031 ACUS01 KWNS 141305 SWODY1 SPC AC 141303 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0803 AM CDT Wed Apr 14 2021 Valid 141300Z - 151200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF SOUTHEAST TEXAS...LOUISIANA AND SOUTHWESTERN MISSISSIPPI... ...SUMMARY... Hail should be the most common threat with severe thunderstorms today over portions of southeast Texas, Louisiana and southwestern Mississippi. ...Synopsis... The upper-air pattern over the CONUS will continue to be dominated by two cyclones and a belt of enhanced, nearly zonal flow to their south. The first cyclone is located over the northern WI/Upper MI/ Lake Superior region, and is forecast to shift east-southeastward across the Upper Great Lakes to Lake Huron by 12Z tomorrow. The western cyclone -- apparent in moisture-channel imagery over the Great Basin and southern parts of the interior Northwest -- will move slowly eastward through the period, while remaining part of a loosely organized Rex configuration with a high over BC. By 12Z, the 500-mb low should reach northern UT. To its south, a weak shortwave trough was evident over AZ and northern Sonora. This feature should move through the broader-scale ridge over the southern High Plains by around 00Z, then reach the Mid-South region by the end of the period. Surface analysis at 11Z depicted a diffuse cold front from western PA across southeastern KY, middle TN, and southeastern AR, becoming somewhat better defined southwestward over the Arklatex to near DRT. The front is forecast to shift slowly southward/southeastward, extending from a weak low over western VA across northern GA, southwestern AL, southern LA, the mid/upper TX Coastal Plain, and north-central Coahuila by 00Z. By 12Z, the low should move/ redevelop just offshore from the Delmarva Peninsula, with cold front over central GA, southeastern AL, and southeastern LA, becoming quasistationary over south-central TX. ...West Gulf Coastal Plain... Scattered thunderstorms are ongoing over a broad swath of the Arklatex, east TX, LA, and the lower Mississippi Valley region, with the potential for at least isolated severe hail. Additional development is expected through this evening, especially on the western and southern fringes of the ongoing activity, as isentropic lift to LFC continues atop a cold pool and near the frontal zone. The cold pool was located north and northeast of an outflow boundary from yesterday's convection, and is being reinforced over much of east TX, AR, and central/northern LA by ongoing activity. The boundary was analyzed between rig stations GRY and GHB over the Gulf, south of the LA coastline, northwestward across the HOU area then diffusely northward to a frontal intersection roughly between UTS-CRS. This boundary should move slowly northward with time over southeast TX and southwestern-coastal south-central LA, gradually leading to an increase in theta-e and more robustly surface-based effective-inflow parcels over that region. The 15% wind and marginal tornado probabilities are more dependent on such inflow parcels, and have been shaped accordingly. Inland diurnal heating also should support a sea-breeze boundary today over mid/upper TX coastal areas, with isolated to widely scattered thunderstorms possible thereon, also offering a wind/hail threat. Atop the cold pool, elevated MUCAPE of 1000-2000 J/kg will be enabled by a nearly saturated layer near 850 mb, 6.5-7.5 deg C/km midlevel lapse rates and a tropopause near 200 mb. Highly variable effective-shear magnitudes are expected, mainly in the 30-45 kt range, supporting a blend of multicell and supercell modes. Clustering also is likely given the weak elevated MUCINH, which would reduce potential hail duration and size somewhat in any given area. However, any cluster/MCS that can build into nearly surface- based parcels closer to the coast may offer a more-organized wind threat on the mesoscale. ...NC/VA... Widely scattered to scattered, high-based thunderstorms should develop late this afternoon into this evening, initially near the low and trailing surface/lee trough, as the leading edge of a plume of low/middle-level ascent reaches the area. Damaging gusts, with isolated severe/50+ kt gusts possible, will be the main concern, though a supercell or two may develop this evening and produce hail as well. Sustained diurnal heating of the boundary layer over this region is expected, given current and forecast clouds trends. This will lead to steep low-level lapse rates with a well-mixed boundary layer, and in turn, support downdraft acceleration in any convection that encounters this air mass before nocturnal/diabatic cooling becomes too stabilizing. The main inhibiting factor for a more-robust/ organized severe event will also be that which contributes to the gust potential: lack of richer low-level moisture. Surface dew points in the mid 40s to low 50s F are common over the region, and may increase somewhat today via moist advection, before afternoon mixing levels them off or even reduces dew points in some areas. Accordingly modified model soundings max out MLCAPE only around 500 J/kg. Low-level shear vectors and hodographs are expected to enlarge with time from midafternoon into early evening in the preconvective environment, as prefrontal mass response strengthens southeast of the increasingly progressive Great Lakes cyclone. Effective SRH may grow into the 200-300 J/kg range while parcels still are surface- based, along with 50-60 kt effective-shear magnitudes. This will support supercell potential, and a risk for at least isolated, marginally severe hail, despite the modest midlevel lapse rates and lack of greater buoyancy. ..Edwards/Broyles.. 04/14/2021 $$
