Severe Storm Outlook Narrative (AC)
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Issued by NWS
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931 ACUS01 KWNS 081933 SWODY1 SPC AC 081932 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0232 PM CDT Wed May 08 2024 Valid 082000Z - 091200Z ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS FROM THE MISSOURI OZARKS EASTWARD INTO SOUTH-CENTRAL KENTUCKY AND MIDDLE TENNESSEE... ...SUMMARY... Severe thunderstorms are likely from parts of the mid Mississippi, Ohio, and Tennessee Valleys into the southern Plains. All severe hazards, including tornadoes, very large to giant hail, and potentially significant damaging winds are possible. Some tornadoes may be strong. ...20z Update... The ongoing forecast remains on track and no changes have been made to the outlook with the 20z update. A broad area of severe thunderstorm and tornado potential remains in place from parts of the southern Plains to the OH/TN Valleys and the Carolinas. For details regarding short term severe concerns see valid Watches 203-208 and Mesoscale Discussions 701, 702, and 704. ..Leitman.. 05/08/2024 .PREV DISCUSSION... /ISSUED 1145 AM CDT Wed May 08 2024/ ...Ozarks to Mid Mississippi, lower Ohio and Tennessee Valleys... A complex, partially mesoscale-driven forecast is apparent for the Ozarks eastward into the lower OH/TN Valleys. A morning thunderstorm complex over TN has draped an outflow boundary --western portion of it advancing northward-- across the lower TN/OH Valleys. A very moist/unstable airmass is located along and south of the boundary across the MS Valley, with surface dewpoints in the upper 60s to lower 70s contributing to a very unstable airmass (2500-4000 J/kg MLCAPE). A strong belt of southwesterly 500-mb flow extending through the base of a mid-upper level low over the north-central High Plains, will remain overhead from the southern Great Plains east-northeast into the Mid South/OH Valley. Ongoing severe storm cluster late this morning will likely expand in convective coverage over eastern MO into western TN/KY through the mid afternoon. Supercells potentially capable of tornadoes, a few of which may be strong, and large to very large hail and significant severe gusts are possible with this activity. Have adjusted (lowered) severe probabilities on the northern periphery of the risk area given expected concentration of thunderstorms and associated severe to favor the corridor in the vicinity of the boundary. Eventual upscale growth is expected later today into tonight as this activity moves east-southeast with an accompanying swath of wind damage. ...Eastern OK to AR, Mid-South and central TX... Thunderstorms should develop by mid-late afternoon near the front and dryline, as a combination of lift along those boundaries and strong surface heating combine with very rich low-level moisture to erode the cap. Once storms initiate, rapid evolution to supercells is expected. Forecast soundings in the OK/AR/MO border area show very large CAPE and elongated hodographs. Large to giant hail and tornadoes will be the primary hazards early in the convective life cycle. Have extended the 10-percent significant tornado probabilities into far southwest MO, northwest AR, and far eastern OK. There is some signal for upscale growth to occur this evening across AR with an MCS moving east across the Mid South. Have adjusted severe-wind probabilities farther south to account for this potential scenario. Farther southwest, lower storm coverage is expected across the TX/Arklatex. However, steep lapse rates and 70s dewpoints will contribute to extreme MLCAPE. Although near-surface flow generally will be 10 kt or less, limiting lowest-km hodographs/shear, effective-shear magnitudes in the 40-55-kt range indicate supercells will be possible. These will be capable of very large/destructive hail exceeding 3 inches in diameter. Clusters or upscale mergers of convection also may offer deep, precip-loaded, hail-cooled downdrafts with locally severe gusts. Cell mergers and interactions with boundaries will factor into tornado potential on the storm scale, since the environmental low-level shear appears on the margins. ...Southern Appalachians/Cumberland Plateau into Carolinas... Ongoing strong to severe thunderstorm cluster over the eastern TN will likely continue to develop to the east-southeast during the afternoon. The airmass will continue to destabilize downstream across the western Carolinas. Scattered strong to severe gusts (55-70 mph) will be capable of wind damage. A few of the stronger cells may also pose a large hail threat. Additional storms are possible east over the Piedmont and into portions of the coastal plain this afternoon into the early evening. An attendant severe risk may accompany the stronger storms. Later tonight, an MCS or several smaller-scale bows are forecast to move east-southeast along the instability gradient forecast to remain draped across the Mid South. Moist/unstable conditions will support a continued risk for damaging gusts moving into northern AL/southern Middle TN into northern GA late tonight. ...Northeastern CONUS... Widely scattered to scattered thunderstorms may develop this afternoon over parts of eastern NY and New England. A diurnally destabilizing airmass will become weakly unstable (500-1000 J/kg MLCAPE) along/ahead of a prefrontal surface trough. Ample deep-layer shear attendant to a Northeast U.S. shortwave trough, will act to organize updrafts. A mix of multicell and modest supercell characteristics are forecast before activity moves over more-stable air and/or offshore, and weakens by early evening. Isolated hail/damaging gusts are the primary expected hazards. ...Eastern SD to eastern IA and vicinity... Widely scattered to scattered thunderstorms are possible from early afternoon in western parts to evening closer to the Mississippi River, with isolated damaging gusts, large hail and possibly a tornado or two. Activity should form in an area of deep-layer lift (including low-level convergence/mass response) related to a vorticity lobe in the southeastern part of the mid/upper-level cyclone. Associated cooling aloft will combine with residual low-level moisture (dewpoints mid 40s to mid 50s F) to yield pockets of around 500-1000 J/kg MLCAPE. Backed low-level winds will aid convergence and storm-relative low-level flow, and may contribute to locally enlarged hodographs. $$