SPC Jun 30, 2020 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook
Day 1 Outlook Image

Day 1 Convective Outlook NWS Storm Prediction Center Norman OK
0749 AM CDT Tue Jun 30 2020 Valid 301300Z - 011200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM
CENTRAL/EASTERN NEBRASKA AND WESTERN IOWA TO EASTERN/CENTRAL NORTH
DAKOTA... ...SUMMARY...
Thunderstorms with large hail and damaging wind are possible this
afternoon through tonight across parts of eastern Nebraska, the
Dakotas and western Minnesota/Iowa. ...Synopsis...
The upper-air pattern over the CONUS will be dominated by slow-
moving cyclones over New England and the northern Rockies region. The latter, substantially larger circulation, will evolve through
the period from a merger of two initially separate vorticity
lobes/circulation centers: the first now over western MT, and the
second over central BC. As the southern lobe moves northward to
southern AB, a basal shortwave trough -- now apparent in moisture-
channel imagery from southeastern ID to northern AZ -- will eject
northeastward and assume negative tilt. By 00Z, this trough should
extend across central MT, eastern WY and eastern CO, with meridional
500-mb flow downstream across much of the Dakotas and western/
central NE. By 12Z, the trough should weaken and maintain negative
tilt, reaching western ND, central/eastern SD and southwestern MN. Meanwhile, a small shortwave trough and embedded MCV should stay cut
off from the prevailing westerlies aloft, and as such, stay
quasistationary over western IL. At the surface, 11Z analysis showed a series of weak lows along an
occluded/cold front (reinforced on the mesoscale by antecedent
outflow) extending from northwestern ND across western SD, western
NE, and east-central/central CO. With additional convective
reinforcement of its baroclinicity, the cold-frontal zone will shift
slowly eastward in the net across the Dakotas and central High
Plains through the period. By 12Z the frontal zone should reach
east-central portions of the Dakotas, NE and KS, to the TX Panhandle
and northeastern NM. A warm to stationary frontal zone was drawn
across west-central to northeastern SD, western IA, northern MO,
southern IL, and middle TN, becoming a cold front across extreme
northern GA and northern NC to a low over Atlantic waters near ACK. A frontal-wave low may develop through the period under the low/
middle-level MCV over western IL, while the accompanying boundary
only experiences slight mesoscale perturbations either way, under
weak mid/upper influences. ...Central/northern Plains region...
Scattered thunderstorms are forecast to develop near the cold front
this afternoon, increasing rapidly in coverage into the evening. Initial, discrete to semi-discrete convection will offer the
greatest potential for large hail. With flow aloft being nearly
parallel to the formative zone of low-level ascent, convection
should aggregate rather readily into quasi-linear mode, with the
predominant severe threat becoming damaging wind. A tornado or two
cannot be ruled out, though with the strongest flow aloft being
displaced behind the cold front, ambient deep shear will be modest,
indicating supercellular modes should be transient and likely
heavy-precip in character. The warm sector will remain very moist, supplemented near the
surface this afternoon by evapotranspiration. The result should be
surface dew points commonly in the 70s F, beneath steep midlevel
lapse rates of 8-9 deg C/km. In tandem with diurnal heating, this
will yield strong buoyancy, with preconvective MLCAPE values
commonly peaking in the 3500-5000 J/kg range from NE northward
across the eastern Dakotas. This, combined with a deep mid/upper-
level layer of modest winds (e.g., 700-250-mb mean flow commonly in
the 20-25-kt range east of the front) will support deep/intense
updrafts, substantial precip loading of downdrafts, and rapid
convective growth after initiation. Somewhat stronger mid/upper
winds over the south-central/southwestern NE area -- closer to the
base of the ejecting shortwave trough and in stronger capping -- may
support somewhat longer-lived, tail-end supercell characteristics
early in the convective cycle. Some potential exists for activity in the middle/southern part of
the convective swath to persist overnight at or near severe levels
across the lower Missouri Valley into parts of IA. This activity
would be supported by lift near a low-level warm front and enhanced
storm-relative flow near the nose of a south-southwesterly to
southwesterly LLJ branch. The balance between convective-scale
forced ascent and stabilizing effects relating to a decoupling of
the foregoing boundary layer will determine severe-wind potential;
however, enough unconditional potential is apparent in convective
guidance across multiple synoptic and convection-allowing progs to
extend that portion of the outlook area southeastward somewhat. ...Lower Ohio Valley to Carolinas...
Widely scattered to scattered thunderstorms -- comprised of discrete
multicell/pulse storms and episodic larger clusters -- are expected
to form across this corridor near and south of the warm/stationary
front. Rich low-level moisture, strong diurnal heating, and lift
along the front and outflow/differential-heating boundaries, will
weaken MLCINH substantially. Sea-breeze boundaries will be a factor
near the coastal sections of the Carolinas as well. The greatest
convective coverage is likely from mid-afternoon into early evening.
Isolated large hail and severe gusts may occur in the most intense
cores, with occasional subsevere gusts also capable of minor damage.
With weakness of both flow aloft and vertical shear, the threat
appears too disorganized and conditional on storm- to mesobeta-scale
processes for more than a broad-brushed marginal outlook to be
maintained at this juncture. ..Edwards/Gleason.. 06/30/2020 

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