260
ACUS01 KWNS 171300
SWODY1
SPC AC 171258

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0758 AM CDT Fri May 17 2024

Valid 171300Z - 181200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
SOUTHEAST...AND PARTS OF THE NORTHERN PLAINS...

...SUMMARY...
Severe thunderstorms may impact parts of the central into eastern
Gulf Coast states and portions of the northern Great Plains this
afternoon through tonight.

...Synopsis...
In mid/upper levels, a progressive, split-flow pattern will shift
eastward across the CONUS.  In the northern stream, a complex trough
over western Canada and the U.S Pacific Northwest should evolve
gradually to a closed cyclone over central parts of AB and SK by the
end of the period.  A basal perturbation -- evident in moisture-
channel imagery over WA -- will move eastward to western ND by 12Z
tomorrow.  This feature will be preceded by 9-12 hours along a
similar path, by a weaker (yet still influential) shortwave trough
now over the northern Rockies.  This perturbation should reach
western ND at or shortly before 00Z.

The main pattern split occurs around the northwest side of a
separate shortwave trough -- extending initially from the Mid-South
west-southwestward across the Red River region of southern OK/north
TX, then southwestward over the Big Bend region.  This trough should
shed its northeastern portion, lose some positive tilt, and reach
near a LIT-TXK-ALI line by 12Z.  A strong belt of west-southwesterly
mid/upper winds will extend from the northwestern Gulf and east TX
across much of the Southeast.  This flow field will host numerous
small-scale vorticity maxima (some convectively induced/magnified).

At the surface, 11Z analysis showed a wavy, quasistationary frontal
zone extending from just off the SC coast across central FL, to the
northeastern Gulf, where it has been overtaken by an MCS and
accompanying outflow extending to just off the MS coastline.  The
front was evident again from a weak low near ARA southwestward over
the mid/upper TX coastal waters to between MFE-LRD.  The western
segment of this boundary should move southeastward through the
remainder of deep south TX by mid/late afternoon.  Meanwhile, the
front-reinforcing outflow boundary from the MCS should move
northward to northeastward and inland today, gradually becoming more
diffuse amidst a broad, ambient, low-level theta-e advection regime.

...Southeastern CONUS...
Ongoing convection capable of isolated severe potential includes:
1.  A long-lived MCS/bow echo moving east-southeastward from the
northeastern Gulf/Apalachee Bay region across coastal northwestern
FL, mainly moving over relatively stable surface air along and north
of the front.  This complex may penetrate strong to isolated severe
gusts to the surface the next 1-2 hours before weakening.  See SPC
mesoscale discussion 813 for near-term details.
2.  Isolated to scattered thunderstorms, elevated atop the cold pool
from the northeastern Gulf complex, with potential for isolated
severe hail for a few more hours.

Thunderstorms in the expanding warm sector, and along the front,
should pose a risk for a few tornadoes, along with large hail and
severe gusts, this afternoon into tonight.  The main uncertainty at
this time is coverage, and by which preferred mechanism the greatest
lift will occur (related concepts).  Warm-frontal passage and
diurnal heating will combine to destabilize the airmass inland from
southwest-northeast today, with upper 60s to mid 70s F surface
dewpoints.  This will yield MLCAPE in the 2000-3500 J/kg range,
beneath 45-60 kt effective-shear magnitudes, once surface-based
parcels are attained.  A more-focused area of severe potential may
develop within the lengthy corridor outlooked, particularly near the
inland-shifting baroclinic zone where low-level shear should be
maximized.  However, mesoscale uncertainties are still too large to
introduce greater unconditional severe probabilities at this time.

...Northern Plains...
Scattered, high-based thunderstorms are expected to develop this
afternoon over the Bighorns and perhaps farther northward/
northeastward over the Plains of southeastern MT.  Activity then
should sweep east-northeastward across the northern Great Plains
into the eastern Dakotas and perhaps northwestern MN tonight before
weakening.  Along that swath, one or more clusters of cold-pool-
driven convection are possible, offering severe gusts.

A deep, well-mixed subcloud layer will support locally intense
downdrafts, which may also include downward momentum transfer from
strong midlevel winds, and which should be the most common in and
near the 15%/"slight" probabilities.  Surface dewpoints initially
analyzed in the 40s to low 50s across the region should lower to the
mid/upper 30s and 40s today, as diurnal heating and mixing reduce
boundary-layer moisture content.  Nonetheless, steep low/middle-
level lapse rates will be fostered by the approaching shortwave
perturbation and surface heating.  Meanwhile, remaining moisture
will support surface-based buoyancy across much of the area, with
MLCAPE 200-700 J/kg being common.  At least loosely organized cold
pools should drive this activity across a broad swath of the Dakotas
before weakening tonight over or near the Red River of the North.

...South TX...
Ongoing areas of thunderstorms over south TX in the SAT-CRP-LRD
triangle, and approaching deep south TX from adjoining MX, are
moving generally east-northeastward across this region.  Activity
will be capable of isolated, episodic large hail through midday,
amid favorable deep shear and elevated buoyancy, before shifting
east of the area.

..Edwards/Goss.. 05/17/2024

$$