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Friday, August 27, 2021

SPC Aug 27, 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 Fri Aug 27 2021 Valid 272000Z - 281200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER IOWA...MINNESOTA AND WISCONSIN...PARTS OF THE NORTHERN HIGH PLAINS...AND OVER A SMALL PART OF PENNSYLVANIA...MARYLAND AND NORTHERN VIRGINIA... ...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 will be possible from northern Iowa and southeastern Minnesota into southwestern and central Wisconsin. Isolated downburst winds will be possible from northern Virginia into southern Pennsylvania. Little change was made to the previous outlook. The main threat is expected to be with storms developing during the late afternoon along the warm front from northern IA into southern MN and into WI. A southerly low-level jet should aid air mass recovery near the boundary, with increasing elevated instability to the north. For additional short-term information see mesoscale discussion 1617. ..Jewell.. 08/27/2021 .PREV DISCUSSION... /ISSUED 1130 AM CDT Fri Aug 27 2021/ ...Upper Mississippi Valley region through tonight... A convectively reinforced front extends from southern WI through central IA to a weak surface low in eastern NE. This boundary is expected to move north into southern MN and central WI as a shortwave trough approaches from the west. Modification of the surface layer is already occurring near the boundary across northern IA where winds are veering to southeasterly along with dewpoints rising into the upper 60s F. However, widespread clouds and ongoing areas of precipitation will slow boundary layer recovery farther north. By late afternoon or early evening, sufficient boundary layer recovery might exist to support surface-based storms developing in vicinity of the warm front as a corridor of deeper ascent accompanying the shortwave trough moves over the region. Up to 2500 J/kg MLCAPE and 45 kt effective bulk shear will support supercells with initial discrete modes. Low-level hodograph size will also be sufficient for a few tornadoes, especially in vicinity of the warm front assuming enough boundary layer recovery occurs across southern MN into WI. Eventually storms should evolve into lines/clusters with an increasing damaging wind threat into the evening before activity weakens later tonight. ...Southeast Montana and western Dakotas this afternoon/evening... In the wake of an ejecting shortwave trough near the MT/ND border, an upstream trough over northern ID will progress eastward today, along with an associated cold front. This front will focus ascent, 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. ...Southern PA through northern Virginia this afternoon/evening... A well defined MCV is evident on satellite imagery along the southwest PA / western MD border moving slowly east. Downstream from this feature, strong heating of the moist boundary layer will result in moderate instability this afternoon with 2000-2500 j/kg MLCAPE likely. Forcing for ascent over the higher terrain as well as within the convergence bands associated with the MCV will contribute to numerous thunderstorms developing today. A belt of slightly stronger winds aloft with up to 20 kt westerly winds in the 700-500 mb layer is evident in point forecast soundings, especially along the southern periphery of the MCV. Some of the storms might eventually evolve into line segments and clusters as they spread east with a threat for isolated strong to damaging wind gusts through early evening. Read more LIVE: spc.noaa.gov
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