U.S. Severe Weather Forecast (Convective Outlook)

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 
south-central/southeast Texas. 

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 
most areas. 

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. 

.Edwards/Peters.. 02/20/2017 


Mesoscale Discussion

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). 

.Peters/Edwards.. 02/20/2017 

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


Latitude...Lon 27779771 28939625 28509582 28049661 27599703 26999714 
25949696 25789738 25979818 26519794 27099768 27389768