Valid 071200Z – 081200Z
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE THIS
AFTERNOON AND EVENING ACROSS PARTS OF THE CENTRAL PLAINS…
…SUMMARY…
Severe thunderstorms are possible late this afternoon and evening
near the central and eastern Kansas/Nebraska state border area.
Strong thunderstorms may also impact surrounding areas of the
Plains, as well as portions of the Pacific Northwest, which could
pose at least some risk for severe hail and wind.
…Synopsis…
Hurricane Dorian is in the process of becoming absorbed into
troughing within the westerlies. At mid-levels, this troughing is
forecast to continue digging across the lower Great Lakes through
the Northeast today, before turning eastward and northeastward, up
the north Atlantic coast. As it does, the low-level circulation
center of Dorian appears likely to pass southeast of Cape Cod early
today, before continuing to accelerate northeastward into the
Canadian Maritimes.
Meanwhile, upstream mid-level ridging, which has been prominent
across much of the western and central U.S., is in the process of
weakening and becoming suppressed to the lower latitudes, as an
initial short wave trough progresses across the northern
intermountain region and Rockies. The perturbation is forecast to
turn southeast of the northern Rockies early today, around the
remnants of the ridging, before continuing across the northern
Plains/mid Missouri Valley region through tonight. It appears that
this will occur ahead of more amplified upstream troughing, which
models indicate will progress inland of the U.S. Pacific coast, in
negatively tilted fashion, late this afternoon through tonight.
The latter feature will be accompanied by a cold front, which may
advance inland and through much of the northern intermountain region
and western portions of the Great Basin by 12Z Sunday.
East of the Rockies, in the wake of the troughing within the
westerlies, a cold front has already reached the lower Ohio/middle
Mississippi Valleys and central Plains. It is forecast to continue
into the Southeast and lower Mississippi Valley later today through
early Sunday, while stalling and retreating northward through the
central Plains during the day today. A narrow corridor of
seasonably moist boundary layer air is being maintained along this
front, and may contribute to moderately large CAPE with insolation
by this afternoon.
…Northern into central Plains/mid Missouri Valley region…
Several areas of scattered strong thunderstorm development appear
possible today, some of which could pose at least some risk for
severe weather, mainly in the peak late afternoon and early evening
instability.
A corridor of strong daytime heating within surface troughing to the
south of the front, across parts of southeastern Colorado into
western Kansas, may provide one focus for vigorous thunderstorm
development late this afternoon. This may be mostly south of the
belt of 30-40 kt west-northwesterly mid-level flow, associated with
the troughing digging to the lee of the northern Rockies, but a hot
and deeply-mixed boundary layer may support localized strong
downbursts near the most vigorous convection through early evening.
The front, enhanced through the day by strong differential surface
heating, may provide another focus for vigorous thunderstorm
development. Late this afternoon, it appears that low-level
convergence and warm advection may become enhanced near the central
Kansas/Nebraska state border area, where large CAPE and deep-layer
shear on the southern fringe of the aforementioned mid-level jet may
support supercell development. It seems probable that this will
pose a risk for severe hail and locally strong surface gusts, with
at least some potential for a tornado, before forcing for ascent,
aided by modest southerly low-level jet strengthening, contributes
to upscale convective growth this evening. This may include a small
organized convective system, with a period of increasing potential
for strong surface gusts eastward toward the Missouri Valley, before
weakening overnight.
Farther north, another zone of stronger differential surface heating
may provide a focus for isolated strong thunderstorm development
east of the Black Hills late this afternoon and evening, aided by
stronger mid-level forcing for ascent associated with the digging
short wave trough. Mixed-layer CAPE across this region may only
become weak to modest, but the environment may still be conducive
for stronger cells to pose at least some risk for severe hail and
localized strong surface gusts.
…Pacific Northwest…
Models suggest that a corridor of weak to moderate CAPE may develop
along/east of the Cascades by this afternoon, with daytime heating.
This should precede the arrival of stronger mid-level forcing for
ascent associated with the inland advancing troughing, which is
expected to support scattered thunderstorm development, aided by
orographic forcing. With perhaps modest deep-layer shear enhancing
convection, stronger cells may pose at least some risk for
marginally severe hail and/or locally strong surface gusts while
tending to propagate northward through this evening.
..Kerr/Elliott.. 09/07/2019
$$