U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 270450 
Storm Prediction Center ac 270449 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1149 PM CDT Wed Apr 26 2017 

Valid 271200z - 281200z 

..there is a marginal risk of severe thunderstorms from portions of 
the Great Lakes region southward to the southeast states... 

..there is a marginal risk of severe thunderstorms across portions 
of southeast Colorado...western/southern Kansas...the Texas/OK 
panhandles...northern OK... 

Isolated severe wind gusts may accompany thunderstorms from portions 
of the Great Lakes region to the southeast states today into this 
evening, and across portions of the central and southern Great 
Plains late this afternoon into this evening. 

Deep surface low pressure is forecast to advance from the vicinity 
of Lake Michigan northward into Ontario, associated with a 
northward-advancing shortwave trough embedded within the eastern rim 
of broadly cyclonic flow covering the central and western states. A 
cold front will arc south from the low to the north-central Gulf 
Coast vicinity and westward. The south-central states portion of the 
front is forecast to weaken and develop northward in response to 
surface Lee cyclogenesis over the Central High plains -- enhanced by 
the emergence of a midlevel speed maximum from the central/southern 
rockies vicinity. 

..portions of the Great Lakes region southward to the southeast 
comparatively stronger deep ascent accompanying the 
northward-advancing shortwave trough is forecast to become 
increasingly displaced to the north of Richer moisture within the 
low-level cyclone's warm sector. This will tend to marginalize the 
severe potential through the period. Nevertheless, storm coverage 
and intensity are forecast to gradually increase through the diurnal 
heating cycle along and ahead of the front, with an extensive 
corridor of eastward- and northeastward-moving clusters of storms 
across the marginal risk area. Isolated damaging wind gusts should 
be the primary hazard with this activity. 

The strongest pre-frontal buoyancy related to the richest low-level 
moisture characterized by middle/upper 60s dewpoints will be 
confined to portions of central SC into Georgia and vicinity. While 
effective shear around 30-40 kt and modest low-level directional 
shear may support a few rotating updrafts, weakening low-level 
ascent will tend to limit updraft sustainability and keep any severe 
risk isolated. 

Farther north, diurnal gains in buoyancy will be muted by cloud 
coverage and more scant moisture return northward to the Great Lakes 
region and vicinity. However, sufficient low/midlevel flow could 
encourage momentum transport capable of supporting locally damaging 
wind gusts. 

Storms across the marginal risk area will weaken during the evening 
as they spread across an area from western New York southward to the SC 

..portions of southeast co, western/southern KS, the Texas/OK 
panhandles, northern OK... 
strengthening deep ascent associated with the aforementioned speed 
maximum, including isentropic ascent related to Lee cyclogenesis, 
will support the development of convective clusters in the 
steep-lapse-rate environment across the High Plains by late 
afternoon. Convection will subsequently spread eastward along a zone 
of frontogenesis through the evening hours. A dearth of moisture 
return will greatly stunt buoyancy. However, long/looping hodographs 
amid sufficient buoyancy and the steep lapse rates may support a few 
bowing convective segments capable of isolated severe wind gusts. 

.Cohen/leitman.. 04/27/2017 


Mesoscale Discussion

acus11 kwns 270636 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 270636 

Mesoscale discussion 0570 
National Weather Service Storm Prediction Center Norman OK 
0136 am CDT Thu Apr 27 2017 

Areas affected...southeast Mississippi...western and central Alabama 

Concerning...Severe Thunderstorm Watch 167...168... 

Valid 270636z - 270830z 

The severe weather threat for Severe Thunderstorm Watch 167, 168 

Summary...a severe threat is expected to continue across parts of 
western and central Alabama. Wind damage and hail will be the 
primary threats. 

Discussion...the latest radar analysis shows a mult-segmented line 
of strong to severe thunderstorms from west of Birmingham, Alabama 
southwestward into southern Mississippi. This line was located just 
ahead of an axis moderate instability with the rap estimating MLCAPE 
values in the 500 to 1500 j/kg range. This combined with large-scale 
ascent associated with an upper-level trough in the lower 
Mississippi Valley will continue to support the line for several 
more hours. The WSR-88D vwp at Birmingham shows strong deep-layer 
shear (0-6 km shear near 60 kt). This combined with surface winds 
from the south-southeast ahead of the line will continue to support 
organized bowing line segments with wind damage and hail potential. 
The more intense updrafts may also rotate and have a potential for 
isolated large hail. Model forecasts gradually veer the winds to the 
south-southwest at the surface across southwest and central Alabama 
later tonight suggesting that the severe threat should become 
marginal. The corridor with the greatest severe threat should exist 
to the south of Birmingham where the combination of shear and 
instability is most favorable. 

.Broyles/Edwards.. 04/27/2017 

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


Latitude...Lon 33428763 32258901 31478963 30808930 31068761 31968650 
33258622 33428763