- Day Three
acus01 kwns 211946
Storm Prediction Center ac 211944
Day 1 convective outlook
National Weather Service Storm Prediction Center Norman OK
0144 PM CST sun Jan 21 2018
Valid 212000z - 221200z
..there is a slight risk of severe thunderstorms from East Texas to
..there is a marginal risk of severe thunderstorms surrounding the
slight risk from southeast Texas to MO...
Severe storms are expected from parts of East Texas/Oklahoma into
Arkansas and northwest Louisiana into tonight. A few tornadoes,
damaging winds, and isolated large hail will be possible.
Changes are not warranted to 1630z outlook.
Large-scale forcing for ascent is quickly spreading across the High
Plains and shallow convection is now developing from north-central
Texas into eastern OK. Earlier thoughts regarding the evolution of
thunderstorms into the evening hours remain.
Refer mesoscale discussion #32 for more information regarding the short-term threat
across this region.
Previous discussion... /issued 1028 am CST sun Jan 21 2018/
..East Texas/OK to AR/la...
Overall minor changes to the slight and marginal risks in this region.
Considered an upgrade to enh centered on Texarkana but continued
concerns over the degree of buoyancy and potential weakness in
forecast hodographs around 700 mb suggest the tornado threat may not
yet warrant greater probabilities.
12z soundings sampled mean mixing ratios of 10-12 g/kg within the
modifying western Gulf air mass that is characterized by surface dew
points of 60-65 f. These dew points should spread as far north as
eastern OK and western/southern Arkansas during the afternoon/evening.
Abundant low cloud cover will limit diabatic surface heating within
the moist sector. However, low-level convergence will intensify
along the dryline during the late afternoon as mid-level height
falls associated with the southern rockies shortwave trough
overspread the boundary. This should yield storms gradually
increasing along/ahead of the dryline across eastern OK into
Forecast soundings show modest low/middle-level lapse rates across
much of the moist sector. Nearly saturated low-level thermodynamic
profiles should result in predominant MLCAPE values of 300-800 j/kg.
Low-level and deep-layer shear will strengthen tonight and become
increasingly favorable for updraft rotation, however some forecast
soundings indicate weaknesses in the hodograph around 700 mb. Given
the positive tilt of the mid/upper wave, the related geometry of
winds aloft should lead to a substantial component of mean flow
parallel to the developing convective corridor. These factors should
contribute to a mixed Mode of semi-discrete cells transitioning to a
largely quasi-linear Mode. Damaging winds should be the primary
hazard, but a few tornadoes are possible, particularly centered from
northeast Texas into southeast OK and southwest Arkansas between 00-06z where
the combination of buoyancy/shear should be relatively maximized.
A broken band of storms will likely expand/develop slowly eastward
towards southern MO through the Sabine valley tonight. The severe
threat should diminish towards the lower MS valley during the early
morning as the warm sector Narrows from north to south ahead of the
convective band, though isolated damaging wind potential may persist
Relative to areas farther south, colder midlevel temperatures will
offset weaker boundary-layer Theta-E to maintain weak buoyancy for
storms, amid an intense warm-advection conveyor. Forecast soundings
suggest MLCAPE will be meager, below 500 j/kg along and south of the
warm front in the 00-06z time frame. A shallow, near-surface stable
layer reinforced by sporadic precip should temper downdraft strength
somewhat. Still, convective downward momentum Transfer and related
isolated damaging winds may penetrate that layer. Low wind
probabilities have been maintained in this regime of favorably
strong deep-layer lift and weak buoyancy immediately preceding the
ejecting low-level cyclone.
acus11 kwns 211914
sels mesoscale discussion
Storm Prediction Center mesoscale discussion 211913
Mesoscale discussion 0032
National Weather Service Storm Prediction Center Norman OK
0113 PM CST sun Jan 21 2018
Areas affected...northeast Texas...eastern Oklahoma through western
Concerning...severe potential...Tornado Watch likely
Valid 211913z - 212115z
Probability of watch issuance...80 percent
Summary...thunderstorms are expected to develop by 21z across
northeast Texas into eastern Oklahoma and subsequently increase in
coverage and intensity. Supercells and bowing line segments with
damaging wind and a few tornadoes should be the primary threats. A
Tornado Watch will probably be needed before 21z.
Discussion...this afternoon a dryline extends from northeast Kansas
through west-central OK to north-central and southwest Texas. Pacific
front will merge with the dryline later this afternoon, and this
boundary will continue through the Southern Plains this evening as a
surface low consolidates over eastern Kansas in association with a
progressive shortwave trough. Modifying Continental-polar air with
low 60s f dewpoints has advected through the warm sector beneath
modest lapse rates where objective analysis shows a corridor of
500-800 j/kg MLCAPE from central and northeast Texas into southeast OK.
WV imagery indicates mid-upper level moistening and clouds resulting
from a band of deeper forcing for ascent moving through eastern OK
and northeast TX, and scattered showers are already developing over
northeast Texas in association with this process. Rap soundings
indicate a capping inversion still in place around 750 mb, but
ascent in the 850-500 mb layer layer will eventually erode the
inversion, contributing to thunderstorm development by 20-21z.
Vertical wind shear will increase as the low-level jet strengthens
within exit region of the approaching mid-upper jet with large 0-1
km hodographs and effective-bulk shear exceeding 40 kt. This
environment should support organized storms including a few discrete
supercells and bowing line segments as storms intensify later this
afternoon and evening.
..please see www.Spc.NOAA.Gov for graphic product...
Latitude...Lon 30929680 34139629 36149588 36619432 35819334 33839335