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Saturday, September 4, 2021

SPC Sep 4, 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 Sat Sep 04 2021 Valid 041300Z - 051200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM MUCH OF OKLAHOMA THROUGH THE OZARKS AND PART OF THE MID MISSISSIPPI VALLEY... ...SUMMARY... Isolated strong to severe thunderstorms will be capable of damaging winds from much of Oklahoma through the Ozarks and part of the mid Mississippi Valley today. ...Synopsis... A substantial mid/upper-level synoptic trough was evident in moisture-channel imagery over central Canada to the northern Plains/Dakotas, anchored by a cyclone over the MB coastline of Hudson Bay. This trough should both amplify and expand in wavelength as it moves east-southeastward through the period, to an axis from eastern Hudson Bay across northern ON, Lake Superior, WI, IA and MO. As a series of shortwave troughs pivots through the trough, some suppression of the persistent southern-CONUS anticyclone is expected across the southern Plains and mid Mississippi Valley, extending the associated cyclonic flow as far south as OK, AR and the Mid-South region by 12Z tomorrow. At the surface, an associated cold front was drawn at 11Z from a low near DBQ southwestward across northern MO, south-central KS, the northern TX Panhandle, and north-central NM. A warm to quasistationary front -- demarcating the back edge of the cooler, drier continental air mass advected across the eastern CONUS behind Ida, was drawn from the low southward across the STL and LIT areas then arching southeastward/eastward across northern LA and the coastal FL Panhandle. By 00Z, the cold front should extend across eastern IL, southern MO, eastern OK, northwest TX, the TX South Plains region, and southeastern NM. ...OK to Ozarks/mid MS Valley... Widely scattered to scattered thunderstorms are possible today into this evening from the Great lakes to NM, along the front and prefrontal outflow/differential-heating boundaries. While a rogue damaging or severe gust cannot be ruled out in that swath, the best-focused area for marginally severe gusts still appears to be from near the Mississippi/Ohio River junction across the Ozarks to OK this afternoon into early evening. Within that subset of the corridor, ongoing convection and cloud cover (mostly over KS/MO) will reinforce the baroclinicity and, in turn, augment lift, as the boundaries impinge on the highest-theta-e portion of the warm sector. This will occur beneath slowly falling mid/upper-level heights and increasingly cyclonic flow (as noted above). The strongest winds aloft mostly should remain behind the front, with the possible exception of the southeastern MO/southern IL region, where the southern fringe of 30-40-kt 500-mb flow may contribute to enough effective shear (35-40-kt magnitudes) for at least transient supercell characteristics to some convection. Eastern extent of potential in that region will be limited by low-theta-e air on the east side of the very slow-moving warm front. Flow aloft will weaken southwestward along the cold frontal/outflow corridor into the southern Plains, supporting more multicellular/ pulse processes, but with stronger surface heating and still-ample boundary-layer moisture (surface dew points mainly 60s to lower 70s) to offset weak midlevel lapse rates. MLCAPE should range from around 2000 J/kg over OK to 500-1000 J/kg over southern IL. Well- mixed subcloud layers will support localized strong-severe downdraft potential from the weaker-shear storms over OK/AR/MO. In the absence of substantial LLJ or mid/upper forcing aside from general height falls, the marginal threat for strong-severe convection will be strongly diurnal in nature, and should wane soon after dark. ..Edwards/Broyles.. 09/04/2021 Read more LIVE: spc.noaa.gov
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