U.S. Severe Weather Forecast (Convective Outlook)

هذا اليوم
Day Three

acus01 kwns 281239 
Storm Prediction Center ac 281237 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0637 am CST Tue Feb 28 2017 

Valid 281300z - 011200z 

..there is an enhanced risk of severe thunderstorms late this 
afternoon into tonight from northern Arkansas northeastward across 
parts of Missouri...Illinois...and Indiana... 

..there is a slight risk of severe thunderstorms surrounding the 
enhanced risk area...from the Arkansas-la-tex to the Ohio Valley... 

..there is a marginal risk of severe thunderstorms immediately 
surrounding the slight risk area... 

Severe thunderstorms are expected to develop from portions of the 
lower Mississippi Valley to the Ohio Valley through tonight. Large 
hail, damaging winds, and a few strong tornadoes are possible. 

A large-scale mid-upper trough over the Great Basin and lower Colorado 
River valley will progress eastward to the MS valley by the end of 
the period. An associated surface cyclone will develop 
northeastward from the MO/Iowa border to lower Michigan overnight, as a cold 
front moves eastward from the plains to the MS valley. Ahead of the 
front, a relatively broad/unstable warm sector is expected from 
parts of the middle MS and lower Ohio valleys southward, where 
vertical shear will increase with time as the mid-upper trough 
approaches from the west. Several rounds of strong/severe storms 
may occur across a broad area, with all severe storm hazards 
possible this afternoon through tonight, though uncertainty persists 
with warm sector development through this evening. 

..northern Arkansas/southern MO to the lower Ohio Valley this evening... 
To the southwest of ongoing elevated storms over northern MS and 
western TN, rich low-level moisture (boundary layer dewpoints of 
65-70 f) is spreading northward from East Texas/la toward eastern 
OK/AR. Though a subtropical cirrus plume may tend to limit surface 
heating in some areas, afternoon temperatures well into the 70s 
along the moist axis will drive MLCAPE values upward to 1500-2500 
j/kg. Simultaneously, vertical shear will become increasingly 
favorable for supercells in the warm sector, with effective bulk 
shear at or above 70 kt, and effective srh of 300-400 m2/s2 expected 
by this evening. 

The primary uncertainty for diurnal convection in the open warm 
sector will be a focus for initiation. Only a weak cap is expected 
this afternoon over the warm sector given soundings modified for 
upper 70s and upper 60s dewpoints, and the primary source of weak 
ascent during the afternoon will be low-level warm advection. 
However, uncertainty remains substantial, as represented by 
inconsistency among multiple convection-allowing model forecasts. 
If storms can form in this regime across central/northern Arkansas and 
southern MO during the afternoon/evening, they will likely become 
primarily discrete supercells and move rapidly northeastward at 
50-60 kt across southeast MO toward southern Illinois. The combination of 
vertical shear, buoyancy, and low-level moisture will conditionally 
support a risk for strong tornadoes, while isolated very large hail 
will also be possible. 

..east of the surface cyclone into Illinois this afternoon/evening... 
A few near-surface-based storms could form this afternoon and 
persist into this evening immediately east of the surface cyclone, 
along the warm front into central/northern Illinois as boundary layer 
dewpoints increase to near 60 f beneath steep midlevel lapse rates. 
Cloud breaks will dictate the degree of surface heating into 
northeast MO, with potentially moderate buoyancy (mlcape 1000-1500 
j/kg) along the north edge of the warm sector into central/northern 
Illinois. If storms form within the north edge of the warm sector, strong 
deep-layer and low-level shear will favor supercells with all severe 
hazards possible. 

..MO/Arkansas to the Ohio Valley overnight... 
As the surface cyclone develops northeastward to the Great Lakes, 
the surface cold front will progress eastward to The Ark-la-miss and 
lower Ohio Valley by Thursday morning. One or more bands of 
convection are likely along or just ahead of the front tonight into 
Wednesday morning, with an accompanying risk for damaging winds and 
hail. The wind and tornado risk will be maximized with embedded 
bowing segments and/or supercell structures within or ahead of the 
convective band/front, in an environment with strong vertical shear 
and sufficient low-level moisture/buoyancy. There will also be some 
risk for more discrete storm development farther south into western 
and central Arkansas late tonight. 

.Thompson/Kerr.. 02/28/2017 


Mesoscale Discussion

acus11 kwns 280810 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 280810 

Mesoscale discussion 0217 
National Weather Service Storm Prediction Center Norman OK 
0210 am CST Tue Feb 28 2017 

Areas affected...parts of southeastern Arkansas...northern/central 
MS...northwestern la 

Concerning...severe potential...watch unlikely 

Valid 280810z - 281045z 

Probability of watch issuance...20 percent 

Summary...isolated to widely scattered strong storm development is 
possible through the 09-12z time frame, accompanied by a risk for 
severe hail. The need for a watch is not currently anticipated, but 
trends will be monitored. 

Discussion...in the presence of rising mid-level (500 mb) heights 
and capping associated with warm elevated mixed layer air (based 
near 700 mb), the initiation of widespread deep convective 
development still seems unlikely. However, model guidance, in 
general, indicates an increase in thunderstorm activity is possible 
through the 09-12z time frame, as far south as the Mississippi Delta 
Region. This appears mostly in response to a forecast strengthening 
of southerly low-level flow (40-50+ kt at 850 mb), in a corridor 
near/east of the Mississippi River. This may be accompanied by 
sufficient strengthening of ascent associated with warm advection to 
overcome inhibition. 

If/when storms form, environment profiles appear conducive to at 
least some severe weather potential. Activity likely will be based 
above a residual stable near surface layer, but steep mid-level 
lapse rates and relatively cool mid/upper levels may be supportive 
of the risk for severe hail in the strongest storms, aided by 
strong, largely unidirectional shear in the cloud bearing layer. 

.Kerr/Thompson.. 02/28/2017 

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


Latitude...Lon 33289229 34629157 34848945 34538884 33248812 32728974 
32539129 33289229