- Day Three
acus01 kwns 271257
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
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.
acus11 kwns 270636
sels mesoscale discussion
Storm Prediction Center mesoscale discussion 270636
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
Summary...a severe threat is expected to continue across parts of
western and central Alabama. Wind damage and hail will be the
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.
..please see www.Spc.NOAA.Gov for graphic product...
Latitude...Lon 33428763 32258901 31478963 30808930 31068761 31968650