The SPC is forecasting ...Severe thunderstorms expected over parts of the lower Mississippi Valley later today and tonight....
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statement about this event. View What is a Watch? clip.
Day 1 Risk
Area (sq. mi.)
Some Larger Population Centers in Risk Area
Little Rock, AR...North Little Rock, AR...Pine Bluff, AR...Hot Springs, AR...Texarkana, TX...
Memphis, TN...Shreveport, LA...Tyler, TX...Longview, TX...Bossier City, LA...
Probability of damaging thunderstorm winds or wind gusts of 50 knots or higher within 25 miles of a point. Hatched Area: 10% of greater probability of wind gusts 65 knots or greater within 25 miles of a point.
Day 1 Wind Risk
Area (sq. mi.)
Some Larger Population Centers in Risk Area
Memphis, TN...Shreveport, LA...Little Rock, AR...Tyler, TX...Longview, TX...
SPC AC 261257
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0757 AM CDT Wed Apr 26 2017
Valid 261300Z - 271200Z
...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS FOR PARTS OF THE
ARKLATEX REGION INTO CENTRAL AND SOUTHEASTERN ARKANSAS...
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ELSEWHERE FROM
PARTS OF EAST TEXAS TO SOUTHEASTERN MISSOURI AND NORTHERN
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK AND EXTENDING INTO NORTHERN PARTS OF EASTERN ILLINOIS
AND WESTERN INDIANA...
Severe thunderstorms are expected across the Lower Mississippi
Valley. Tornadoes, large hail, and damaging winds are most probable
from northeast Texas across parts of Arkansas and northern Louisiana
through afternoon. Isolated severe thunderstorms may extend as far
north as Illinois and into the central Gulf States during the
The mid/upper-level pattern will remain characterized by a
synoptic-scale trough over portions of the Rockies and Great Plains
regions. Associated cyclonic flow aloft will spread eastward overs
the Mississippi Valley, upper Great Lakes and lower/middle Ohio
Valery through the period, traversed by several shortwaves of
variable amplitude. The primary/basal shortwave trough -- now
evident in moisture-channel imagery over the southern High Plains to
the Big Bend region of TX -- is forecast to pivot eastward across TX
and OK through the day, reaching southeastern KS, eastern OK, and
northeast through south-central TX by 00Z. Overnight, this feature
will weaken and eject northeastward to IL and western TN.
Meanwhile, a weaker perturbation approaching the Pacific Northwest
coast will move southeastward and inland through the period,
extending from the northern Rockies to near the Four Corners by 12Z.
A 150-170-kt 250-mb jet will follow this perturbation inland. Ahead
of that shortwave trough, a broad area of cold air aloft and weak
low-level moisture will support a general thunderstorm potential
over parts of the northern Rockies and Great Basin.
At the surface, a cold front extended from a surface low near LSE
across eastern IA, southwestward across MO, southeastern OK, extreme
north-central TX, and west-central TX. The surface low should
migrate erratically northeastward across WI and the northern Lake
Michigan/eastern Upper Michigan area, reaching eastern Lake Superior
or adjoining portions of ON by the end of the period. The cold
front by 00Z should extend from southeastern WI across northern/
central IL, eastern AR, western LA, to the mid/upper TX coast. By
12Z, the front should extend from western Lower MI across IN,
central KY, middle TN, northern AL, southeastern MS, southeastern
LA, and the northeastern Gulf.
...East TX to Mid-South...
A band of thunderstorms currently over parts of southeastern OK and
western AR should encounter favorably moist low-level/warm-sector
air that will destabilize through the morning, amidst strengthening
deep shear. The main hazards through the rest of the morning will
be hail and damaging gusts; however, a tornado or two cannot be
ruled out. For more near-term information, see SPC watch 161 and
related mesoscale discussions.
The warm sector is forecast to destabilize through the afternoon,
offering increasing buoyancy juxtaposed with strengthening deep
shear and favorable low-level hodographs. Surface dew points
commonly in the mid/upper 60s F and steep midlevel lapse rates will
contribute to MLCAPE 1500-2500 J/kg, amidst 45-60 kt effective-shear
Some uncertainty remains regarding density of convection within the
line and its effect on severe type/coverage. A pearl-necklace
configuration of embedded supercells/bows would support more of a
tornado risk, as has been outlooked, whereas more solid coverage
would transition the threat to mainly wind with embedded/QLCS
mesovortices offering some tornado concern. With early development
over the DFW Metroplex this morning indicating sufficient large-
scale lift for prefrontal storms, and with the potential for
line-embedded supercells moving into a favorable parameter space for
a tornado threat, will maintain the moderate-risk and enhanced-risk
equivalent tornado probabilities for now, but each shifted slightly
northward in deference to two features:
1. Afternoon surface winds -- relatively backed (southerly to
southeasterly) flow progged to occur north of I-20 and primarily
into AR, with somewhat veered winds from I-20 southward in east TX
and western LA, and
2. The outflow boundary from earlier convection draped across
north-Central AR, which may act both as a focus and northern
delimiter for substantial severe potential in that state as it
slowly retreats northward ahead of the near-frontal storms.
The severe threat with the main band of convection may extend as far
east as central to eastern AL and middle TN overnight, given forced
ascent on the convective boundary, intensifying gradient flow above
the surface related to the ejecting mid/upper perturbation, and
favorable low-level moisture. Weakening of both lapse rates and
boundary-layer instability also is forecast with eastward extent
across AL/TN/KY, accounting for the diminishing probabilities for
very late in the period.
...Mid Mississippi Valley, IL, IN...
The northern extension of the convective regime discussed above may
produce damaging gusts over parts of this region today, with
isolated hail and a conditional/marginal tornado probability as
well. Confidence remains low, and uncertainty strong, regarding
sufficient destabilization with northward extent across this area to
support more than a slight to marginal categorical gradation in
equivalent wind probabilities today, despite the strengthening
deep-layer wind fields and infusion of at least somewhat supportive
low-level moisture. The concerns mainly revolve around weak
low/middle-level lapse rates and resultant weak buoyancy, including
restrictions on diurnal destabilization imposed by
1. Direct effects of cloud cover streaming off convection farther
2. Low-level trajectories emanating at least partly from areas of
convective outflow that have surged eastward over eastern/southern
MO and northern AR.
However, the potential for an organized band of
wind-damage-producing convection to reach at least the lowest part
of the Ohio Valley appears sufficient to include that area in a 15%
outlook for now, which may need to be extended further as mesoscale/
destabilization trends warrant.
NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z