Subj : DAY1SVR: Day 1 Convective Outlook To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jul 27 2023 12:52:39 ACUS01 KWNS 271252 SWODY1 SPC AC 271250 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0750 AM CDT Thu Jul 27 2023 Valid 271300Z - 281200Z ....THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE UPPER MISSISSIPPI VALLEY REGION...AND IN PORTIONS OF THE NORTHEAST... ....SUMMARY... Severe gusts and hail (some potentially significant and unusually intense) are possible this afternoon and evening in parts of the Minnesota and Wisconsin. Damaging to severe gusts also may occur today with thunderstorms in parts of the Mid-Atlantic/New England regions, and the central High Plains. ....Synopsis... A persistent mid/upper-level anticyclone will remain centered over NM through the period, but with some eastward amplification of associated ridging across the southern Plains, Ozarks and Mid-South region. To its north, a nearly zonal flow belt will persist from the Interior Northwest (downstream from an offshore cyclone) across the northern Plains/Upper Midwest, Great Lakes and Northeast. This flow belt will be perturbed mainly by a positively tilted shortwave trough -- evident in moisture-channel imagery over the Lower Great Lakes and parts of OH. On last night's 00Z 500-mb analysis, this trough already had begun weakening, with 40-m greater magnitude of height rises behind it than of falls ahead. Deamplification will continue through today as the trough moves eastward across the northern Mid-Atlantic and New England. By 00Z, the positive tilt will increase as the northern part outpaces the southern part, with the 500-mb trough and accompanying vorticity ribbon reaching the ME coastline, southern New England, southeastern NY and eastern PA. At the surface, 11Z analysis depicted a surface low over southeastern ON, with a prefrontal troughs over OH/eastern KY and central parts of PA. A weak cold front extended from the low across northern IN. These features should shift eastward across the Northeast through the period, but mostly will be outpaced gradually by the progression of the mid/upper trough. The western limb of the front was diffuse over northern IL to western IA, to another low in the PIR/9V9 area, but should shift northeastward through the day across the upper Mississippi Valley region. ....Upper Mississippi Valley and vicinity... Scattered, perhaps locally numerous thunderstorms may develop over this region along/ahead of the front this afternoon into evening. While still relatively discrete, thunderstorms should pose a threat for severe to significant (2+ inch diameter) hail and isolated severe gusts. With time, one or two bowing clusters may develop, evolving upscale to offer a better-organized potential for severe gusts on the mesoscale. Any such cluster may offer localized gusts near 75 mph. Rich low-level moisture will be widespread across the region, with upper 60s to low 70s F surface dewpoints already common across much of MN/IA and expected to spread northeastward over much of WI as well. In concert with afternoon surface heating and steep midlevel lapse rates (around 7.5 deg C/km), this will support peak MLCAPE in the 3000-4500 J/kg range in MN/IA (locally higher), decreasing to a still-favorable 2500-3500 J/kg in parts of northern/western WI and the MN Arrowhead where deep-layer moisture and lapse rates each will be somewhat lower. With the belt of mid/upper westerlies over the area, cloud-layer shear will be strong, and effective-shear magnitudes of 30-40 kt will support a blend of organized multicells and at least intermittent supercells. Shear and lift each will decrease substantially with southward extent across IA, with convective coverage uncertain, but likely lower. A conditionally greater potential for early hail and/or subsequent severe wind in a cold-pool-driven MCS may develop over portions of this region, especially MN and/or WI. However, with the lack of substantial mid/upper-level forcing, and somewhat nebulous foci in low levels, too much mesoscale uncertainty exists for greater unconditional probabilities this outlook cycle (especially spatially, but to some extent timing-wise as well). ....Northeast to central Appalachians/Ohio Valley... Scattered thunderstorms will develop near the front/trough later this morning and into the afternoon, shifting eastward across the outlook area, and posing a threat for damaging to isolated severe gusts. Somewhat enlarged hodographs and stronger vertical shear over parts of New England indicate a slight tornado potential as well. The warm sector across the region begins the diurnal cycle characterized by widespread favorable moisture, with mid-60s to low-70s F surface dewpoints common. Although weakening of the mid/upper trough, and its early passage, will limit overall deep- layer mass response, sufficient large-scale lift still should occur immediately preceding it to support a broken belt of convection, amid a combination of low-level lift near the trough, variable diurnal heating (dependent on cloud cover), and weak ambient MLCINH. Peak/preconvective MLCAPE in the 1500-2000 J/kg range should develop from the Chesapeake Bay region northeastward across southern and perhaps central New England, decreasing northeastward over southern ME. Deep-layer flow and vertical shear generally should be stronger with northward extent, with effective-shear magnitudes generally less than 30 kt from eastern PA/NJ southward, and 35-45 kt (supporting some supercell potential) over central/eastern New England. Farther southwest across the central Appalachians and Ohio Valley region, weaker large-scale support and deep-tropospheric wind fields behind the trough will tend to keep convection more poorly organized, though thunderstorm coverage may become locally numerous amid stronger heating and still weak MLCINH. Isolated damaging gusts are the main concern, though the threat for severe (50+ kt) gusts appears marginal at best. ....Central High Plains... Isolated to scattered thunderstorms are expected to develop this afternoon, predominantly over higher terrain along the eastern Front Range and foothills, and Laramie Range vicinity, moving eastward with the prevailing flow around the northern part of the synoptic- scale anticyclone. Isolated severe gusts are the main concern. Sufficient moisture and buoyancy to survive deep mixing across the adjoining Plains, and support relative concentrations of convection and storm longevity, still appear to be concentrated over parts of the eastern WY/NE Panhandle/Black Hills area, and portions of eastern CO toward extreme western KS. Strong surface heating will lead to a very deep boundary layer with high LCL (between 700-500 mb) and values of DCAPE exceeding twice the 500-1000 J/kg MLCAPE apparent in forecast soundings. Lack of greater magnitude of both moisture and winds aloft preclude more than a marginal unconditional outlook for these areas, at this time. ...Edwards/Mosier.. 07/27/2023 $$ = = = 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) .