SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0657 AM CST Sun Nov 29 2020 Valid 291300Z - 301200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF THE CENTRAL GULF COAST TO THE EASTERN CAROLINAS... ...SUMMARY... Thunderstorms may produce a few tornadoes and areas of strong-severe gusts today and tonight, from parts of the central Gulf Coast to the eastern Carolinas. ...Synopsis... An initially split-flow pattern across the central CONUS will become dominated by a northern-stream trough, now evident in moisture- channel imagery from northwestern ON across the Dakotas to eastern WY. This trough will remain positively tilted, but also will strengthen, as it moves southeastward across the mid/upper Mississippi Valley and central Plains through tonight. By 12Z tomorrow, the 500-mb trough should extend near an axis from MQT-MSN- STL-TXK-DAL. In response to these developments, the compact mid/ upper cyclone now centered over southwestern OK will move eastward to southwestern AR by 00Z, by when it should be devolving to an open-wave trough. This perturbation will deamplify considerably tonight, as it ejects across the Tennessee Valley region to the south-central Appalachians. At the surface, a frontal-wave low analyzed at 11Z over the Gulf southeast of GLS and S of the BPT area, which should deepen substantially today and tonight as it moves northeastward and inland. The low should cross southern portions of LA/MS today, to a 00Z position over southwestern AL, continuing to deepen and move northeastward overnight to a 12Z position near TRI. The trailing cold front should reach the MOB/PNS area by 00Z, then by 12Z, extend from the low across the western Carolinas, eastern GA, the FL coastal bend, and the eastern to south-central Gulf. East of the low, a warm to quasistationary front was analyzed just south of the LA coastline and eastward over the northeastern Gulf, across the northern FL Peninsula, through a weak low north-northeast of XMR, then northeastward offshore from GA/SC. This boundary will shift inland over southeastern LA and southern MS this morning, and irregularly northward farther east through the period. Its definition and baroclinicity over parts of AL/GA/SC/NC may be muddled on the mesoscale by areas of precip/outflow. By 00Z, the synoptic warm front generally should extend across southern portions of AL/GA to near coastal parts of SC and southeastern NC. By 12Z, the warm front may reach southern parts of the Chesapeake Bay/ Delmarva area, but still with areas of precip likely on both sides to disrupt the surface theta-e/instability fields. ...Gulf Coast to southern GA... Scattered to locally numerous thunderstorms should develop this afternoon and evening from near the low southward along and just ahead of the cold front, and eastward along and north of the warm front. Given the likely weak MLCINH amidst strengthening large- scale lift, discrete to clustered warm-sector convection also may form by this evening over the Gulf and move ashore across the FL Panhandle. A few tornadoes and sporadic damaging to severe gusts appear to be the main concerns. As the mid/upper low/troughs approach the Southeast, and the synoptic-scale height gradient intensifies over the region, flow will increase throughout the troposphere above the surface (e.g., through the 850-250-mb layer). This will spread strong deep shear and growing hodographs across an incompletely (yet in some places sufficiently) modified Gulf return-flow air mass. Much uncertainty remains about how far north/inland the favorable surface-based buoyancy will extend, given 1. The warm front may not be sharply defined inland, especially with areas of precip and convection on both sides, 2. Extensive clouds and scattered areas of inland precip will mute diurnal/diabatic heating and keep low-level lapse rates modest, and 3. Areas of convection remain possible over parts of the Gulf, potentially restricting advectively driven inland destabilization in some areas. Still, given the strength of the low-level and deep shear forecast, it appears prudent to maintain a northward-gradating slope of "slight" to "marginal" levels of unconditional wind and tornado probabilities, with highly variable threat levels possible on the mesoscale. The regime should shift eastward across the outlook area, reaching GA tonight and northwestern FL the last few hours of the period. However, veering surface flow ahead of the front over FL may begin to limit convergence and low-level shear before the convective band arrives. ...Eastern Carolinas/VA Tidewater... Convective coverage should increase from early this evening onward, in a northward-expanding regime of scattered thunderstorms. This process will be related to strengthening low-level moisture and large-scale ascent ahead of the low-level cyclone and mid/upper troughs, amidst weak CINH. Along and south of the warm front, some of this activity will become rooted in the boundary layer, offering a damaging-wind and tornado threat. As with the Gulf Coast regime, a good deal of uncertainty remains on the inland extent of favorable destabilization along and south of the warm front, as the deepening surface cyclone shifts up the Appalachians to the west. The associated isallobaric response will result in strengthening low-level shear, enlarging hodographs and increasing SRH as the LLJ increases tonight, as well as at least marginally favorable theta-e advection across the coastal plains to the sounds and Outer Banks. A partly modified version of the air mass now evident south of the front over the Gulf Stream (east of GA/FL) should advect northward with and following what should be a rather diffuse warm-frontal passage. The greatest northward and inland extent of favorable boundary-layer theta-e for surface-based convection may occur in the last few hours of the period over eastern NC and perhaps a part of southeastern VA, following a swath of pre-warm-frontal precip. In any event, modest low/middle-level lapse rates will render this a low-CAPE, strengthening-shear scenario, with MLCAPE mostly less than 800 J/kg, and diminishing with inland extent. This will support a likely messy melange of blended convective modes from discrete to densely clustered, in the form of multicells, small bows and at least a few supercells. ..Edwards/Broyles.. 11/29/2020
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