U.S. Severe Weather Forecast (Convective Outlook)

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Storm Prediction Center ac 271255 


Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0755 am CDT Thu Apr 27 2017 


Valid 271300z - 281200z 


..there is a marginal risk of severe thunderstorms from parts of 
Alabama and Georgia to parts of the Great Lakes... 


..there is a marginal risk of severe thunderstorms from 
southeastern Colorado across parts of Kansas...northwestern 
Oklahoma...and parts of the Oklahoma and Texas panhandles... 


... 
Isolated severe wind gusts may accompany thunderstorms from portions 
of the Great Lakes region to the southeast states today into this 
evening, and across portions of the central and southern Great 
Plains late this afternoon into this evening. 


... 
In mid/upper levels, mean troughing will persist across the 
west-central U.S., As one leading perturbation ejects away, and 
others reinforce the associated cyclonic flow and height deficit. 
That leading perturbation -- currently located from a vorticity Max 
over northern Illinois southward across MS -- will move to WV, Ohio and 
eastern lower Michigan by 00z, phasing with a 500-mb low now over the 
northwest angle region of Minnesota. Thereafter, it will eject 
northeastward into Canada, as synoptic-scale amplification occurs to 
the troughing over The Rockies and High Plains. 


At the surface, the 11z analysis showed a low over the western shore 
of Lake Michigan between mke-grb, with cold front to southern in, 
middle TN, southeastern MS, and the northwestern Gulf. The low will 
eject northeastward across James Bay through tonight. By 00z the 
cold front is expected to reach western NY, WV, the Tennessee/NC border 
region, and southern al, becoming diffuse and quasistationary over 
the northwestern Gulf. By 12z the cold front should extend from the 
Hudson Valley region across western NC, losing definition farther 
southwest. Meanwhile, cyclogenesis will occur today over the 
southeastern Colorado and northern Texas/OK panhandles region, at the 
intersection of a frontogenetic zone and a Lee trough that extends 
from there northwestward to central/southwestern Montana. The resulting 
frontal-wave low should migrate to the Guy-hhf corridor by 00z, then 
east-northeastward to south-central/southeastern Kansas overnight, with 
a cold front southwestward across the Texas South Plains region by 12z. 


..Great Lakes to GA/al... 
Widely scattered thunderstorms in broken bands and small clusters 
are expected to develop through afternoon along and ahead of the 
surface cold front, across the western rim of the outlook area, 
before moving eastward to northeastward. The main concern will be 
isolated damaging gusts. 


An ongoing, broken/ragged band of non-severe convection and precip 
from eastern Kentucky across eastern TN, northern/western Georgia and parts of 
southern Alabama still may be capable of an isolated damaging gust -- 
especially along the southern end where rich low-level moisture 
still characterizes the low-level inflow region. Clouds and precip 
associated with this activity will hinder destabilization across 
affected areas and downstream in mid/upper levels, into the central 
Appalachians and upper Ohio Valley. Still, a combination of patchy 
surface heating and low-level warm/moist advection will yield areas 
of at least marginally favorable buoyancy in the prefrontal boundary 
layer. 


Meanwhile, large-scale ascent/cooling preceding the ejecting 
shortwave trough will aid in destabilization over northern parts of 
the outlook area, generally from the central Appalachians to the 
Ohio Valley and Great Lakes, partly offsetting the weaker low-level 
Theta-E expected there. Meanwhile, though the large-scale upper 
support is lifting away from the area, favorable low-level Theta-E 
will persist across the Carolinas/Georgia portion of the outlook, along 
with favorable deep/speed shear to support a few organized 
multicells, line segments, and small bows, with transient supercell 
structures also possible. Isolated severe gusts of 50+ kt may occur 
with convection in this corridor, primarily from midday through 
afternoon, along with other convective gusts that are subsevere but 
capable of damage to trees and utilities. 


The severe threat will not be uniform across this corridor. For 
now, smaller-scale uncertainties within the broad marginal-risk area 
preclude a more concentrated area of greater unconditional severe 
probabilities; however, a 15%/slight-risk equivalent area may be 
added during the day as mesoscale trends warrant. 


..south-Central Plains region... 
Aforementioned amplification of the synoptic trough, and the passage 
of several associated lower-amplitude shortwaves/vorticity maxima, 
will lead to a combination of strengthening deep-layer ascent and 
cooling aloft through several related processes. Those include: 
low-level warm advection, low/middle-level frontogenetic/ 
cyclogenetic forcing, lift beneath the left-exit region of the 
110-130-kt upper-jet core, and passing shots of DCVA. In sum, these 
should act on residual (but still sufficient) moisture to lead to 
afternoon, surface-based, high-based thunderstorm development over 
the High Plains of Colorado into parts of the Texas/OK panhandles and 
southwestern Kansas. Activity should move rapidly eastward across this 
region through early evening, when it will encounter progressively 
more stable near-surface inflow air and weaken. 


Forecast soundings suggest steep low/middle-level lapse rates and 
weak cinh, even for temps in the 60s f and dew points in the 30s, 
with MLCAPE reaching the 300-500 j/kg range. Though lack of 
moisture precludes a more-substantial, better-organized severe 
threat, the presence of strong flow and well-mixed subcloud layers 
supports the potential for at least isolated severe gusts from any 
sustained convection that develops. 


.Edwards/Broyles.. 04/27/2017 


$$ 

Mesoscale Discussion


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Storm Prediction Center mesoscale discussion 270636 
alz000-msz000-270830- 


Mesoscale discussion 0570 
National Weather Service Storm Prediction Center Norman OK 
0136 am CDT Thu Apr 27 2017 


Areas affected...southeast Mississippi...western and central Alabama 


Concerning...Severe Thunderstorm Watch 167...168... 


Valid 270636z - 270830z 


The severe weather threat for Severe Thunderstorm Watch 167, 168 
continues. 


Summary...a severe threat is expected to continue across parts of 
western and central Alabama. Wind damage and hail will be the 
primary threats. 


Discussion...the latest radar analysis shows a mult-segmented line 
of strong to severe thunderstorms from west of Birmingham, Alabama 
southwestward into southern Mississippi. This line was located just 
ahead of an axis moderate instability with the rap estimating MLCAPE 
values in the 500 to 1500 j/kg range. This combined with large-scale 
ascent associated with an upper-level trough in the lower 
Mississippi Valley will continue to support the line for several 
more hours. The WSR-88D vwp at Birmingham shows strong deep-layer 
shear (0-6 km shear near 60 kt). This combined with surface winds 
from the south-southeast ahead of the line will continue to support 
organized bowing line segments with wind damage and hail potential. 
The more intense updrafts may also rotate and have a potential for 
isolated large hail. Model forecasts gradually veer the winds to the 
south-southwest at the surface across southwest and central Alabama 
later tonight suggesting that the severe threat should become 
marginal. The corridor with the greatest severe threat should exist 
to the south of Birmingham where the combination of shear and 
instability is most favorable. 


.Broyles/Edwards.. 04/27/2017 


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


Attention...WFO...bmx...mob...Jan...lix... 


Latitude...Lon 33428763 32258901 31478963 30808930 31068761 31968650 
33258622 33428763