Long-Track Tornado Outlook: 17 December 2023 - Florida Peninsula

 A surface cyclone is forecast to develop over the Gulf of Mexico during Saturday (12/16/2023) as a 250 mb jet streak rounds the base of a trough and becomes coupled with the right rear quadrant of a second jet feature located farther north over the eastern CONUS. The low then tracks northeast across the Florida Peninsula during early Sunday morning (12/17/2023) and a very moist airmass with dewpoints in the 60s to near 70F rapidly spreads northward across the southern and central Peninsula. Flow right above the surface is forecast to be in excess of 50 kt, which will yield very large low-level hodograph structure favorable for rotating updrafts. Lapse rates will be weak and buoyancy will likely be limited to MLCAPE values around 500 J/kg. Convective mode could subsequently be messy, ranging from clusters of cells to short lines and perhaps even a QLCS as the strongest zone of surface forcing progresses inland across the Peninsula during Sunday morning. Nevertheless, the tornado pathlength algorithm I'm testing signals the potential for long-track tornadoes, with predicted pathlengths during the 09-12Z time period exceeding 100 km. Updraft helicity swaths begin to show up in the NAMNEST during the 11-15Z time period over the southern Peninsula, including the Miami metro area, and the predicted pathlength at 11Z using a NAMNEST forecast sounding near Miami is 43 km.

NAM 250 mb winds/heights valid at 12Z 12/17/2023. Note the coupled jet structure developing between the southern and northern stream speed max.

NAM 925 mb winds/heights valid 12Z 12/17/2023. Speeds over the central Peninsula are 50+ kt out of the south.

NAM surface temperature/wind/MSLP valid 12Z 12/17/2023. The surface low near coastal Georgia tracked northeastward from the Gulf of Mexico during the preceding 24 hours.

Surface theta-e valid 12Z 12/17/2023. Moist air surges north with the ejecting surface low, though lapse rates remain weak.

NAMNEST forecast sounding valid for the central Peninsula region at 13Z 12/17/2023. This sounding yields a predicted tornado pathlength in excess of 100 km.

NAMNEST 6-hour updraft helicity swath valid 14Z 12/17/2023. A prominent swath occurs in the vicinity of the Miami metro area.

NAMNEST forecast sounding valid in the vicinity of the Miami metro area at 11Z 12/17/2023. This sounding profile yields a predicted tornado pathlength of 43 km.