SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0752 AM CDT Thu Jul 18 2024
Valid 181300Z - 191200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS PORTIONS OF NORTH
CAROLINA AND EXTREME SOUTHEASTERN VIRGINIA...
...SUMMARY...
The greatest concentration of damaging gusts today should be over
portions of North Carolina and extreme southeastern Virginia.
...Synopsis...
The upper-air pattern will remain dominated by mean ridging over the
West (from AZ-AB and northward to the Arctic) and troughing in the
East. The large-scale ridge will be anchored by a high slowly
retrograding westward from the Four Corners toward the southern
Great Basin. Just upstream from the ridge, a strong shortwave
trough was apparent over the ID Panhandle and adjoining parts of BC.
This feature should move slowly eastward through the mean ridge
today, reaching northwestern MT and southern AB by 00Z, then turning
southeastward over central MT tonight.
A strong cyclone over northern tip of QC will dig southward over
northern QC through the period, with little movement to the mean
trough extending southwestward across the lower/mid Mississippi
Valley. Within the associated cyclonic flow, a positively tilted
synoptic-scale trough -- evident in moisture-channel imagery from
central/southwestern QC across the neck of ON to Lower MI and IN --
should pivot eastward and become more positively tilted as the
northern part accelerates. By the end of the period, the trailing
part of this trough should extend across eastern New England to NJ.
A subtle, positively tilted perturbation -- now over eastern parts
of KY/TN -- should move east-northeastward to eastern VA and
northeastern NC today.
In between the longwave trough and ridge, northwesterly to northerly
flow aloft will prevail across the Great Plains, with small/low-
amplitude shortwaves and vorticity lobes exiting the mean-ridge
position over the northern Rockies. Difluent upper-level flow will
extend across most of the Plains States, but should be especially
prominent from the central High Plains southeastward to the TX Gulf
Coast.
Surface analysis at 11Z showed a cold front from southeastern QC
across the Hudson Valley to central/western VA, middle TN, central
AR, north-central TX, the TX Permian Basin, and central NM. This
boundary should move slowly southeastward and southward across the
Atlantic and Gulf Coast States today, reaching near a line from
ORF-SPA-MGM-PIB-SAT by 00Z. The segment of the front from south TX
to AL should stall overnight, while the remainder drifts east-
southeastward. A lee trough was drawn from southeastern MT across
eastern WY/CO/NM just est of the mountains, and should remain near
its present position through the day.
...NC/VA and vicinity...
Thunderstorms may develop from midday through this afternoon in the
higher terrain of western NC and southwestern VA, as well as weak
surface troughing to the east, and the front on the north side of
the outlook. While at least isolated damaging gusts and large hail
will be possible over a broad area from the high-terrain genesis
zone to the Tidewater and northern SC, the greatest concentration
and organization of convection (and potential for damaging to severe
gusts) should be over portions of central/eastern NC south of the
front, and perhaps extreme southeastern VA near the NC border along
the front.
Activity will be aided by weak large-scale ascent preceding the
southern Appalachians shortwave trough, acting in concert with
string diurnal heating to steepen low/middle level lapse rates. A
deep summertime troposphere and rich low-level moisture (dewpoints
commonly in the 70s over and east of the Piedmont) will further
contribute to favorable buoyancy, with peak MLCAPE reaching the
2000-3000 J/kg range over the "slight risk" area of greatest
unconditional severe probabilities. 25-35-kt westerly to
west-southwesterly flow aloft should encourage storm motion into
that optimal moisture/CAPE, with upscale aggregation possible into
forward-propagating clusters or a squall line. That would maximize
damaging-wind potential until activity moves offshore this evening.
...Central/southern High Plains...
Another afternoon of scattered to widely scattered thunderstorm
development is likely near the lee trough, and over higher terrain
near the eastern rim of the Rockies. Additional foci may include
parts of the Cheyenne/Palmer/Raton Mesa ridges, and an
outflow/differential heating boundary from ongoing/non-severe
convection over the TX Panhandle/northeastern NM region.
Thunderstorms should move mainly to the south, with some
southeastward or even southwestward tracks (for rightward-deviant
cells) possible. Isolated severe gusts and large hail will be
possible, with small-scale clustering potentially increasing severe
probabilities locally.
From the lee trough eastward, low-level flow will be weak, but with
a predominant easterly surface component, helping to maximize both
low-level lift and (through strong veering with height) deep shear.
Effective-shear magnitudes of 35-45 kt and somewhat long, nearly
straight low-level hodographs are progged east of the lee trough,
supporting the potential for a blend of multicell and supercell
modes. However, modest mid/upper winds are forecast, keeping storm
motion slow outside of areas where cold pool aggregate and
accelerate storm-translation vectors well past those suggested by
ambient winds. The corridor of riches low-level moisture will cover
about the eastern half of the outlook, within which peak/
preconvective MLCAPE of 1000-2000 J/kg will be possible. However,
MLCINH also will increase with eastward extent, with time this
evening, and with lower surface elevation. As such, activity should
become weaker and more isolated tonight.
...Portions of MT...
Widely scattered to scattered, high-based thunderstorms are expected
to develop this afternoon over higher terrain of west-central MT, as
three processes overlap:
1. Preceding the shortwave trough, large-scale DCVA/ascent/cooling
aloft over spreads the area;
2. Diurnal heating preferentially erodes low-level MLCINH over
higher elevations, and builds deep/well-mixed subcloud layers with
40s to low 50s F surface dewpoints, beneath around 300-700 J/kg
MLCAPE;
3. Modest low-level convergence continues near a lee trough just
east of the mountains, with weak closed-low development possible at
the surface over north-central MT.
As this activity shifts eastward, isolated severe gusts may occur.
Aggregating outflows may permit an arc of strong/locally severe
outflow to organize, with convection along or behind the leading
edge. Lack of more robust moisture/buoyancy should limit the
overall threat, and contribute to enough stabilization of the
foregoing boundary layer to weaken the activity during the evening.
..Edwards/Broyles.. 07/18/2024
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