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Source: US National Oceanic and Atmospheric Administration

SPC AC 271953

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0253 PM CDT Fri Sep 27 2019

Valid 272000Z – 281200Z

…THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM NORTHEAST
SOUTHEAST IOWA…NORTHEAST MISSOURI INTO NORTHERN ILLINOIS…

…SUMMARY…
Scattered to numerous severe thunderstorms are expected late this
afternoon into the evening from northern Illinois, southern Iowa,
northern Missouri and eastern Kansas. The primary threats are large
hail and damaging wind, but a few tornadoes remain possible.

…Midwest through central Kansas…

Destabilization is occurring from KS, northern MO into southern IA
in wake of a large MCS ongoing from northern IL into lower MI. As of
mid-afternoon, objective analysis indicates a corridor of 2000-2500
J/kg MLCAPE from KS into northern MO and southern IA, but a warm
elevated mixed layer has contributed to a cap in most of the warm
sector. Water Vapor and objective analysis data show a weak,
low-amplitude shortwave trough moving through the central Plains.
Deeper forcing for ascent attending this feature will interact with
the frontal zone, and thunderstorms should develop along and north
of the front later this afternoon and evening from KS into northern
MO and southern IA and spread east into northern IL. Effective-bulk
shear from 35-45 kt within the frontal zone should support a few
supercells with large hail as the initial storm mode. Many of these
storms will be elevated above the stable surface layer, but any
storms developing along the leading edge of the front will be near
surface based. Tendency has been for low-level winds to veer to
southwesterly in pre-frontal warm sector, reducing the size of
low-level hodographs, but 0-1 km storm relative helicity from
150-200 m2/s2 remains sufficient for low-level mesocyclones with any
surface-based supercells. Largest low-level hodographs exist on the
cool side of an outflow boundary over northern IL where winds remain
backed to southeasterly. Some surface-layer recovery is expected,
mainly along the western periphery of this boundary, supporting the
best isolated tornado threat near and just east of the southeast IA
/ IL border. Otherwise, tendency should be for storms to eventually
congeal into clusters/lines in post-frontal zone during the evening.
Reference swomcd 2026 for more information.

..Dial.. 09/27/2019

.PREV DISCUSSION… /ISSUED 1130 AM CDT Fri Sep 27 2019/

…KS to Lower MI…
Elevated convection is ongoing within a low-level warm advection
regime across the Mid-MO Valley and northern IL. Robust
boundary-layer heating is underway in the wake of this morning
activity beneath an elevated mixed layer that is attempting to
advect northeast from the central Great Plains/Lower MO Valley. In
conjunction with surface dew points increasing into the upper 60s to
lower 70s surface dew points, a strongly unstable air mass with
MLCAPE of 2500-3500 J/kg should develop ahead of a
southwest-northeast cold front from eastern KS to the IA/IL/MO
border area.

Despite near-neutral mid-level height tendency, guidance is fairly
consistent with the presence of a mid-level speed max developing
across northeast KS this afternoon and reaching the IA/IL border
area this evening. While the bulk of initial convective development
may be atop a more stable boundary-layer from central IA to southern
WI, sufficient convergence should exist for storm development along
the front and into the warm sector from its intersection with the
differential heating boundary in northern IL to the dryline
intersection in south-central KS. The greatest convective coverage
should reside near the IA/IL/MO border region with more scattered
coverage back into eastern KS.

Low to deep-layer wind profiles will favor potential for several
supercells during the early evening which will probably consolidate
into multiple organized clusters through late evening. Large hail
will be the most probable hazard with scattered severe wind gusts
expected as well. Given rich boundary-layer moisture and an enlarged
low-level hodograph, certainly a tornado risk will exist with any
supercells just ahead of the front. 12Z HREF guidance and related
CAMs are indicative of higher-end tornado coverage. However, with
cyclolysis expected, the overall synoptic pattern should be a
limiting factor. Still, given the potential for at least a few
tornadoes, will double-upgrade from less than 2 percent to a 5
percent area with this outlook.

…Southeast NM and west TX…
A low-amplitude shortwave trough near the AZ/Sonora border should
approach the TX Panhandle. Scattered storms should develop this
evening into the overnight. Modest buoyancy, the bulk of which
should remain below the strong speed shear from the mid to upper
levels, will probably marginalize the overall severe hail and wind
risk.

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