SPC 1630Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1129 AM CDT Sun Aug 22 2021 Valid 221630Z - 231200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE THIS AFTERNOON INTO TONIGHT ACROSS PARTS OF SOUTHERN NORTH DAKOTA...CENTRAL AND EASTERN SOUTH DAKOTA...CENTRAL AND NORTHEASTERN NEBRASKA AND ADJACENT PORTIONS OF SOUTHWESTERN MINNESOTA AND NORTHEASTERN IOWA... ...SUMMARY... Scattered strong to severe thunderstorm development is possible late this afternoon into tonight across parts of the northern Great Plains and mid Missouri Valley region. ...Synopsis... Westerlies remain generally progressive across the northern mid-latitude Pacific through the western and central Canadian/U.S. border area, into northeastern Canada, with the most prominent embedded perturbation forecast to advance east-northeast of Hudson Bay, accompanied by an associated large and deep surface cyclone. Another fairly significant upstream perturbation is forecast to continue digging across southern British Columbia into the western international border area. In between, weaker troughing, with at least a couple of embedded smaller-scale impulses, appears likely to accelerate northeast of the Rockies, within/northeast of the crest of amplifying larger-scale ridging in advance of the digging upstream short wave trough. Still, the latter perturbation has been accompanied by modest surface cyclogenesis across eastern Montana into western North Dakota. This surface cyclone is forecast to migrate northeastward into southern Manitoba by early this evening. However, models suggest that seasonably cool and dry air associated with the cold front trailing the Hudson Bay cyclone will be slow to erode northeast of the middle Missouri Valley through the Upper Midwest. It appears that this air mass may be reinforced across parts of the eastern Dakotas by cooling associated with initial warm advection driven convection, in advance of the approaching mid-level perturbation. Meanwhile, in the southern mid- and subtropical latitudes, mid-level ridging will remain prominent across much of the central and southern tier of the U.S., into the western Atlantic. However, weak troughing will linger across much of the Northeast into the southern mid Atlantic Seaboard, with the remnants of Henri continuing to merge into this troughing, slowly migrating north-northwestward, inland across southern New England today through tonight. ...Northern Great Plains/mid Missouri Valley... Stronger mid/upper support is forecast to overspread the Dakotas today through tonight, while low-level moisture return supportive of more substantive boundary-layer destabilization remains generally confined to the vicinity of lee surface troughing and a developing warm front across parts the central Great Plains into mid/lower Missouri Valley. However, at least modest destabilization (mixed-layer CAPE on the order of 500-1500 J/kg) appears possible within a corridor along the lee surface troughing, to the south of the surface low, across central/eastern South Dakota into portions of North Dakota by early this evening, aided by mid/level cooling. It is possible that this corridor of boundary-layer destabilization may remain fairly narrow, in the wake of the warm advection convection, but it still may provide a window for scattered vigorous thunderstorm development, including a few supercells, in the presence of strong deep-layer shear. Mid-level cooling and forcing for ascent to weaken inhibition southward into Nebraska/Kansas border vicinity, where shear will be weaker but mixed-layer CAPE considerably larger, remains more unclear. However, various model output continues to suggest that the initiation of at least isolated convective development is possible late this afternoon and evening. If this occurs, the environment probably will be conducive to the risk for supercells and/or perhaps the evolution of a small organizing severe storm cluster. ...Parts of southeastern New England... Among other factors, a boundary-layer air mass initially characterized by relatively modest dew points has inhibited the potential for convection capable of producing tornadoes. This may not change substantively, as the mid-level warm core spreads inland across southeastern New England, coincident with shrinking low-level hodographs, through mid to late afternoon. If trends go as currently anticipated, low probabilities for tornadoes may be removed in the 20Z update. ..Kerr/Lyons.. 08/22/2021 Read more LIVE: spc.noaa.gov
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