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Thursday, October 14, 2021

SPC Oct 14, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0748 AM CDT Thu Oct 14 2021 Valid 141300Z - 151200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM NORTHEASTERN ARKANSAS TO THE LOWER OHIO VALLEY REGION...AND SEPARATELY ACROSS PORTIONS OF OKLAHOMA... ...SUMMARY... A few strong/marginally severe thunderstorms are possible today from northeastern Arkansas to the lower Ohio Valley region, and later this evening across portions of Oklahoma. ...Synopsis... While remaining highly amplified, the pattern aloft will shift this period, primarily related to the longstanding mid/upper-level troughing now centered over the Rockies. An amplifying shortwave trough -- initially evident in moisture-channel imagery over eastern OR and northern NV -- will move southeastward to the Four Corners vicinity by 00Z. This trough then should pivot eastward across the central/southern Rockies to the adjoining High Plains by 12Z tomorrow. Downstream, a compact cyclone now centered near the northwestern corner of MN will eject roughly northward on a wobbly path toward northern MB by the end of the period. In the southern- stream southwest-flow field southeast of the major trough, a perturbation related to the remnants of Pamela aloft will eject from south TX across the Tennessee Valley and weaken considerably. The surface analysis at 11Z showed a cold front related to the ejecting MN/ND/MB cyclone, extending across portions of eastern WI, northwestern IL and central MO, becoming quasistationary southwestward to a weak frontal-wave low over the TX Panhandle. The front was preceded by a swath of multilayered clouds, precip and some embedded thunderstorms from the southern Lake Michigan area southwestward across MO to south TX. By 00Z, the cold front should reach parts of Lower MI, central IN and southern IL. The boundary should stall today and then move slowly northward as a warm front across parts of southwestern MO and southeastern KS, to a frontal-wave low over south-central KS, then to another frontal-wave low over northwest TX. By 12Z, the KS low should migrate roughly eastward over MO and southern IL, with the trailing boundary again a cold front across southwestern MO, eastern/southern OK, and west-central/southwest TX. ...AR to the lower Ohio Valley region... Widely scattered to scattered thunderstorms are expected to move northeastward across this region today, both in the existing, slowly eastward-shifting plume and developing anew near its leading edges (where best access to diurnally destabilized boundary-layer air should be). Damaging to isolated severe gusts are possible, and a marginal/brief tornado potential is apparent. A corridor of moisture advection/transport will move over the area today, sourced from the mid/upper remnants of Pamela over south TX, as well as rich moisture from the Gulf in low levels. The result should be a weakly capped boundary layer with modest but still somewhat unstable low/middle-level lapse rates, and strong upper/ anvil-level flow. Forecast soundings show effective-shear magnitudes around 30-40 kt, suggesting mainly variably organized multicells, with sporadic/transient supercell potential. Cloud cover will keep diurnal heating from yielding steep low-level lapse rates, but MLCAPE in the 500-1000 J/kg range will be supported by the abundant moisture. Elsewhere in the main precip/convective plume from south TX to Lower MI, a strong/damaging gust or brief tornado cannot be ruled out; however, with weaker lift and storm coverage south, and weak instability north, the threat is too conditional, isolated and poorly focused for a large categorical severe area. ...OK... Capping may delay substantial thunderstorm initiation and growth until after 00Z -- perhaps even as late as 03-06z. However, isolated to scattered thunderstorms this evening into tonight will offer a threat for hail and strong gusts. Marginal, weak tornado potential also is apparent along/ahead of the front, given the strengthening deep shear and available moisture. Large-scale lift should increase over the region this afternoon and tonight with the approach of the main mid/upper trough, reinforced by the digging shortwave perturbation. The returning warm sector has been altered somewhat unfavorably by extensive precip/convection upstream to the south, yet still should contain enough theta-e to support strong/isolated severe potential as it is lifted along and over the surface front. Even with a shallow/near-surface stable layer apparent in forecast soundings, MLCAPE of 500-1200 J/kg appears possible in the warm sector, along with similar values of elevated MUCAPE within about 50-100 nm behind the boundary. 500-250-mb layer winds will strengthen over this area from late afternoon into tonight, contributing to 50-60-kt effective-shear magnitudes in support of supercell potential. Limiting factors for the severe threat will include lack of greater low- and middle-level lapse rates tonight, limiting buoyancy and near-surface acceleration potential for both updrafts and downdrafts. ..Edwards/Gleason.. 10/14/2021 Read more LIVE: spc.noaa.gov