Day 4-8 Outlook
Day 4-8 Convective Outlook NWS Storm Prediction Center Norman OK 0357 AM CST Sun Dec 27 2020 Valid 301200Z - 041200Z ...DISCUSSION... ...Wednesday/Day 4 and Thursday/Day 5... The medium-range models move an upper-level trough across the southern Rockies and central High Plains on Wednesday. At the surface, a cold front is forecast to advance southeastward into the mid Mississippi Valley and Arklatex. Thunderstorms appear likely to develop along and ahead of the front during the day due to weak destabilization and strong large-scale ascent. Moisture advection is forecast to continue during the day across the lower Mississippi Valley in the presence of a pronounced low-level jet. In spite of weak instability, very strong wind profiles associated with the low to mid-level jet structure, could enable thunderstorms that obtain a severe threat. Cells that initiate well to the east of the front across the moist sector will have potential to become supercells. The main threat would be for a few tornadoes and isolated wind damage. The potential appears great enough to add a 15 percent contour for parts of the lower Mississippi Valley. On Thursday, an upper-level low is forecast to develop in the southern Plains. In response, the low-level jet in the central Gulf Coast states is forecast to strengthen. Thunderstorms will be possible in the Tennessee Valley and central Gulf Coast states along the axis of the low-level jet and further to the west, along the cold front. Instability is forecast to remain weak from Wednesday into Thursday. However, strong wind profiles should offset this limitation, with the possibility of a severe threat continuing into Thursday. The main severe weather potential would be for an isolated tornado and wind damage threat with clusters that organize during the day. The wind damage threat could become more widespread if a squall-line can develop across the region. A 15 percent contour has been added to parts of the central Gulf Coast for Thursday to account for this possibility. ...Friday/Day 6 to Sunday/Day 8... Southwest mid-level flow is forecast to be maintained from the Southeast to the Eastern Seaboard on Friday. A cold front is forecast to move eastward across the eastern Gulf Coast States and Carolinas. Thunderstorm development will be possible along and ahead of the front. Surface dewpoints in the 60s F along with weak instability and strong deep-layer shear could be enough for an isolated severe threat. On Saturday and Sunday, the system is forecast to move eastward to the Eastern Seaboard as surface high pressure dominates across much of the continental United States. For this reason, thunderstorm potential is expected to be minimal over the weekend in most areas.
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