Subj : DAY1SVR: Day 1 Convective Outlook To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Apr 28 2023 12:52:46 ACUS01 KWNS 281252 SWODY1 SPC AC 281250 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0750 AM CDT Fri Apr 28 2023 Valid 281300Z - 291200Z ....THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF NORTH...CENTRAL AND SOUTH TEXAS... ....SUMMARY... Destructive hail (some at least as large as baseballs), thunderstorm gusts potentially exceeding 75 mph, and a couple tornadoes are possible this afternoon and evening from north Texas southwestward to parts of the Rio Grande Valley. ....Synopsis... The large-scale pattern over North America will remain characterized by high-amplitude ridging just inland from the Pacific Coast, and a longwave trough over east-central portions of the U.S. and Canada. In the latter's broad cyclonic flow field, four main shortwave perturbations will affect convective potential this period: 1. An ejecting trough -- apparent in moisture-channel imagery from Lower MI across eastern parts of KY/TN, and forecast to weaken gradually before it reaches the Lower Great Lakes and Mid-Atlantic regions around 12Z tomorrow. 2. A compact shortwave trough -- initially located over the central Rockies -- which will amplify considerably as it follows an arc southeastward across the central/southern High Plains today. By the end of the period, the trough should extend from northeastern OK southwestward across the DRT area and Coahuila, with a closed 500-mb low developing near DAL. 3. A northern-stream perturbation -- now located near the northern MB/SK border, and projected to dig southeastward to the eastern Dakotas/western MN region by 12Z. Near 12Z or early day-2, it should merge with... 4. A foregoing, slow-moving trough now over the Dakotas. As large-scale ascent aloft related to this feature overspreads a plume of modest low-level moisture in parts of the upper Mississippi Valley, isolated to widely scattered thunderstorms may occur this afternoon and evening. At the surface, 11Z analysis showed a wavy warm front across eastern/central NC, central SC and central GA, forecast to move northward somewhat today ahead of an occluded/cold front analyzed from OH across eastern parts of KY/TN, to central MS. The latter boundary became quasistationary and poorly defined, through previously precip-cooled air, westward to a low near DYS. A separate cold front was drawn across central KS, western OK, northwest TX, to near INK, then over extreme southern NM. Through this evening this cold front will overtake a dryline initially drawn across central TX to near DRT. By 00Z, this cold front should extend from southeastern IA to southeastern OK, then across central TX to near DRT. By 12Z, the cold front should reach central/ southern AR, where a frontal-wave low may form ahead of the mid/upper perturbation, then south-southwestward across parts of western LA to the northwestern Gulf. ....TX... This afternoon, severe hail is expected across parts of north- central and central TX, with some very large/destructive hail possible (3+ inches). A couple tornadoes also may occur. From late afternoon through the evening, expansive convective growth should result in a severe-wind threat, with some significant (65+ kt) gusts possible. Scattered thunderstorms -- including a few supercells -- are expected to develop this afternoon along/ahead of the front. Greatest initial convective coverage and intensity are expected in a corridor of relatively maximized convergence and backed surface winds between the low and the northern rim of the "just in time" returning moist plume. Given the observed translation of the cold front -- which has been consistently faster than in most progs -- ideal moisture return appears limited to the area from around DFW southward before clouds, precip and convection truncate its poleward advance. As such, outlook lines have been shunted southward somewhat, but with some potential left near the Red River to account for hail potential from elevated thunderstorms. The Metroplex appears again to be in the gradient of the severe threat, with increases substantially with southward extent across the area. Though near-surface winds may be weak, pronounced veering with height and large 2-3-km hodographs are expected, amidst 50-60-kt effective-shear vectors. Diurnal heating, low/middle-level lapse rates greater than 8 deg C/km, and surface dewpoints in the 60s F, will drive peak MLCAPE into the 2000-3000 J/kg range in the ACT-AUS corridor, and even larger (perhaps approaching 4000 J/kg) over parts of south TX where heating/moisture are greater (but vertical shear somewhat weaker). Hail models applied to forecast soundings consistently yield 3+ inch max diameter, with some 4+ inch analogs. With a well-mixed boundary layer expected in areas receiving a few hours of heating, severe downdrafts are possible from supercells as well as from the subsequent upscale clustering expected over central/south TX. Convection should develop farther south near the front across central/south TX, and perhaps over the eastern slopes of the Serranias del Burro in Coahuila (with the latter potentially aggregating upscale and moving eastward into TX). This upscale growth should lead to one or two organized, bowing MCSs, potentially merging, with enough cold-pool development (amid increasing mid/upper winds) to drive a substantial severe-wind threat over parts of central and south TX. With the warm sector being narrow in the north and larger near I-10, the wind threat should last longer over southern areas this evening, and wind probabilities accordingly have been expanded somewhat in that region. ....FL/GA... An outflow boundary from the prior day's convection has settled across southernmost mainland Fl and the upper keys/Florida Bay region, arching northwestward toward a slowly weakening/shrinking MCS over the east-central Gulf. That weakening trend should continue through the remainder of the morning as what's left of the complex encounters further convectively processed boundary-layer air. Clouds and perhaps a dissipating precip area from that activity will limit diurnal destabilization across much of west- central/southwestern FL today. However, enough heating is possible to enable an East Coast sea-breeze boundary from the MIA area northward, as well as north of the MCS cloud plume over parts of northern FL and southern GA. In those areas, widely scattered to scattered thunderstorms should form this afternoon, supported by low-level lift along outflow, differential-heating and sea-breeze boundaries. Though midlevel temperatures will not be as cool as the past few days, marginal shear on the southern rim of the ejecting midlevel wave's influence may support a blend of organized multicell and transient supercell structures, with attendant risk of isolated severe hail/gusts. ....Mid-Atlantic region to Upper Ohio Valley... Multiple rounds of convection are expected across this region -- mainly involving early thunderstorms near the warm front over parts of the Carolinas, then afternoon convection from NC into the upper Ohio Valley nearer to the remnant cold core region of the ejecting mid/upper trough. Isolated large hail and a few damaging to severe gusts are possible. Buoyancy will increase through the day behind the early warm-frontal activity, as both diurnal heating and midlevel cooling occur, amid modest but sufficient boundary-layer moisture. With the mid/upper trough deamplifying, effective-shear magnitudes generally should remain below 35 kt, but a few organized multicells and transient supercells near the occluded/cold front should offer localized strong downdrafts and marginally severe hail. ...Edwards/Mosier.. 04/28/2023 $$ = = = To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. 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