SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0756 AM CDT Fri May 20 2022
Valid 201300Z - 211200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
LOWER MICHIGAN...THE NORTHERN MID-ATLANTIC...AND A SMALL PART OF THE
SOUTHERN PLAINS...
...SUMMARY...
Severe thunderstorms are expected to be most concentrated over
portions of Lower Michigan, the northern Mid-Atlantic, and a small
part of the southern Plains.
...Synopsis...
In mid/upper levels, positively tilted mean troughing will continue
from a cyclone over Hudson Bay southwestward across the northern
Plains and northern Great Basin region. A tightening of the height
gradient aloft and commensurate increase in southwest flow is
expected from the southern Rockies to the upper Great Lakes, as
heights rise over the Eastern CONUS. Within those height rises,
however, a shortwave trough and accompanying MCV -- arising from
south-central High Plains convection a couple days ago and
persisting now across OH -- will cross PA today, before reaching the
southern NY/NJ region around 00Z, and eastward over southern New
England, Long/Block Islands, and the adjoining Atlantic tonight.
The 11Z surface analysis showed a seasonally strong cold front from
a triple point over southern WI southwestward through a low over
south-central KS, then over northwestern OK, the TX Panhandle, and
northeastern NM. A wavy warm front -- somewhat diffuse west of the
Appalachians thanks to outflow from MCV-related convection to its
south -- was drawn from that triple point across central Lower MI,
northeastern OH, northeastern WV, and northern VA. This warm front
should move northward through Lower MI today prior to cold frontal
passage, while also moving northward over southern/eastern PA and
NJ.
By 00Z the cold front should extend from Lower MI to near a line
from ORD-STL-TUL-SPS-LVS. A dryline should intersect the front over
northwest TX and extend south-southwestward to the Rio Grande
shortly upstream from DRT. By 12Z, the front should reach
northwestern OH, southern IL, north-central TX, and southeastern to
central NM, overtaking the retreating dryline over west-central TX.
Relative maxima in severe potential will occur in two nodes near the
front, over Lower MI and the Red River region, with isolated, likely
nocturnal severe potential from mostly elevated convection near the
front in between.
...Lower MI...
Scattered thunderstorms are expected to develop this afternoon --
perhaps in multiple episodes - ahead of the surface cold front, from
west of central/northern Lake Michigan across central/northern Lower
MI. The most favorable buoyancy/shear parameter space for severe
thunderstorms -- including a few supercells with a tornado threat,
damaging winds and large to significant hail -- will be across
central/northern Lower MI.
A combination of convergence near the cold front and boundary-layer
heating in the warm sector will contribute to thunderstorm
development, along with outflow/differential-heating boundaries on
the mesobeta scale. Low-level warm/moist advection will aid in
airmass recovery behind a pool of outflow produced by earlier
MCV/MCS passage to the south of the area. Surface dew points
commonly in the 60s F, diurnal heating, favorable low/middle-level
lapse rates, and a deep troposphere will contribute to MLCAPE
building into the 2000-3000 J/kg range, amidst supercell-favorable
shear. Hail models applied to, and analog data closely associated
with, these thermodynamic and wind profiles suggest any supercells
will pose a threat of 2+ inch hail; therefore, a significant-hail
area was added. Density of the tornado threat is more uncertain,
with time series of forecast soundings suggesting some veering of
near-surface winds and shrinking of hodographs right as the cap
breaks. Still, a mesobeta-scale area of greater potential may
develop, and observational trends and 12Z guidance will be monitored
for considering ENH-level probabilities in succeeding outlooks.
...Northern Mid-Atlantic...
Scattered thunderstorms -- some severe -- should develop just ahead
of the MCV and along/south of the warm front, as the MCV moves
across PA today. Damaging to severe gusts, isolated large hail, and
a few tornadoes are possible. Large-scale lift on a mesoscale
spatial scale will immediately precede the MCV, as will a corridor
of relatively maximized low-level shear and enhanced 700-500-mb
flow. These and relatively backed winds near the warm-frontal zone
will contribute to favorable vertical shear for supercell and
especially bow-echo organization.
Meanwhile, moisture/theta-e advection and diurnal heating -- behind
a swath of precursory morning clouds/convection now crossing the
region -- should destabilize the boundary layer and weaken MLCINH
sufficiently to support the severe threat. Peak preconvective
MLCAPE in the 1500-2500 J/kg range is possible in central PA,
decreasing eastward toward the Jersey Shore. However, even with
lesser buoyancy, a cold-pool-driven, compact MCS may develop and
persist eastward to near the coastline, sustained by forced ascent
of at least marginally unstable parcels on the leading edge.
...Southern OK/north TX...
Scattered strong-severe thunderstorms should develop this afternoon
and persist into evening near the front and east of the dryline,
over parts of southern OK and adjoining north TX. Severe hail/gusts
will be possible. A favorably unstable moist warm-sector boundary
layer will develop, with surface dewpoints in the upper 60s to near
70 F, and strong heating yielding a well-mixed subcloud layer
despite the moisture content. MLCAPE of 3500-4500 J/kg appears
probable, much of it in layers suitable for both hail growth and
lightning generation. Low-level and deep shear will be weak,
limiting modes to clustered, multicellular, and perhaps temporarily
cold-pool-forced in character. With weak mid/upper winds expected,
rapidly collapsing cores with considerable precip loading and severe
gusts are possible. Overall, the threat should diminish after dark,
though a few strong-severe storms may persist well into the evening
given available moisture and steep low/middle-level lapse rates away
from convectively modified air masses.
..Edwards/Kerr.. 05/20/2022
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