U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 281636 
Storm Prediction Center ac 281635 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1135 am CDT Fri Apr 28 2017 

Valid 281630z - 291200z 

..there is an enhanced risk of severe thunderstorms OK to Tennessee 

..there is a slight risk of severe thunderstorms surrounding the 
enhanced risk... 

..there is a marginal risk of severe thunderstorms southern Great 
Plains to central Appalachians... 

Numerous severe storms are expected today, initially across parts of 
the Ohio and Tennessee valleys this afternoon, and across the 
southern Great Plains and Ozarks by tonight. Very large hail, 
tornadoes, and damaging winds are anticipated. 

Potential exists for multiple severe episodes this afternoon into 
tonight across the enhanced risk characterized by increasing 
moisture, moderate to strong buoyancy, and ample vertical shear. 
However, some of this risk is conditional and confidence is below 
average in mesoscale details regarding severe coverage. 

..TN/Ohio valleys... 
A mid-level impulse over eastern NE should decay during the period 
as an upstream shortwave trough pivots over The Four Corners into 
AZ/nm. This will effectively yield rising heights over a large 
portion of a poleward-advecting warm sector emanating from the Gulf. 
This warm sector will remain characterized by rather rich mean 
mixing ratios with upper 60s to middle 70s surface dew points 
becoming established from the Ohio Valley on south as a warm front 
passes north through this evening. 

Within the low-level warm-advection regime, elevated convection is 
ongoing from NE/Iowa into MO/IL. Additional activity may develop 
farther south/east closer to the advancing warm front towards late 
afternoon. Low-level hodographs will be comparatively large within 
this corridor in conjunction with 40-50 kt 850-mb flow. This setup 
could support mixed modes of discrete supercells and upscale-growing 
bows capable of tornadoes, large hail, and damaging winds. These 
threats will probably remain along and north of the warm front, with 
capping increasingly pronounced with southern extent in the warm 

..southern Great Plains to the Ozarks... 
Similar to areas farther east, an increasingly rich low-level air 
mass is steadily returning, but beneath a more substantial 700-mb 
cap. This should inhibit sustained storm development until about 
03z. Along the northwest periphery of the warm sector, a 
consolidating surface cyclone over northwest Texas amid near-neutral 
mid-level height change should result in strengthening frontal 
convergence and yield storm development over OK into far northwest 
OK. Storm coverage should become widespread overnight along and 
north of the quasi-stationary front eastward across the Ozarks. 

Supercell-favorable wind profiles and steep mid-level lapse rates 
should support several rotating storms with large hail (some 
significant) as the primary hazard. Damaging winds and a couple 
tornadoes will also be possible, particularly with cells that can be 
sustained along the quasi-stationary front. The overall evolution 
into a broad clustering band with embedded line segments amid an 
anafrontal setup should yield a predominant hail and some wind risk, 
continuing on an increasingly isolated basis into the overnight. 

.Grams/Mosier.. 04/28/2017 


Mesoscale Discussion

acus11 kwns 272326 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 272326 

Mesoscale discussion 0576 
National Weather Service Storm Prediction Center Norman OK 
0626 PM CDT Thu Apr 27 2017 

Areas affected...southeastern Alabama into central Georgia 

Concerning...Tornado Watch 169... 

Valid 272326z - 280030z 

The severe weather threat for Tornado Watch 169 continues. 

Summary...severe weather potential is expected to become 
increasingly negligible through 8-11 PM EDT . A new severe weather 
watch is not anticipated, and Tornado Watch 169 may be allowed to 
expire as previously scheduled at 8 PM EDT. 

Discussion...thunderstorm development persists from near Troy Alabama 
through the Columbus Georgia area into areas south of Peach Tree city GA. 
This is occurring within a lingering corridor of moderate boundary 
layer instability, east southeast of a stalling/weakening frontal 
zone. Localized enhanced surface convergence and weak lower/mid 
tropospheric warm advection may be contributing to forcing for this 
activity, and vertical shear near a 30 kt southwesterly 850 mb jet 
axis (beneath 40-50 kt westerly/cyclonic 500 mb flow) remains 
strong. However, diurnal boundary layer cooling coupled with a 
warming mid-level environment is expected to result in diminishing 
convective trends across the region through 00-03z. 

.Kerr.. 04/27/2017 

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


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