SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0642 AM CST Sun Nov 08 2020 Valid 081300Z - 091200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER THE SOUTHERN FLORIDA PENINSULA AND KEYS... ...SUMMARY... A tornado threat may develop from late this afternoon through tonight across south Florida and the Keys, as Tropical Storm Eta approaches. ...Synopsis... A high-amplitude mid/upper-level pattern will exist over the CONUS through the period, anchored by a big, complex, cold-core cyclone over the West, and an anticyclone moving slowly east-southeastward over/off the Mid-Atlantic region. To the south and southwest of the anticyclone, an elongated, quasistationary mid/upper trough and cyclonic-shear zone is evident in moisture-channel imagery from the Arklatex across the central Gulf to near Tropical Storm Eta. The Gulf/Arklatex part of this perturbation will weaken through the period, as the eastern part merges with Eta. The resultant combined cyclone will become the dominant vorticity component of the trough, effectively forming the southern, cut-off member of a Rex-like pattern configuration, in tandem with the synoptic anticyclone to the north-northeast. At the surface, 11Z analysis showed a pronounced cyclone over the SD/MT/WY border area, with cold front south-southwestward across east-central CO, central NM, and northwestern MX. The cyclone should move northeastward across Lake Manitoba late tonight, as the front proceeds eastward across the High Plains and portions of the Upper Midwest. By 12Z, the front should extend near a line from INL-ONL-GLD-TCC-ELP, generally moving slower with southward extent. A dryline now developing over western portions of NE/KS/TX will sharpen gradually, but also, will be overtaken in the central High Plains by that cold front. General thunderstorms are possible late in the period near the front, in the outlined area, but with inadequate moisture/instability for an organized severe threat. ...Eta: South FL, Keys... Increasingly influenced by (and part of) the aforementioned mid/ upper perturbation, T.S. Eta is forecast by NHC to exit central Cuba later this morning and turn northwestward to westward over the Straits and Keys of FL through the period. As it does so, this cyclone is forecast to spread a strengthening low-level wind field (in space and time) across south FL and the Keys, with enlarging low-level hodographs. Under current NHC progs, the most-favored sector of the vortex, north through northeast of center, will shift from east-west across the outlook area this evening and overnight, apace with the westward component of Eta's translation vector. Hodographs will enlarge and become more-favorably oriented during that process. Still, concerns about low-level instability and convective character -- especially with a good deal of dry air aloft being entrained into the blended system per moisture-channel imagery -- preclude more than a marginal unconditional probability of tornadoes at this juncture. Refer to the latest NHC advisories and maps for tropical-related watches/warnings, as well as track/intensity forecasts for Eta. ..Edwards/Kerr.. 11/08/2020
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