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Source: US National Oceanic and Atmospheric Administration

SPC AC 241257

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0757 AM CDT Sun May 24 2020

Valid 241300Z – 251200Z

…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM SOUTH TEXAS
TO EASTERN COLORADO AND THE UPPER MISSISSIPPI VALLEY…

…SUMMARY…
Severe thunderstorms, with large hail and damaging wind, are
possible today into this evening, in a broad swath from south Texas
to eastern Colorado and the upper Mississippi Valley.

…Synopsis…
In mid/upper levels, mean troughing is forecast to shift eastward
over the Rockies through the period, as numerous shortwaves pivot
through the associated cyclonic-flow field. This process will
involve height falls across the Great Plains, and eastward spread of
southwesterlies across most of the nation between the Rockies and
Mississippi Valley. However, loosening of the height gradient will
occur, conterminous with the eastward shift, resulting in little or
no net increase in deep/speed shear over most of the central CONUS.
An embedded shortwave trough — now located over UT, should shift
eastward across CO through the period.

Farther south, a basal/southern-stream perturbation — well-evident
in moisture-channel imagery over the AZ/NM border region southward
over northwestern MX — will pivot eastward to the Permian Basin by
00Z. This feature should acquire convectively generated vorticity
in the latter half of the period and amplify as it moves toward
central/south-central TX. Another southern-stream shortwave trough
was apparent from central/southern AL southward across the Gulf to
the eastern Yucatan Peninsula/western Yucatan Channel region. This
trough, and associated/embedded vorticity maxima, will move slowly
eastward to northeastward through the period.

Surface analysis showed a low near HON with cold front southwestward
to a weak low near HLC, then across parts of the OK Panhandle and
northeastern NM. This front is expected to progress eastward to the
upper Mississippi Valley, central KS, and southeastern NM through
the period. A dryline will oscillate eastward today to the TX
Panhandle and Permian Basin region, southward into Coahuila.

…Southern Plains…
As the basal shortwave trough approaches, large-scale ascent will
increase over the dryline this afternoon. This will coincide with a
rapid decrease in MLCINH from strong diurnal heating of a favorably
moist air mass. The result should be scattered to numerous
thunderstorms forming near the dryline, in a north-south swath from
the TX Panhandle to the Serranias del Burro of northern Coahuila.
Severe gusts and hail — some of the hail being significant/2+
inches in diameter — are possible in the first few hours. The
threat should transition to predominantly wind as the activity
aggregates into a more dense line with embedded bow/LEWP formations
and possibly localized rear-inflow jet development. Convection
should become dominated by forward propagation and move eastward to
the lower Plains of western OK, northwest TX, the Big Country,
Concho Valley, Edwards Plateau, Hill Country, and Rio Grande Valley
between DRT-LRD. Convection growing upscale from high-terrain
Mexican development may extend as far as parts of deep south TX and
the middle/lower TX Coastal Plain tonight, while still at or near
severe levels.

Weak midlevel flow will limit deep shear, with effective shear
generally under 25 kt over most of the area, indicating
predominantly multicellular characteristics. However, storm-
scale/boundary interactions amidst the strong buoyancy may
contribute to isolated, heavy-precip supercellular modes while
convection still is relatively separated in western parts of the
outlook area. Surface dew points should recover from south to north
behind the previous day’s outflow, reaching the 60s east of the
dryline, with 70s southward through the Rio Grande and Coastal
Plain. With steep midlevel lapse rates and a deep troposphere, peak
preconvective MLCAPE values of 3500-4500 J/kg will be common.

…Eastern/northern KS to upper Mississippi Valley…
An ongoing MCS should continue to be largely subsevere for a few
more hours as it moves along and north of a warm front analyzed
across southern IA, with a boundary trailing from the tail end of
the complex into southeastern NE and northern KS. The boundary
should decelerate through the remainder of the morning.

Scattered thunderstorms are expected to form this afternoon
along/ahead of the cold front and residual boundary from the morning
convection, offering the potential for large hail and damaging
gusts. Activity should develop in a plume of favorable low-level
moisture and diabatic surface heating, amidst diurnally weakening
MLCINH. Steep midlevel lapse rates and surface dew points generally
in the 60s F will contribute to 2500-3000 J/kg MLCAPE, but with
modest low/middle-level flow limiting vertical shear. The
predominant mode should be multicellular, with brief supercell
characteristics possible. MCVs moving northeastward from their
current positions over northern and central OK may strengthen
vertical shear and UVV on the mesoscale, in support of convective/
severe potential across this area. However, by contrast, related
swaths of cloud cover and precip will inhibit diurnal heating.
Considerable localized variability in convective intensity and
severe potential will exist within the corridor from KS to IA.

…Central High Plains…
Widely scattered to scattered thunderstorms should form from midday
into early afternoon across portions of the eastern mountains,
foothills, and adjoining higher terrain of the Palmer Divide and
Raton Mesa. Isolated severe downdrafts are possible with the
best-organized convection within a longer plume of precip.

Post-frontal northerlies will veer to some component of easterly
across eastern CO through the afternoon, north of a weak low over
the Raton Mesa area, as the mid/upper-level shortwave trough slowly
approaches. Some progs indicating a confluence/convergence axis
near the Arkansas River or Palmer Divide. Associated upslope flow,
along with heating of higher terrain and accompanying erosion of
MLCINH, will support convective development from midday into the
afternoon. The continued easterly components will aid storm-
relative low-level flow, and strong directional shear will set up
beneath mid/upper southwesterlies, yielding 40-45-kt effective-shear
magnitudes. Buoyancy will be restricted by a lack of more-robust
low-level theta-e; however, modified RAOBs and forecast sounding
suggest effectively surface-based inflow for at least a few hours,
as activity propagates off the higher terrain and over the eastern
CO Plains. 300-600 J/kg MLCAPE is possible atop a diurnally
well-mixed subcloud layer, in support of localized strong-severe
gusts.

…FL Keys…
Areas of precip and embedded thunderstorms are expected to affect
the Keys/southern Everglades region through the period, offering
primarily a heavy rain threat (see WPC excessive-rain outlooks for
more guidance on that potential). A marginal tornado threat also
may develop tonight, within a very moist, low-LCL environment with
increasing low-level shear. As the shortwave trough slowly crosses
the east-central/northeastern Gulf, preceded by divergence aloft,
and a low-level tropical wave drifts westward over the northwest
Caribbean/Yucatan region (per NHC tropical weather discussion),
low-level easterly gradient flow is expected to increase, especially
just above the surface. Time series of forecast soundings suggest
that resultant enlarging of curved low-level hodographs may yield
enough SRH to support rotation in some cells tonight.

..Edwards/Smith.. 05/24/2020

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