SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0655 AM CST Wed Dec 16 2020 Valid 161300Z - 171200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF COASTAL NORTH CAROLINA... ...SUMMARY... Severe thunderstorms are possible across parts of coastal North Carolina, and the Gulf coastal areas of west-central/northwestern Florida. ...Synopsis... In mid/upper levels, the split-flow pattern -- now regulated by the strong trough extending from eastern IA to south-central TX -- will become more unified as the trough moves east and deamplifies slightly. Still, a potent shortwave trough should extend by 00Z across portions of OH eastern KY/TN, to near CSG. The trough should become even more compact and move over the NJ/NYC area between 09-12Z. At 11Z, surface lows were drawn just south of the AL Coast, and over Atlantic shelf waters southeast of CHS. A warm front extended from each low, southeastward to east-southwestward over waters of the northeastern Gulf and Atlantic, respectively. A secondary warm front/marine-modification boundary, north of the synoptic front and near the west edge of the Gulf Stream, extended from the Atlantic low northeastward, parallel to the southeastern NC coast then east of HSE near buoy 41001. The latter boundary will move more slowly until the main warm front overtakes it from the deepening low eastward. The Gulf low should move inland obliquely across the central/ eastern FL Panhandle today and weaken, while the Gulf warm-frontal segment moves eastward and only gradually diffuses. Meanwhile the Atlantic low will move north-northeastward over eastern NC and evolve into the dominant, deepening cyclone center over the southern Delmarva region by 00Z. The main cold front will extend from there across the FL coastal bend region and over the eastern Gulf, with a combined warm front over Atlantic waters east of the Hampton Roads region. By 12Z, the primary low should be deeper and occluded offshore from the Mid-Atlantic, with the cold front moving southeastward over central FL. ...Eastern NC... Scattered thunderstorms should move northeastward over the outlook area this afternoon, offering threats for damaging gusts and a few tornadoes. The outlook follows the expected path of the warm sector, east through south of the surface low across easternmost portions of NC. Boundary-layer theta-e in relatively convectively uncontaminated parts of the warm sector should contribute to the development of surface-based effective-inflow parcels with 500-800 J/kg MLCAPE. This is most probable over the Outer Banks and sounds. Favorable destabilization is more uncertain/marginal with westward/ inland extent, given more prior precipitation in the low-level trajectories. Deep-tropospheric wind fields should increase throughout the day as the mid/upper trough and associated strong height gradients approach. This, along with backed low-level winds along and south of the warm front, will yield favorable shear for supercells (e.g., effective-shear magnitudes 45-55 kt and effective SRH 150-300 J/kg, in forecast soundings with surface-based parcels). The threat should ramp up around midday/18Z and diminish markedly after about 00Z. ...Coastal FL Panhandle to Tampa Bay region... An ongoing area of precip -- with isolated, embedded, nonsevere thunderstorms north of the surface warm front -- will cross the eastern Panhandle and northern FL this morning through midday. Behind that, the northern and eastern fringes of a narrow, marginally destabilized slice of warm-sector air will brush along coastal areas of the central/eastern FL Panhandle, coastal bend, and northwestern FL. Greater inland penetration of favorably buoyant air is possible this afternoon over parts of central FL. In all cases, weak deep-layer lapse rates near the decaying warm-frontal zone should keep MLCAPE generally below 1000 J/kg. Though this is not optimal for FL severe, some onshore progression of supercells and/or bowing segments from the Gulf may occur while shear still is favorable, with a conditional threat for a damaging to severe gust or tornado before activity weakens inland. Prefrontal surface flow will veer through the afternoon, amidst larger-scale mass response to the NC coastal cyclogenesis. With time, this gradually will reduce hodograph size and low-level vector shear, and with that, supercell potential. Destabilization should be more marginal with eastward/inland extent across northern FL, especially before the surface veering occurs to advect higher- theta-e Gulf air farther inland. ..Edwards/Smith.. 12/16/2020
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