SPC 0700Z Day 2 Outlook
Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1155 PM CST Wed Feb 24 2021 Valid 261200Z - 271200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS MAINLY FRIDAY EVENING ACROSS PARTS OF SOUTHEASTERN ARKANSAS...NORTHERN MISSISSIPPI...NORTHWESTERN ALABAMA AND PORTIONS OF WESTERN AND MIDDLE TENNESSEE... ...SUMMARY... A few strong thunderstorms are possible in a corridor across parts of the northwestern Mississippi Delta region and adjacent southeastern Arkansas into parts of middle Tennessee Friday evening, accompanied by at least some risk for severe weather. ...Synopsis... The southern mid-latitude eastern Pacific blocking ridge may undergo some suppression during this period, as the first in a series of significant short wave troughs progresses into/around its northwestern periphery. However, ridging will remain a prominent influence across much of the eastern Pacific. At mid-levels, models indicate that the broad ridge axis within the westerlies may take on more of a positive tilt across the northeastern Pacific, with ridging building into the British Columbia and adjacent Pacific Northwest coast by 12Z Saturday. Downstream flow will take on an increasingly northerly component across the interior West, with a significant embedded digging short wave trough contributing to larger-scale trough amplification across the Intermountain West and Rockies. As this occurs, one preceding short wave impulse is forecast to accelerate east of the northern Rockies, toward the Great Lakes region. At least broadly cyclonic mid-level flow is expected to prevail across much of the interior of the U.S., but models indicate that subtropical ridging will begin to build northeastward across the Gulf of Mexico into portions of the Southeast. And mid-level ridging in the westerlies will shift ahead of the lead short wave trough, across much of the Northeast. It appears that an initially vigorous short wave impulse progressing through the Ozark Plateau by 12Z Friday, may undergo considerable deformation while accelerating northeastward toward the northern Mid Atlantic coast region. In lower levels, the leading edge of southward advancing Arctic air may spread across the central Canadian/U.S. border area by late Friday night. Otherwise, a warming south-southwesterly to west-southwesterly return flow appears likely to continue to develop east of the Rockies, in the wake of expansive cold surface ridging shifting east of the Atlantic Seaboard. It does appear that one remnant frontal zone, initially extending across the Gulf into south Atlantic coast states, may tend to surge north-northeastward, beneath a 40-70 kt southerly 850 mb jet axis across the Tennessee Valley toward the Cumberland Plateau. ...Ark-La-Tex into Cumberland Plateau... Supported by forcing associated with the impulse progressing across the Ozark Plateau, thunderstorm activity may be ongoing along the frontal zone in a corridor across the Ark-La-Tex into parts of northern Alabama early Friday. This activity is expected to gradually diminish/decrease in coverage through the day, but some thunderstorm development may persist and spread with the mid-level impulse toward the Cumberland Plateau, before diminishing further. In the wake of this activity, forcing associated with the short wave emerging from the northern Rockies may contribute to renewed thunderstorm development to the west and northwest later Friday through Friday night. Stronger storms may tend to focus along a developing area of enhanced lower/mid tropospheric warm advection near the northwestern Mississippi Delta region by early Friday evening. This probably will be on the leading edge of better low-level moisture return, in the presence of modestly steep mid-level lapse rates, which may contribute to CAPE of 250-500 J/kg. Coupled with the strong low-level shear, it is possible that this environment could become conducive to the evolution of supercell structures while developing northeastward with low-level jet, toward middle Tennessee later Friday night. Latest NAM forecast soundings suggest that this will be rooted above a residual near-surface stable layer, but this may be shallow enough to allow for some risk for potentially damaging surface gusts, in addition to potential for small to marginally severe hail. ...Columbia Plateau/Snake River Basin vicinity... Insolation beneath a 500 mb cold pocket to -35 C may contribute to weak boundary-layer destabilization and convective development across a sizable portion of the northern intermountain region Friday. While thermodynamic profiles may be largely below freezing, it appears that they could still become conducive to some lightning production, mainly where the boundary layer is able to warm above freezing in the lower elevations. ..Kerr.. 02/25/2021
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