U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 230600 
Storm Prediction Center ac 230558 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1258 am CDT Mon Oct 23 2017 

Valid 231200z - 241200z 

..there is a slight risk of severe thunderstorms across portions of 
the Carolinas and into central Virginia... 

..there is a marginal risk of severe thunderstorms from the 
Delaware-Maryland-Virginia area south to southeast Georgia and west to the 

Strong/locally severe storms are expected from eastern Georgia to 
the mid-Atlantic Monday afternoon into the overnight hours. The 
primary threat will be damaging wind gusts, along with potential for 
a few tornadoes. 

A large upper cyclone with broad/surrounding cyclonic flow is 
progged to become established across the central and eastern U.S. 
Today and tonight. This will occur as two features -- a low moving 
across the mid MS/lower Ohio/Tennessee valleys early in the period and a 
short-wave trough digging southeast across the northern and Central 
Plains -- gradually coalesce into one larger-scale low. Either side 
of this large low/trough, highly amplified ridges will likewise 
evolve -- one gradually encompassing much of the west, and a second 
extending from the western Atlantic north across New England into 
eastern Canada. 

At the surface...low pressure initially over the Tennessee Valley 
area is forecast to shift northward and deepen substantially with 
time, reaching the upper Great Lakes as a 985 mb low early Tuesday 
morning. As the low deepens northward, a trailing cold front is 
forecast to shift east across the southern and central Appalachians 
through the afternoon, and reaching the middle and southern Atlantic 
coasts and crossing North Florida late in the period. 

..portions of Virginia/the Carolinas and surrounding areas... 
Scattered to widespread showers and thunderstorms are expected to be 
ongoing near/ahead of the front at the start of the period over the 
mid south and southeast, spreading northeastward with time into the 
Carolinas and Virginia prior to frontal passage. Models exhibit 
substantial differences regarding coverage of the pre-frontal 
convection, but at this time it appears that the degree of 
cloudiness/precipitation will be substantial enough to maintain weak 
lapse rates and hinder diurnal heating to at least some degree. As 
such, cape will likely remain limited in many areas to around 500 
j/kg or less -- and in turn therefore limiting overall intensity of 
convection/degree of severe risk. 

With that said, shear profiles should gradually strengthen across 
the region, likely becoming quite supportive of rotating updrafts, 
as 40 to 50 kt southwesterly flow at mid levels spreads atop 
isallobarically backed/enhanced low-level southeasterlies. Thus -- 
with favorable shear but uncertainty with respect to available 
instability, the severe forecast remains conditional upon initiation 
of deeper/robust updrafts. 

At this time, it appears that focused ascent near the front itself 
may promote a band of stronger/forced updrafts on the western edge 
of ongoing/scattered pre-frontal convection. While some tornado 
risk will exist within the stronger pre-frontal cells, greater 
severe potential -- to include damaging winds in addition to a few 
tornadoes -- will likely evolve within the frontal convective band. 
This band should evolve/intensify by early to mid afternoon, and 
then shift northeastward across the mid-Atlantic and southeast 
states through the evening and into the overnight hours. Some risk 
for wind -- with a narrow band of frontal convection that may 
produce little lightning -- may spread across the central 
Appalachians and possibly north of the Mason/Dixon line overnight, 
though greatest severe threat should reside over the Carolinas and 
parts of Virginia as indicated by the slight risk area until 
convection gradually weakens -- and eventually moves offshore -- 
through latter stages of the period. 

.Goss.. 10/23/2017 


Mesoscale Discussion

acus11 kwns 221900 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 221900 

Mesoscale discussion 1736 
National Weather Service Storm Prediction Center Norman OK 
0200 PM CDT sun Oct 22 2017 

Areas affected...portions of southern Alabama and th western Florida 

Concerning...severe potential...watch unlikely 

Valid 221900z - 222030z 

Probability of watch issuance...20 percent 

Summary...a few strong wind gusts and a brief/weak tornado or two 
may occur across southern Alabama into the western Florida Panhandle 
through the afternoon and early evening. A watch is not expected at 
this time. 

Discussion...heating ahead of a cluster of storms currently located 
over southwest Alabama has allowed for weak destabilization (less than 
1000 j/kg) across the Florida Panhandle into southern Alabama. While 
midlevel lapse rates are unremarkable, convection is likely being 
enhanced by an embedded mesoscale convective vortex (noted earlier in the morning over 
southeast ms). Near the apex of the bowing structure over Baldwin 
County, Alabama some stronger wind gusts will be possible as the storm 
cluster tracks east-northeast as low level lapse rates have 
steepened sufficiently. Additionally, a narrow zone of effective srh 
values around 200 m2/s2 will shift northeast across southern Alabama and 
the Florida Panhandle. A few briefly rotating cells have already been 
noted moving inland east of Mobile and this trend should continue as 
the system tracks northeast. Given the overall marginal nature of 
the thermodynamic and kinematic environment and transient/weak 
nature of any rotating cells, a watch is not expected at this time. 

.Leitman/Thompson.. 10/22/2017 

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


Latitude...Lon 30048707 30118764 30288797 30888803 31248785 31518752 
32018668 32128597 32108547 31988518 31588504 30878512 
30098554 29888573 30048707