U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 200543 
Storm Prediction Center ac 200541 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1241 am CDT Sat Oct 20 2018 

Valid 201200z - 211200z 

..no severe thunderstorm areas forecast... 

the risk for severe thunderstorms appears negligible across the U.S. 
Saturday through Saturday night. 

A vigorous short-wave trough -- embedded in broader cyclonic flow 
over the eastern U.S. -- Is forecast to dig rapidly southeastward 
from the western upper Great Lakes area initially, across the lower 
lakes and Midwest/Ohio Valley through the day, and then to the New 
England and mid-Atlantic coasts by period's end. 

As this system digs/advances, a strong cold front -- trailing from a 
deep low over Quebec, will shift rapidly 
eastward/southeastward/southward with time, crossing the Atlantic 
and Gulf Coast states before shifting offshore. 

Meanwhile, background ridging will prevail over the west, though a 
weak low will linger in the vicinity of the California coast through 
the period. 

Showers -- and possibly a thunderstorm or two -- may evolve over 
portions of southern and western Arizona and into the Colorado River 
valley vicinity. More numerous showers and a few afternoon 
thunderstorms are also expected ahead of the cold front -- mainly 
from off the Atlantic coast west-southwest across the Gulf of Mexico 
and Gulf coastal region. 

Finally, low-topped showers and scattered thunderstorms are also 
expected to develop diurnally, across the lower Great Lakes and 
upper Ohio Valley region. Cold air aloft near the core of the 
digging short-wave trough will provide up to a couple hundred j/kg 
cape, generally below 500 mb. Moderately strong west-southwesterly 
flow at low-levels may support gusty winds with a few stronger 
cells, and very small hail may also occur. However, the cape/shear 
parameter space anticipated during the afternoon does not appear 
sufficient at this time to warrant upgrade to even low (5%) severe 
probability. Convection should diminish rapidly in intensity during 
the evening. 

.Goss/Cook.. 10/20/2018 


Mesoscale Discussion

acus11 kwns 162228 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 162228 

Mesoscale discussion 1587 
National Weather Service Storm Prediction Center Norman OK 
0528 PM CDT Tue Oct 16 2018 

Areas affected...portions of NC/SC 

Concerning...severe potential...watch unlikely 

Valid 162228z - 170100z 

Probability of watch issuance...5 percent 

Summary...strong/gusty winds may occur on an isolated basis through 
the evening. Watch issuance is unlikely. 

Discussion...glancing influence of large-scale ascent associated 
with an upper-level jet over the mid-Atlantic/northeast coupled with 
modest low-level convergence along a stalled front should support 
isolated to scattered convective development through the evening 
across the southern half of NC and parts of SC. This generally 
east-west oriented front will likely serve as a focus for 
thunderstorms, with a couple attempts at convective initiation noted 
recently near gsp in upstate SC. A meso-low noted on 22z surface 
analysis near Fay may also subtly enhance low-level convergence 
along the front in southern NC. 

Some cloud breaks to the south of the front have contributed to 
temperatures in the low 80s at 2228z. A moist-level airmass is also 
present across the warm sector, with dewpoints generally in the 
upper 60s to lower 70s. Daytime heating and plentiful moisture are 
supporting MLCAPE around 250-1000 j/kg, with poor mid-level lapse 
rates limiting even greater destabilization. 40-50+ kt of 
west-southwesterly mid-level flow is present across much of the 
Carolinas, and a strengthening wind profile with height is 
contributing to around 40-50 kt of effective bulk shear. Mid-level 
rotation within thunderstorm updrafts will likely occur given this 
degree of shear, and a couple transient supercells cannot be ruled 
out. A small line segment or thunderstorm cluster will probably 
consolidate along the front as it moves eastward through the 

Even with nocturnal cooling commencing soon, already steepened 
low-level lapse rates should encourage efficient momentum Transfer 
of convective downdrafts to the surface. Isolated strong/gusty winds 
primarily capable of tree damage appear to be the main severe 
threat, but very isolated hail may also occur. Regardless, the 
thermodynamic environment will remain quite marginal, and watch 
issuance is unlikely at this time. 

.Gleason/Edwards.. 10/16/2018 

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


Latitude...Lon 34358191 34878185 35328123 35558022 35557834 35157592 
34857618 34577645 34587693 34367746 34007779 33817793 
33837819 33767854 33437895 34358191