Severe Storm Outlook Narrative (AC)
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Issued by NWS
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603 ACUS01 KWNS 141301 SWODY1 SPC AC 141259 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0759 AM CDT Sun Jul 14 2024 Valid 141300Z - 151200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE CORRIDOR FROM THE NORTHERN PLAINS TO THE UPPER MIDWEST AND OHIO VALLEY... ...SUMMARY... Strong to severe thunderstorms are expected from the northern Plains into the Upper Midwest and Ohio Valley. ...Synopsis... The mid/upper-level pattern will remain dominated by mean troughing from the Great Lakes southwestward across TX, and an anticyclone over the Four Corners States, though each should deamplify through the period. A belt of relatively maximized, west-northwesterly to northwesterly flow will persist from the northern Rockies to the Great Lakes, with strong difluence on its southern and southeastern fringes from the central Plains to parts of the mid-Mississippi and Ohio Valleys. Convectively generated/enhanced perturbations now over Lower MI, southern WI and southeastern SD will traverse the flow belt across the southern Great Lakes region through the period. Upstream, a strong shortwave trough was evident in moisture-channel imagery over parts of northern/western AB. This feature should move southeastward to southeastern AB and southern SK by 00Z, then reach the Red River (of the North) Valley near the end of the period. At the surface, features relevant to today`s convective potential mostly were outflow-related at 11Z, south of what had been a synoptic warm to stationary front. These will be discussed in greater detail below. As the outlook areas are predicated on multiple interactions between existing and future boundaries and MCS activity, the outlook lines may shift considerably through the day as mesoscale trends and newer guidance focus potential better. ...Northern Plains... One or more clusters of severe thunderstorms may develop over the region this afternoon or tonight, with potential for severe gusts. This includes the possibility of significant (65+ kt) gusts. However, confidence has decreased in placement or timing of any specific significant-gust area. The airmass in and around the one from the previous outlook has been directly and substantially overturned by a large and significantly severe overnight MCS. Convectively parameterized ("synoptic") guidance and convection-allowing progs are widely dispersed in location, size, and to lesser extent timing of MCS upscale development over this area. The general consensus appears to be for MCS potential over ND, north of the main corridor disrupted by the major complex of last night. This may be the most reasonable solution synoptically, given the increasing large-scale ascent likely tonight from the approaching Canadian shortwave trough, and potential for residual moisture not directly affected by the prior MCS to suffice in its inflow region. Still, questions remain regarding how much associated convectively processed air will affect inflow trajectories of activity farther north. The outflow boundary from that complex -- which generated intense pressure perturbations and a broad rear-inflow jet -- has surged all the way into central IA, southeastern NE and northern KS -- well south of substantial flow aloft and south of supportive upper-air features. Given these intervening developments, the unconditional "slight" area has been trimmed northward more in line with potential for an overnight complex in ND, whose severity and longevity itself is uncertain. ...Upper Mississippi Valley region to Lower Great Lakes States... This outlook area covers two potential MCS/severe episodes, though considerable uncertainty remains as to which may be more dominant in terms of total report distribution. Skeletal remnants of an earlier MCS, with remaining convection non-severe in nature, was apparent over portions of northwestern OH, northern IN and southern Lower MI. The outflow boundary from this activity was drawn across IN/IL/IA near a LAF-MLI-CID line, where it was being overtaken on the west by outflow from a complex over southern WI and northern IL, that was penetrating an airmass stabilized by the earlier activity, but undergoing modification from low-level warm/moist advection. As this complex or its associated outflow arc moves southeastward into less-modified, more strongly diurnally heated boundary layer near and north of the oldest outflow boundary, additional strengthening appears increasingly probable, with damaging to severe gusts the main concern. Regardless of how severe it becomes later, the ongoing complex, in turn, should continue to deposit outflow across northern IL and eastern IA, reinforcing the baroclinic zone in that region, with some northeastward shift possible through the day amid modest but steady low-level warm advection. When the remnant pressure/theta perturbation from the old SD complex interacts with this boundary this afternoon, during a period of peak diurnal heating and weakest MLCINH, additional thunderstorms should develop -- with fairly quick upscale evolution into yet another MCS possible. As with its predecessors, any such complex should move parallel to, mainly along and north of the residual outflow boundary and generally with the mid/upper-level flow, offering damaging gusts and isolated large hail. The upstream boundary layer that will source inflow for these potential complexes is characterized by rich moisture, with surface dewpoints remaining commonly in the upper 60s to low 70s F (locally higher) along and south of the outflow boundaries, supporting a corridor of MLCAPE ranging from near 4000 J/kg in the IA/northern IL area to 2000-3000 J/kg over parts of OH/IN. ...AZ... Widely scattered thunderstorms are expected to develop this afternoon -- initially over higher terrain where diurnal heating preferentially erodes MLCINH fastest -- then move generally westward or west-southwestward to lower terrain and deeper boundary layers. Isolated severe gusts are possible from the most vigorous cells, atop a deep subcloud layer characterized by nearly dry adiabatic lapse rates from surface to LFC. This motion should be supported by a layer of substantial easterly component roughly from 700-300 mb. Enough low-level moisture should remain over the desert floors through the heating/mixing cycle to support 500-1500 J/kg MLCAPE, and 1000-1500 J/kg DCAPE. ..Edwards/Broyles.. 07/14/2024 $$