Subj : DAY2SVR: Marginal Risk To : ALL From : Mike Powell Date : Mon Sep 11 2023 21:09:00 ACUS02 KWNS 111717 SWODY2 SPC AC 111715 Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1215 PM CDT Mon Sep 11 2023 Valid 121200Z - 131200Z ....THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF SOUTHERN ARIZONA... ....SUMMARY... Isolated severe thunderstorm wind gusts and a couple instances of hail are possible across parts of southern Arizona Tuesday afternoon into early evening. ....Southern Arizona... Several shortwave impulses are forecast to migrate across the southwestern U.S. on Tuesday. Modest westerly deep-layer flow is forecast, with effective shear magnitudes around 25-30 kt evident in most point forecast soundings across southern AZ. Meanwhile, surface dewpoints in the upper 50s to low 60s F and modestly steep midlevel lapse rates, will support MLCAPE to around 1000-1500 J/kg. Strong heating will result in deep boundary-layer mixing and steepening of low-level lapse rates within the sub-cloud layer. While CAMs guidance varies in thunderstorm coverage, at least a couple of organized cells appear possible across southwest/south-central AZ, with an attendant risk for severe gusts. Midlevel lapse rates around 7-7.5 C/km and sufficient instability in the freezing layer suggest a couple instances of near-1 inch hail also will be possible in strongest cells. ....Upper Ohio and Mid-Atlantic vicinity... An upper trough oriented over the upper Great Lakes to the Lower MS Valley Tuesday morning will pivot eastward toward the Appalachians by Wednesday morning. A belt of enhanced southwesterly flow, mainly above 700 mb, will overspread portions of the upper OH Valley into southern Ontario and Quebec in association with this trough. At the surface, a cold front is forecast to extend from northwest OH to central TX Tuesday morning, shifting south and east through the period to become oriented from the Hudson Valley through the NC/VA Piedmont and the northern Gulf of Mexico by Wednesday morning. Ahead of the cold front, generally low 60s F surface dewpoints will be in place. Cloud cover and potentially areas of warm advection showers are expected across parts of the Ohio Valley into the Mid-Atlantic vicinity. This will limit stronger heating, and coupled with poor midlevel lapse rates, instability will be low (less than 500 J/kg MLCAPE for the most part through early evening). Somewhat low-topped convection is expected to develop within strong forcing near the cold front from eastern OH into western PA by late afternoon. While effective shear magnitudes around 45 kt are forecast, 850-700 mb flow will remain modest, around 15-25 kt. Most of the stronger flow contributing to higher effective shear magnitudes is forecast above 600 mb and closer to the top of the cloud bearing layer. Given a lack of stronger flow in the lower (below 700 mb) troposphere, severe potential appears low across the Upper Ohio Valley vicinity. Further south/east across the Chesapeake Bay/Mid-Atlantic vicinity, stronger low-level moisture will be in place (mid 60s F dewpoints). Stronger heating of this more moist airmass will result in somewhat greater instability (500-1000 J/kg MLCAPE) compared to further northwest. However, vertical shear will be somewhat less, and large-scale ascent weaker as the region remains further removed from the surface low over Ontario/Quebec and the core of the jet streak moving through the upper trough. The cold front also will move into the region after peak heating, during the evening. The poor alignment of modest instability/weaker shear with an ill-timed frontal passage should limit severe potential. ...Leitman.. 09/11/2023 $$ --- * SLMR 2.1a * A distant ship, smoke on the horizon.... * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .