Day 1 Severe Weather Outlook

Valid 191300Z – 201200Z


Thunderstorms associated with Tropical Storm Nestor may produce a
few tornadoes, along with isolated damaging wind, from the central
Florida Peninsula to the Carolina Coast.

In mid/upper levels, synoptic ridging will shift eastward across the
remainder of the Great Lakes, Ohio Valley and Mid-Atlantic regions,
reaching central QC and New England by 12Z. Upstream, broadly
cyclonic flow will cover an increasing proportion of the CONUS. A
strong shortwave trough — now evident in moisture-channel imagery
over the Pacific Northwest — will dig east-southeastward and
develop a closed 500-mb low around the end of the period, near the
Black Hills. As this occurs, a corresponding surface low will
deepen, with a cold front moving out of the central Rockies to the
adjoining High Plains late tonight.

Another shortwave trough will eject from the central/southern Plains
toward the Ohio Valley and southern/central Appalachians. As this
occurs, Tropical Storm Nestor is forecast by NHC to move
northeastward across GA and SC to southern NC by the end of the

…FL to coastal Carolinas…
The potential for episodic supercells — and related threat for a
few tornadoes and isolated damaging gusts — will continue over much
of central/northern FL through the remaining forenoon hours. See
tornado watch 675 and related mesoscale discussions for the latest
near-term guidance.

This threat will be episodic and discontinuous spatially, and will
exist both:
1. Near the diffuse inland part of the surface warm front (where
backed flow and SRH are maximized, but instability will be on the
marginal side due to extensive antecedent precip), and
2. Eastward and southward across the warm sector, where SRH is
somewhat less (but still favorable), and lift will be maximized in
convective/convergence bands arching through the middle-outer
eastern semicircle of Nestor. Isolated, discrete to semi-discrete
supercells also may occasionally develop in this regime, as has
occurred during prior overnight hours. Though cloud cover and
precip will remain extensive inland for several more hours, at
least, this potential may be maximized anywhere that sustained
insolation can boost CAPE and strip away whatever MLCINH remains.

Supercell potential will wind down across FL from southwest to
northeast this afternoon into evening, as dry slotting aloft and
veering low-level flow with time reduce thunderstorm coverage and
low-level shear, respectively. Meanwhile, and continuing through
the overnight hours, the risk will shift northeastward and spread
slightly inland with the increasingly sheared warm sector across
coastal sections of GA and the Carolinas. Uncertainty lingers as to
how far, but more inland leeway has been drawn into the marginal/2%
and slight/5% tornado lines in deference to the strong onshore/
inland theta-e advection probable in this scenario. This is in
keeping with most guidance regarding at least weak MLCAPE along and
southeast of the warm front. This front will continue to be a
buoyancy-limiting boundary, with favorable shear on both sides but
supportive CAPE along and southeast of it.

See NHC advisories for latest tropical watches/warnings and
track/intensity forecasts regarding Nestor.

..Edwards/Dean.. 10/19/2019


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