SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0753 AM CDT Wed Aug 18 2021
Valid 181300Z - 191200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS IN PARTS OF THE
INLAND MID-ATLANTIC AND CENTRAL APPALACHIANS REGION...
...SUMMARY...
Tornado potential exists today in parts of the inland Mid-Atlantic
and central Appalachians region, over the eastern sector of Tropical
Depression Fred.
...Synopsis...
The main mid/upper-level features for this period will be the trough
related to the remnants of Fred, as well as a large, pronounced
cold-core cyclone that has developed over the interior Northwest.
The latter's 500-mb low -- initially located over the PUW/LWS area
-- is forecast to move/redevelop southeastward to the northeastern
corner of NV by 12Z tomorrow. By that time, associated cyclonic
flow will cover most of the western CONUS, with mid/upper-level
trough from the northern Rockies, through the low, to the lower
Colorado River Valley.
At the surface, 11Z analysis showed a wavy cold front from
southeastern MB to a low near DIK, then roughly southwestward across
central WY, northwestern UT, and eastern/southern NV. The low
should remain over western ND most of the day, then move
northeastward along the baroclinic zone to northeastern ND or
southeastern MB by 12Z tomorrow. In the meantime, the cold front
will move southeastward to the western Dakotas, southeastern WY, and
central/southwestern UT by 00Z, then overnight to the central
Dakotas, while decelerating over southern WY and UT.
...Interior Mid-Atlantic/central Appalachians...
As noted in WPC advisories, Tropical Depression Fred -- initially
near the OH/WV border -- is expected to proceed northeastward over
the central Appalachians region through the day, while continuing a
gradual weakening trend. Nonetheless, a threat for tornadoes and
damaging/isolated severe thunderstorm gusts will develop the next
few hours into midday (see SPC mesoscale discussion 1550), and shift
northeastward over the inland Mid-Atlantic today, before weakening
overnight.
Despite the weak surface wind speeds over most of the area, a
corridor of enhanced gradient flow near the top of the boundary
layer (i.e., around 850 mb) will help to maintain favorable
low-level hodographs and shear vectors in the eastern semicircle of
the remnant circulation. This flow field will overlap very rich
low-level moisture (surface dewpoints commonly in the 70s F).
Theta-e advection, moisture transport and pockets of at least subtle
diabatic/diurnal heating will contribute enough destabilization to
offset weak midlevel lapse rates for MLCAPE 500-1500 J/kg (highest
in southern/eastern areas of the outlook). Meanwhile, effective SRH
200-400 J/kg should shift northeastward in step with the translation
of Fred, most probably superimposed with favorable buoyancy in and
near the "slight risk" outlook area. Tornado potential is more
conditional/marginal with southward extent into greater instability
but weaker shear, and vice versa to the north and northeast.
...Intermountain West...
Widely scattered to scattered thunderstorms are expected to cross
the outlook area episodically through the period, peaking during
afternoon into early overnight hours. Isolated severe gusts and
hail will be possible.
Height falls, strengthening flow aloft/deep shear, and increasing
large-scale ascent preceding the mid/upper cyclone are forecast over
western/northern UT through the day, and in the evening farther east
into more of southwestern WY and central UT, in support of storm
organization. Forecast soundings indicate as much as 40-45-kt
effective-shear magnitudes may develop over the SLC area, near the
northwestern fringes of the favorable buoyancy, with largely
unidirectional deep-layer flow. Frontal lift and diurnal heating/
reduction of MLCINH in the warm sector will contribute to
development, as will upslope sides of elevated terrain, and lift on
outflow/differential/heating boundaries from prior convection.
Favorable low-level moisture will characterize the warm sector, with
PW of 0.75-1.2 inches already commonly noted via GPS sensors. The
ongoing presence of cloud cover and precip over much of the area
remains a complicating factor, likely to delay substantial heating
for several hours. A mesoscale area of greater severe threat may
develop today within the broader outlook, but potential appears too
conditional for such a delineation at this time.
...Mid-South/Tennessee Valley...
Scattered to locally numerous multicellular thunderstorms are
expected to develop and move generally northeastward across the
outlook area from midday into parts of this evening, offering
locally damaging/isolated severe gusts.
A weak but readily apparent southern-stream perturbation -- in
moisture-channel imagery over south-central/southeastern OK -- is
expected to eject northeastward across AR toward the lower Ohio
Valley through the period. This feature should provide both
large-scale lift and slightly enhanced midlevel winds/deep shear on
the mesoscale to its east and southeast, supporting some convective
organization and perhaps relatively dense coverage today. Rich
low-level moisture -- with surface dew points commonly in the upper
60s to mid 70s F -- and diurnal heating will overcome weak MLCINH in
support of 1500-2000 J/kg MLCAPE, suitable for both:
1. Intensification of holdover convection from this morning, and
2. Formation anew along outflow/differential-heating boundaries.
Upscale growth/clustering is possible, with potential for
concentration of strong to marginally severe thunderstorms on the
mesobeta and smaller scales.
..Edwards/Kerr.. 08/18/2021
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