SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0652 AM CST Wed Jan 27 2021 Valid 271300Z - 281200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS TODAY ACROSS SOUTHERN GEORGIA AND EXTREME NORTHERN FLORIDA... ...SUMMARY... Thunderstorms will pose a marginal severe-weather threat today across southern Georgia and extreme northern Florida. ...Synopsis... In mid/upper levels, a high-amplitude trough will remain just offshore from the Pacific Coast through the period, as one accompanying cyclone now offshore from OR ejects northeast and is replaced by another developing into essentially the same area by 12Z tomorrow. Associated cold air aloft will contribute to marginal instability for thunder near portions of the CA Coast. Downstream, a strong shortwave trough was apparent in moisture-channel imagery over portions of KS/OK. This perturbation will deamplify somewhat -- but still remain well-defined -- as it accelerates eastward across the Mid-South to the Tennessee Valley today. By the end of the period, the trough should reach the eastern Carolinas. As that occurs, a weak surface low -- analyzed at 11Z near MAI -- should ripple east-northeastward along what is now a quasistationary frontal zone extending from southeastern LA across southern GA. The low should move offshore around 00Z and merge with another cyclone developing in the cool sector over SC. The combined cyclone should deepen substantially late tonight as it crosses Atlantic waters progressively farther offshore from GA and the Carolinas. Behind the low this evening, the boundary should accelerate southeastward across the outlook area as a strong cold front, reaching south FL by 12Z. ...Southern GA/northern FL, eastern FL Panhandle... Widely scattered to scattered thunderstorms are expected to move eastward across the area, mainly ahead of the surface front. A few embedded/transient supercells are possible. A marginal damaging- wind/tornado threat exists through late afternoon. This regime will be bracketed on the north by relatively stable air north of the frontal zone. The shortwave trough and related DCVA/cooling aloft will pass well north of the area, minimizing direct upper support and keeping deep-layer lapse rates weak. This will keep MLCAPE around 500 J/kg or less in most areas. Still, associated tightening of height gradients will strengthen deep/speed shear over the area, contributing to effective-shear magnitudes of 45-55 kt, despite veered near-surface winds. This flow geometry will limit low-level shear and lift, though: 1. Weak MLCINH should enable convective initiation with available frontal and near-cyclone lift anyway, and 2. Forecast soundings do indicate enough hodograph curvature to support pockets of marginal (around 150 J/kg) effective SRH, especially midday over parts of the eastern FL Panhandle and southern GA. The southwesterly surface flow also will advect better-modified Gulf air with greater moisture across the area, supporting greater eastward longevity of convection. Weaker boundary-layer buoyancy and deep-layer lift are forecast with southward extent and time this evening over northern FL, reducing the severe potential. ..Edwards/Gleason.. 01/27/2021
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