SPC AC 021319
Day 1 Convective Outlook CORR 1
NWS Storm Prediction Center Norman OK
0819 AM CDT Wed Sep 02 2020
Valid 021300Z - 031200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM PORTIONS OF
THE NORTHEAST/MID-ATLANTIC INTO THE OHIO VALLEY/MID-SOUTH...AND OVER
PARTS OF CENTRAL/SOUTH-CENTRAL TEXAS...
CORRECTED FOR HAIL GRAPHIC
...SUMMARY...
Isolated severe thunderstorms are possible today from parts of the
Northeast/Mid-Atlantic into the Ohio Valley/Mid-South, and over
parts of central/south-central Texas.
...Synopsis...
A large, strong cyclone over north-central Canada will influence the
northern-stream pattern across the northern U.S., where very broadly
cyclonic flow is anticipated. A prominent shortwave trough -- now
evident in moisture-channel imagery over Lake Superior and adjoining
northwestern ON -- will move eastward across southern and eastern QC
through the period. Meanwhile, the strongest trailing perturbation
-- currently located over portions of north-central SK/BC -- will
move rapidly southeastward and reach MN by the end of the period.
To the south, a small but well-defined cyclonic gyre is located over
the TX Panhandle, South Plains region. The associated vorticity max
should pivot eastward across north TX through 12Z, by which time the
attached positively tilted trough should extend from the western
Ozarks to northern Coahuila. A series of lower-amplitude vorticity
maxima -- some convectively generated/enhanced -- will traverse the
weakly confluent, downstream midlevel southwesterlies from the
lower/mid Mississippi Valley to the central Appalachians.
At the surface, 11Z analysis showed a cold front across portions of
Lake Huron, Lower MI, western IL, to central MO, becoming
quasistationary from there across southern KS, northwestern OK,
northwest TX, and southeastern NM. This front is expected to shift
to southwestern ON, western/central NY, western PA and southern IN
by 0Z, with little to no movement elsewhere. The front should
weaken through 12Z while reaching eastern New England, the Delmarva
region and eastern KY. Meanwhile a stronger cold front --
associated with the digging western Canadian shortwave trough, will
move across the northern Plains and Upper Midwest, reaching WI,
western IA, and northeastern CO by 12Z.
...Northeast CONUS to Mid-South...
Widely scattered to scattered thunderstorms are expected to develop
in a complex blend of overlapping episodes from the Northeast across
the Mid-Atlantic and central Appalachians to the Mid-South.
Isolated severe wind is expected, and a tornado or two cannot be
ruled out.
Development mainly will be diurnal and focused on several processes,
including:
* Though the strongest forcing for ascent will be in Canada, a zone
of weak large-scale lift may spread over the area near the southern
edge of influence from the shortwave trough, combined with modest
frontal forcing, over parts of the Northeast (mainly NY/PA);
* Prefrontal troughing/weak convergence in low levels, combined with
diurnal heating of higher terrain across the northern/central
Appalachians, with activity spreading eastward toward the Hudson/
Champlain Valleys in the north and Mid-Atlantic coastal plain
further south;
* Lift on mesoscale boundaries such as outflows and differential-
heating zones area-wide;
* Mesoscale lift/shear enhancements related to MCVs ejecting
east-northeastward from northern KY toward the Mid-Atlantic, and
from the Ozarks toward the Mid-South and KY.
Areas of precip and cloud cover will render a rather patchy or
streaky planar buoyancy field today. Still, with rich moisture over
most of the warm sector, areas of sustained diurnal heating should
offset modest mid/upper lapse rates enough for areas of 500-1500
J/kg peak MLCAPE over higher terrain, and 2000-3000 J/kg over parts
of the Mid-South and east of the Blue Ridge in VA. Low-level and
deep-layer shear generally should increase northward into areas of
weaker overall instability, but may favor supercell potential as far
south as central/southern VA. Effective SRH of 150-300 J/kg may
develop intermittently from there through northern NY, especially
near and east of the prefrontal trough where surface flow is most
backed. Some part of that corridor may require an upgrade to
unconditional probabilities once mesoscale placement, coverage, and
timing/forcing uncertainties are better resolved.
...Central/south-central TX...
Widely scattered to scattered thunderstorms are expected to develop
near outflow boundaries this afternoon across parts of the Hill
Country region and move eastward to southeastward, with the most
intense cells offering isolated large hail, and damaging wind in
water-loaded downdrafts. Some clustering and upscale aggregation of
convection may increase the wind threat locally.
Dispersal of morning clouds/precip will lead to sustained heating
through the day, and erosion of MLCINH, especially near outflow/
differential-heating boundaries. Rich surface moisture -- with dew
points in the upper 60s to near 80 F -- already is in place, and
should not be reduced much by vertical mixing while capping does
hold into the afternoon. Areas of longest-duration direct
insolation will attain MLCAPE locally exceeding 3000 J/kg in a deep
troposphere, while 1500-3000 J/kg MLCAPE should be common elsewhere.
Though low-level winds and shear generally will be weak, localized
hodograph/vorticity augmentation is possible along outflow
boundaries (particularly any that can stall for much of the day with
strong heating on both sides). As such, the dominant storm mode
should be multicellular, with short-lived supercell characteristics,
and a general heavy-precip character to the storms, consistent with
heavy-rainfall concerns expressed in WPC's excessive rainfall
discussion.
..Edwards/Kerr.. 09/02/2020
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