SPC AC 251949
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0249 PM CDT Mon Jun 25 2018
Valid 252000Z - 261200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE THIS
AFTERNOON AND EVENING ACROSS MUCH OF THE LOWER MISSOURI VALLEY
Isolated severe thunderstorms are possible late this afternoon and
evening across much of the lower Missouri Valley, while other severe
thunderstorms are expected across the southern Appalachians vicinity
and Carolinas during the afternoon and evening.
...20Z Outlook Update..
Some changes to categorical and probabilistic lines have been made,
mainly across the lower Missouri Valley region to account for
ongoing trends concerning destabilization and apparent convective
Strongest destabilization, characterized by moderate CAPE in excess
of 2000 J/kg has been largely focused across parts of central and
southern Missouri into adjacent portions of the lower Missouri
Valley. And there appears a signal in latest model output,
including recent runs of the high-resolution Rapid Refresh, that the
warm frontal zone near through east southeast of the St. Louis MO
area may become a focus for strong/severe thunderstorm development
late this afternoon and evening. Deep layer shear appears more than
sufficient for organized convective development, including isolated
supercells and an upscale growing convective system. Supporting
this idea, deepening convective development is now evident to the
west and southwest of the St. Louis area, where 2 hourly 2 mb
surface pressure falls have been observed in 19Z surface data.
Otherwise, along the warm frontal zone to the northwest,
destabilization has remained more modest across northern Missouri
into southern Iowa, with persistent rain continuing to inhibit
boundary layer destabilization across western Iowa into the middle
Missouri Valley. However, forcing for ascent aided by the
approaching closed low could still provide support for strong/severe
thunderstorm development where the boundary layer has destabilized.
This may include the risk for a tornado or two, though stronger
low-level shear appears to be in the process of becoming focused to
the cool/stable boundary layer side of the frontal zone. If strong
storm development can commence within the next hour or two, this
potential may be maximized near the Missouri/Iowa border vicinity.
.PREV DISCUSSION... /ISSUED 1104 AM CDT Mon Jun 25 2018/
...IA/MO area this afternoon/evening...
A closed midlevel low over the southwest NE/northwest KS border will
drift generally eastward toward IA/MO by the end of the period, as
an upstream shortwave trough crosses the northern Rockies. At the
surface, a weak cyclone will move northward from northeast KS to
eastern NE as an embedded speed max rotates around the NE closed
low. A diffuse warm front will move northward with the surface
cyclone, and a belt of southeasterly low-level flow (30-40 kt at 850
mb) will be maintained along the warm front. Assuming enough cloud
breaks occur to allow destabilization along the fringe of the
ongoing band of rain and thicker clouds, there will be some
potential for supercells and a couple of tornadoes this
afternoon/evening across southern/central IA.
Farther south, a corridor of low-mid 70s boundary-layer dewpoints
will be maintained along the MS River, to the south of a synoptic
front and southwest of early convection and a weak MCV moving
eastward over the lower OH Valley. Some cloud breaks by this
afternoon will allow surface temperatures to warm well into the 80s,
resulting in afternoon MLCAPE approaching 2500-3000 J/kg. Some
enhancement to midlevel flow (30-40 kt) is expected with the
approach of a vorticity lobe rotating around the southeastern
periphery of the NE closed low, which will favor organized
multicells and some supercell structures as convection develops
southward in advance of the diffuse wind shift across MO. Some
clustering of the storms will be possible across east central and
southeast MO, where the damaging wind probabilities have been
...Southern Appalachians into the Carolinas this
A series of remnant MCVs will move from KY toward VA/NC around the
southwestern periphery of a midlevel trough over New England.
Surface heating is underway across the Carolinas in the wake of
overnight convection and along/south of the thicker cloud band and
slow-moving synoptic front from eastern KY to north central NC. As
low-level destabilization continues, thunderstorm development is
expected this afternoon from northeastern TN and southeastern KY
across western and central NC in the zone of differential heating
and near the southern fringe of the remnant MCVs now crossing
southwestern VA and KY/TN. Multicell clusters will be the primary
convective mode, with the potential for occasional wind damage with
downbursts through this evening as storms coalesce on outflows and
move southeastward toward the coast.
...Central OK to southwest MO this afternoon into tonight...
The remnants of overnight convection linger this morning across
central OK, though this convection should weaken by midday. Gradual
thinning of cloud debris and modification of the outflow air mass
from overnight storms will result in destabilization up to the I-44
corridor, just southeast of the quasi-stationary front in OK.
Renewed thunderstorm development will be possible late this
afternoon continuing into tonight along the boundary, where moderate
buoyancy and the southern periphery of the stronger mid-upper flow
will support a mix of multicell clusters and marginal supercells
capable of producing isolated damaging gusts and large hail.
...Southwest into north central MT this evening...
A pronounced midlevel trough and embedded jet streak crossed eastern
WA early this morning, while the larger-scale trough will move
eastward over the northern Rockies later today through tonight.
Low-level moisture is somewhat limited this morning, though strong
surface heating and residual boundary-layer dewpoints around 50 F
will support weak surface-based CAPE this afternoon. Some
high-based convection is expected to form over the higher terrain of
southwestern MT late this afternoon and subsequently spread
northeastward across west central into north central MT late this
evening through early tonight. An increase in midlevel flow and
inverted-v profiles in the low levels will support a threat for
isolated strong/damaging outflow winds and marginally severe hail
for a few hours this evening.
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