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Tuesday, July 13, 2021

SPC Jul 13, 2021 2000 UTC Day 1 Convective Outlook

SPC 2000Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0302 PM CDT Tue Jul 13 2021 Valid 132000Z - 141200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE NORTHERN AND CENTRAL HIGH PLAINS... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OUTSIDE THE ENHANCED AREA IN THE NORTHERN AND CENTRAL PLAINS...AND IN PARTS OF THE CENTRAL APPALACHIANS... ...SUMMARY... Thunderstorms capable of severe wind and hail are possible late this afternoon into the evening from southern Montana across northern and eastern Wyoming and the Black Hills area of South Dakota. Activity may spread farther east through southern South Dakota and northern Nebraska into tonight. Thunderstorms capable of locally damaging wind and a brief tornado will also be possible this afternoon across a portion of the Northeast. Sporadic strong storms may occur over parts of the Southwest as well as the Florida Peninsula. ...DISCUSSION... The first change to the outlook is to expand the enhanced risk area across parts of the central and northern High Plains. The HRRR suggests that a linear MCS will move quickly east-southeastward from northeast Wyoming into southwest South Dakota and northern Nebraska from late afternoon into the mid evening. Significant wind gusts in excess of 70 knots will be possible along the leading edge of the MCS. The second change to the outlook is to add a 2 percent tornado from far southeast Montana east-southeastward into the northern Nebraska, along the track of the MCS. Otherwise, small adjustments were made to the thunder and probability lines but none that where significant. ..Broyles.. 07/13/2021 .PREV DISCUSSION... /ISSUED 1126 AM CDT Tue Jul 13 2021/ ...Northern Rockies into the Central Plains... Within an evolving easterly upslope-flow regime, low-level dewpoints from the upper 40s to mid 50s F will be maintained beneath steep lapse rates, supporting MLCAPE from 1000-1500 J/kg as the boundary layer destabilizes this afternoon. Forcing for ascent associated with a low-amplitude shortwave trough now moving through western MT will contribute to a favorable environment for storms to develop over the higher terrain of southern MT into northern WY, and this activity will intensify as it spreads east into the High Plains during the evening. Effective bulk shear from 45-50 kt will promote some supercells with initial development before activity evolves into an organized MCS farther east toward the High Plains this evening. Large hail and damaging wind including potential for some significant severe gusts will be the main threats. ...Northeast States... A warm front extends through eastern NY into Ontario, while a cold front stretches from a surface low near Lake Michigan southward through eastern IL. The most unstable portion of the warm sector is expected to evolve from PA through western and central NY where ample boundary layer heating is already occurring within a boundary layer characterized by dewpoints in the low 70s F. MLCAPE from 1500-2000 J/kg is likely in this corridor. Meanwhile the gradient between upper high pressure off the east coast and a shortwave trough moving northeast through the OH valley is resulting in a belt of moderate mid-level flow and up to 30-40 kt 0-6 km shear. This environment will support both multicells and possibly some supercell structures, evolving into lines/clusters with time as storms develop in the weakly capped warm sector. Isolated damaging wind and some hail will be the main threats, though a tornado or two are also possible, especially with storms interacting with warm front. ...Florida... Mid-level temperatures are warmer than yesterday. Nevertheless, moderate instability and a destabilizing boundary layer will support potential for a few strong wind gusts as storms develop along west and northwest-moving sea breeze boundaries. ...Southwest States... Similar to the last several days, a hot and relatively moist (dewpoints in the 50s F) boundary layer will result in sufficient instability for afternoon/evening thunderstorms from southern NV into central and southern AZ. The upper wind fields will be rather weak and erratic, but several CAM solutions indicate the potential for storms persisting into the evening and spreading into the lower deserts. This would promote a risk of locally damaging wind gusts. Read more LIVE: spc.noaa.gov
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