Valid 091300Z – 101200Z
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
UPPER MIDWEST AND UPPER MISSISSIPPI VALLEY REGIONS…
…SUMMARY…
Severe thunderstorms appear most probable this afternoon and evening
over parts of the Upper Midwest and upper Mississippi Valley
regions.
…Synopsis…
In mid/upper levels, the CONUS subset of the longwave pattern will
amplify as mean troughing becomes more prominent in the West,
anchored by a slow-moving synoptic cyclone over the Pacific
Northwest. That feature, in turn, will remain part of a Rex block,
with the anticyclonic member located over northwestern Canada.
Meanwhile, a separate regime of ridging will amplify across the
mid/upper Mississippi Valley and upper Great Lakes regions. In
between, a leading shortwave trough — with several embedded or
closely associated vorticity maxima — is evident in moisture-
channel imagery from north-central MT across eastern WY to central
CO. This perturbation should pivot northeastward, with the basal
vorticity lobe(s) ejecting across the central Dakotas around 00Z and
reaching that part of ON near the MN border by 12Z. A low-amplitude
shortwave trough — initially located over portions of Lake Erie and
OH — will move southeastward across portions of PA, WV and MD,
reaching NJ and the Delmarva Peninsula around 00Z, before moving
offshore.
At the surface, 11Z analysis showed a wavy/quasistationary frontal
zone from southeastern VA across central NC, central GA, and
northeastern MS, becoming a diffuse, weakening warm-frontal segment
across northwestern AR and central KS. A warm front was drawn from
northern NE across portions of IA to western IL. As a broad/weak
area of low pressure shifts eastward across SD today, ahead of the
shortwave trough aloft, the warm front will move northeastward
across the upper Mississippi Valley region, modulated on the
mesoscale by areas of baroclinicity related to clouds and precip.
Meanwhile, the frontal segment across VA and NC may buckle northward
across northern NC and southern VA some more, but on broader scales
will exhibit little net movement.
…Upper Midwest and upper Mississippi Valley regions…
Isolated to scattered thunderstorms, in episodic fashion, are
possible across a broad area from eastern SD and eastern NE across
the upper Mississippi Valley this afternoon through tonight. The
more sustained/organized among this convection will pose a risk of
large hail and severe gusts, and a few tornadoes cannot be ruled
out.
Considerable uncertainties linger from the prior outlook cycle
regarding both strength of destabilization south of the warm front
(with considerable antecedent cloud cover), and timing and density
of convection in the region near the warm front. Still, potential
does appear lower across central SD and greater over parts of IA/MN.
Low-level shear will be favorable near the surface warm front,
where backed near-surface winds enlarge low-level hodographs beneath
a southwesterly LLJ that will strengthen and shift eastward through
the evening. Conditionally, the kinematic profile within and
proximally south of the warm-frontal zone (over portions of IA and
southern MN) will become favorable for supercells and at least a
slight tornado risk, in addition to associated wind and hail.
Forecast soundings suggest effective SRH 240-400 J/kg and MLCAPE
1500-2500 J/kg available to any relatively sustained convection
moving into this region, as forecast by a few CAMS and in a more
smudged/generic sense by a couple synoptic models.
…Central Plains…
Isolated to scattered thunderstorms should develop late this
afternoon and early evening in a swath extending southwestward from
the main outlook area across eastern NE and northern/western KS,
near a moisture/buoyancy axis. Well-mixed subcloud layers will
support gust potential, and isolated severe hail also may occur.
Forecast soundings suggest that strong surface heating will occur
this afternoon behind the ongoing areas of clouds/precip this
morning, combining with favorable boundary-layer moisture to boost
MLCAPE into the 1500-2000 J/kg range (locally higher) across much of
this corridor, with minimal MLCINH. Mid/upper flow and deep shear
each will be weaker than in areas farther north and northeast, with
effective-shear magnitudes generally remaining below 35 kt. The
dominant mode should be multicellular, with discrete storms and
clustering each possible. The overall severe threat should diminish
late evening into the overnight hours, in the absence of mid/upper-
level support and beneath confluent flow aloft, as the boundary
layer nocturnally stabilizes.
…NC/VA…
Widely scattered thunderstorms should develop this afternoon from
the higher terrain near the Blue Ridge of western VA, southeastward
across northeastern NC. This activity will form amidst a
combination of diurnal surface heating and related steepening of
low-level lapse rates and weakening of MLCINH, with some aid from
frontal lift. Conterminous increasing of large-scale ascent/cooling
aloft, ahead of the shortwave trough, also will nurture a favorable
convective environment. Thunderstorms should move southeastward to
east-southeastward, offering damaging gusts and isolated severe
hail.
Areas of ongoing convection from southern VA across extreme
northeastern NC, and adjoining Atlantic waters southeast of the
Hampton Roads, appear to be reinforcing the baroclinic zone across
the region. The associated instability gradient will help to focus
and confine the northeastern extent of substantial convective
potential this afternoon, while warm-sector development also may
occur amidst weak capping. Favorably rich low-level moisture (e.g.,
surface dewpoints upper 60s to mid 70s F and PW 1.5-2 inches) should
contribute to MLCAPE ranging from around 1000-1500 J/kg near the
front, where lift will be best-organized, to around 2000-3000 J/kg
over eastern NC’s lower elevations in the warm sector. Low-level
winds will be weak. Still, over the VA part of the outlook area,
backed near-surface flow and somewhat greater mid/upper winds will
foster enough vertical shear to support well-organized multicells
and/or transient/marginal supercell characteristics. Farther south
where CAPE is larger, water-loaded downdrafts may approach severe
levels.
..Edwards/Dial.. 09/09/2019
$$