Apr 26, 2017 1300 UTC Day 1 Convective Outlook
Updated: Wed Apr 26 12:57:24 UTC 2017 (20170426 1300Z Day 1 shapefile | 20170426 1300Z Day 1 KML)
Probabilistic to Categorical Outlook Conversion Table
Public Severe Weather Outlook

The SPC is forecasting ...Severe thunderstorms expected over parts of the lower Mississippi Valley later today and tonight....   Please read the latest public statement about this event.  View What is a Watch? clip.

Categorical Graphic
20170426 1300 UTC Day 1 Outlook Graphic
Day 1 Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
MODERATE 22,696 1,228,007 Little Rock, AR...North Little Rock, AR...Pine Bluff, AR...Hot Springs, AR...Texarkana, TX...
ENHANCED 78,559 4,819,734 Memphis, TN...Shreveport, LA...Tyler, TX...Longview, TX...Bossier City, LA...
SLIGHT 126,639 8,404,534 Birmingham, AL...Garland, TX...Jackson, MS...Mesquite, TX...Clarksville, TN...
MARGINAL 121,390 24,107,516 Dallas, TX...Indianapolis, IN...Fort Worth, TX...Nashville, TN...Arlington, TX...
Probabilistic Tornado Graphic
20170426 1300 UTC Day 1 Tornado Probabilities Graphic
Probability of a tornado within 25 miles of a point.
Hatched Area: 10% or greater probability of EF2 - EF5 tornadoes within 25 miles of a point.
Day 1 Tornado Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
SIG SEVERE 34,204 1,822,183 Little Rock, AR...North Little Rock, AR...Pine Bluff, AR...Conway, AR...Hot Springs, AR...
15 % 22,561 1,223,811 Little Rock, AR...North Little Rock, AR...Pine Bluff, AR...Hot Springs, AR...Texarkana, TX...
10 % 29,435 1,993,005 Memphis, TN...Shreveport, LA...Longview, TX...Bossier City, LA...Monroe, LA...
5 % 78,876 4,843,051 Jackson, MS...Tyler, TX...Jackson, TN...Jonesboro, AR...Alexandria, LA...
2 % 88,372 5,107,526 Baton Rouge, LA...Lafayette, LA...Tuscaloosa, AL...Lake Charles, LA...Decatur, AL...
Probabilistic Damaging Wind Graphic
20170426 1300 UTC Day 1 Damaging Wind Probabilities Graphic
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.) Area Pop. Some Larger Population Centers in Risk Area
30 % 101,058 6,021,377 Memphis, TN...Shreveport, LA...Little Rock, AR...Tyler, TX...Longview, TX...
15 % 126,734 8,339,914 Birmingham, AL...Garland, TX...Jackson, MS...Mesquite, TX...Clarksville, TN...
5 % 122,252 24,291,025 Dallas, TX...Indianapolis, IN...Fort Worth, TX...Nashville, TN...Arlington, TX...
Probabilistic Large Hail Graphic
20170426 1300 UTC Day 1 Large Hail Probabilities Graphic
Probability of hail 1" or larger within 25 miles of a point.
Hatched Area: 10% or greater probability of hail 2" or larger within 25 miles of a point.
Day 1 Hail Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
SIG SEVERE 46,662 2,870,435 Shreveport, LA...Little Rock, AR...Tyler, TX...Longview, TX...Bossier City, LA...
30 % 29,008 1,623,490 Shreveport, LA...Longview, TX...Bossier City, LA...Pine Bluff, AR...Monroe, LA...
15 % 131,007 8,404,639 Memphis, TN...Garland, TX...Little Rock, AR...Jackson, MS...Mesquite, TX...
5 % 96,690 13,813,300 Dallas, TX...Fort Worth, TX...Arlington, TX...St. Louis, MO...Plano, 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
   MISSISSIPPI...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   ENHANCED RISK...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   SLIGHT RISK  AND EXTENDING INTO NORTHERN PARTS OF EASTERN ILLINOIS
   AND WESTERN INDIANA...

   ...SUMMARY...
   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
   overnight hours.

   ...Synopsis...
   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
   magnitudes. 

   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
   southwest and
   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.

   ..Edwards/Mosier.. 04/26/2017

   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z