SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0744 AM CDT Fri Oct 23 2020 Valid 231300Z - 241200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER SOUTHEASTERN LOWER MICHIGAN...PARTS OF WESTERN/NORTHERN OHIO...AND NORTHEASTERN INDIANA... ...SUMMARY... Threat for a few severe storms capable of damaging wind, hail and perhaps a tornado or two will exist this afternoon into the early evening over a portion of the Great Lakes and Ohio Valley. ...Synopsis... In mid/upper levels, broadly cyclonic flow will persist across approximately the northern 1/2 of the CONUS, with two substantial/ embedded shortwave troughs this period: 1. A perturbation now located over portions of SD and NE, forecast to deamplify somewhat as it ejects northeastward across WI toward eastern Upper/northern Lower MI through the daylight hours. The trough should reach western QC by 12Z tomorrow. 2. A trough currently evident in moisture-channel imagery over northern BC and parts of the AK Panhandle, which will move southeastward down the length of BC and the Canadian Rockies to the interior Northwest near the international border. At the surface, 11Z analysis depicted a low near MBL with cold front southwestward near ORD, SGF, and MLC, to north-central TX, the Permian Basin, and south-central/central NM. The low should move northeastward across Lake Huron, the neck of ON, and southwestern to northeastern QC through the period, ahead of the aforementioned eastern shortwave trough. By 00Z, the trailing cold front should reach southern ON, western OH, western KY, northwestern MS, ad the TX coastal plain. By 12Z the front should extend from northeastern NY or northern VT across southern PA, eastern TN, and AL, to the west-central Gulf. ...Southeastern MI/northwestern OH to Ohio Valley... Scattered to numerous thunderstorms are forecast to form mainly along the cold front during midday to early afternoon from Lower MI at least as far southwestward as the Arklatex region, moving eastward over the adjoining Great Lakes and Ohio Valley States and Mid-South to lower Mississippi Valley in the afternoon. While a strong gust cannot be ruled out south of the existing outlook areas, the most favorable parameter space for severe wind/hail and a threat of a tornado or two still appears to be near the northern part of this frontal passage, across southeastern MI, eastern IN and western/northern OH. Only minor expansive adjustments were made accordingly to account for observational and model trends seen the past several hours, and no substantial alternations to the prior area nor reasoning are required at this stage. By the time the front and its relative max in lift reach suitably destabilizing, minimally capped boundary-layer air in the 17-20Z time frame, diurnal heating and moist advection will contribute to around 1500-2000 J/kg prefrontal/preconvective MLCAPE from the Mid-South to Lower MI. Deep-layer wind fields and shear generally will increase with northward extent over the cold front and adjoining warm sector. Forecast soundings show effective-shear magnitudes ranging from only around 15-20 kt near MEM to around 30 kt near IND and 40-45 kt near DTW. SRH will be limited to some extent by a westerly surface wind component, but still may reach 150-300 J/kg over the "slight risk" area in association with a 45- 55-kt LLJ. Rather messy, quasi-linear modes will predominate, though any sustained, discrete to semi-discrete convection in the same area may become supercellular, with concordant local enhancement of hail/tornado threat. Convection should weaken this evening and with eastward extent as it encounters progressively less-favorable inflow-layer theta-e. ..Edwards/Mosier.. 10/23/2020
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