to 
account for current trends in model output and observational data. 
Considerable insolation across the Carolinas into southern Virginia 
appears to have contributed to cape of 1000-1500 j/kg along and 
south of a lingering frontal zone extending across the southern 
Delaware-Maryland-Virginia peninsula and southern Virginia...into the mountains of 
southwest Virginia and North Carolina. A relatively cool/stable 
near surface layer now present to the north of the front seems 
likely to hold through tonight across much of northern 
Virginia/central and eastern Maryland...with any surface pressure 
falls to the Lee of the Appalachians forecast to remain weak and 
confined to the western Carolinas late this afternoon...perhaps 
northward into the Piedmont of central Virginia by late evening. 

Substantive middle-level height falls...the middle-level cold core and 
core of stronger winds aloft associated with closed low now digging 
into the middle Ohio Valley appear likely to remain confined to 
areas near and west of the crest of the central Appalachians. 
However...deep layer shear currently appears sufficient for 
organized storm development...including supercells...where the 
boundary layer has already destabilized across the Virginia and 
Carolina Piedmont. And it still appears possible that an area of 
middle-level forcing for ascent pivoting around the periphery of the 
closed low could augment storm development to the east of the Blue 
Ridge mountains by the 00-03z time frame. Although low-level wind 
fields/hodographs are currently rather modest to weak...model output 
suggests a coinciding strengthening of southerly 850 mb winds along 
the frontal zone across south central Virginia... to around 30+ knots. 
Associated enlargement of low-level hodographs could be accompanied 
by some increase in tornadic potential before boundary layer 
instability wanes overnight. 

.Kerr.. 09/28/2016 

Previous discussion... /issued 1128 am CDT Wednesday Sep 28 2016/ 

..mid Atlantic region... 
A large upper low is currently centered near Chicago...tracking 
southward into the middle MS and Ohio valleys. Cyclonically curved and 
moderately strong middle-level winds extend from the base of the trough 
into the middle-Atlantic region...where a moist and unstable air mass 
is in place. Increasing daytime heating over parts of Maryland/VA/NC is 
leading to pockets of moderate cape and the likelihood of afternoon 
thunderstorm development. 

Early initiation may occur over parts of eastern Virginia into northeast 
NC. This activity may include rotating storms capable of locally 
damaging wind gusts and perhaps a tornado or two. Please refer to 
mesoscale discussion number 1738 for further details on this region. 

By middle-late afternoon...scattered thunderstorms are possible 
throughout the slight risk area. Most model solutions suggest that 
southerly low level winds strengthen just after dark...which would 
in turn increase the possibility of a few tornadoes in the strongest 
cells. Otherwise...damaging winds appear to be the main threat. 

..OH valley... 
The core of the upper low will track across parts of Illinois/ind/OH this 
afternoon...coincident with a pocket of cold middle level temperatures 
and vorticity aloft. Visible satellite trends suggest that little 
in the way of heating will occur ahead of the surface 
boundary...limiting destabilization. While scattered thunderstorms 
are expected in this region...the risk of large hail appears to be 
below 5 percent. Nevertheless small hail and funnel clouds are 
possible in the stronger cores this afternoon. 


Mesoscale Discussion

acus11 kwns 282303 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 282302 

Mesoscale discussion 1741 
National Weather Service Storm Prediction Center Norman OK 
0602 PM CDT Wednesday Sep 28 2016 

Areas affected...NC...VA 

Concerning...Severe Thunderstorm Watch 488... 

Valid 282302z - 290030z 

The severe weather threat for Severe Thunderstorm Watch 488 

Summary...severe thunderstorms will focus across the southeastern 
half of ww488. Large hail remains the primary severe risk. 

Discussion...scattered severe thunderstorms are now focused across 
the southeastern half of ww488. The strongest instability favorably 
correlates with this convection along a corridor from clt...northeastward to 
ric. Latest mrms hail algorithm suggests numerous near-severe to 
severe hail cores along the aforementioned corridor. Given the 
areal coverage of convection it appears instability should be 
overturned within the next few hours...especially with onset of 
diurnal cooling. 

.Darrow.. 09/28/2016 

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


Latitude...Lon 36528162 38817617 36797617 34498162 36528162