- 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
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.
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.
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
..please see www.Spc.NOAA.Gov for graphic product...
Latitude...Lon 31169751 31459650 31779565 31449514 30989541 30569655