SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1226 AM CDT Thu Jul 29 2021 Valid 291200Z - 301200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER A PORTION OF THE MIDDLE ATLANTIC REGION... ...SUMMARY... Thunderstorms associated with wind damage and a risk for a tornado, will be possible from the Ohio Valley, central Appalachians and Mid-Atlantic states on Thursday. ...Northeast through Middle Atlantic area... Upper pattern in this region will be characterized by a broad trough today. MCV/shortwave trough embedded within the synoptic trough will move from the lower Great Lakes to the Middle Atlantic. At the surface a warm front will move northward through the Middle Atlantic, while a cold front advances southeast, reaching PA and the central Appalachians by evening. Thunderstorms should be in progress at the start of the period in association with the MCV over the southern portion of the Great Lakes. Other storms may be ongoing in the warm-advection regime north of the warm front over a portion of the Middle Atlantic. Widespread multi-layer clouds may precede the warm front. However, the warm sector should become moderately unstable during the afternoon with 1000-2000 J/kg MLCAPE supported by the advection of richer low-level moisture and pockets of diabatic heating. Thunderstorms are expected to develop ahead of the cold front in association with ascent attending the southeast-moving MCV as well as along the southeast-advancing cold front. This area will reside within belt of stronger mid-upper flow associated with the synoptic trough supporting 40-45 kt effective bulk shear. Therefore, some supercells may evolve along with line segments with bowing structures as activity develops southeast during the afternoon into the evening. Primary threat appears to be damaging wind and possibly some hail, though low-level hodographs will be sufficient for a few tornadoes, especially if sufficient instability can develop in vicinity of the warm front. ...Ohio Valley through the Midwest region... Trailing portion of the cold front will extend westward through the Ohio Valley and into the Midwest during the afternoon. The pre-frontal warm sector should become moderately to strongly unstable with 2000-3500 J/kg MLCAPE possible as the moist boundary layer warms. Storms will likely develop along the cold front and possibly along any remnant pre-frontal outflow boundaries and spread southward. Winds aloft will decrease with western extent toward the Midwest, but 25-40 kt effective bulk shear will be sufficient for some organized structures, especially given expected degree of instability. Both multicell lines and a few supercell structures will be possible in this regime with damaging wind and hail the main threats. ...Central High Plains region... Easterly upslope flow will become established in the post-frontal region of the central High Plains during the afternoon resulting in corridor of moderate instability from eastern WY into NE. The proximity to the upper ridge suggests forcing aloft will remain weak. However, storms will likely develop over the higher terrain and spread east during the late afternoon into the evening, possibly evolving into a small MCS. Other more isolated storms might develop farther east along the stalled front. Up to 35-40 kt effective bulk shear will support some organized structures with isolated damaging wind and hail the main threats from late afternoon into the evening. ..Dial/Bentley.. 07/29/2021 Read more LIVE: spc.noaa.gov
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