Subj : DAY1SVR: Day 1 Convective Outlook To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Apr 21 2023 13:02:52 ACUS01 KWNS 211302 SWODY1 SPC AC 211301 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0801 AM CDT Fri Apr 21 2023 Valid 211300Z - 221200Z ....THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF DEEP SOUTH TEXAS... ....SUMMARY... Isolated severe thunderstorms will be possible today across parts of south Texas, the lower Mississippi Valley region and upper Ohio Valley/lower Great Lakes. Very large hail is possible over deep south Texas. ....Synopsis... A slowly progressive mid/upper-level cyclone -- initially centered over MN -- will drift eastward to the Lake Superior vicinity through the period, while anchoring a high-amplitude trough extending to its south. Associated cyclonic flow will cover most of the CONUS between the Rockies and Atlantic Coast, west of a ridge moving eastward from the northern Mid-Atlantic region to New England. At the surface, 11Z analysis showed an occluded low over northeastern MN -- related to the mid/upper-level cyclone centered nearby. An occluded front was drawn from there to northern OH, with cold front across western KY, western TN, southern AR, and south-central TX. By 00Z, this front is forecast to move to western parts of MY/PA, southern OH, eastern KY, middle/western TN, western MS, central LA, and deep south TX. Surface northerlies preceding the front over south TX, as well as substantial, antecedent convective outflow now present over much of southwestern LA and south-central/southeast TX, may make identification of the front difficult for most of today over those regions. A warm front extended across northeastern KY, WV, northern VA, and DE, and should move/redevelop northward over the Lower Great Lakes and parts of NY through the period. ....South TX to lower Mississippi Valley... As indicated in the previous outlook, uncertainty regarding impacts of prior/overnight convection continue to influence the severe potential today, though the threat has become somewhat better defined. A strong-severe MCS moved southeastward across much of south TX and western LA overnight, and persists this morning across southeastern LA (in non-severe form) and the northwestern Gulf south of western LA and the middle/upper Texas Coast, with some well-organized, possibly severe convection still apparent over open Gulf waters. The passage of this MCS has stabilized the prefrontal boundary layer substantially across much of the middle/upper TX Coastal Plain into southwestern/western LA. Though near the mid/upper trough, this area will represent a relative minimum in low-level buoyancy and airmass-recovery potential today. As such, a relative minimum in severe potential still is apparent between south Texas and the Mississippi Valley, but has been expanded eastward into the most intensely affected airmass. Widely scattered to scattered thunderstorms should develop this afternoon near the front and its interaction with outflow/differential-heating boundaries related to earlier convection. The two most probable areas for this to lead to a severe threat still appear to be south-central to deep south TX, and the lower Mississippi Valley. Over south TX, convective coverage is more uncertain, but a conditional significant (2+ inch diameter) hail threat exists for any sustained convection that can form. Confidence in at least isolated destructive hail has increased enough to warrant an upgrade to the unconditional probabilities for that hazard, and introduction of an embedded significant "hatched" area. The outflow boundary from morning activity and/or the cold front will provide lift in a progressively weaker MLCINH environment, even as frontal forcing becomes more subtle amid the northerly component on both sides. The airmass over deep south TX should destabilize strongly from diurnal heating, with dewpoints in the upper 60s to low 70s F, roughly 8 deg c/km midlevel lapse rates, and a deep troposphere supporting equilibrium levels higher than the 200-mb pressure level. This should support MLCAPE in the 3500-4500 J/kg range. Low-level flow/shear will be modest, but with enough effective shear (35-45 kt) to support at least transient supercell structures. 100-110-kt anvil-level westerly flow is expected in the subtropical jet, with a substantial component off the expected southeast- to south-southeast-directed storm motion. Meanwhile, scattered thunderstorms are expected to develop near the front today, mainly north through northeast of areas most strongly stabilized by the current cold pool. Some recovery should occur with distance from the most intensively affected air mass (i.e., northern LA into MS) to permit sustained, strong to isolated severe convection in a frontal band from this afternoon into early evening. Damaging wind (with isolated, marginally severe gusts) may occur, along with isolated severe hail and a marginal tornado threat. Being more distant from EML effects, MLCINH should be weaker, but also, with considerably smaller midlevel lapse rates than over south TX. With low/mid 60s F surface dewpoints being common, this should lead to peak/preconvective MLCAPE in the 500-1000 J/kg range, locally maximized where cloud breaks from the morning MCS can be sustained longest. Veering of flow with height, and strong midlevel westerlies, should contribute to around 40-55-kt effective-shear magnitude (influenced strongly by CAPE depth), and low-level hodograph curvature/size supporting 150-300 J/kg effective SRH. Severe potential should diminish late this evening into overnight as the front encounters weakening instability. ....Upper Ohio Valley/lower Great Lakes... Widely scattered thunderstorms are expected to develop near the surface front this afternoon and move northeastward, moving slightly to the right of parallel to the front itself, but likely remaining near the boundary due to the latter's motion. The main concern will be damaging wind. A narrow corridor of surface-based buoyancy will develop ahead of the front, aided by moist advection and modest diurnal destabilization. Steep low-level lapse rates will support evaporative potential in downdrafts, beneath 100-500 J/kg MLCAPE. Though low-level shear and deeper-layer directional shear will be small, sufficient speed shear will exist in the profile to support effective-shear magnitudes around 40-50 kt to aid in convective organization. ...Edwards/Leitman.. 04/21/2023 $$ = = = To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .