Valid 071200Z – 081200Z
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS TODAY INTO TONIGHT
ACROSS PARTS OF THE MID ATLANTIC COAST STATES INTO WESTERN NEW
ENGLAND…ACROSS PARTS OF THE UPPER MIDWEST…AND ACROSS PARTS OF
THE CENTRAL PLAINS…
…SUMMARY…
Severe thunderstorms are possible today into tonight across parts of
the Mid Atlantic Coast states into portions of western New England,
across parts of the Upper Midwest, and across parts of the central
Plains. These will pose are risk for large hail and potentially
damaging wind gusts.
…Synopsis…
Blocking appears likely to remain prominent across the mid-latitude
eastern Pacific through this period, with a long fetch of
northwesterly mid/upper flow downstream, across Alaska and the Yukon
Territory through the central Canadian/U.S. border vicinity. On the
leading edge of this regime, a large short wave trough and embedded
mid-level closed low are forecast to continue digging southeastward,
across northern Ontario and the upper Great Lakes region. Models
indicate that this will be accompanied by another notable cold
front, which is forecast to advance southeastward across much of the
northern Plains, Upper Midwest and Great Lakes region by daybreak
Thursday.
This will be preceded by another cold front, associated with
large-scale troughing within a weaker branch of westerlies at lower
latitudes, which is expected to continue gradually shifting east of
the Mississippi Valley through the Atlantic Seaboard. The lead
front may progress southeastward into the Mid Atlantic coast, and
more slowly southward through the Tennessee Valley/Ozark Plateau
region. It may be reinforced by convective outflow, before stalling
across eastern portions of the central Plains.
A mid/upper subtropical high, centered over the Four Corners states,
may begin to weaken, but large-scale ridging is expected to remain a
prominent influence across much of the Intermountain West/Rockies
and southern Plains.
Subtle short wave perturbations progressing around the periphery of
the ridging, into the southern periphery of the broad troughing over
the Tennessee Valley/eastern Gulf states, may provide the support
and focus for organizing clusters of thunderstorms today through
tonight, in the presence of moderate to strong instability.
However, the predictability of these features remains relatively
low, resulting in only marginal severe probabilities, except across
eastern portions of the central Plains. Otherwise, forcing for
ascent associated with the more prominent short waves, and
destabilization ahead of their associated cold fronts, appear to
provide more certain support for severe thunderstorm development.
…Mid Atlantic Coast states into western New England…
Although the plume of tropical moisture has advected ahead an
initial short wave trough, well east of the Atlantic Seaboard (aside
from the southern Florida Peninsula), low-level moisture appears to
remain sufficient to support moderate CAPE along pre-frontal surface
troughing near/east of the Appalachians. As destabilization
commences with daytime heating, 20-40 kt south/southwesterly flow in
the 850-500 mb layer is expected to overspread the region and
contribute to at least marginally sufficient shear to organize
convection and support a risk for potentially damaging wind gusts
and some hail. This will be aided by forcing for ascent associated
with several smaller-scale perturbations. This may include one
mid-level cyclonic vorticity maximum and associated developing area
of low pressure, that could contribute to at least low probabilities
for a tornado or two across parts of Upstate New York into the
Hudson/Champlain Valleys.
…Upper Midwest…
Mid-level cooling and forcing for ascent in the exit region of a 50
kt mid-level jet propagating around the southern periphery of the
vigorous digging upper trough will provide support for thunderstorm
development late this afternoon or evening. With models generally
suggest CAPE at least on the order of 1000-2000 J/kg across parts of
eastern Minnesota into Wisconsin and portions of upper Michigan, the
environment is expected to become conducive to isolated supercells
and small organizing storm clusters posing a risk for severe hail
and locally strong surface gusts.
…Eastern portions of the central Plains…
Uncertainty exists concerning the extent and intensity of a possible
early period cluster of storms now developing across parts of
central/eastern Nebraska. However, models generally indicate that
an associated outflow boundary/zone of stronger differential surface
heating will provide a potential focus for organized severe
thunderstorm development late this afternoon into tonight. Large
CAPE along and south of the boundary, coupled with favorable shear
beneath at least moderate northwest mid-level flow, may become
supportive of isolated supercells by early this evening. The nose
of a strengthening southerly low-level jet (including 30-40 kt at
850 mb) may provide the focus for an upscale growing and organizing
convective system across northeastern Kansas late this evening into
the overnight hours, which could pose a risk for strong surface
gusts.
..Kerr/Dean.. 08/07/2019
$$