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Friday, July 30, 2021

SPC Jul 30, 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 Fri Jul 30 2021 Valid 301300Z - 311200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF CENTRAL NEBRASKA TO SOUTHWESTERN IOWA AND NORTHERN MISSOURI... ...SUMMARY... Severe thunderstorms appear most probable late this afternoon into tonight from parts of central Nebraska to southwestern Iowa and northern Missouri. ...Synopsis... In mid/upper levels, a large-scale trough will remain over the northeastern 1/4 of the CONUS through the period. As a series of shortwave troughs traverses the Upper Midwest, Great Lakes and Ohio Valley regions, that part of that trough's cyclonic-flow field will amplify, with greater curvature. Meanwhile, the persistent anticyclone to its southwest -- from the north-central Rockies to the Mid-South -- will weaken and elongate further. On last night's 00Z 500-mb chart, multiple highs were analyzed within the anticyclone. This nodular character to the height maxima should become more pronounced with time as the overall anticyclone gradually erodes. One shortwave perturbation was noted in moisture-channel imagery over the northeastern WY to central SD region, associated with convection now to its east over SD. This feature -- with an embedded convectively generated/enhanced vorticity max now apparent between PIR-MBG and another north of PHP -- is forecast to pivot east-southeastward across SD and northeastern NE today, reaching the southeastern IA/western IL/northeastern MO region by 12Z tomorrow. The 11Z surface analysis showed a low over the Gulf of Maine, with cold front southwestward across the Delmarva Peninsula, north- central VA, WV, and northern KY, becoming quasistationary over western MO, northeastern KS, and southeastern/western NE. The cold front is expected to move southeastward through the central/southern Appalachians, adjoining Piedmont and coastal plain/Tidewater area through the period. By 00Z, the front should extend near the NC/VA line, with a weak frontal-wave low possible between ORF-DAN, then across northern portions of GA/AL/MS, becoming a stationary to warm front northwestward over western MO and southern/central NE, then a cold front again southwestward over central CO. By 12Z, the front should reach southeastern NC, central GA, northern MS, and eastern AR, to a low between north-central KS and northwestern MO, then southwestward to southeastern CO. ...Central Plains... Isolated to widely scattered thunderstorms should develop late this afternoon near the frontal inflection point and moisture/buoyancy axis over central to north-central NE, as boundary-layer temperatures and lapse rates are diurnally maximized near the mid/upper-level shortwave trough. Damaging gusts and large hail are expected. Some tornado threat may develop also, conditional on maturation and maintenance of supercell(s) before activity grows upscale. With time into this evening, clustering of convection is expected with a transition more toward a wind threat, and large hail still an concern as well. Activity should move southeastward -- largely parallel to the front and through a favorably moist/buoyant corridor -- continuing a threat for severe gusts and large hail, before weakening late tonight in southeastern parts of the outlook area. Surface dew points in the upper 60s to low 70s F have become common as far west as the high Plains of southwestern NE, northwestern KS and the eastern NE Panhandle, with outflow from the SD/WY activity acting as an effective cold front over the NE Panhandle and southeastern WY. The associated moisture (and CAPE) axis should arch from the western/central Sandhills across southeastern NE and central MO this afternoon, with diabatic heating and somewhat steep midlevel lapse rates helping to drive MLCAPE into the 2000-3000 J/kg range in a roughly 80-120-nm-wide corridor centered on that axis. Similar MUCAPE values will extend somewhat farther north to near the SD line, where convective inflow parcels may be somewhat elevated. Low-level and deep shear each will be maximized along and north of the front, with relatively backed flow contributing to effective SRH 200-350 J/kg. Despite modest mid/upper flow, effective-shear magnitudes may reach 30-40 kt. This parameter space will support heavy-precipitation supercells with potentially quick merger/ transition to clustered, messier modes. This will lead to the potential for forward propagation into favorable near-surface storm-relative winds and along the instability gradient and moist axis. ...Carolinas/extreme southeastern VA... Isolated to widely scattered thunderstorms should develop this afternoon near the front, and over higher terrain in the mountains of northwestern SC. Diurnal heating may be delayed or reduced for several hours by anvil debris emanating from ongoing convection across eastern TN -- especially over the Carolinas. Less of such influence is expected near the Hampton Roads/northern NC frontal segment, to which the outlook was adjusted to accommodate convective potential there. Still, heating should be sufficient (amidst weak MLCINH and rich boundary-layer moisture) to offset modest midlevel lapse rates and support MLCAPE mainly in the 1000-2000 J/kg range. Weak and veered low-level winds will help to limit both hodograph size and deep shear, though cloud-layer shear will benefit from strong anvil-level flow. As such, damaging to marginally severe gusts will be the main concern, with activity diminishing through the evening. ..Edwards/Leitman.. 07/30/2021 Read more LIVE: spc.noaa.gov
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