SPC 1630Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1130 AM CDT Wed Sep 23 2020 Valid 231630Z - 241200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF LOUISIANA/MISSISSIPPI... ...SUMMARY... A tornado or two may occur this afternoon and evening across southern portions of Louisiana/Mississippi. A few strong to severe storms with hail and gusty winds are possible this evening from northern Minnesota into Wisconsin and western Upper Michigan. A few strong storms may also affect coastal Washington this evening/tonight. ...Southern Louisiana/southern Mississippi... The post-tropical remnants of Beta will continue to move northeastward and gradually phase with a slow-moving shortwave trough over the Ozarks/middle Mississippi Valley. The surface low near the southeast Texas/southwest Louisiana border at late morning will move northeast and reach western/central Mississippi tonight, while a low-preceding warm front and moisture-rich warm sector (low/mid 70s F surface dewpoints) will gradually develop northward across southeast Louisiana/southern Mississippi through the afternoon and tonight. Some cloud breaks have been noted and convection will continue developing within several convergence bands east of the surface low/remnant center. The main risk for a couple of tornadoes will be in a relatively narrow corridor this afternoon and evening as storms move northward and interact with the warm front where 0-1 km hodograph size will be maximized in conjunction with an easterly component to the near-surface winds. ...Northern Minnesota/western Wisconsin/western Upper Michigan... Prior forecast reasoning remains largely unchanged. Steep 7.5-8 C/km mid-level lapse rates will develop eastward through the upper Mississippi Valley in advance of a low-amplitude shortwave trough. This will contribute to modest instability with 1000-1500 J/kg MUCAPE in vicinity of an east-west oriented baroclinic zone, even with somewhat limited low-level moisture. It is likely that storms will develop this evening as deeper ascent accompanying the shortwave trough interacts with the boundary. Effective bulk shear might become sufficient for a few storms to develop modest mid-level updraft rotation which, in conjunction with the thermodynamic environment, will support some threat for severe hail. There may be some tendency for storms to congeal into a small MCS capable of producing a few strong wind gusts as well. ...Coastal Washington... Stronger height falls will occur later today with the strongest cooling aloft and destabilization expected after 00Z across western Washington. A leading band of precipitation associated with warm advection will give way to discrete cells moving rapidly east/inland off the ocean. While only a few hundred J/kg MLCAPE will exist, strong large-scale lift as well as long hodographs may favor a few long-lived cells, possibly with rotation which will contribute to a threat for gusty winds and small hail. ..Guyer/Leitman.. 09/23/2020
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