SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0734 AM CDT Fri Aug 27 2021 Valid 271300Z - 281200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON INTO EARLY TONIGHT FOR PARTS OF THE UPPER MS VALLEY AND THE NORTHERN HIGH PLAINS... ...SUMMARY... Scattered storms are expected today from eastern Montana into the upper Mississippi Valley. Hail and damaging winds will be the main threats, though a tornado or two will be possible from northern Iowa and southeastern Minnesota into southwestern Wisconsin. ...Upper MS Valley through tonight... Widespread overnight convection has weakened since about 09z, though the effective front is located across central IA as a result of convective outflow. This boundary will tend to move northward toward the MN/IA border this afternoon in response to advection and surface heating/mixing, though the influence of the morning convection casts uncertainty on the boundary location this afternoon. Forcing for ascent this afternoon will be somewhat nebulous, though a couple of remnant MCVs located over NE/SD this morning will move east-northeastward roughly toward northern IA and southern MN this afternoon/evening. Assuming sufficient recovery occurs in the wake of the morning storms, strong buoyancy (MLCAPE in excess of 2500 J/kg) is expected along and south of the boundary by mid-late afternoon. If a couple of storms or small clusters can form along the boundary, the environment will favor supercells and some tornado threat given large low-level hodographs with warm advection (effective SRH in excess of 250 m2/s2) and effective bulk shear of 35-45 kt. This threat remains highly conditional, so will maintain low/2% tornado probabilities. Otherwise, damaging winds and large hail can be expected with any storm clusters along the boundary into early tonight. ...Southeast MT into the Dakotas this afternoon/evening... In the wake of an ejecting shortwave trough near the MT/ND border, and upstream trough over northern ID will progress eastward today, along with an associated cold front. This front will focus ascent today, and thunderstorm development is expected this afternoon across southeast MT. Though low-level moisture will be rather modest (boundary-layer dewpoints in the 50s) and MLCAPE will likely remain at or below 1000 J/kg, strong deep-layer shear will favor organized storms/clusters capable of producing occasional severe outflow gusts and isolated large hail through late evening. ...Southeast PA to northern VA this afternoon/evening... Storm coverage is expected to be a little greater today compared to yesterday from near the Blue Ridge eastward, along a weak surface trough and immediately east of a weak midlevel perturbation over WV. Though flow/vertical shear will remain weak, thermodynamic profiles suggest some potential for strong downbursts and isolated wind damage this afternoon, based on precipitation-loaded downdrafts in an environment with MLCAPE of 1500-2500 J/kg and DCAPE of 750-1000 J/kg. ...Lower Keys through early Saturday... Tropical Storm Ida is expected to strengthen while moving northwestward across the western tip of Cuba into the southeastern Gulf of Mexico by early Saturday. The storm is somewhat asymmetric at the moment as a result of some southwesterly shear aloft, with the primary convection/bands north and northeast of the center. Still, it is not clear the outer (northeastern) edge of the envelope for mini-supercells will spread as far northeast as the Dry Tortugas and Lower Keys. Thus, low tornado probabilities will not be added in this update, but this area will be monitored as the structure/intensity of Ida become more clear. ..Thompson/Broyles.. 08/27/2021 Read more LIVE: spc.noaa.gov
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