Subj : DAY1SVR: Day 1 Convective Outlook To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Aug 12 2023 12:50:11 ACUS01 KWNS 121250 SWODY1 SPC AC 121248 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0748 AM CDT Sat Aug 12 2023 Valid 121300Z - 131200Z ....THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM EASTERN OHIO ACROSS WESTERN/NORTHERN PENNSYLVANIA AND PORTIONS OF WESTERN/SOUTHERN NEW YORK... ....THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SURROUNDING THE ENHANCED AREA...AND ALSO OVER PARTS OF THE TENNESSEE VALLEY AND SOUTH-CENTRAL GREAT PLAINS... ....SUMMARY... The most concentrated area of severe-thunderstorm threats today extends from northeast Ohio across parts of Pennsylvania and into central New York, with damaging to severe wind expected, along with large hail and a tornado or two possible. ....Synopsis... In mid/upper levels, a zonally elongated anticyclone will remain over much of TX and the western Gulf Coast States, with ridging eastward offshore from JAX. A cut-off cyclone -- initially centered just offshore from southern CA -- will meander near its present position through most of the remainder of the period. In between those features, a series of low-amplitude shortwaves with embedded cyclonic vorticity maxima will traverse the southerly/southwesterly flow aloft, from northwestern MX to the central High Plains, where they will join the southern part of the prevailing westerlies. In the CONUS subset of the northern-stream belt, the western portion will amplify through tomorrow morning. A shortwave trough will dig southeastward from western Canada to parts of SK/MT by 00Z, then strengthen and move to eastern MT and WY by 12Z tomorrow. Downstream, another strong shortwave trough was apparent in moisture-channel imagery over Lake Superior, adjoining parts of northwestern ON, and south-southeastward over parts of Lower MI. This perturbation should shift eastward by 00Z, reaching eastern ON, the Lower Great Lakes, and parts of western NY to PA. Preceding the Great Lakes shortwave trough, a related, occluded surface cyclone was analyzed at 11Z over eastern Lake Superior, with cold front extending from a triple point over Lake Huron southwestward through southern Lower MI, northern IL, northern MO, east-central KS, and northeastern NM. By 00Z, the main surface cyclone should elongate west/east as the occlusion process persists. The cold front should reach northern OH, south-central IL, and southwestern MO, becoming quasistationary or perhaps retreating slightly northward over the area from northern/western OK across the northern TX Panhandle and Raton Mesa. Outflow boundaries will precede the front over portions of OK, MO, and the lower Ohio Valley. ....Northeast... Scattered thunderstorms should develop later this morning into afternoon, over the mid/upper Ohio Valley and Lower Great Lakes regions. This activity should pose an increasing threat for hail, damaging to severe gusts, and a few tornadoes as it moves eastward across the outlook area, with the greatest threats in and near the "enhanced" corridor. This will occur as large-scale ascent, deep-layer wind profiles and vertical shear all strengthen ahead of the Great Lakes shortwave trough, and perhaps ahead of an eastward-moving MCV or convectively enhanced vorticity lobe from morning activity over IL. These processes, occurring along and ahead of the surface cold front, may support multiple rounds of convection. Activity will impinge on a diurnally destabilizing and favorably moist boundary layer, commonly characterized by surface dewpoints in the upper 50s to upper 60s F. In addition to heating, moist advection ahead of the convection should help to: 1. Strengthen buoyancy over areas of PA/NY now containing more modest theta-e than in the Ohio Valley, and 2. Offset vertical boundary-layer mixing enough to maintain or even increase preconvective dewpoints farther west. MLCAPE in the 1500-2000 J/kg range should become common, in a supercell-supporting kinematic parameter space of 40-55-kt effective-shear magnitudes. Though near-surface flow may be weak, veering with height and hodograph curvature should be greatest near the NY/PA border and into western NY, with effective SRH in the 150-300 J/kg range. Analog soundings and a 2-D hail model applied to forecast soundings each suggest significant-severe hail may occur, with the risk of a few such reports now appearing large enough to draw an unconditional area. Otherwise, the wind and tornado threats largely appear to be similar to those described in the previous outlook, with minor adjustments made for more-recent progs of convective trends. ....South-central Plains... Widely scattered to scattered thunderstorms should develop this afternoon over higher terrain, from the eastern slopes of the Sangre de Cristos into the Raton Mesa region, and perhaps over intensely heated Plains areas of the northwestern TX Panhandle and northeastern NM. Activity then should move eastward and coalesce into one or more complexes, with severe wind the main concern, and isolated large hail possible. Development and initial growth should occur as these processes overlap: 1. Moisture/theta-e advection from lower elevations, amidst the easterly to east-northeasterly flow north of the front; 2. Upslope mechanical lift, augmented by... 3. Strong heating, which will combine with moistening to remove remaining MLCINH preferentially on the higher elevations; 4. Shots of large-scale ascent aloft related to aforementioned southwest-flow perturbations. Activity will move over a well-mixed, deep boundary layer, with progressively hotter surface temperatures southward until CAPE becomes negligible south of the frontal zone due to boundary-layer dryness. The optimal corridor for convective maintenance and upscale growth appears to be from southeastern CO and the OK/ northern TX Panhandles across southern KS and northern OK, where just enough flow aloft exists to encourage eastward-directed, forward-propagational cold-pool surges into a very unstable boundary layer, with preconvective MLCAPE in the 1500-3000 J/kg range. Larger CAPE may exist farther north across central/western KS, but MLCINH will increase with northward extent under an EML. One exception may be across the northeastern CO/northwestern KS/southwestern NE region, where forecast soundings and convective progs suggest a short window of opportunity for high-based convection with isolated severe potential. In KS/OK, severe-gust potential should decrease with eastward extent tonight as activity encounters a gradually stabilizing boundary layer, but uncertainty exists over cold-pool strength vs. CINH and how fast the threat will diminish. ....Southeast... Scattered to locally numerous thunderstorms may develop this afternoon over parts of the Tennessee and lower Ohio Valley regions, and perhaps the Mid-South. Damaging to locally severe gusts will be the main concern. Outflow and differential-heating boundaries -- both from residue of ongoing/morning convection farther north, and over this region -- should provide foci for afternoon initiation as strong heating and rich low-level moisture minimize MLCINH. Given the strong mesoscale dependence of these features through the remainder of the morning into early afternoon, enough uncertainty remains in their position and strength that outlook lines over this region may need substantive adjustment again before the convective event starts. The air mass over the region will be located largely well south of the strongest mid/upper-level flow, but with rich low-level moisture (surface dewpoints commonly upper 60s to mid 70s F). Based on a mildly modified 12Z BMX RAOB and model soundings, this will support widespread 3000-4000 J/kg MLCAPE with negligible MLCINH, southeast of both the front and any areas of persistent cloud cover lingering from morning convection. The deeply buoyant thermodynamic profile and lack of inhibition will led to potential for clusters of strongly water-loaded downdrafts and mesobeta-scale cold pools to yield relative concentrations of damaging gusts. ....North-central Plains... Isolated to scattered thunderstorms should develop this afternoon over higher terrain in and near the Black Hills, then move generally southeastward across the Plains. Isolated severe gusts and hail will be possible. With southeasterly surface winds underlying strengthening mid/ upper-level westerlies, vertical shear will be favorable for some supercell evolution, with effective-shear magnitudes in the 40-50-kt range. While not as intensely heated nor moist as areas farther south over the south-central Plains, a corridor of maximized low- level moisture will extend northward through western NE to western SD. This will act in concert with diabatic heating of the elevated terrain to remove MLCINH and support development. MLCAPE in the 500-1500 J/kg range (locally higher) should develop atop a deep, well-mixed boundary layer supporting maintenance of both strong- severe downdrafts and hailstones to the surface. Severe potential should diminish overnight and with southeastward extent, as the inflow layer stabilizes and becomes more elevated. ...Edwards/Dean.. 08/12/2023 $$ = = = 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) .