Administrators NorthGeorgiaWX Posted July 12 Administrators Posted July 12 SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0755 AM CDT Fri Jul 12 2024 Valid 121300Z - 131200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE OZARKS TO LOWER OHIO VALLEY...AND OVER THE NORTHERN AND CENTRAL GREAT PLAINS... ...SUMMARY... Isolated severe wind and hail will be possible this afternoon from the Ozarks to lower Ohio Valley, and over the northern and central Great Plains from late afternoon into tonight. ...Synopsis... Deamplification of the northern-stream pattern is forecast in mid/ upper levels, as a persistent trough over the Great Lakes and vicinity moves eastward and weakens substantially. A height weakness will continue to extend from that area southwestward across the mid Mississippi Valley, Arklatex, and south TX, with several embedded/slow-moving vorticity lobes. Meanwhile, a broad area of difluent, zonal to northwesterly flow should continue over the central/northern Plains region, northeast of a prominent anticyclone shifting slowly eastward across the Great Basin and Four Corners regions. At the surface, the 1Z analysis showed weak lows over northeastern MT and southwestern SD, along a quasistationary to slow-moving warm front drawn from southern AB to western SD, central/southeastern NE, central MO, and southern IL. The slightly better-defined MT low should move slowly eastward to northwestern ND by early evening, while the frontal zone to its southeast drifts eastward and northward over central SD, eastern NE, northeastern KS, and central/ northern MO. A lee trough -- initially drawn from western SD to eastern CO and the TX Panhandle -- should meander erratically near its present position today, with some adjustment by falling pressures related to strong surface heating. ...Northern/central Plains... Isolated to widely scattered thunderstorms are moving southeastward over portions of southeastern SD to central N this morning, in a zone of weak DCVA aloft superimposed on a broader area of low-level warm/moist advection above the surface. While an isolated, marginally severe hail report cannot be ruled out in another couple hours, the overall decreasing trend should continue as the LLJ likewise weakens through the remainder of the morning. Isolated to scattered, high-based thunderstorms are expected to develop over western parts of the outlook area near the lee trough today, within a broad area of strong diurnal heating. Isolated development also may occur farther east near the warm front and greater low-level moisture, though weaker lift and stronger MLCINH are expected in that corridor than near the lee trough. Activity west of the front will move atop a deeply well-mixed boundary layer suitable for strong-severe downdrafts. Surface dewpoints mixing well down into the 40s to mid 50s will result in decreasing MLCAPE as MLCINH also weakens, with around 500-1000 J/kg expected in the most-probable area for storm formation. Buoyancy will increase eastward as mixing weakens, but capping strengthens, with a corridor of 60s F dewpoints and 2000-3000 J/kg MLCAPE along and just east of the front. Considerable uncertainty exists how much of the lee- trough activity can aggregate cold pools and potentially utilize forced ascent eastward into the greater moisture, for longer and better organization. Outside the lee trough, forcing for convective-scale lift appears nebulous and poorly focused across the region, with considerable uncertainty remaining on preferred potential corridor(s) for greater severe concentration. One possibility is closer to a northern- stream mid/upper trough north of the international border, but still with uncertainty on the southern extent into the CONUS of any resulting MCS. These uncertainties are reflected in a wide range of convective distributions and intensities across the various convection-allowing guidance, as well as implied from precip fields in the deterministic synoptic models and their ensembles. As such, a broad marginal area is maintained for this outlook cycle, with embedded upgrade(s) possible if mesoscale trends and 12Z/later guidance come into clearer focus. ...Ozarks to lower Ohio Valley... Isolated to widely scattered, non-severe thunderstorms are ongoing over parts of southern MO, northwestern AR and extreme eastern OK, in a field of low-level WAA and weak DCVA. A related increase in convective coverage and intensity is possible farther east and southeast across portions of this region as the foregoing boundary layer diurnally destabilizes, with marginally severe gusts or hail possible in the strongest cells. One of the vorticity lobes embedded in the height weakness aloft, over eastern KS to central/eastern MO, is being enhanced by an MCV from prior/overnight convection now over southwestern MO. This feature will move slowly eastward today and may contribute to relatively maximized thunderstorm coverage in an increasingly unstable airmass from the Ozarks to lower Ohio Valley this afternoon. Rich low-level moisture (surface dewpoints commonly upper 60s to low 70s F) and diurnal heating should offset modest mid/upper-level lapse rates to yield peak MLCAPE in the 2000-3000 J/kg range. Weak low/middle-level flow and modest shear should limit organization enough that severe potential should remain isolated and poorly organized. ..Edwards/Broyles.. 07/12/2024 Read more View the full article Quote
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