SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0757 AM CDT Thu Apr 08 2021 Valid 081300Z - 091200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM PARTS OF THE FLORIDA PANHANDLE ACROSS THE SOUTHERN APPALACHIANS TO PARTS OF THE GREAT LAKES REGION... ...SUMMARY... Isolated severe thunderstorms are possible today from parts of the Florida Panhandle across the southern Appalachians to parts of the Great lakes region. ...Synopsis... Split flow in mid/upper levels will persist over the central CONUS, related to a pronounced cyclone now centered over northwestern MO. As a strong/upstream shortwave trough moves southeastward from the Pacific Northwest across the northern Rockies, the cyclone will eject northeastward. By 00Z, the 500-mb low should be located over eastern IA, moving slowly northward to northeastward toward western WI by the end of the period. At the surface, 11Z analysis showed a low over north-central IA, with occluded/cold front arching eastward over the DBQ area then southeastward across central IL, western portions of KY/TN, western MS, and southwestern LA, becoming quasistationary across the lower/middle TC Coastal Plain to near LRD. The low should meander around northern IA and southern MN through most of the period as the mid/upper cyclone approaches. Along an inverted trough extending northward from the surface low, a second closed low may form between 09-12Z over northern MN. The front will move eastward across IN and parts of OH and Lower MI through the period, while also crossing into eastern TN/KY and northwestern GA, and decelerating/stalling over MS/AL. The trailing part of the boundary will become a warm front moving northward over parts of central/east TX today and tonight, and then over LA/MS overnight. ...Great Lakes to southern Appalachians... Widely scattered to scattered thunderstorms are expected to form from late morning through afternoon along/ahead of the front, in at least a broken arc, initially from the southern Lake Michigan area across southern IN to middle/eastern TN. Isolated damaging to severe gusts will be the main concern, though a tornado cannot be ruled out. Overall, most of the richest low-level moisture and highest theta-e will be displaced in the Gulf Coast States, south of the area of strongest deep-layer forcing (both large-scale warm advection, deeper low-level mass convergence nearer the cyclone, and frontal lift). The greatest potential for overlap will be across the corridor roughly between CVG-CHA, but with mean-wind/deep-shear vectors having a substantial component parallel to the convective axis. As such, and with effective shear on the margins (vector magnitudes 30-40 kt over much of the area), the main modes will be quasi-linear and multicellular, but with brief supercell modes and embedded mesocirculations possible. The warm sector will become more narrow and incompletely modified with northward extent, only somewhat offset by colder air aloft in generating buoyancy. MLCAPE should be in the 500-1000 J/kg range in the western portions of the outlook area, where greater diurnal and midlevel destabilization each are possible, and across some parts of eastern TN/KY where moisture is greater. Convection should diminish in intensity and coverage with eastward extent late this afternoon into evening, upon encountering even more-meager instability. ...AL/GA/FL Panhandle... Farther south from the southernmost Appalachians (AL/GA) and Gulf Coast, times series of forecast soundings reasonably depict weakening low-level hodographs with time through the day, as boundary-layer instability strengthens from diabatic surface heating. With the deep-layer cyclone already well-removed from this region and moving away, little appreciable upper support is expected, and the main influence on convective development will be the front (with modest lift) and mesoscale boundaries. A somewhat favorably moist prefrontal air mass will remain in place, with surface dew points in the 60s F and pockets of sustained heating enabling 1000-2000 J/kg MLCAPE. This will occur beneath the eastern/northeastern edge of the mid/upper speed max (500-250-mb layer) supporting 45-55-kt effective-shear magnitudes. As such, any sustained convection that can develop may become at least briefly supercellular and/or assume bowing/LEWP configurations, locally yielding a severe threat. However, overall convective coverage is in question, keeping the unconditional threat marginal. ..Edwards/Dial.. 04/08/2021
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