U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 181955 
Storm Prediction Center ac 181953 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0253 PM CDT sun Mar 18 2018 

Valid 182000z - 191200z 

..there is a slight risk of severe thunderstorms from parts of 
north-central Texas east-southeastward across the arklatex...Gulf 
Coast states and southern Georgia... 

..there is a marginal risk of severe thunderstorms from parts of 
the southern and Central Plains east-southeastward into the 
arklatex...lower Mississippi Valley and southeast... 

Thunderstorms with large hail and a tornado risk are possible 
especially across north and East Texas late this afternoon into 
evening, with additional severe thunderstorms possible across other 
parts of the south-Central Plains to southeast states. 

Several changes have been made to the outlook for this issuance. The 
first change is to remove the slight risk area from near the Red 
River in northern Texas/southern Oklahoma extending eastward across 
most of Arkansas into north-central Mississippi. Instability is very 
weak along this corridor and the stronger instability is forecast to 
remain to the south. The second change to the outlook is to expand 
the slight risk area westward into north-central Texas where cumulus 
is developing along an axis of moderate instability. The current 
thinking is that surface-based convective initiation will take place 
just to the west of the Dallas/Fort Worth Metro, see mesoscale discussion 0133. 
Moderate instability and strong deep-layer shear will support a 
conditional threat for supercell formation and hailstones greater 
than 2 inches in diameter. The tornado and hail probabilities have 
been realigned along the corridor with the greatest potential for 
supercell development. The final change is to remove the marginal 
from parts of north-central Kansas into southeast Kansas, northern 
Arkansas and northern Mississippi where instability is not expected 
to be strong enough to support marginal severe threat. 

.Broyles.. 03/18/2018 

Previous discussion... /issued 1156 am CDT sun Mar 18 2018/ 

..North/East Texas and far southern Oklahoma to arklatex... 
The potential exists for locally intense storm development later 
this afternoon/early evening, but a number of questions remain 
regarding the likelihood and locations/coverage of deep convective 
development. Along these lines, various convection-allowing models 
have shown semi-dramatic sub-regional spatial shifts between 00z and 
12z of preferred severe corridors later today, while various hrrr 
runs have also exhibited considerable run-to-run variability 
(supercells vs. Essentially no development) this morning. 

Scattered showers and some thunderstorms along with multi-layer 
cloud cover remain prevalent across the region at midday, related to 
east/northeastward-transitioning southern-stream forcing for ascent 
embedded with a strong (100+ kt at 250 mb) subtropical jet stream. 
In the wake of this scattered precipitation/thicker clouds, cloud 
breaks and some additional low-level moist influx should allow for 
relatively rapid destabilization especially across 
north-central/east-central Texas to the near/east of a dryline and 
along/south of an east/southeastward-extending front. 

But given some hindered heating through early afternoon and the 
early timing of the aforementioned southern-stream impulse, the 
likelihood and extent of deep convective development later today 
remains questionable, particularly given that areas such as 
north-central Texas may be influenced by subsidence aloft during 
peak heating. Accordingly, the overall scenario seemingly remains 
largely conditional, with the notion that any sustained 
surface-rooted development could evolve into supercells capable of 
very large hail and some tornado risk in the presence of steep 
mid-level lapse rates, moderate buoyancy, and favorable hodographs 
for large diameter (potentially significant 2+ inch) hail. 

Further outlook refinements/adjustments can be expected with the 20z 

..lower Mississippi Valley to southeast states... 
A weak impulse or two (and/or mcv) should semi-focus thunderstorm 
clusters across the region today, such as is occurring across 
southern Mississippi at midday. While buoyancy will not be overly 
strong, 40+ kt effective shear could support some bowing segments 
and possibly even a transient supercell or two. Damaging winds and 
possibly some hail should be the primary concerns. 

..portions of Texas Panhandle/western Oklahoma into Kansas... 
At least some severe-weather potential should exist later this 
afternoon through early evening with anticipated low-topped storm 
development across the region. Very limited moisture will exist 
within/north of a narrowing warm sector, but steep lapse rates and 
moderately strong wind profiles could support some strong to severe 
storms across the Texas Panhandle/northwest Oklahoma northeastward 
into southern Kansas. 


Mesoscale Discussion

acus11 kwns 182328 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 182327 

Mesoscale discussion 0136 
National Weather Service Storm Prediction Center Norman OK 
0627 PM CDT sun Mar 18 2018 

Areas affected...eastern Oklahoma and Texas panhandles and western 

Concerning...severe potential...watch unlikely 

Valid 182327z - 190200z 

Probability of watch issuance...5 percent 

Summary...a few thunderstorms might become capable of producing 
strong wind gusts and hail this evening from the eastern Texas and 
Oklahoma panhandles into far western OK. Overall threat appears too 
marginal for a ww. 

Discussion...early this evening a narrow corridor of marginal 
instability (500 j/kg mucape) exists over the eastern Texas and OK 
panhandles east of a dryline supported by the advection of steep 
mid-level lapse rates above modest low-level moisture. High based 
shallow convection is developing from southwest Kansas into the 
north-central Texas Panhandle within zone of ascent accompanying a 
progressive shortwave trough. As this band of ascent continues east 
and interacts with the more unstable environment east of dryline, 
additional, slightly more robust updrafts might develop. Some of 
this activity might become capable of producing hail and a few 
strong wind gusts, but the thermodynamic environment appears too 
marginal for a more substantial severe threat. 

.Dial/Thompson.. 03/18/2018 

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


Latitude...Lon 34999992 35050048 35820083 36920078 37099991 36319954 
35339956 34999992