Subj : DAY1SVR: Day 1 Convective Outlook To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Aug 16 2023 12:59:07 ACUS01 KWNS 161250 SWODY1 SPC AC 161249 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0749 AM CDT Wed Aug 16 2023 Valid 161300Z - 171200Z ....THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE UPPER MISSISSIPPI VALLEY TO THE BOUNDARY WATERS AND LAKE SUPERIOR REGIONS... ....SUMMARY... Severe thunderstorm wind and hail are expected across parts of the Upper Mississippi Valley to the Boundary Waters and Lake Superior regions from late afternoon into evening. ....Synopsis... Mid/upper-level ridging was noted from a high over the San Juan Mountains northwestward across the Pacific Northwest, and Southeastward across parts of southwest/central TX. To its west, the center of a broad, cut-off cyclone that has drifted away from the CA coastline will move erratically over the Pacific roughly 250-350 nm west of MRY. In the northern stream, an increasingly well-defined shortwave trough -- apparent in moisture-channel imagery over southern SK -- will dig southeastward and amplify to synoptic scale through the period. By 00Z, the trough should extend from far northwestern ON across northwestern MN to northeastern SD. By 12Z, it should extend from west-central parts of coastal ON across western Lake Superior, WI, and IA, with associated strong/cyclonic flow covering most of the Great Lakes. Downstream, the northern part of a strong shortwave trough now over NY and the inland Mid-Atlantic region will eject over New England and weaken today, while slower-moving vorticity lobes inhabit weak, strongly difluent mid/upper flow over the southern Appalachians and vicinity. The 11Z surface analysis showed a low over Atlantic waters east of NJ, with cold front southwestward over northeastern NC, central SC, central GA, the western FL Panhandle, shelf waters just south of the LA coastline, and parts of south TX. Over VA and the Carolinas, this front should undergo net inland retreat through 12Z tomorrow, amid low-level mass response to the approaching, intensifying mid/upper trough. The remainder of this front, over the Gulf Coast States, should become quasistationary and move erratically near its present position through the rest of the period. A cold front related to the northern-stream trough was drawn across northwestern ND and northern MT, and should move quickly through the northern Plains and upper Mississippi Valley today and tonight, with the main surface low staying in Canada. By 12Z, the front should extend over central/eastern parts of Lake Superior, Upper MI and WI, southwestward across eastern IA, northwestern MO and central KS. ....Upper Mississippi Valley to the Boundary Waters and Lake Superior regions... A line of thunderstorms should develop this afternoon near the front -- initially over Canada, but building across the Boundary Waters region into portions of northeastern/eastern MN and northwestern WI with time. Activity should move eastward to southeastward across the outlook area, offering mainly occasional strong-severe gusts and large hail. Gradual diurnal heating and low-level moisture/theta-e advection should underlie the leading portion of DCVA/cooling aloft preceding the mid/upper trough. Still, with considerable MLCINH likely to remain over the warm sector, and some westerly component of surface winds ahead of the front, the bulk of convective forcing should be very near the front. Activity will encounter a narrow corridor of 1500-2500 J/kg MLCAPE, supported by steepening low/middle-level lapse rates and dewpoints in the upper 50s to mid 60s F. Though the boundary may compel linear forcing, sufficient vertical shear will be present for supercells, with 50-60 kt effective-shear magnitudes common. A strengthening prefrontal LLJ also will contribute to effective SRH exceeding 250 J/kg. Increasing capping, weaker large-scale lift and shallower frontal forcing with southward extent will limit potential south of about the latitude of northern IA and southern WI, based on a strong consensus of convective progs and model forecast soundings. Nocturnal low-level stabilization will reduce severe potential with eastward extent toward the central/eastern U.P. and Lake Michigan. ....Southeast... Scattered thunderstorms in clusters are expected to develop along/ ahead of the front, mainly this afternoon, over a large area from the central/southern FL Peninsula to the northeastern Gulf Coast and coastal plain of GA/SC. Though this activity will be embedded in weak deep-layer flow and minimal vertical shear, damaging to isolated severe wet downbursts are possible in the most intense cells. Subtle large-scale ascent may be noted on the downwind side of midlevel vorticity maxima over part of this region. In low levels, strong heating of a very moist and minimally inhibited boundary layer -- characterized by dewpoints and PW commonly 70s F and 2 inches, respectively -- will underlie a high tropopause. This should lead to peak preconvective MLCAPE in the 2000-3000 J/kg range, in support of strong-downdraft potential. The threat may be locally maximized near outflow and/or sea-breeze boundary intersections and interactions with erratically moving convection. ....Western CONUS... Isolated to scattered thunderstorms are expected to develop throughout the afternoon and into early evening, over a vast area from the Front Range and much of NM across much of the Colorado Plateau and Great Basin to most of the higher ranges in CA. Activity will develop in a broad plume of low/middle-level moisture from the Southwest Monsoon. The northward and westward spread of this moisture has been encouraged by the difluent, substantially poleward-directed midlevel flow east of the Pacific cyclone over the past several days, preceding the reintensification of regional ridging aloft. Strong local heating and weak MLCINH in higher elevations will support development there preferentially, with some activity propagating over adjacent lower terrain (desert basins, other valleys, and in CO, possibly the I-25 corridor. In those lower elevations, well-heated/mixed boundary layers may support brief/isolate potential for localized strong-severe downbursts. However, any severe potential appears too isolated and nebulously focused for an unconditional outlook area at this time. ...Edwards/Jewell.. 08/16/2023 $$ = = = 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) .