SPC AC 181257
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0757 AM CDT Sat May 18 2024
Valid 181300Z - 191200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF
NORTHERN FLORIDA AND THE FLORIDA PANHANDLE TO PORTIONS OF THE
SOUTHERN ATLANTIC COAST...
...SUMMARY...
The greatest severe-thunderstorm potential today appears to be from
the Florida Panhandle eastward to the southern Atlantic Coast, with
damaging gusts the main concern.
...Synopsis...
Distinct northern- and southern-stream features over nearly opposite
parts of the CONUS will continue to exert the greatest influence on
severe-thunderstorm potential. In the North, a well-developed,
zonal to cyclonic mid/upper-level jet core was evident from the
coastal Northwest to the northern Plains. Some amplification in
that belt is likely overnight, as a shortwave trough digs
southeastward across the Pacific Northwest.
Downstream, a compact but strong shortwave trough -- evident in
moisture-channel imagery over western ND and southeastern SK --
should move east-northeastward to southern MB and northern MN by
00Z, forming a closed 500-mb low over the Lake Winnipeg vicinity.
The attached trough should eject northeastward to northwestern ON by
12Z tomorrow. The associated surface cold front -- drawn at 11Z
over eastern MN to near FSD, EAR, GLD and PUB -- should reach Lake
Superior, northern WI, central IA, and south-central KS by 00Z,
stalling across southwestern KS into eastern CO. By 12Z, the front
should reach northern Lower MI, northern to west-central IL, and
northern MO, while moving northward as a warm front near the I-70
corridor in KS.
Meanwhile, a positively tilted mid/upper-level shortwave trough --
with several associated vorticity maxima -- was evident in
moisture-channel imagery from KY across the Mid-South to
south-central TX. The main vorticity lobe -- now over the
northeastern AR/western TN area -- should shift eastward to middle
TN by 00Z, with trough southwestward to TX shelf waters of the Gulf.
By 12Z, the trough should extend from the western Carolinas across
southern AL. The associated surface low -- analyzed at 11Z near MEM
-- should move eastward today and devolve into a weak trough. An
outflow-reinforced marine front was analyzed over shelf waters off
the middle Texas Coast to just southeast of the Mississippi River
mouth to near PNS, then across southern parts of GA and SC. This
boundary should drift eastward over the north-central Gulf Coast
region and across GA.
...Southeast...
Scattered thunderstorms in clusters -- including occasional/embedded
supercells -- have been persistent this morning just offshore from
the LA/MS/AL coastline to near the western FL Panhandle, reinforcing
the marine/outflow boundary on the southern margins of that
convection. That, and persistent rainfall north of the boundary
across southeastern LA to the AL Gulf Coast, have led to decreasing
unconditional severe potential over those areas.
Instead, the greatest threat today should be with the eastern/inland
extent of the loosely organized Gulf convective plume, which extends
along and north of the boundary over the FL Panhandle and southern
GA. Damaging wind will be the main concern, with a tornado or two
possible, and isolated large hail. This region should continue to
exhibit the greatest low-level moisture/instability through the day
as the convection undergoes a net eastward shift, with surface
dewpoints commonly in the 70s F, areas of cloud-tempered diurnal
heating, weak MLCINH and MLCAPE reaching the 2000-2500 J/kg range.
Deep shear will favor organized convection, with effective-shear
vectors continuing to be commonly in the 40-50-kt range -- albeit
aligned with a substantial component parallel to the main swath of
convective lift. As such, mode should remain somewhat messy,
clustered to linear, with some embedded supercell/bow structures
possible.
Elsewhere, less-organized convection (mainly due to weaker shear)
should occur near the peninsular FL Atlantic Coast, and ahead of the
midlevel vorticity lobe and surface low/trough, from the Mid-South
across the Tennessee Valley. Isolated damaging gusts and marginally
severe hail will be possible.
...Midwest...
Thunderstorms should develop from mid/late afternoon into early
evening along the cold front -- with coverage diminishing from
scattered across the Lake Superior and WI/northeastern IA regions to
widely scattered or isolated from southern IA to portions of KS.
Damaging gusts will be the main concern, though any sustained,
relatively discrete cell may rotate and produce locally large hail
as well.
Development should occur later over the MO/KS segment than farther
north, due to the presence of capping related to an EML. The
strongest flow aloft and deep shear should remain behind the surface
front, with effective-shear magnitudes generally ranging from around
45-50 kt over the MN Arrowhead to less than 30 kt over southeastern
KS. Low/middle-level lapse rates and boundary-layer moisture also
should increase with southward extent, but the opposite will be true
for deep-layer lift/forcing. A narrow corridor of favorable MLCAPE
should develop today ahead of the front, ranging from around 1000
J/kg either side of western Lake Superior to 2000 J/kg over east-
central KS. These offsetting factors and the probable quick
evolution to quasi-linear mode in northern areas -- combined with a
narrow east-west and temporal window for the most vigorous
convection before nocturnal stabilization contribute to weakening --
support keeping unconditional probabilities at marginal levels for
this update.
...Central High Plains...
Widely scattered, high-based, somewhat skeletal thunderstorms are
expected to develop over the mountains and foothills of central/
south-central CO this afternoon near the front, and move
east-northeastward over the adjoining High Plains from late
afternoon into parts of tonight. The main concern will be isolated
strong-severe downbursts. Activity will be supported initially by
diurnal heating of higher terrain, with marginal but adequate
moisture to support convection at those altitudes. While strong
gusts are possible in the mountains, the activity should move atop a
deep, well-mixed subcloud layer over the downshear central High
Plains. That deep boundary layer will be characterized by nearly
dry-adiabatic lapse rates, and enough moisture to support 200-500
J/kg MLCAPE. Eastward extent of the threat is uncertain, but in
general, should diminish overnight as nocturnal stabilizing of the
near-surface profile proceeds.
..Edwards/Goss.. 05/18/2024
NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z
SPC AC 180550
Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1250 AM CDT Sat May 18 2024
Valid 191200Z - 201200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
OF WESTERN AND CENTRAL KANSAS...
...SUMMARY...
Severe thunderstorms are most likely across parts of Kansas Sunday
afternoon and evening. Significant damaging gusts, large to very
large hail, and a few tornadoes all appear possible.
...NE/KS/OK...
A weak upper shortwave impulse over NE Sunday morning will quickly
lift northeast. Some convection may be ongoing across NE/KS during
the morning, but is forecast to quickly weaken/shift east. This
early activity may suppress severe potential across northeast NE
during the afternoon as airmass recovery is uncertain. By afternoon,
another upper shortwave trough is forecast to eject from the
southern/central Rockies into KS, and then the Lower MO Valley by
Monday morning. This secondary shortwave trough will be the focus
for the primary severe risk during the afternoon/evening.
A surface dryline is forecast to extend southward from near the
NE/KS/CO border into western KS and then near the OK/TX border. To
the east of the dryline, dewpoints into the low/mid 60s are expected
across OK into central/eastern KS (somewhat lower across northern KS
and southern NE). Very steep midlevel lapse rates (greater than 8
C/km) will be in place. Capping will suppress convection until
mid/late afternoon, when large-scale ascent increases amid strong
heating and continued warm/moist advection. High-based storms along
the dryline will initially pose a risk of large hail and damaging
gusts. Forecast guidance (including HREF and CAMs) show a strong
signal for upscale growth into an intense bow is possible across KS.
Upscale growth is possible via consolidating outflow and an
increasing low-level jet during the late afternoon/early evening.
Significant gusts will be possible if this scenario evolves as
expected. Large to very hail and a few tornadoes also will be
possible. Hail greater than 2 inches in diameter will be possible
early in convective evolution, but eastward extent of this risk is
uncertain and dependent on storm mode.
Severe potential southward along the dryline into western OK is more
uncertain and conditional. If a storm can develop and be maintained,
all severe hazards would be possible, particularly very large hail
and damaging gusts.
...SD vicinity...
Southerly low-level flow will transport mid/upper 50s F dewpoints
northward into northern NE/SD and southern ND. A cold front is
forecast to develop eastward during the late Sunday afternoon into
Sunday night. Steep midlevel lapse rates will be in place, but an
EML will likely limit a greater severe risk. Nevertheless, MUCAPE to
around 1500 J/kg will support elevated convection ahead of the cold
front. Isolated hail will be the main risk. However, if an organized
line of storms can develop, some gusty winds also are possible.
...Lower MO Valley toward the Mid-MS Valley...
Convection from NE/KS may dissipate over the area during the
morning. However, a warm front is expected to slowly lift northward
through the day, allowing from some airmass recovery and
destabilization. Convection may redevelop near this boundary during
the afternoon. This activity may produce marginally severe hail and
gusty winds.
...FL...
An upper trough from the Mid-Atlantic to the northern Gulf will
shift east over the Atlantic on Sunday. A belt of enhanced
west/southwesterly flow (30-40 kt at 700 mb) will overspread the FL
peninsula ahead of the trough. Mid to upper 60s F surface dewpoints
and strong heating will contribute to MLCAPE values around 1500-2500
J/kg. Steep low-level lapse rates will support isolated strong gusts
in water-loaded downdrafts. Elongated/straight forecast hodographs
and modest midlevel lapse rates (6.5-7 C/km) suggest isolated large
hail also is possible. Effective shear around 30 kt suggests
organized updrafts may be somewhat transient. If confidence
increases regarding coverage of organized severe convection, higher
probabilities may be needed in subsequent outlooks.
..Leitman.. 05/18/2024
WUUS02 PTSDY2
SPC AC 180727
Day 3 Convective Outlook
NWS Storm Prediction Center Norman OK
0227 AM CDT Sat May 18 2024
Valid 201200Z - 211200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF
NEBRASKA...KANSAS...SOUTHWEST IOWA AND NORTHWEST MISSOURI...
...SUMMARY...
Severe thunderstorms are possible across parts of Nebraska and
Kansas into western Iowa and northwest Missouri Monday afternoon
into Monday night. More isolated strong to severe storms may extend
into parts of the Middle Mississippi Valley and southern Wisconsin.
...Synopsis...
An upper shortwave trough is expected to be located over the Lower
MO Valley early Monday. This trough will lift northeast across the
upper Great Lakes through Monday night. Meanwhile, a broad area of
southwesterly flow will continued across the southern/central Plains
east of a deepening upper trough over the western U.S. Deep-layer
flow will become more amplified and stronger across the central
Plains after 00z, as the western trough ejects east toward the High
Plains by early Tuesday.
At the surface, a cold front will be located from the eastern
Dakotas into central NE. This front will continue to shift east
through the evening before stalling from southern MN into eastern NE
and central KS. Southerly low-level flow ahead of the front will
result in a broad area of low to mid 60s F dew points from the
Mid/Upper MS Valley into the central/southern Plains.
...KS/NE into southwest IA/northwest MO...
Large-scale ascent will be nebulous through the afternoon, and
capping will likely suppress convection through at least
mid-afternoon. Thereafter, vertical shear will increase along with
stronger ascent spreading into the Plains during the
evening/overnight as the western trough ejects east. Thunderstorm
clusters may initiate by early evening, with the potential for MCS
development during the nighttime hours. Convective initiation and
evolution is a bit uncertain given several rounds of convection
expected for portions of the region prior to Monday evening.
However, the overall large-scale pattern supports severe convection
capable of all hazards into early Tuesday morning.
...Lower MO Valley to WI...
Convection may be ongoing Monday morning from the MO River into
central IA, northward into northern WI. How this convection evolves
is a bit uncertain. However, some severe potential appears possible
into the afternoon, as at least modest destabilization occurs amid
somewhat favorable vertical shear. Hail and strong gusts will be
possible with this activity through Monday evening.
..Leitman.. 05/18/2024
WUUS03 PTSDY3
NOTE: THE NEXT DAY 3 OUTLOOK IS SCHEDULED BY 0730Z
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