SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0757 AM CDT Thu May 23 2024
Valid 231300Z - 241200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
THE CENTRAL PLAINS...
...SUMMARY...
Severe thunderstorms are possible across a broad swath of the Great
Plains this afternoon into tonight, with a few tornadoes and
isolated very large hail. A concentration of severe winds -- some
75 mph or more -- is expected across a portion of the central Plains
this evening into tonight.
...Synopsis...
In mid/upper levels, a progressive flow belt will cover much of the
CONUS, to the southeast through southwest of a persistent, somewhat
flattened, quasistationary cyclone over the Canadian Prairie
Provinces. This cyclone is loosely attached to another that is
located over northern ON, and forecast to devolve into an open-wave
trough as it moves into western/southern QC by 12Z tomorrow. The
southern part of a preceding vorticity lobe will brush the St.
Lawrence Valley and northern New England through 00Z.
Meanwhile, another mid/upper low -- now over the northern Rockies --
should become part of a strong, open-wave trough by 00Z from eastern
MT to eastern UT. This trough should pivot eastward to the Dakotas
and NE by 12Z. To its south, a series of low-amplitude
perturbations and vorticity lobes (some convectively generated/
enhanced) occupied the extensive west-southwest flow field from the
Desert Southwest to the Appalachians. One of these -- now over
southern OK and north TX -- is influencing ongoing convection from
southeastern OK across AR -- and will move across the Mid-South and
Tennessee Valley regions into tonight. Behind it, similar
perturbations are apparent in moisture-channel imagery over
Chihuahua, and over southeastern AZ/southwestern NM. These should
reach central and northwest TX by 00Z.
At the surface, 11Z analysis showed a low over east-central/
southeastern WY near TOR, with synoptic warm front over northern SD
and southern MN. As the shortwave trough approaches, the low should
deepen sharply and move to near PIR by 00Z, with cold front over
western NE and northeastern CO. By 12Z, the low should reach
eastern ND, with the cold front to northwestern IA, southeastern NE,
south-central KS, and the TX South Plains. A warm to
quasistationary front was drawn across the Red River region of TX/
OK, to the lower/middle Ohio Valley, becoming a cold front over
parts of PA/upstate NY. This boundary will move eastward through
the northern Appalachians, New England and much of the Mid-Atlantic
through the period, while diffusely shifting northward amid areas of
convection over the Mid-South and southern Plains. A dryline --
initially analyzed from the Big Bend region of TX across extreme
eastern NM -- will develop northward over western parts of KS/NE
today while the airmass to the east moistens. This boundary also
will shift eastward into the eastern TX Panhandle near the OK
border, as well as northwest/west-central TX through the afternoon.
...Central/northern Plains...
Scattered thunderstorms are expected this afternoon and evening
along parts of the warm and cold front near the surface low, with
supercells possible in the first 2-3 hours. Damaging gusts, large
to very large hail and a few tornadoes are possible. With
relatively strong deep-layer and frontal forcing superimposed ahead
of the progressive Rockies shortwave trough, an organized band of
strong-severe thunderstorms should evolve along/ahead of the cold
front and move southeastward across parts of the Dakotas and NE,
eventually backbuilding into northern KS tonight as the cold front
overtakes the dryline. This will cause the severe threat to
transition more toward wind, with a swath of significant wind (65+
kt) potentially accompanying the QLCS. A few tornadoes also are
possible with embedded mesocirculations near its edge.
Though not optimally moist for the time of year, a corridor of 50s
to low 60s surface dewpoints should become common this afternoon
between the warm and cold fronts, combining with diurnal heating and
steep midlevel lapse rates to support 1500-3000 J/kg MLCAPE. 45-55
kt effective-shear magnitudes should be common, with larger values
possible near the warm front and low (but also, weaker instability).
This will support both early supercells and persistence of the
upscale MCS into parts of the central Plains and lower Missouri
Valley regions. The southern end of well-organized, severe-wind
potential over KS may be limited by residual outflow air advecting
into the area from the south, while the eastern/northern ends are
more uncertain. Although no change was made to the area of greatest
wind probabilities this cycle, some reshaping of the "enhanced" area
may be needed through the day as informed by 12Z and later guidance,
as well as mesoscale analytic trends. However, with longer
persistence of the MCS possible across IA/MO tonight, parts of the
"slight" and "marginal" equivalent probabilities have been expanded
eastward.
...Southern Plains...
The width of the convectively undisturbed warm sector ahead of the
southern Plains dryline is still uncertain, as considerable
variability exists in progs for upscale growth of convection now
manifest as a small but growing cluster over the Edwards
Plateau/Hill Country/Concho Valley regions. This activity may
produce sporadic large hail and damaging wind as well, through the
remainder of the morning, as it proceeds across portions of central
TX, with wind potential conditionally increasing where aggregation
of cold pools can drive strongest forward propagation.
Outflow and related theta-e deficits from any resulting complex will
be advected northward into north TX and OK, with ambient theta-e
decreasing northward into KS. Still, a corridor of favorable
moisture should advect around the west (back) side of any such cold
pool today and support isolated to widely scattered dryline
development from northwest TX into parts of western OK/KS. Any
sustained convection arising from that process may become
supercellular, with all severe modes possible, and very large/
damaging hail a strong possibility. The least-modified air by
earlier activity will have surface dewpoints in the upper 60s to low
70s F beneath EML-related steep lapse rates, fostering 3000-4000
J/kg MLCAPE. Long low-level hodographs will favor splitting storms,
especially in the first couple hours after initiation, with some
LLJ-aided enlargement possible in the 00-03Z time frame before
parcels begin to stabilize unfavorably near the surface.
...Mid-South to Mid-Atlantic and New England...
Isolated to scattered thunderstorms -- sometimes in small bands or
clusters -- will persist through this afternoon along/ahead of the
front from the Northeast to the lower Mississippi Valley. Areas of
relative concentration within this belt will depend strongly on
mesoscale boundary interactions ahead of the front (outflow and
differential heating, as well as a prefrontal surface trough in the
Northeast). Midlevel lapse rates, intensity of diurnal heating, and
boundary-layer moisture each should generally decrease with
northeastward extent. This should contribute to peak/preconvective
MLCAPE in the 2000-3000 J/kg range around the lower/middle
Mississippi and lower Ohio Valleys to 500-1500 J/kg over parts of
the Mid-Atlantic and Northeast. Somewhat favorable deep shear
amidst nearly unidirectional wind profiles will support potential
for occasional damaging to isolated severe gusts throughout the
Atlantic Coast States part of the corridor. A patchy distribution
of marginal severe (hail and wind), with some mesobeta-scale
concentrations possible, is expected across the remainder of the
"marginal risk" swath.
..Edwards/Goss.. 05/23/2024
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