Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0255 PM CDT Thu Aug 20 2026
Valid 202000Z - 211200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF
THE MID-ATLANTIC...PARTS OF THE SOUTHEAST...AND FROM THE CENTRAL
PLAINS INTO THE UPPER MISSISSIPPI VALLEY...
...SUMMARY...
Scattered severe storms are possible this afternoon into this
evening across parts of the Mid Atlantic, posing a risk for a couple
of tornadoes and damaging wind gusts. Large hail and isolated wind
damage will be possible late today into tonight with severe storms
from Nebraska into northwest Wisconsin. Scattered damaging gusts
are possible over parts of the Deep South this afternoon into the
early evening.
...20z Update...
The previous forecast largely remains on track with only minor
adjustments made based on recent convective trends and recent
guidance. Regional radar mosaics show the development of discrete
and semi-discrete supercells across the northern MD into southern PA
ahead of a mid-level perturbation and weak cold front. The recent 18
UTC IAD sounding sampled modest buoyancy (around 1000 J/kg MLCAPE),
which seems to be more marginal compared to recent RAP mesoanalysis
MLCAPE depictions. Nonetheless, strong shear was sampled by this
RAOB, including just under 200 m2/s2 effective SRH. This environment
likely exists downstream into far southeast PA and western NJ and
warranted a slight northward expansion of the CIG1 tornado contour.
Confidence in convective coverage with southwestward extent into
southern VA and NC remains limited based on trends in
radar/satellite trends and latest high-res guidance. Other forecast
thoughts outlined in the previous discussion below remain on track.
...Northern MS/AL...
Convection is developing along/ahead of a weak surface front and
mid-level wave across southeast AR into northern MS. This aligns
well with recent guidance, especially RRFS solutions, which depict
increasing thunderstorm coverage through early evening. Wind and
hail probabilities have been adjusted slightly across northern
AL/southern TN to reflect these solutions and capture areas where
air mass recovery continues.
...Northern MN...
A surface low is becoming apparent in surface observations and GOES
visible imagery across northern MN with early stages of convection
noted over the past 45 minutes. With 64-66 F dewpoints spreading
north towards north-central MN, some potential exists for more
robust convection than previously depicted by guidance. Recent
RRFS/HRRR time-lagged ensembles have also trended slightly north
with the strongest UH signals, which matches these recent
observations and warranted a northward expansion of probabilities.
Elsewhere, the forecast outlined below remains on track. See MCD
#2053 for additional short-term details regarding the near-term
severe threat across north-central NE.
..Moore.. 08/20/2026
.PREV DISCUSSION... /ISSUED 1133 AM CDT Thu Aug 20 2026/
...Central Appalachians into the Mid-Atlantic...
Radar mosaic late this morning showed a couple of weak convectively
related vorticity maxima moving east across the central and southern
Appalachians (i.e., southwest PA, eastern KY). One of these lead
disturbances was sampled by the WSR-88D VAD at Jackson, KY, with an
upstream shortwave trough over the OH Valley moving towards the
Mid-Atlantic states.
The airmass across the Mid-Atlantic into the Carolinas will continue
to heat amid patchy but thinning cloud cover. Model guidance shows
moderate buoyancy developing by early to mid afternoon from the
southern Appalachians northward into the northern Mid-Atlantic
region. A belt of stronger 700-500 mb westerly flow (30-45 kt) will
overspread the northern half of VA northward into PA/NJ. Beneath
this stronger flow, southerly low-level flow veering and
strengthening with height will support hodographs favorable for
organized severe storms, including supercells and organized line
segments. The latest model guidance shows a few simulated
supercells developing in the area centered over MD and the upper
Chesapeake Bay vicinity during the 20-00z timeframe. This scenario
seems plausible given some low-level enlargement to the hodograph
and effective shear around 40 kt. A tendency for upscale growth
into a line segment is expected as this activity moves east towards
the coast during the evening. A couple of supercell tornadoes are
possible with this activity, in addition to a line-embedded
mesovortex and scattered damaging gusts.
...Carolinas into the TN Valley and lower MS Valley...
Ample low-level moisture and strong heating will result in an
unstable airmass from the Carolinas into much of the Southeast.
Low-level convergence along a cold front that will extend from the
western Carolinas into AR this afternoon will result in scattered
thunderstorm coverage as 2500-4000 J/kg MLCAPE develops by mid
afternoon. High PW (in excess of 2 inches) and steep 0-3 km lapse
rates and modest westerly to west-northwesterly flow will favor
damaging downbursts with the stronger southeast-moving clusters.
...Central Plains into the Upper MS Valley...
A tongue of relatively rich low-level moisture (mainly 60s F surface
dewpoints) will extend from the central Great Plains northward ahead
of a cold front, which is forecast to be draped from western MN into
western NE this afternoon. Thunderstorm initiation is anticipated
first across the Upper MS Valley before expanding southwestward.
Moderate northwesterly flow aloft will support supercell wind
profiles, with elongated/straight hodographs, and the threat for all
severe hazards. Large to very large hail (i.e. greater than 2 inch
diameter) will be the primary severe hazard, particularly during the
early evening as the storms move into greater instability. Isolated
strong to severe gusts (i.e. 55 to 75 mph) are possible as well.
Given the supercellular storm mode and low-level southerly flow, a
low-probability tornado risk also exists. Some risk for isolated
damaging gusts may persist into the early overnight across southern
NE and northern KS if clustering can
occur.
...AZ...
Not much change from prior forecast thinking in that isolated to
widely scattered storms are forecast but a generally lower risk for
severe gusts is anticipated later today into this evening. Steep
lapse rates and inverted-V profiles will support an isolated risk
for severe gusts with the more intense downdrafts.
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