Day 1 Severe Weather Outlook

Valid 211300Z – 221200Z


The severe-thunderstorm threat appears most concentrated today over
portions of the Northeast.

In mid/upper levels, a relatively low-amplitude pattern will persist
across most of the CONUS, with the main belt of westerlies covering
about the northern 1/3 from the Pacific Northwest to New England. A
synoptic-scale cyclone — initially over northeastern ON and
adjoining Hudson Bay waters — will pivot slowly southeastward and
eastward to nearby parts of QC through the period. An attendant
positively tilted trough will move southeastward then eastward over
the upper Great Lakes, followed by another, somewhat stronger trough
across northern MN and Lake Superior late tonight. Downstream, a
shortwave trough was evident in moisture-channel imagery across Lake
Ontario, western NY, and the upper Ohio Valley. This feature will
move eastward across the Mid-Atlantic region and New England through
03Z. In weaker westerlies aloft, MCVs and related vorticity lobes
from prior overnight convection in the central Plains will move
eastward over parts of the lower Missouri and mid Mississippi Valley
regions today into this evening.

At the surface, the 11Z analysis depicted a cold front from Lake
Huron to central Lake Michigan, south-central IA, southeastern NE,
northwestern KS, and portions of east-central/southeastern CO. By
00Z, this front should reach southeastern ON, southern Lower MI,
north-central MO, central KS, to near the eastern CO/NM line. By
12Z the front should extend from northern NY to southwestern OH,
southern IL, north-central/northeastern OK, the TX Panhandle, and
northeastern NM. Over the central/southern Plains and mid
Mississippi Valley, the front will be preceded by several convective
outflow boundaries of variable baroclinicity. A warm front —
initially drawn near the southern New England coast and across
southeastern NY/northeastern PA area — will shift northeastward
across much of the remainder of eastern NY and New England through
the period.

Large-scale DCVA/ascent and warm advection, each preceding the
aforementioned mid/upper-level shortwave trough, will spread over a
favorably moist and destabilizing warm sector today in support of
multiple rounds of convection, with sporadic damaging to severe
gusts. Severe hail also is possible from the most intense/deepest
cells, especially across portions of eastern NY and New England
where the buoyancy/shear parameter space will best support organized
multicells and isolated supercells within the convective regime.

In addition to the influences aloft, low-level destabilization will
occur as the warm-frontal zone moves northeastward, moisture
advection occurs, and pockets of persistent diurnal heating occur
around and between areas of antecedent cloud cover. These processes
should result in preconvective MLCAPE commonly in the 1000-1500 J/KG
range. In the absence of an EML or other mechanisms to support
strong capping, MLCINH will weaken rapidly through the morning into
midday, rendering a situation where only subtle boundary-layer
forcing (e.g., surface heating, weak low-level convergence zones,
outflow/differential-heating boundaries) will be needed to generate
deep convection. In general, vertical shear should strengthen and
instability will weaken with northward extent, though a broad area
of overlap favorable for severe defines the 15%/slight-risk area.
With somewhat backed low-level winds in the warm-frontal zone, and a
southerly component just to its south in the warm sector, hodographs
also may enlarge enough to yield isolated mesocyclones and a
marginal tornado threat, generally overlapping the greatest hail
potential, mainly over portions of New England.

…Central High Plains and Rockies/Foothills…
Isolated to widely scattered thunderstorms should develop this
afternoon over the eastern mountains and foothills from northern WY
across central/southern CO, offering the potential for isolated
severe hail and gusts. Activity is expected predominantly in the
post-frontal/upslope region where lift is maximized by a combination
of upslope flow and diabatic surface heating of the higher terrain
(minimizing MLCINH). Midlevel flow is forecast to be modest;
however, vertical shear will be stronger than over lower elevations
ahead of the front because of the backed boundary-layer winds and
relatively extended low-level hodographs. As such, a mixture of
supercellular and multicellular structures are possible.

Isolated to numerous thunderstorms are possible this afternoon and
evening along and ahead of the front, some of which may produce
severe gusts and hail. Large areas of antecedent cloud cover and
precip will disperse, but in doing so, will delay diurnal
destabilization across much of KS/MO. Outflow and differential-
heating boundaries left behind by this activity, and their
intersection(s) with the frontal zone, should be primary foci for
convective development and intensification this afternoon and
evening. Where the boundary layer can heat well, and is relatively
undisturbed by prior convective activity, steep lapse rates and
surface dew points in the upper 60s to upper 70s F will support
pockets of 3000-4000 J/kg MLCAPE, perhaps locally higher.

Considerable uncertainties linger regarding the ultimate location,
orientation and strength of the prefrontal boundaries. One or two
relative concentrations of severe potential may develop as
convection aggregates around areas of greatest diurnal
destabilization and boundary-related forcing. However, mesoscale
uncertainties preclude assigning any greater unconditional
probabilities with the broader marginal risk, at this early
juncture. Weak mid/upper-level winds, and related lack of
more-robust deep shear, also will be limiting factors for a
better-organized severe threat.

..Edwards/Dean.. 08/21/2019


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