12AM Day 2 Convective Outlook for Friday, February 20. THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS LATE FRIDAY EVENING INTO FRIDAY NIGHT ACROSS PARTS OF NORTHERN MISSISSIPPI ALABAMA GEORGIA AND PERHAPS ADJACENT PORTIONS OF SOUTHERN TENNESSEE
SUMMARY
Widely scattered, but generally weak, thunderstorm activity is possible Friday east of the lower Great Lakes into Mid Atlantic and Southeast. A more substantive increase and clustering of thunderstorms appears possible across parts of the northern Gulf Coast states late Friday evening into early Saturday, which may pose at least some risk for severe hail and localized strong, but generally sub-severe wind gusts.
Discussion
Models continue to indicate that mid/upper flow will undergo renewed amplification across the central into eastern mid-latitude Pacific through this period. This is likely to include a significant short wave trough digging south-southeast of the Gulf of Alaska (roughly along 140W longitude), to the east of building ridging across and north-northwest of the Aleutians. Farther east, mid-level ridging is forecast to build inland of the Pacific coast, with remnant larger-scale downstream troughing developing eastward across the Rockies through Mississippi Valley.
There remains spread among model output concerning the continuing strength of a short wave perturbation, which earlier emerged from this troughing, after it reaches southwestern portions of the Great Lakes region by the beginning of the period. It is generally forecast to become sheared and weaken as it progresses to the south of a blocking mid-level high centered near southern Hudson Bay, and downstream troughing digging into the Canadian Maritimes and northern New England. However, it appears possible that it may maintain considerable strength east-northeastward across the lower Great Lakes region through the day Friday, while its associated occluding surface cyclone weakens across Michigan.
It continues to appear that secondary surface cyclogenesis will be slow/subdued from the lee of the Blue Ridge across and east-northeast of the northern Mid Atlantic, before perhaps undergoing more substantive deepening offshore Friday night. The trailing surface frontal zone probably will tend to stall across the Carolinas, while retreating northward near the southern edge of the strong westerlies across the Gulf Coast states by 12Z Saturday, in advance of a short wave perturbation emerging from the Southwest.
Further low-level moistening associated with return flow off the Gulf is probable along and south of the front. However, warm layers aloft, near the northwestern periphery of mid/upper subtropical ridging centered across the Bahamas/Caribbean, may tend inhibit destabilization and thunderstorm development through much of Friday into Friday evening.
Gulf Coast States
While mid/upper forcing for ascent associated with the short wave trough emerging from the Southwest may remain well to the cool side of the surface frontal zone, it appears that strengthening lower/mid-tropospheric warm advection will support potential for increasing thunderstorm development by late Friday evening and continue overnight. There appears a consensus among latest model output that this will focus across parts of northern Mississippi, Alabama and Georgia by 12Z Saturday. While this is likely to become rooted above a cool, stable near-surface layer, forecast soundings indicate that an initially warm/dry capping layer above the moist layer will contribute to convectively unstable thermodynamic profiles, which may become conducive to severe hail, in the presence of strong cloud bearing layer shear. Potential for severe gusts appears low, but as activity strengthens and perhaps organizes overnight, gusty surface winds may accompany activity.