Subj : DAY2SVR: Nws Storm Predic To : All From : Mike Powell Date : Wed May 17 2023 15:41:00 ACUS02 KWNS 171726 SWODY2 SPC AC 171724 Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1224 PM CDT Wed May 17 2023 Valid 181200Z - 191200Z ....THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THURSDAY AFTERNOON AND EVENING ACROSS PARTS OF THE SOUTHERN HIGH PLAINS...AND PORTIONS OF THE UPPER MIDWEST... ....SUMMARY... Strong thunderstorms may impact parts of the southern Great Plains and the Upper Midwest on Thursday. Some of these may pose at least some risk for severe weather. ....Synopsis... Within the prevailing split flow across the Pacific, a broad and deep mid-level low is forecast to evolve in association with strong cyclogenesis to the southeast of the Aleutians during this period. Downstream of this feature, weak mid-level ridging is forecast to build inland of the U.S. and Canadian Pacific coast, while low amplitude troughing lingers in a separate stream near/west of Baja California. Farther east, weak, broadly confluent mid-level flow will generally prevail across much of the U.S. However, an initially vigorous mid-level low may continue digging south of the central Canadian/U.S. border, gradually elongating across the Upper Midwest into middle Missouri Valley by late Thursday night. Downstream of this feature low amplitude ridging is forecast to shift across and east of the lower Great Lakes, in the wake of a vigorous impulse accelerating east of the Canadian Maritimes. At the same time, downstream of the Baja mid-level troughing, short wave ridging may tend to build across the northern Mexican Plateau and southern Great Plains into the lower Mississippi Valley. However, this may be slowed by a smaller-scale perturbation of subtropical eastern Pacific origins, which is forecast to gradually progress around its northern periphery. Farther east, broad, weak mid-level troughing is forecast to continue slowly migrating eastward across the Southeast. In the wake of the stronger lead northern branch perturbation, and the weaker lead southern branch perturbation, boundary-layer moisture across the southern Great Plains into surface troughing to the lee of the southern Rockies will initially be seasonably modest. Substantive moisture advection appears unlikely, but evapotranspiration may contribute to moistening across this region during the day, as well as ahead of a significant cold front accompanying the northern branch low digging south of the international border. A significant preceding cold front appears likely to stall and weaken across parts of the southern Atlantic coast into Tennessee Valley vicinity. ....Southern High Plains... While models suggest that mid-level lapse rates will be initially modest across much of the region, some gradual steepening with lower/mid-tropospheric warming is forecast through the period. During the day, it appears that the boundary-layer, becoming deeply mixed with daytime heating, may become sufficiently unstable to support scattered thunderstorm posing a risk for marginally severe hail and wind. Deep-layer wind fields and shear appear likely to remain generally weak, but it is possible that a 30-40 kt west-southwesterly jet streak in the 700-500 mb layer (accompanying the subtropical perturbation) could enhance convection as it spreads across the southern Texas Panhandle/South Plains into Red River vicinity Thursday evening. An outflow boundary from prior convection, or at least a zone of stronger differential surface heating across the Texas Panhandle into southwestern Oklahoma/western North Texas, may become a focus for stronger lower/mid tropospheric warm advection by Thursday evening. This could support an isolated supercell structure or two, and eventually an upscale growing cluster which could be accompanied by strong surface gusts for a period late Thursday evening, before forcing shifts to the cool side of the boundary and convective intensities wane overnight. ....Upper Midwest... The cold core of the approaching mid-level low/trough is forecast to remain displaced to the northwest of the surface warm sector. Coupled with the modest warm sector boundary-layer moisture, maximum instability is forecast to remain weak and largely confined to a narrow pre-frontal corridor. However, it appears that this may be sufficient to support a developing line of storms by late Thursday afternoon across parts of north central through southwest Wisconsin and adjacent portions of Minnesota/Iowa, aided by favorable large-scale forcing for ascent. This may coincide with sufficient deep-layer shear beneath 40-50 kt southwesterly mid-level flow, to support a few supercell structures posing at least some risk for severe hail and wind gusts, before convection weakens after dark while spreading southeastward. ...Kerr.. 05/17/2023 $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .