SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0746 AM CDT Thu Oct 22 2020

Valid 221300Z - 231200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
EASTERN IOWA AND SMALL PARTS OF ADJOINING ILLINOIS AND WISCONSIN...

...SUMMARY...
Severe thunderstorms with hail and damaging gusts are possible this
afternoon and early evening over parts of eastern Iowa and small
parts of adjoining Illinois and Wisconsin.  A tornado threat may
develop as well.

...Synopsis...
In mid/upper levels, broadly cyclonic flow over the western CONUS
and High Plains will shift eastward to cover most of the Plains
States and Mississippi Valley through the period.  The dominant
shortwave involved is evident in moisture-channel imagery over the
Yellowstone region of MT/WY/ID.  This perturbation should pivot
eastward by 12Z tomorrow to western MN, southeastern SD and
central/southwestern NE.  A small but pronounced southern-stream
perturbation over western NM is forecast to reach the eastern NM
line with TX by 00Z, then southwestern OK and northwest TX near the
end of the period.

At the surface, 11Z analysis depicted a wavy, quasistationary front
from southern New England to northern IN, becoming a warm front
across central IL, northern MO and northeastern/west-central KS, to
a combined lee-side/frontal-wave low over southeastern CO.  A
dryline was drawn from that low across the western OK Panhandle and
extreme eastern NM, to the TX Big Bend region.  The low should split
into two components:
1.  The western one shifting southward across the south-central High
Plains in step with the propagation of the intersection of the cold
front and lee trough.
2.  A frontal-wave rippling northeastward across northern KS and
southeastern NE today, with a closed low forming over IA this
afternoon.

The eastern low should move northeastward across WI, Lake Michigan,
northern Lower Michigan, and northwestern Lake Huron overnight.  The
warm-frontal zone to its east likewise will cross the eastern upper
Great Lakes and the lower Great Lakes.  By 00Z, the cold front
should extend from the low across northeastern KS and the OK
Panhandle.  By 12Z, the cold front should reach northern/western IL,
central/southwestern MO, southern OK, the Permian Basin area of west
TX, and south-central NM.  The front will overtake the dryline from
north-south.

...IA and vicinity...
Scattered thunderstorms should form this afternoon along and behind
the cold front, and near the surface low, offering a threat for
locally damaging to severe gusts, sporadic hail, and a conditional
tornado threat.

Elevated/morning convection across this region should move away
through midday, behind which the warm sector and its 60s F surface
dew points -- now over portions of KS/MO -- will spread
northeastward across the outlook area.  Convection will become
probable this afternoon as deep lift increases (including direct
frontal forcing and cyclogenesis-related low-level mass
convergence), and the warm sector destabilizes from a combination of
diurnal heating and theta-e advection.  A deep troposphere (EL above
the 200-mb level) also will contribute to favorably large buoyancy,
despite a lack of optimal lapse rates aloft, with preconvective
warm-sector MLCAPE values reaching the 1000-2000 J/kg,
locally/briefly higher.

Strengthening mid/upper winds and relatively backed low-level flow
near the low and warm front will yield supercell-favorable
effective-shear magnitudes of 35-45 kt and effective SRH in the
250-500 J/kg range, based on forecast soundings.  However, with
forcing ramping up quickly, largely front-parallel flow aloft, and
CINH being substantially weakened over much of the area essentially
at once, relatively discrete storm types may be short-lived. 
Sustained supercell evolution/longevity is in question.  The
forecast parameter space in a small area near and east of the low
otherwise would favor a greater unconditional tornado probability,
if there were: 1) more evidence for favorable storm mode, and 2)
less potential for the cold front to undercut convection forming
along it.  Related mesoscale trends will be monitored through the
day.

Otherwise, thunderstorms are expected to develop this evening and
tonight along and behind the cold front as far south as eastern OK,
with isolated, marginally severe hail possible.

...West TX...
Isolated to widely scattered thunderstorms may develop late this
afternoon near the dryline, west of the Caprock, and move eastward
off the Caprock while offering isolated severe gusts/hail. 
Large-scale ascent related to approaching southern/stream shortwave
trough, combined with an antecedent EML, will contribute favorable
midlevel lapse rates.  Strong diurnal heating and the related 
well-mixed boundary layer will minimize MLCINH for a couple hours
across the outlook area, contributing to peak MLCAPE in the
1500-2000 J/kg range.  Despite a lack of stronger mid/upper-level
flow, directional shear will be pronounced, contributing to 30-40-kt
effective-shear magnitudes.  Multicells, with transient supercell
characteristics, will be possible.  Nocturnal cooling/stabilization
of the boundary layer beneath the EML will lead to weakening of the
convective/severe threat eastward across the Low Rolling Plains this
evening.

..Edwards/Leitman.. 10/22/2020

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