Day 1 Severe Weather Outlook

Valid 011300Z – 021200Z


Severe thunderstorms capable of large hail and strong wind gusts are
possible across the central Plains this afternoon and evening.

A longstanding mid/upper-level high will remain over the southern
High Plains, with associated anticyclone covering a large portion of
the Plains, Rockies and Great Basin regions. Mean ridging will
amplify north of the high across the northern Plains and west-
central/northwestern Canada. However, a series of small
perturbations — now apparent in moisture-channel imagery over
portions of UT, ID and WY — will offset that larger-scale
amplification as they penetrate the mean ridge over WY and the
Dakotas this period. Downstream, a persistent mean trough will
remain anchored over the lower Great Lakes, Ohio Valley and
Southeast, extending to a slow-moving, weak upper cyclone over the
central/north-central Gulf.

In between, a pronounced northwesterly to northerly mid/upper flow
regime will cover much of the Plains States and Mississippi Valley.
An area of convectively generated vorticity, related to the prior
overnight convection over eastern KS and northeastern OK, may travel
southward across the Arklatex region toward the Sabine Valley,
aiding in potential for strong storms this afternoon in parts of
east TX or western LA. However, this potential appears weakly
focused in low levels and too conditional for severe probabilities
at this time.

The 11Z surface analysis showed a cold front from southeastern QC
across southeastern lake Erie, becoming quasistationary over central
OH, western KY, and central MO, then a warm front to a low over
western KS. Another, outflow-reinforced baroclinic zone extended
from west-central KS southeastward to northeastern OK. Lee
troughing was evident over the central/northern High Plains from
MT-CO. The KS low should drift southeastward through the period,
while the frontal zone north of the outflow air remains rather
diffuse, and the outflow boundary acts as a more-important source of
boundary-layer baroclinicity. A weak trough will extend from the
low north-northwestward over western NE to the lee troughing of the
northern Plains.

…Central Plains…
Scattered thunderstorms are forecast to develop mid/late this
afternoon over portions of the Front Range, adjoining eastern-slope
foothills, and perhaps the Palmer and Cheyenne Ridges. This should
occur as strong diabatic surface heating preferentially erodes
MLCINH over the higher terrain, in the presence of favorable low-
level moisture. Those factors, along with large-scale lift south/
southeast of the shortwave trough(s), should contribute to steep
low/middle-level lapse rates and development of 1000-2000 J/kg
preconvective MLCAPE. Bulk shear will be modest (e.g., effective-
shear vectors around 30-35 kt), given the weak low/middle-level
winds over most of the area, though strong directional shear is
expected. This indicates a predominantly multicellular storm mode,
with hail and some wind at first, then potential for upscale
aggregation and cold-pool growth. That would increase the organized
convective-wind threat with eastward extent across the High Plains
this evening, until convection encounters progressively more static
stability tonight into parts of KS.

Farther east, isolated to scattered convection should continue to
occur episodically across the eastern KS/northeastern OK corridor,
in a regime of low-level warm advection and moisture transport, off
the east rim of the EML. A good deal of the low-level lift will
occur over outflow from prior rounds of thunderstorms that have
trained across this region. The most intense cores may produce
severe hail or gusts, but overall severe potential should remain
isolated and somewhat conditional on mesoscale processes. Isolated
strong-severe convection also may develop for a short time window
between these regimes, over parts of central/southern KS and perhaps
northern OK, as strong heating weakens CINH near an outflow boundary
and surface trough.

…Northern Plains…
Widely scattered to isolated thunderstorms are possible across a
corridor extending from the 15%/slight-risk area northward through
eastern MT and the western Dakotas, near and east of the lee trough.
At least isolated severe hail/gusts are possible. Sufficient
low-level moisture and deep shear should be present for organized
convection, though strength and duration of favorable boundary-layer
diabatic heating is in question due to the extensive cloud cover
downstream from the shortwave perturbations. Still, large-scale
ascent related to the westernmost of those features should still be
over parts of the region in support of convection across this
corridor, during afternoon and early evening, when MLCINH is
relatively minimized. Some modal clustering/upscale growth may
occur to concentrate severe potential further on the mesoscale for a
few hours, though too much uncertainty remains in timing/location of
such processes to assign a small area of greater unconditional
probabilities at this time.

..Edwards/Dial.. 08/01/2019


Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.