U.S. Severe Weather Forecast (Convective Outlook)

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Day Three

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Storm Prediction Center ac 261257 


Day 1 convective outlook 
National Weather Service 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... 


... 
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. 


... 
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 Texas -- is forecast to pivot eastward across Texas 
and OK through the day, reaching southeastern KS, eastern OK, and 
northeast through south-central Texas by 00z. Overnight, this feature 
will weaken and eject northeastward to Illinois and western Tennessee. 
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 Texas. 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 Texas coast. By 
12z, the front should extend from western lower Michigan across in, 
central KY, middle TN, northern al, southeastern MS, southeastern 
la, and the northeastern Gulf. 


..East Texas to mid-south... 
A band of thunderstorms currently over parts of southeastern OK and 
western Arkansas 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 Storm Prediction Center 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 Texas 
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 Alabama and middle Tennessee 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 Alabama/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 Arkansas. 
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 


$$ 




Mesoscale Discussion


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Storm Prediction Center mesoscale discussion 261439 
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Mesoscale discussion 0560 
National Weather Service Storm Prediction Center Norman OK 
0939 am CDT Wed Apr 26 2017 


Areas affected...southeastern MO...much of central and eastern 
Arkansas...far northwestern la...northeast Texas 


Concerning...severe potential...Tornado Watch likely 


Valid 261439z - 261515z 


Probability of watch issuance...95 percent 


Summary...a Tornado Watch will be issued shortly for much of central 
and eastern Arkansas into northeast Texas. An isolated severe threat will 
also extend into southeast MO. 


Discussion...the air mass over Arkansas will continue to destabilize over 
the next few hours. While the risk for tornadoes appears limited in 
the near term, the increasing instability/strong shear will be 
favorable for storm-scale rotation and an increasing risk for 
low-level mesocyclones capable of tornadoes. The greatest risk for 
strong tornadoes will probably develop later across southern Arkansas 
coincident with peak heating during the afternoon and evening hours. 


.Smith/Hart.. 04/26/2017 


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


Attention...WFO...pah...Meg...lsx...lzk...sgf...shv...fwd... 


Latitude...Lon 33149539 37039181 37328987 36768944 35689016 32609280 
32279423 32439514 33149539