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Wednesday, October 13, 2021

SPC Oct 13, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0742 AM CDT Wed Oct 13 2021 Valid 131300Z - 141200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE EASTERN DAKOTAS/MINNESOTA AREA...AND FROM SOUTH TEXAS TO MISSOURI... ...SUMMARY... Marginal severe-weather potential (in the form of thunderstorm wind and tornado threat) is apparent today over parts of the eastern Dakotas/Minnesota area, and from south Texas to Missouri. ...Synopsis... In mid/upper levels, a longwave trough will remain over the western CONUS. Its cyclonic-flow field will be traversed by several shortwave to synoptic-scale perturbations of varying intensities. For this convective forecast, two are the most important: 1. A closed cyclone -- initially centered near CDR -- forecast to eject northeastward to southern/eastern ND by 00Z, then to southern MB by the end of the period. 2. The mid/upper-level perturbation and midlevel vorticity field associated with eastern Pacific Hurricane Pamela, now east of the southern tip of Baja near the mainland coast of Mexico. The mid/ upper portion should outrace the low-level center northeastward in the longwave's cyclonic-flow field, as the low-level circulation weakens rapidly today in the northern Mexican mountains, per latest NHC forecast. At the surface, the 11Z analysis showed the surface manifestation of the cyclone centered over southwestern SD, with occluded/cold front arching southeastward over north-central NE to east-central KS and central OK (behind a squall line), then southwestward over northwest TX to near the Big Bend. By 00Z the cold front should reach eastern IA and central/southwestern MO, decelerating to quasistationary (under nearly parallel flow aloft) across northeastern through southwestern OK to west-central/southwest TX. ...OK/KS/MO through midday... See SPC mesoscale discussion 1853 for more details on near-term severe threats with a squall line initially located over southeastern KS into east-central OK. Isolated gusts near severe limits or a brief/leading-edge tornado cannot be ruled out for the rest of the morning and into midday, as the convective band moves through a narrow corridor of relatively minimized MLCINH and LLJ-enhanced hodographs. However, as the strongest mid/upper forcing related to the cyclone aloft moves away from the area, the LLJ will weaken, and low-level convergence should diminish under front-parallel deep-layer flow. ...South-central to north-central TX, southeastern OK... Scattered to locally numerous thunderstorms are expected to develop over the area along/ahead of the front, and spread northeastward across the area from Mexico through the period. This activity also may offer marginal potential for damaging to low-end severe gusts or a tornado. The dominant hazard should be from heavy rain; see WPC excessive-rainfall discussions for more details on that. A deep, rich layer of low-level moisture will remain over this region, with surface dew points in the 70s F and PW values commonly 1.75-2.25 inches. Moisture aloft and large-scale forcing for ascent each will increase through the period as the mid/upper-level remnants of Pamela approach the area. This process will both temper diurnal heating (with abundant cloud cover) and reduce capping from the remnants of a modest EML, noted on prior 00Z and latest 12Z RAOBs and 700-mb charts from 13/00Z. The net result should be gradually diminishing MLCINH amidst a plume of 1500-2000 J/kg MLCAPE. Forecast soundings suggest modest low-level shear, but 40-45-kt effective-shear vectors. As such, mixed and mostly messy modes are likely, with mainly multicell characteristics, but isolated supercells possible. ...Eastern Dakotas/western MN... An arc of low-topped showers and thunderstorms may develop just ahead of the deep-layer cyclone this afternoon, offering marginal damaging-gust and brief-tornado potential. Backed low-level flow in a regime of strong isallobaric forcing will enlarge hodographs in a narrow corridor near the arc of strongest low-level lift, supporting some potential for storm-scale rotation. DCVA/cooling aloft ahead of the ejecting cyclone will steepen midlevel lapse rates and contribute to weak MLCINH, especially where even relatively short-lived diurnal surface heating may take place. The limiting factor -- and main area of uncertainty -- will be amount of supportive low-level destabilization possible behind the extensive area of early/midday convection. Even with relatively cool surface temperatures in the upper 50s to near 60 F in forecast soundings, a narrow area of 200-700 J/kg MLCAPE may develop. Any severe potential should decrease markedly after dark. ..Edwards/Gleason.. 10/13/2021 Read more LIVE: spc.noaa.gov