Administrators NorthGeorgiaWX Posted July 29 Administrators Posted July 29 SPC 1630Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1130 AM CDT Mon Jul 29 2024 Valid 291630Z - 301200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE NORTHERN/CENTRAL PLAINS...AND PORTIONS OF THE MIDWEST/LOWER OHIO VALLEY... ...SUMMARY... Scattered strong to severe thunderstorms may occur across parts of the northern/central Plains this afternoon and evening, and also over portions of the Midwest and Lower Ohio Valley mainly this evening through the early overnight hours. ...Northern/Central Plains into the Upper Midwest... Various 12Z observed soundings across the northern/central High Plains show shallow/limited low-level moisture in the wake of prior convection and a weak front, with PWAT values generally less than 1 inch. Still, sufficient moisture to support weak to moderate instability should be in place through this afternoon along/east of a weak surface trough near the border of ND/SD and MT/WY. Large-scale ascent should remain fairly modest in a mostly zonal mid/upper-level westerly flow regime, with a low-amplitude mid-level shortwave trough over southern Saskatchewan/Manitoba and ND expected to move eastward through the period. Another weak mid-level perturbation evident on water vapor satellite imagery late this morning should also move eastward from the northern Rockies across the adjacent High Plains through this evening. Both of these features should aid isolated to scattered thunderstorms developing across parts of ND/SD and western NE by late afternoon, generally along/east of the weak surface trough. Modest low-level flow is expected to veer to westerly and gradually strengthen with height to around 30-40 kt through mid levels. Similar values of deep-layer shear will support organized updrafts, including the potential for a few supercells. Large hail generally 1-2 inches in diameter will be possible with any sustained, discrete convection through the early evening. With a long, generally straight hodograph forecast at mid/upper levels, splitting supercells may occur. Still, overall coverage of intense thunderstorms remains somewhat uncertain given the modest large-scale ascent. The potential for convection to grow upscale this evening into one or more bowing clusters also appears rather uncertain given the weak low-level mass response forecast. If a cluster can develop, then severe/damaging winds, with some potentially up to 75 mph given rather steep low/mid-level lapse rates, would become a greater threat with time through the evening hours across central/eastern ND/SD and northern NE. An isolated severe wind threat may persist with eastward extent into southern MN and IA tonight with any cluster that can persist along an axis of greater instability. ...Midwest/Lower Ohio Valley into the Tennessee Valley/Southeast... A large area of convection from an earlier MCS across the Midwest has devolved into an area of disorganized showers and thunderstorms late this morning across the lower OH Valley vicinity. Extensive outflow from this activity has surged southward to the Ozarks to parts of western KY based on recent visible satellite and radar imagery. But, airmass recovery is already well underway across parts of central MO, with southwesterly surface/low-level winds advecting a very moist airmass northward towards the mid MS Valley and western portions of the Midwest/OH Valley. Convective evolution across these areas later today into tonight remains rather uncertain given the early-day convection. Even so, most guidance continues to remain insistent that a moderately to strongly unstable environment will become re-established across parts of eastern MO into southern/central IL/IN by early evening. If this destabilization can occur, then potential for intense thunderstorms, including supercells, and a bowing cluster could be realized. Forcing for ascent for this robust convection will likely be tied to southwesterly low-level warm/moist advection, and a mid-level vorticity maximum forecast to move eastward from WI/IL to the Great Lakes region. Sufficiently strong northwesterly mid/upper-level flow and veering winds through the column will support generally 25-35 kt of deep-layer shear and updraft organization. It appears possible that initial supercells with an isolated large hail threat may quickly transition to a linear cluster with greater severe/damaging wind potential with time this evening/tonight across the lower OH Valley and vicinity. Sufficient low-level shear should also be present to support some threat for a couple of tornadoes with any semi-discrete thunderstorms, although the mode may quickly become messy this evening with multiple mergers/interactions. Given the continued uncertainty in convective evolution later today into tonight, only small expansions have been made to the Slight Risk across this region. A broader area of isolated strong to damaging wind potential remains apparent across parts of the TN Valley into the Southeast, where moderate to strong instability and marginal deep-layer shear may support loosely organized clusters spreading generally southeastward this afternoon/evening. ...Mid-Atlantic... Modestly enhanced northwesterly mid/upper-level flow will overspread parts of the eastern Mid-Atlantic and Delmarva Peninsula on the western periphery of a mid-level cyclone that will continue to advance northward across New England today. As the boundary layer across this region warms and becomes weakly to moderately unstable, a few strong thunderstorms could develop within this regime during the afternoon. Locally moderate instability and effective shear around 30-35 kt should support some potential for isolated damaging gusts and marginally severe hail with the more robust cores. ..Gleason/Moore.. 07/29/2024 Read more View the full article Quote
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