Convective Outlook

SPC 1300Z Day 1 Outlook
Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0752 AM CDT Wed May 22 2019

Valid 221300Z - 231200Z


The greatest potential for severe thunderstorms is this afternoon
into evening, from western Missouri to southeastern Kansas and
northeastern Oklahoma.  Tornadoes, large/damaging hail and severe
gusts all are possible.

The period begins with a large area of mid/upper-level cyclonic flow
covering most of the central/western CONUS, anchored by embedded
lows initially evident in moisture-channel imagery over the north-
central Plains and the Great Basin.  The eastern of the two lows is
occluded in low levels and nearly stacked, and is forecast to weaken
while ejecting northeastward toward Lake Superior through 12Z. 
Meanwhile the western low should pivot slowly around the southern/
eastern Great Basin, while assuming less-positive tilt.  In between,
shortwave ridging and height rises will build over the
southern/central Plains. 

At the surface, 11Z analysis showed the surface manifestation of the
eastern mid/upper cyclone, with a low over southeastern SD and
occluded front southeastward to the EVV area.  A wavy warm front was
drawn from there across southern KY and eastern TN, to southern NC
and northern SC.  This warm front will move northward across the
Ohio Valley to the lower Great Lakes through the period.  A
stationary front extended from the triple point southwestward over
the Mid-South, becoming a warm front across north-central to central
TX.  This boundary will become diffuse and spread northward over OK
and AR.  A secondary cold front extended from the low across eastern
KS and central/western OK. This boundary should stall then retreat
northward over OK today, ultimately merging with the remains of the
front to its south.  The combined front should become better-defined
this afternoon and evening across southeastern KS and northern OK. 
A forecast veering wind shift behind the front -- related to the
departure of the deep-tropospheric low -- will make it more
convergent this afternoon, while warm-sector theta-e advection
contributes to strengthening the associated baroclinic gradient. 

...Southern Plains...
Scattered to numerous thunderstorms are expected to develop
gradually from mid afternoon through this evening along the
strengthening frontal zone over the southeastern KS/southwestern MO
and northeastern OK region.  A more-isolated/conditional threat is
evident farther west and late afternoon into evening along the front
over north-central OK, and southward into the warm sector this
afternoon in central/eastern OK.  Any sustained, relatively discrete
convection could become supercellular and offer the risk of
tornadoes, large to very large hail, and severe gusts.  Isolated
strong-severe convection may also develop late overnight near the
dryline as it retreats across the south-central High Plains. 

Even with background height rises and lack of substantial mid/upper
forcing, intensifying thermodynamic support from below will
contribute to a favorable warm-sector parameter space this
afternoon.  As the boundary layer recovers northward through OK and
around the western Ozarks vicinity, the air mass will destabilize
from a combination of moist/warm advection and diabatic surface
heating.  Given the reservoir of available moisture upstream, the
return-flow regime should become characterized by surface dew points
upper 60s to low 70s F, mean mixing ratios strengthening to the
16-18 g/kg range, and PW 1.5-2 inches.  This process will reduce
MLCINH to negligible levels by early/mid afternoon, in a northward-
shifting way, with MLCAPE reaching the 2500-4000 J/kg range.  The
LLJ will help to maintain large hodographs with effective SRH above
200 J/kg, amidst 50-60-kt effective-shear magnitude.

The more-certain area for convection, and ultimately greatest
cumulative concentration, should be with the corridor of low-level
frontogenetic forcing and convergence, where severe probabilities
remain greatest.  Farther south across the warm sector, CAM progs of
gradual convective/supercell genesis are uncertain but possible. 
Based on character and motion of shallower/pre-supercellular
elements in high-res guidance, any such development appears related
to initial elevated convection in the warm-advection regime becoming
surface-based, as the boundary layer moistens, deepens and
destabilizes beneath, 0-3-km lapse rates steepen, and CINH erodes. 
However, increasing boundary-layer theta-e may be offset somewhat,
as other progs (e.g., NAM, RAP) also show warming near 700 mb
related to poleward advection of northern portions of a residual
Mexican EML.  Special afternoon sounding releases from SGF/OUN are
planned to sample this airmass evolution diagnostically, given that
the absence of substantial boundaries south of the front indicates
crucial processes could occur mainly above the surface.  

...Mid Mississippi Valley to WI...
Additional strong-severe convection should develop near the front
northeastward across the mid Mississippi Valley, and perhaps as far
as southeastern WI/northern IL.  This threat will peak late
afternoon into early evening, as the warm sector destabilizes and
moistens, also with vertical shear supporting supercell and
organized multicell potential.  A nearly separate regime of
destabilization, related less to low-level theta-e advection and
more to cooling aloft ahead of the ejecting/occluded low,
conditionally may support isolated/brief/marginal severe farther
north across parts of eastern/northern WI this afternoon. 

...OH and vicinity...
Isolated to widely scattered thunderstorms are possible near the
warm front and southward into the warm sector this afternoon, near a
diurnally destabilizing theta-e axis.  The strongest cells may
produce marginally severe hail and gusts.  Low-level hodographs and
deep shear will be favorable for organized multicells or even
isolated supercells, especially near the warm front where near-
surface flow is most backed.  However, the lack of substantial lapse
rates aloft and of deep-layer forcing casts convective
maintenance/coverage into doubt.

..Edwards/Leitman.. 05/22/2019

Read more

Updated 22 May 2019 | 8:00 am