SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1248 AM CDT Thu Jul 20 2023 Valid 201200Z - 211200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM EASTERN COLORADO AND WESTERN KANSAS INTO THE OKLAHOMA/TEXAS PANHANDLES AND NORTHWEST OKLAHOMA... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF THE GREAT LAKES/MIDWEST...THE TENNESSEE VALLEY AND SOUTHEAST... ...SUMMARY... Scattered severe thunderstorms are expected across parts of the central/southern High Plains, Great Lakes/Ohio Valley, and the Tennessee Valley into parts of the Southeast. The greatest concentration of severe wind may occur over parts of eastern Colorado and western Kansas into northwest Oklahoma. ...Central/Southern Plains Vicinity... A shortwave impulse is forecast to eject from the central Rockies into the central Plains this afternoon/evening. This will result in increasing northwesterly mid/upper flow, while a modest southerly low-level jet develops during the evening across the southern High Plains. At the surface, a cold front will sag southward and stall across northeast NM, and extend eastward across the TX Panhandle into northern OK. Easterly post-frontal upslope flow will maintain a band of 60s F dewpoints along and north of the frontal zone into eastern CO/western KS. Thunderstorms will develop in upslope flow near higher terrain and increase in coverage/intensity as they shift east/southeast from late afternoon into the evening. With time, a bowing MCS is expected to develop across far eastern CO or western KS and shift east/southeast into tonight along a strong instability gradient/baroclinic zone oriented near the OK/KS border vicinity. Initial semi-discrete cells developing over CO into KS will pose a risk for large hail. Once clustering and upscale development occurs, the risk for a swath of damaging wind (with some gusts greater than 75 mph) will increase from far eastern CO/western KS into northwest OK. The greatest potential for significant wind appears to be from around 23z through 06z. ...Great Lakes/Ohio Valley toward NY/NJ... An upper trough will shift east across much of the Midwest today and become oriented from western NY/PA to coastal NC by Friday morning. This will result in a large swath of enhanced mid/upper low flow ahead of an eastward-advancing surface cold front and atop a seasonally moist airmass. By midday, the cold front is forecast to be oriented from northern Lower MI into northern IL/MO. Mid 60s to low 70s F dewpoints beneath a pocket of steeper midlevel lapse rates will support a corridor of strong destabilization (MLCAPE to 3000 J/kg) from near IN into southern Lower MI and western OH. Some early day convection is possible in a warm advection regime ahead of the approaching trough and surface front. This activity may limit overall thunderstorm coverage later in the day, precluding higher severe probabilities. Though early day convection still may pose an isolated risk for large hail/strong gusts. An additional band of severe thunderstorms is expected during the afternoon along the cold front. Effective shear magnitudes around 35 kt will favor transient supercells. Elongated forecast hodographs and large instability suggest large hail will be possible. A few instances of 2+ inch diameter hail appears possible across a small area from southeast Lower MI into northeast IN/northwest OH. As forcing for ascent lifts northeast toward PA/NY, convection will increase during the evening and overnight hours. A band of thunderstorms posing a risk for sporadic strong gusts will spread eastward across PA/NY overnight, and toward the I-95 corridor by Friday morning. ...TN Valley vicinity toward GA/SC... An MCV/convectively enhanced vorticity maxima are forecast to migrate through northwesterly flow from the TN vicinity toward the SC coast today into this evening. CAMs and ensemble guidance is in good agreements that one or more clusters of storms, possibly developing into a forward-propagating MCS will spread southeast across the region. A very moist and unstable airmass amid 30-40 kt midlevel west/northwest flow should support damaging wind potential across the region. ..Leitman/Wendt.. 07/20/2023
