SPC AC 051250
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0750 AM CDT Sun Jul 05 2020
Valid 051300Z - 061200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM NORTHEASTERN
WYOMING ACROSS PARTS OF THE DAKOTAS TO NORTHWESTERN MINNESOTA...
...SUMMARY...
The greatest potential for severe thunderstorms should be this
afternoon and evening, from northeastern Wyoming across parts of the
Dakotas to northwestern Minnesota.
...Synopsis...
In mid/upper levels, a belt of nearly zonal/west-southwest flow will
extend obliquely across the northern border, from the Pacific
Northwest to northwestern ON. An embedded, positively tilted
shortwave trough -- located initially over central/southwestern MT
-- will move east-northeastward to southwestern MB, western ND and
southeastern MT by 00Z, then weaken as it ejects across eastern MB
overnight.
Farther south, a broad, zonally elongated anticyclone will persist
from NM, far west TX and northern MX westward across similar
latitudes of the northeastern Pacific. On its northeastern rim, a
slow-moving shortwave trough over eastern CO and the TX/OK
Panhandles, including an MCV now evident over the northeastern TX
Panhandle -- should move slowly southeastward to southward. A
broad, weak cyclone will longer over the central Gulf Coast.
Related cyclonic flow and embedded, mostly convectively induced
vorticity maxima will persist from the Ozarks across the
northern/central/eastern Gulf to portions of FL/GA.
A strong shortwave trough -- now apparent in moisture-channel
imagery over central/northern QC -- is forecast to dig southeastward
toward New England today, forming a closed 500-mb low that should
pass close to the northern tip of ME tonight. The low should reach
NB by the end of the period, with trough southwestward across
southern New England.
At the surface, 11Z analysis showed a weak cold front across
southwestern MB to northeastern WY, somewhat diffused behind an area
of convective outflow that covers much of the western Dakotas.
After the front moves eastward today, the combined baroclinic zone
should reach northeastern ND, central SD, and east-central/south-
central WY by 00Z. The front then should move somewhat farther
southeastward but weaken through the remainder of the period.
...Northeastern WY to northwestern MN...
Scattered thunderstorms are expected to develop this afternoon along
and near the front over the Dakotas, moving into a very moist and
unstable warm sector, while widely scattered thunderstorms form on
the higher terrain of southern MT and northern WY and move generally
eastward behind the front. In both regimes, supercells and
organized multicells will be possible, offering large hail and
severe gusts. Tornado potential is substantially more conditional
and isolated, but cannot be ruled out.
In the eastern/central Dakotas, satellite imagery shows cloud cover
from earlier convection is breaking up. This trend should continue
through the day, allowing areas of sustained insolation amidst
surface dew points in the upper 60s to low 70s F ahead of the
outflow/front. This will contribute to strong instability, with
peak preconvective MLCAPE reaching 3500-4500 J/kg. Low- to
middle-level flow will be modest, but convective organization should
be aided by ventilating anvil-level winds, as 200-500-mb layer flow
strengthens with the approach of the mid/upper trough. Isolated
very large/damaging hail cannot be ruled out relatively early in the
convective cycle, before activity congeals into clusters; however,
uncertainties on location and duration of relatively discrete modes
mesoscale and duration uncertainties preclude a sig-10% hail area at
this time.
Moisture will be more limited behind the front, but still sufficient
amidst steep low/middle-level lapse rates to support surface-based
convection away from the mountains. Activity should move into a
well-mixed and diurnally heated boundary layer supporting
maintenance of hail/gusts to the surface. The northeasterly flow
behind the front will elongate hodographs and augment the deep
shear, beneath stronger mid/upper-level winds than experienced by
frontal convection farther east. Forecast soundings show effective-
shear magnitudes potentially reaching the 50-60-kt range. When
combined with MLCAPE in the 800-1500 J/kg range (locally/briefly
higher), the environment is well within the parameter space
supporting supercell mode.
...Central/southern High Plains...
Isolated to widely scattered thunderstorms may develop this
afternoon over the higher terrain of northeastern NM and central CO.
Additional development is possible over adjoining High Plains --
particularly near outflow and differential-heating boundaries -- as
diurnal heating and modest cooling aloft steepen lapse rates and
weaken MLCINH near the shortwave trough and MCV. Isolated severe
gusts and hail are possible, though low/middle-level flow and shear
will be weak, limiting overall organization of the threat. Also, an
extensive area of clouds, precip and outflow has spread across a
large part of the southern High Plains, to west-central/northwest TX
and the Permian Basin. This feature is related to yesterday's
convection over northeastern NM, southeastern CO, and the TX/OK
Panhandles. As a result, lower theta-e, longer diabatic-
destabilization times, and somewhat poorer moisture quality will be
available to convection forming and moving off the higher terrain
today.
...New England...
Widely scattered to scattered thunderstorms should develop this
afternoon -- perhaps in a couple of episodes -- near a subtle
surface trough and convergence line across the northern and western
parts of the outlook area. Isolated hail and damaging to marginally
severe gusts should be the main concern.
Though near-surface flow will be weak, MLCINH will be as well, with
only subtle kinematic boundary-layer forcing needed for convective
lift in concert with diurnal heating. This will occur as the strong
mid/upper shortwave approaches from QC, with associated cooling/
destabilization aloft contributing to a favorable convective
environment. Sufficient low-level moisture is in lace to support a
narrow corridor of 500-1000 J/kg MLCAPE in northern ME, increasing
to 1000-1500 J/kg MLCAPE south-southwestward across the eastern
remainder of New England. Though low-level shear will be
constrained by the weak speeds, veering with height will contribute
to 30-40-kt effective-shear magnitudes, and approximately straight
low/middle-level hodographs. This indicates potential for a blend
of multicell and somewhat-organized supercell modes.
..Edwards/Broyles.. 07/05/2020
NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z
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