Severe Storm Outlook Narrative (AC)
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Issued by NWS
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137 ACUS01 KWNS 041938 SWODY1 SPC AC 041936 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0236 PM CDT Thu Jul 04 2024 Valid 042000Z - 051200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE UPPER MISSISSIPPI VALLEY...AND FROM THE SOUTHERN PLAINS TO THE OHIO VALLEY... ...SUMMARY... Scattered severe thunderstorms will be possible through this evening across parts of the upper Mississippi Valley, and from the southern Plains to the lower Ohio Valley. ...20z Update... Severe probabilities have been trimmed across parts of MN/IA in-line with the current location of the surface cold front. Otherwise, only minor changes have been made to the 10 percent general thunderstorm line. Overall forecast thinking remains similar to the previous outlook. However, coverage of severe storms across parts of southern IL into the Ohio Valley (outside of WW 505) is uncertain the remainder of the period. However, a very moist and unstable airmass resides across the area. Additional storms may develop this evening/overnight along the surface front, and any other storms that can develop through evening would pose severe gust potential, precluding a downgrade in probabilities at this time. For information on short term severe potential across the Upper MS Valley and the southern Plains, reference MCDS 1534 and 1535. ..Leitman.. 07/04/2024 .PREV DISCUSSION... /ISSUED 1130 AM CDT Thu Jul 04 2024/ ...Synopsis... An upper trough over the northern/central Plains will move generally eastward today across the Upper Midwest and mid MS Valley. A separate, convectively augmented mid-level vorticity maximum is present over southern IA/northern MO late this morning. This MCV and associated convection across the OH Valley will move east-northeastward across the OH Valley/Midwest through this evening. At the surface, a weak low should develop eastward across parts of the Upper Midwest in tandem with the large-scale upper trough. Another weak surface low should also track northeastward over the OH Valley with the remnant MCV. A convectively reinforced front draped from the southern Plains northeastward into the mid MS and OH Valleys will likely serve as a focus for strong to severe thunderstorms this afternoon and evening. ...Mid Mississippi Valley into the Ohio Valley and Mid-Atlantic... Mostly elevated thunderstorms are ongoing late this morning across the lower OH Valley. The rather moist low-level airmass along/south of a front will continue to gradually destabilize through the afternoon. Although mid-level lapse rates will remain poor, moderate instability has already developed downstream of activity moving across southern IN and western KY. Additional intensification of this convection will probably occur through early afternoon across parts of central/eastern KY and perhaps southern OH ahead of the MCV. Even though stronger mid-level flow may tend to remain displaced to the north of the front, modest but sufficient deep-layer shear should be present to support multicell clustering. Scattered damaging winds occasionally reaching severe thresholds should be the main threat with this activity as it spreads eastward across the lower OH Valley this afternoon and evening. Low-level flow is forecast to remain generally modest and veered to southwesterly. Still, sufficient speed shear will be present to support some threat for updraft rotation and perhaps a tornado or two, especially if any supercells can develop and persist. Isolated hail may also occur with more robust cores. Confidence in strong convection moving off the Blue Ridge Mountains in the Mid-Atlantic is relatively low. But, any thunderstorms that do form across this region could produce locally damaging winds. ...Southern Plains to the Ozarks... The portion of the cold front extending across the southern Plains into the Ozarks is forecast to make only slow southward progress through this evening. Even with some mid/high-level clouds persisting across these areas, robust daytime heating along/south of the front will likely encourage surface temperatures to warm into the 90s and low 100s. Even though low/mid-level flow will remain fairly weak, rather large surface temperature/dewpoint spreads (up to 30-40 degrees F) and steepened low-level lapse rates should foster a threat for mainly severe/damaging winds with any thunderstorms that can develop this afternoon and evening. Robust convection appears probable along/near the front by 22-00Z, and there appears to be some potential for an organized cluster to develop over the Ozarks/southern MO vicinity and sweep east-southeastward through the late evening. ...Upper Mississippi Valley... A focused area of ascent associated with the upper trough will overspread southern MN and northern IA today. While cloud cover remains prevalent over much of this region late this morning, there has been some recent clearing on the western edge of the cloud deck. A relatively small area of modest destabilization is still anticipated, with MLCAPE around 1000-1500 J/kg possible. A 45-60 kt mid-level jet attendant to the upper trough will overspread the developing warm sector through the afternoon. This jet will aid sufficiently strong deep-layer shear to support supercells with an associated threat for severe hail. Some clustering/upscale growth may eventually occur, which would support a greater threat for severe/damaging winds. This activity should weaken by late evening into WI and northern IL as it encounters a less unstable airmass. $$