SPC 2000Z Day 1 Outlook
Day 1 Convective Outlook CORR 1
NWS Storm Prediction Center Norman OK
0314 PM CDT Sun Aug 21 2022
Valid 212000Z - 221200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR PARTS OF THE
OHIO VALLEY...
CORRECTED FOR THUNDER LINE ACROSS SOUTH TEXAS
...SUMMARY...
Isolated strong to briefly severe thunderstorms will continue into
late this afternoon, with the greatest relative threat across parts
of the Ohio Valley.
...20Z Update...
The northern portion of the Slight Risk across OH has been trimmed
in the wake of ongoing convection, though a threat for localized
damaging wind may continue with additional rounds of storms late
this afternoon into the early evening. The Marginal Risk has been
trimmed across parts of north TX, NC, west TN, and northern MS/AL,
based on observational trends and the progression of ongoing
convection and related boundaries. Portions of coastal NC have been
added to the Marginal Risk, where isolated damaging wind gusts will
be possible ahead of an eastward-moving line of storms. See MCD 1708
for more information on the threat across coastal NC.
Otherwise, no changes have been made, with a Slight Risk remaining
in place across parts of the Ohio Valley. See the previous
discussion below for more details.
..Dean.. 08/21/2022
.PREV DISCUSSION... /ISSUED 1120 AM CDT Sun Aug 21 2022/
...Synopsis...
Mid/upper ridging remains generally suppressed across the southern
mid- and subtropical latitudes of the eastern Pacific into parts of
the Southwest, and across the northern Mexican Plateau and Gulf
Basin into the Atlantic. Much of the U.S. will remain under the
influence of a weak branch of westerlies, well downstream of large,
deep troughing centered over the mid-latitude Pacific, roughly
between 140-170 W. This regime includes mean ridging across the
Pacific coast into the Rockies, with an embedded short wave trough
approaching the Pacific Northwest coast. Downstream, broad mean
troughing prevails east of the Rockies, with the most substantive
embedded short wave troughing forecast to continue slowly digging
east-southeast of the mid/upper Mississippi Valley toward the
Appalachians. Another weaker perturbation likely will very slowly
progress across the southern Great Plains.
Milder and, particularly, drier air has already infiltrated much of
the northern Great Plains into Upper Midwest, and will gradually
overspread much of the Ohio Valley, while a similar environment
remains entrenched across much of New England. Seasonably high
moisture content will continue to gradually become increasingly
confined to parts of the Southwest, and Gulf into southern and mid
Atlantic Coast states, by the end of the period.
...East of the Rockies...
The lingering seasonably moist air, coupled with weak mid-level
inhibition and large-scale forcing for ascent, is already
contributing to considerable pre-frontal thunderstorm development
across parts of the southern Great Plains into the Ohio Valley,
lower Great Lakes region and Mid Atlantic. This will continue, with
a further increase in storm coverage with boundary-layer heating
through late afternoon.
While the remnants of elevated mixed-layer air might enhance
potential for localized downbursts across parts of north central and
northeastern Texas by late afternoon, lower/mid tropospheric lapse
rates across most areas are generally weak. Deep-layer mean flow
and shear is also rather weak, but heavy precipitation loading and
downward mixing of 20-30 kt southwesterly to westerly flow in the
850-500 mb layer, to the south and east of the digging short wave
trough, might be enough to contribute to locally damaging downbursts
in stronger storms, mainly across parts of the Ohio Valley.
...Southwest...
Boundary-layer heating and mixing are forecast to remain somewhat
modest for the time of year, with the stronger heating and deeper
mixing mostly confined to the lower Colorado Valley. However, this
may be sufficient, given the relatively moist conditions, to support
sizable CAPE by late this afternoon. Aided by 10-20 kt northerly
mid-level flow, if thunderstorms can cluster/consolidate
sufficiently along the Rim late this afternoon, there appears to be
potential for convection to gradually propagate off the higher
terrain (and particularly toward the Colorado Valley) accompanied by
a risk for strong surface gusts into this evening.
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