U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 280541 
Storm Prediction Center ac 280539 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1139 PM CST Mon Feb 27 2017 

Valid 281200z - 011200z 

..there is an enhanced risk of severe thunderstorms from Arkansas to 

..there is a slight risk of severe thunderstorms from eastern OK to 

..there is a marginal risk of severe thunderstorms from northeast 
Texas to western PA... 

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


Ridging appears to be responsible for delaying meaningful convection 
across the MS valley late this evening. However, low level jet is beginning 
to increase across MO/Illinois and isolated thunderstorms are evolving 
within a corridor from the bootheel of MO into central Illinois. This 
activity should gradually expand in areal coverage as warm advection 
lifts north toward the Great Lakes region. Given these trends, 
there is increasing confidence that much of the Southern 
Plains/lower MS valley will remain convection-free into the 
afternoon hours. As a result, significant boundary-layer recovery 
is expected into the Ohio Valley prior to frontal passage. 

Late evening model guidance continues to suggest a corridor of 
strong southwesterly flow aloft will extend from the southern 
rockies, into the Great Lakes with 500mb flow expected to increase 
in excess of 80kt by early afternoon across western portions of the 
severe risk area. Large-scale height falls will overspread the 
Mississippi Valley and several convective scenarios may ultimately 
evolve within a broad moistening warm sector. 

1. Warm-advection corridor (northern il): 

Strengthening low level jet over Illinois will aid northward advance of higher 
quality air mass as surface dew points rise through the 40s into the 
50s as far north as northern Illinois by late morning. Warm advection is 
expected to induce scattered strong/severe thunderstorms early in 
the period as warm front lifts northward in advance of the primary 
surface wave. Convection may be aided by a weak mid-level 
disturbance embedded within the stronger southwesterly flow. 
Initial activity will be elevated in nature and hail is the primary 
risk with these storms. 

2. Warm sector (ar/MO/il): 

28/00z sounding from oun exhibited a steep-lapse-rate environment 
with substantial low-level moisture. Strong capping and 
neutral-weak subsidence will allow this air mass to advect northeast 
such that surface dew points should rise into the lower 60s across 
much of the warm sector south of I-70. Breaks in cloud cover during 
the day should allow surface parcels to approach their convective 
temperatures from eastern OK into southwestern MO by mid-late 
afternoon. Latest thinking is isolated supercells may evolve well 
ahead of the cold front along nose of a secondary branch of low level jet that 
will strengthening across Arkansas during the day. It's not clear how 
much storm coverage will be noted across this region but 
environmental shear and increasing instability/buoyancy favor robust 
supercells. Forecast soundings suggest very large hail could 
accompany this activity and a few strong tornadoes are possible, 
especially if discrete structures evolve as it appears they may. 

3. Cold front: 

Large-scale forcing will contribute to a convectively active cold 
front by early evening as the wind shift surges into an increasingly 
moist/unstable air mass from eastern Iowa into northern MO. 
Thunderstorms are expected to mature into a strong squall line that 
will surge east across the mid MS valley into the Ohio Valley as 500mb 
flow increases to near 100kt by 01/12z. Damaging winds should be 
noted with this frontal convection and tornadoes may also be 
embedded along the line. Eastward momentum should easily allow this 
activity to spread across much of Ohio by sunrise Wednesday morning. 

.Darrow/picca.. 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