- Day Three
acus01 kwns 201257
Storm Prediction Center ac 201255
Day 1 convective outlook
National Weather Service Storm Prediction Center Norman OK
0655 am CST Mon Feb 20 2017
Valid 201300z - 211200z
..there is a marginal risk of severe thunderstorms from southeast
Texas across central and southern Louisiana...
Thunderstorms will move across parts of the Upper Texas coastal
plain and central and southern Louisiana through tomorrow morning.
Sporadic, isolated damaging gusts are possible and a brief tornado
or two cannot be ruled out.
A high-amplitude synoptic pattern is present over the central and
eastern U.S., With a progressive ridge from the eastern Gulf to
Hudson Bay, and upstream trough arching from southern ab down the
High Plains to northern and central mx. By the end of the period,
the ridge should extend from Cuba to the Carolinas and northern qc,
while the trough curves from northwestern on across WI and the
Ozarks to southeast Texas and east-central/south-central mx. A basal
shortwave perturbation - now evident in moisture-channel imagery
from the South Plains and lower Pecos River area southward across
northern mx, will proceed east-northeastward to near a
fyv-ggg-gls-bro line by 12z.
At the surface, a weak cold front -- now extending from the eastern
Dakotas across northwestern Kansas to northeastern nm -- should move to
Lake Superior, the Ozarks, and East Texas by 12z. Influence of this
boundary on sensible convective threat will be minimal, however,
given the presence of a well-defined, strongly baroclinic,
convectively generated, outflow/convergence boundary to its east.
That boundary -- located initially over western AR, northwestern la
and East Texas -- will proceed slowly eastward across AR, la and the
remainder of southeast Texas through the period, as additional
convection develops primarily along and behind it.
..southeast Texas/la outlook area...
Earlier bands of strong to severe thunderstorms over OK and East Texas
have weakened considerably. Still, farther south, the convective
boundary is being reinforced and forced eastward by a line of
strong/sporadically severe thunderstorms from the hou Metro
south-southwestward across the coastal waters to the bro area.
Overall net motion of the convection across the outlook area should
be slow, given the strongly meridional component of flow aloft.
However, favorably strong deep/speed shear will aid in occasional
organization of segments of the band into small-scale, accelerating
lewp/Bow structures capable of locally damaging gusts and embedded
bookend/mesovortices. Such structures may develop preferentially
where the line of thunderstorms intercepts a prior outflow boundary,
as has been observed the past several hours over
Effective-shear magnitudes of 45-55 kt are present in forecast
soundings ahead of the primary convective plume through most of the
period, as the basal mid/upper-level shortwave trough and associated
height gradient shift eastward. Irregular diurnal heating beneath
patches of anvil cirrus will destabilize the foregoing air mass
gradually through early afternoon. This, combined with mid-60s to
near 70 f surface dew points, should offset weak mid/upper-level
lapse rates enough to boost MLCAPE into the 800-1500 j/kg range over
With time tonight, the convective regime will move farther eastward
across eastern la into a progressively less-favorable/lower-Theta-E
air mass across northeast la and MS. As this occurs, overall
convective organization and severe potential should lessen further.
acus11 kwns 201032
sels mesoscale discussion
Storm Prediction Center mesoscale discussion 201032
Mesoscale discussion 0197
National Weather Service Storm Prediction Center Norman OK
0432 am CST Mon Feb 20 2017
Areas affected...deep south Texas
Concerning...severe potential...watch unlikely
Valid 201032z - 201330z
Probability of watch issuance...20 percent
Summary...locally strong wind gusts and/or a brief tornado will be
possible through 12-14z across deep south Texas to portions of the
middle Texas coast. Given expected sparse coverage of the overall
severe-weather threat, a watch is not anticipated into the early
morning as storms move toward the Texas Gulf Coast.
Discussion...another forward-propagating mesoscale convective system has developed across
deep south Texas early this morning (since about 08z per
radar/satellite trends), with the band of storms extending from Live
Oak and Jim Wells counties to the lower Rio Grande River/
international border in Hidalgo County and into northern tamaulipas
Mexico. Surface analysis showed a convective outflow boundary
extending northeast from northwest Kenedy County to just inland of
the middle Texas coast, and then north-northeast to the ongoing mesoscale convective system
moving through East Texas. The environment south of this boundary
from the middle Texas coast to deep south Texas remains moderately
unstable with strong effective bulk shear vectors oriented parallel
to the surface boundary and the north-south oriented band of storms
in deep south Texas.
These storms appear to be located along the leading edge of forcing
for ascent and height falls attendant to the large-scale trough
moving east from the Texas Big Bend region and northern Mexico.
This forcing has weakened the cap across deep south Texas allowing
for the storms to form within the moderate instability. Given deep
meridional flow across the discussion area, propagation of the band
of storms will remain slow toward the east, while some portions of
the band/qlcs track to the northeast at 25-30 kt. Locally strong
wind gusts will be possible, while effective srh exceeding 200 m2/s2
suggest a tornado cannot be ruled out (especially with mesovortices
within the qlcs).
..please see www.Spc.NOAA.Gov for graphic product...
Latitude...Lon 27779771 28939625 28509582 28049661 27599703 26999714
25949696 25789738 25979818 26519794 27099768 27389768