Day 1 Severe Weather Outlook

Valid 041300Z – 051200Z

…THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS TODAY INTO THIS
EVENING IN THE ARKLATEX AND MID-SOUTH REGIONS…AND LATE OVERNIGHT
OVER THE UPPER TEXAS COASTAL PLAIN…

…SUMMARY…
Marginal severe-thunderstorm potential exists today into this
evening in the Arklatex and Mid-South regions, and late overnight
over the upper Texas coastal plain.

…Synopsis…
The CONUS upper-air pattern will be dominated this period by a
positively tilted synoptic-scale trough — initially analyzed from
the Dakotas to the Four Corners region and central Baja. Several
smaller-scale/shortwave troughs and vorticity lobes will pivot
through the associated cyclonic-flow field as the trough shifts
slowly eastward. By 12Z tomorrow, the trough should extend near
HON-IML-ABQ-FHU then over northwestern MX and central/southern Baja.
To its east, a minor/elongated perturbation — now apparent in
moisture-channel imagery over western OK and northwest/west-central
TX — will eject northeastward and weaken gradually.

At the surface, 11Z analysis showed a cold front from western NY
across western OH, southern IL, the MO/AR Ozarks, southeastern OK,
west-central TX and southeastern NM. By 00Z, the front should
extend from the central Appalachians across the Mid-South near MEM,
through a weak frontal wave (and possibly a closed low) over
southwestern AR, to east and south-central TX. By the end of the
period, the front should reach central VA, middle TN, northwestern
MS, western LA, near the mid/upper TX coast, to deep south TX.

…Arklatex to Tennessee Valley/Mid-South…
Widely scattered thunderstorms are expected to develop on both sides
of the front throughout the period, becoming scattered near the
front this afternoon and evening across parts of the Mid-South and
AR. The main convective corridor should shift eastward across
southwestern/middle TN and northern MS/northwestern AL overnight.
Some of the most intense convection will pose a threat for damaging
to marginally severe gusts, and a tornado cannot be ruled out.

Instability will be a limiting factor, as mid/upper-level lapse
rates remain weak, and low-level lapse rates are tempered by
abundant clouds and prefrontal precip. Still, sufficient boundary-
layer theta-e advection should occur to render areas of MLCAPE
ranging from 800-1200 J/kg across southern AR to around 500 J/kg
near MEM. Kinematically, time series of forecast soundings
reasonably suggest shrinkage of low-level hodographs through the
afternoon, ahead of the cold front. This will occur as:
1. The leading shortwave trough departs the area, and
2. The early-day LLJ weakens and shifts eastward, and flow from the
surface through the boundary layer veers and generally weakens with
time.
Still, even with modest near-surface winds and nearly unidirectional
deep-layer flow, sufficient speed shear will persist to keep
effective-shear magnitudes in the 40-50-kt range.

…Southeast TX late…
During the last few hours of the period (roughly 09Z-12Z), widely
scattered thunderstorms may develop over this region on either side
of the front, in a regime of strengthening moisture, low-level warm
advection and isentropic/frontal lift. While not ideally steep for
severe hail, lapse rates may be increased to around 7 deg C/km,
amidst the early stages of cooling aloft with the approaching
synoptic trough, and overlying the warm-advection regime. Time
series of forecast soundings suggest increasing elevated buoyancy
overnight, rooted near 850 mb. MUCAPE may exceed 1000 J/kg near the
end of the period, with values 300-800 J/kg common after 06Z. Deep-
layer wind profiles will be favorable, with effective-shear
magnitudes reaching the 50-65-kt range.

..Edwards/Kerr.. 02/04/2020

$$

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