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Monday, September 13, 2021

SPC Sep 13, 2021 2000 UTC Day 1 Convective Outlook

SPC 2000Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0241 PM CDT Mon Sep 13 2021 Valid 132000Z - 141200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE UPPER MIDWEST INTO THE GREAT LAKES AND A PORTION OF THE NORTHEAST STATES... ...SUMMARY... Scattered strong to severe storms remain possible from eastern South Dakota and southern Minnesota/northern Iowa into Wisconsin late this afternoon and tonight, in addition to parts of southern New York, northern Pennsylvania and New Jersey. A couple of tornadoes could occur along the Texas Coast. ...Discussion... No significant changes have been made to previous forecast. ..Dial.. 09/13/2021 .PREV DISCUSSION... /ISSUED 1128 AM CDT Mon Sep 13 2021/ ...Upper Midwest area through tonight... As a midlevel shortwave trough near the WY/NE/SD border moves east-northeastward toward the Upper MS Valley by tonight, surface cyclogenesis is expected along a stalled surface front that now extends across central IA and eastern NE. This front will move slowly northward today with heating/mixing, but the main northward movement is expected tonight in response to the deepening surface cyclone. Regional 12z soundings revealed a relatively warm elevated mixed layer from LBF-OAX-DVN, which suggests the boundary layer will likely remain capped along the front through the day. Much of the convection today is expected to remain elevated across eastern SD, to the cool side of the front, where isolated large hail will be possible. Late this evening into tonight, as the surface cyclone begins to deepen and the midlevel trough approaches eastern SD, some convection rooted closer to the surface will be possible, immediately in advance of the cyclone and midlevel trough. Wind profiles will increase as will vertical shear, but there will also be some lingering low-level static stability/convective inhibition near the warm front. Thus, the nature of the convection tonight is somewhat in question, and will keep tornado probabilities low for these reasons. Otherwise, a mix of line segments and possibly supercells will be capable of producing occasional damaging gusts and large hail tonight. ...Lower MI to southern NY/northern PA through tonight... A stalled front is draped from northeast PA to southern Lower MI, and this boundary will move little through the day. Aloft, a belt of 40-50 kt westerlies persists, with an embedded perturbation/remnant MCV now moving over southwest Lower MI. An ongoing cluster of elevated storms with the MCV will move across Lower MI through early afternoon, though the environment does not look particularly favorable for damaging winds to reach the ground. The remnants of this storm cluster/MCV could persist long enough to pose a threat for damaging winds/large hail along the front into northern PA, where the storms could be rooted close to the surface with moderate buoyancy, steep midlevel lapse rates, and effective bulk shear near 50 kt. ...Middle and upper TX coast through tonight... Tropical Storm Nicholas continues to organize across the northwest Gulf of Mexico and is forecast to make landfall this evening near Matagorda Bay (see latest NHC advisories for details). Strengthening of wind profiles is expected as Nicholas moves northward and deepens, and there should be sufficient low-level shear/hodograph curvature to support the threat for a couple of tornadoes with outer band supercells starting this afternoon and continuing into tonight northeast and east of the center. Read more LIVE: spc.noaa.gov
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