Subj : DAY1SVR: Day 1 Convective Outlook To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Apr 27 2023 12:55:41 ACUS01 KWNS 271255 SWODY1 SPC AC 271254 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0754 AM CDT Thu Apr 27 2023 Valid 271300Z - 281200Z ....THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE GULF COAST STATES...FLORIDA PENINSULA AND MID-SOUTH.... ....SUMMARY... Severe storms are expected along much of the Gulf Coast Thursday, from portions of south Texas and southern Louisiana into the Florida Panhandle. Damaging gusts, large hail, and a few tornadoes will all be possible. Severe thunderstorms are also expected across the eastern Florida Peninsula and parts of the Mid-South. Hail and wind are the primary concerns with this activity. ....Synopsis... In mid/upper levels, a broadly cyclonic pattern will be maintained over the central CONUS, as one substantial shortwave perturbation exits the larger-scale trough, and another digs southeastward down the northern/central Rockies and expands. The leading trough is evident in moisture-channel imagery near an STJ-FSI-PRS axis, with vorticity max evident near OKC. This feature should deamplify gradually as it moves eastward over the Ozarks, AR and northeast TX by 00Z, then reaches southern IN, central/eastern KY and eastern TN by 12Z tomorrow. At the surface, 11Z analysis showed a weak low over southern AR, with cold front to another low near LFK, then southwestward across the middle TX Coast to near LRD. A warm front was drawn over southeastern MS and southwestern AL, across the coastal FL Panhandle and northern FL -- and should move northward/inland through the day. The Gulf cold front should proceed southward across deep south TX and southeastward over LA today, preceded by convective complex(es) that should shift the effective boundary well into the FL Panhandle region and north-central Gulf by 00Z. This will leave behind a weak surface low and residual front moving eastward across AR and parts of the Mid-South. A low related to the northern-stream shortwave trough was drawn over northeastern MT, with a cold front arching across central/ northwestern MT. The northern Plains low is expected to reach the HON/BKX area by 00Z, with cold front southwestward across central NE and eastern CO. ....Central Gulf Coast and vicinity... A blend of severe thunderstorm modes -- MCS, warm-sector and warm-frontal supercell(s), and multicellular clusters -- will lead to potential for all severe hazards today as the activity shifts eastward along the Gulf Coastal Plain -- mainly along and south of the front. Isolated large hail or marginally severe gusts may occur north of the front as well. For near-term concerns, see SPC severe thunderstorm watch 172 and related mesoscale discussions. Through the afternoon, a low-level convergence zone -- associated with activity now over portions of MS/LA -- is expected to consolidate convection across the outlook area and adjoining Gulf, resulting in a quasi-linear MCS with damaging wind and isolated large hail possible. A few tornadoes also may occur, especially in the relatively high-vorticity/helicity environment where the activity intercepts the northward-drifting warm-frontal zone. Rich low-level moisture -- with surface dewpoints commonly in the upper 60s to low 70s F, will support 1500-2500 J/kg peak MLCAPE (locally higher), beneath strong mid/upper-level westerlies supporting 45-55-kt effective-shear magnitudes. Flow aloft will have a substantial component across the convective line, such that embedded supercells, bow/LEWP formations or mesovortices will be possible. If mesoscale and/or guidance trends more reliably indicate continuity of organized severe potential with this line into more of GA and northern FL, the 15% areas may need to be connected in an ensuing outlook update. ....FL Peninsula, southeastern GA... Scattered thunderstorms are expected to develop again today -- mainly along the West Coast sea breeze as it penetrates well inland, but with some activity also firing off on the East Coast sea breeze before moving offshore. Also, in northern FL/GA, surface-based convection may be supported by a mesoscale warm-advection zone preceding the central Gulf Coast MCS activity. Additional thunderstorms in between these areas may form as outflow boundaries develop and interact through the afternoon and into early evening. Damaging winds and large hail each are expected with the most-intense convection. Organized multicells and a few supercells are possible, amidst 35-45-kt effective-shear magnitudes and mostly small low-level hodographs (though some backed flow near the immediate East Coast may locally enlarge hodographs there). Midlevel temperatures will remain cold enough (e.g., -10 to -12 C at 500 mb) into afternoon, when overlapping boundary-layer moisture supporting mid 60s to low 70s F surface dewpoints, to support preconvective MLCAPE in the 1500-2500 J/kg range. Locally well-mixed boundary layers, in areas receiving the most-sustained insolation, will support the strongest downdraft potential. Strong/isolated severe gusts and hail also may occur over northern FL as the central Gulf Coast MCS moves into the region this evening, though with the air mass stabilizing inland from outflow and diabatic cooling, this threat appears more marginal/conditional. ....South TX... Isolated to scattered thunderstorms this afternoon will pose a threat for severe hail -- some potentially significant and destructive at over 2 inches in diameter -- along with severe downdrafts. Convective coverage is uncertain, given the northerly surface wind component south of the front and potential related limitations on frontal convergence, but the conditional potential is strong enough to keep 15% hail/wind and 10% significant-hail probabilities active. As the outflow-reinforced front continues southward through the outlook area today, it will encounter a strongly diurnally destabilizing air mass, beneath steep, EML-driven midlevel lapse rates. The combination of that with ample boundary layer moisture (surface dewpoints commonly upper 60s to mid 70s F) will lead to a strong instability, with peak MLCAPE in the 2500-3500 J/kg range. Northerly surface-flow components should prevail even ahead of the front, with modest low-level speeds, keeping hodographs small. However, enough mid/upper flow will be present associated with the subtropical jet to support 40-45-kt effective-shear magnitudes, and some supercell threat. Activity should exit the region into northeastern MX and the Gulf by about 00Z. ....AR, Mid-South region... Widely scattered to scattered thunderstorms are expected to move eastward and east-northeastward across the area this afternoon and evening, posing a threat for sporadic large hail and isolated damaging to severe gusts. Strong cooling aloft is expected as the mid/upper trough, and vorticity lobe now over central OK, approach the region. Though cut off from optimal low-level theta-e/moisture by frontal and convective processes farther south, sufficient residual moisture (surface dewpoints mainly in the mid 50s to near 60 F) should combine with pockets of diurnal heating, to support 1000-1500 J/kg MLCAPE. Weak low-level flow -- gradually strengthening into midlevels with little directional shear -- yields long, rather straight hodographs in forecast soundings, amidst 35-45-kt effective-shear magnitudes. This suggests a mixed-mode episode of multicells and as few supercells (some splitting) are possible. ....Southern WY to north-central High Plains... Widely scattered to scattered thunderstorms should form today near the surface cold front and move southeastward across the outlook area, offering isolated severe gusts. This region will be under both difluent flow aloft and a broad plume of large-scale ascent, related to midlevel DCVA preceding the shortwave trough, and a well-defined left-exit region of the cyclonically curved 500-250-mb jet. Convection should become surface-based and intensify as the front impinges on a diurnally destabilizing, weakly moist boundary layer. Diurnal heating, beneath cold (and cooling) midlevel temperatures, will steepen lapse rates and remove MLCINH from midday into the afternoon. Even though moisture will be rather limited -- with surface dewpoints commonly in the 30s F -- MLCAPE of around 200-600 J/kg should develop atop a well-mixed boundary layer. Thunderstorm-gust potential should diminish markedly after sunset as the boundary layer cools and SBCAPE is removed. ...Edwards/Leitman.. 04/27/2023 $$ = = = 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) .