U.S. Severe Weather Forecast (Sääkatsaus)

Day Three

acus01 kwns 280556 
Storm Prediction Center ac 280555 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1255 am CDT Fri Apr 28 2017 

Valid 281200z - 291200z 

..there is an enhanced risk of severe thunderstorms across parts of 
far North Texas...Oklahoma...southern Missouri...northern 
Arkansas...southern Illinois...southern Indiana...western 
Kentucky...and western Tennessee... 

..there is a slight risk of severe thunderstorms surrounding the 
enhanced risk area...and including portions of southwest Ohio and 
central Kentucky... 

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

Numerous severe storms will be possible on Friday from a portion of 
the Southern Plains into the lower to middle Mississippi, Ohio and 
Tennessee valleys. 

An upper jet (evident in water vapor imagery across the California 
vicinity) continues nosing southeastward with time on the back side 
of a large western U.S. Trough. This will result in digging -- and 
eventual closed low development within the trough over The Four 
Corners states. In response, downstream ridging is forecast to 
evolve over southeast portions of the country. 

At the surface, a roughly west-to-east baroclinic zone lying across 
the Gulf Coast states and westward into Texas at the start of the 
period will advance gradually northward with time. The front will 
shift toward/into the Ohio Valley area later in the period, becoming 
quasi-stationary through the second half of the period. The front 
will focus the main zone of convective/severe potential from this 
afternoon through the end of the period. 

..Ohio and Tennessee valleys west-southwest to Oklahoma and North 
potential for widespread severe weather is evident across the region 
later today and tonight, though the forecast remains 
complex/somewhat uncertain at this time due to fairly substantial 
spatial differences regarding the zone of greatest convective 

Generally speaking, a broad area of warm/moist advection will be 
underway at the start of the period -- and will continue through 
Saturday morning, from the central and Southern Plains east across 
the Ohio Valley area, ahead of the large/digging western U.S. 
Trough. Convection should be at a minimum early in the period, but 
should increase during the afternoon as diurnal heating in 
conjunction with boundary-layer moistening near and south of the 
surface front results in moderate destabilization. While capping 
may largely hinder storm development, operational models and some 
cam runs develop isolated to scattered storms across the 
Tennessee/lower and mid Ohio Valley area by mid afternoon. With the 
degree of instability anticipated, and shear profiles which 
veer/increase favorably with height, potential for supercells -- and 
associated risks for very large hail and locally damaging winds as 
well as a couple of tornadoes -- is evident. However, 
aforementioned questions regarding if, and how many, storms develop 
within the warm sector suggest initiating only 5% tornado 
probability at this time. If signs become apparent that more 
numerous afternoon warm sector storms will develop, greater risk for 
tornadoes would warrant upgraded probability in later outlooks. 

Storm coverage is forecast to increase substantially during the 
evening and overnight from the Oklahoma vicinity east-northeast into 
the Ohio Valley -- with convection primarily elevated within 
isentropic ascent/warm advection north of the primary synoptic 
front. Supercell wind profiles above the inversion layer and steep 
lapse rates throughout the troposphere would support 
intense/rotating storms, with very large hail the primary risk. 
Again -- the zone of greatest convective coverage remains uncertain, 
with the NAM (both the operational run and high-res cam output) 
remain generally farther north with convection than other models. 
As such, adjustment in the enh area and associated higher 
probabilities may be required in later forecasts. In any case, 
widespread storms should continue through the overnight hours. 

.Goss/cohen.. 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... 


Latitude...Lon 32168591 33188449 33648383 33488240 32468383 31528580 
31398641 31838642 32168591