Valid 131630Z – 141200Z
…THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS EASTERN CO AND
WESTERN KS…
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SOUTHERN VA TO
NORTHERN SC WEST ACROSS THE TN VALLEY…
…SUMMARY…
Significant severe hail and wind gusts are possible across the
central High Plains during the late afternoon to mid evening.
Scattered damaging winds are possible from the Tennessee Valley to
the Carolinas and southern Virginia through early evening.
…Central High Plains…
Weak warm-advection driven convection is ongoing across
northwest/north-central NE. To its south, ample low-level moisture
is prevalent. The difference between the NAM/RAP in the degree of
boundary-layer mixing within the warm/moist sector is unusual in
that the NAM is actually quite a bit more mixed at peak heating than
the RAP (typically its the reverse). Surface dew points will
probably mix to 60-65 F, contributing to a confined corridor of
MLCAPE of 2000-3000 J/kg across the CO/KS/NE border area beneath
rather steep mid-level lapse rates. Surface-based storm development
is expected around 21Z near this tri-state region as convergence
strengthens at the intersection of the lee trough and the
southward-sagging cold front. Initial storms will likely be discrete
supercells capable of producing isolated very large hail and perhaps
a tornado or two. Convection should grow upscale into a southward
propagating cluster into a weak nocturnal low-level jet across far
eastern CO/western KS, with a continuing threat for severe wind
gusts into early tonight.
…TN Valley to the Carolinas/southern VA…
A separated belt of enhanced 700-mb westerlies should remain
centered on the KY to VA area through early evening. Areas of
decaying stratiform rain remnants of early morning MCSs are
generally located within this belt of stronger flow, largely south
of 12Z HREF members. Convection is ongoing an arc across eastern TN
into far southwest VA along the remnant outflow. Ample heating is
occurring downstream across the Carolinas and it is possible that
this convection intensifies (see MCD 1722). With flow weakening with
southern extent, the degree of organization becomes increasingly
uncertain, but at least a few multicell clusters should develop with
a risk for scattered damaging winds across southern Virginia and
parts of the Carolinas.
Additional clusters should also form west across the TN Valley along
the remnant outflow towards late afternoon as large buoyancy
develops amid upper 70s surface dew points. Somewhat weaker
effective shear from 15-25 kt should foster multicell clusters with
a risk for isolated to scattered damaging winds.
…Upper MS Valley…
A shortwave trough along the Red River Valley should dampen as it
tracks into WI tonight. Ascent preceding this trough will coincide
with a pocket of stronger surface heating and low-level convergence
along a weak northwest-southeast oriented front, which will support
scattered thunderstorm development towards late afternoon. A
confined corridor of MLCAPE from 1000-1500 J/kg may potentially
coincide with effective bulk shear near 40 kt. However, modest
mid-level lapse rates should temper overall storm intensity despite
the expectation of a few transient supercells. Isolated damaging
winds and severe hail will likely be the main threats. A brief
tornado may also occur with storms moving southeast along the
boundary, where low-level shear and vertical vorticity will be
larger.
..Grams/Mosier.. 08/13/2019
$$