U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 181249 
Storm Prediction Center ac 181247 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0747 am CDT sun Mar 18 2018 

Valid 181300z - 191200z 

..there is a slight risk of severe thunderstorms from north-central 
Texas and the Red River area to southwestern Georgia... 

..there is a marginal risk of severe thunderstorms surrounding the 
slight risk and extending to central and southern Kansas... 

Afternoon thunderstorms with very large, damaging hail and a tornado 
risk are possible over North Texas and far southern Oklahoma. 
Severe gusts and isolated large hail may occur from there through 
parts of the southeast. Isolated severe storms also may form this 
evening and tonight over central and southern Kansas and parts of 

A complex, generally progressive upper-air pattern with lowering 
amplitude on the synoptic scale is expected through the period, as 
the western-U.S. Cyclone breaks down into several distinct 
shortwaves, each moving different directions. The most relevant of 
those, for this forecast, is evident in moisture-channel imagery 
over Arizona. This perturbation should assume negative tilt by 00z, when 
it is located from central Colorado to eastern nm, with a 500-mb low 
forming in a vorticity maximum near the southern Sangre de Cristos. 
By 12z, a compact cyclone should form around that low near the 
eastern Kansas/OK border, with associated shortwave trough extending to 
northeast Texas. 

At the surface, troughing with weak/embedded lows, now over 
northern/eastern co, will consolidate into a strong cyclone over 
east-central/southeastern Colorado by 00z. This cyclone then should pivot 
southeastward across southwestern/south-central KS, becoming nearly 
stacked under the 500-mb low between ict-cfv by 12z. A dryline 
initially just west of a drt-sww line will shift eastward to The 
Hill Country and areas between abi-Special Weather Statement today. Meanwhile a nearly 
stationary frontal zone across northern la and north-central/ 
northwest Texas will merge gradually with a marine frontal zone now 
located over south-central/southeast TX, south of which dew points 
upper 60s to near 70 f are common. The combined frontal zone will 
be the primary focus for the severe threat this afternoon/evening, 
east of its intersection with the dryline. 

..north-central TX, Red River region to arklatex... 
A few supercells are expected to develop this afternoon near the 
triple point and along/east of the dryline, right-moving generally 
eastward (with isolated northeastward left-splitters also possible). 
All forms of severe are possible. Multi-County swaths of damaging 
hail have occurred here with similar scenarios during recent years, 
and such potential exists again today. 

Near and south of the warm front, steep low/middle-level lapse 
rates, and adequate water content with mean mixing ratios 11-13 g/kg 
occupying the inflow layer, will combine with areas of diabatic 
surface heating to yield MLCAPE 1500-2500 j/kg, locally near 3000 
j/kg. SBCAPE will drop sharply with northward extent away from the 
warm front, near the Red River, though elevated cape appears 
sufficient to sustain some threat into extreme southern OK. A 
substantial portion of the buoyancy, as well as mesocyclonic 
augmentation to lift aloft, will reside in optimal hail-growth 
layers. Accordingly, and in this case justifiably, hail-model and 
analog applications to forecast soundings each suggest 
significant/2-inch hail is probable, and hailstones of 3+ inches in 
diameter possible, with any mature, relatively discrete supercells. 
Lower local and higher srh/0-1-km shear in the frontal zone between 
I-20 and the Red River also indicate at least slight tornado 
potential, though still-uncertain small-scale factors (such as 
storm-spacing/interference considerations and downdraft 
characteristics) will regulate the longevity and intensity of 
tornado risk as hodographs enlarge late afternoon into evening. 
Some of the activity should weaken as it moves eastward over and 
part the arklatex region this evening, however potential exists for 
some upscale growth and movement into the rest of the slight-risk 
area as discussed below. 

If diagnostic/mesoscale trends, as well as subsequent progs using 
morning radiosonde observation data, continue to affirm the relatively tight 
afternoon/early-evening focus that a consensus of overnight progs 
have exhibited, a corridor of greater hail probabilities may be 
needed in a succeeding outlook. 

... Mississippi Valley and southeast... 
widely scattered to scattered thunderstorms in clusters will offer 
the risk of damaging gusts and isolated large hail through tonight. 

The convective potential on this corridor appears rather 
poorly/broadly focused and inconsistently forecast by assorted 
models -- which is not surprising given the broad/diffuse nature of 
the baroclinicity, likelihood of weak cinh in much of the area, and 
subtle nature of the large-scale support aloft. Still, a few areas 
of thunderstorms with severe wind/hail potential are expected to 
form through the period, initially in association with the area of 
upward vertical velocity contributing to the precip/convection now over portions of 
MS/la. Mesoscale to localized boundaries -- including outflows, 
differential-heating zones and sea breezes -- also may aid 
development. Patchwork fields of maximized diabatic 
destabilization/buoyancy are expected to develop under thin/open 
areas in the cloud cover, while the northward shift of the 
Gulf/marine air contributes to growing Theta-E. Additional air-mass 
recovery is expected behind the effects of precip swath as well, 
late this afternoon into evening, in the form of warm/moist 

Lapse rates aloft and buoyancy will be on the low side east of the 
Mississippi River, with MLCAPE under 1000 j/kg in most areas except 
for portions of southern MS/al/la where somewhat larger midlevel 
lapse rates overlie 60s to near 70 f surface dew points. 40-50 kt 
effective-shear vectors suggest some potential for supercell and/or 
bowing organization. Tonight, one or two clusters of thunderstorms 
evolving from Southern Plains convection may enter the region as 
well, also with a risk of severe gusts and isolated large hail. 

..KS/northern OK... 
Scattered thunderstorms may develop late this afternoon or early 
evening over parts of southwestern Kansas and/or the OK/northern Texas 
panhandles and move northeastward, offering the potential for hail. 
Strong-severe gusts also are possible, though relative lack of both 
buoyancy and mid/upper flow (under the difluent region of the 
shortwave perturbation), along with a nocturnal stable layer near 
the surface, indicate the wind risk is too conditional/isolated for 
more than marginal probabilities at this time. The convective 
potential will be supported by a narrow, southeast/northwest- 
oriented corridor of at least marginally favorable buoyancy wrapping 
into the sector near, east and southeast of the surface low, along 
with strong low-level convergence and maximized mid/upper-level lift 
ahead of the shortwave trough. 

The risk further southeast into central and eastern OK appears more 
uncertain and conditional, though any sustained convection this 
evening and tonight likewise will pose a risk of severe hail/gusts. 

.Edwards/Mosier.. 03/18/2018 


Mesoscale Discussion

acus11 kwns 180135 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 180135 

Mesoscale discussion 0132 
National Weather Service Storm Prediction Center Norman OK 
0835 PM CDT Sat Mar 17 2018 

Areas affected...east central Texas 

Concerning...Severe Thunderstorm Watch 9... 

Valid 180135z - 180230z 

The severe weather threat for Severe Thunderstorm Watch 9 continues. 

Summary...isolated damaging wind and large hail remain possible over 
east central Texas through 04z, but overall trend should be for 
storms to weaken after 03z. 

Discussion...as of mid evening, storms persist over the eastern part 
of central Texas from east of kileen to west of Lufkin. The strongest 
storms are developing along an outflow boundary interacting with a 
modest southerly low-level jet. A couple of supercell structures 
continue to be observed with large hail the main threat. With onset 
of nocturnal cooling, updrafts should become increasingly elevated 
with a gradual weakening trend expected during the next couple of 

.Dial.. 03/18/2018 

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


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