Probability of damaging thunderstorm winds or wind gusts of 50 knots or higher within 25 miles of a point. Hatched Area: 10% of greater probability of wind gusts 65 knots or greater within 25 miles of a point.
Oklahoma City, OK...Tulsa, OK...Little Rock, AR...Billings, MT...Broken Arrow, OK...
SPC AC 131630
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1130 AM CDT Thu Aug 13 2020
Valid 131630Z - 141200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS BY THIS EVENING
ACROSS PARTS OF NORTHWESTERN MINNESOTA AND PARTS OF ADJACENT EASTERN
NORTH DAKOTA...AND THIS EVENING THROUGH THE OVERNIGHT HOURS ACROSS
PARTS OF FAR SOUTHEASTERN MONTANA THROUGH THE NORTH AND SOUTH DAKOTA
STATE BORDER VICINITY...
A couple of clusters of organized thunderstorms may develop across
parts of the northern Great Plains late this afternoon through
tonight, accompanied by a risk for severe wind gusts and hail.
A belt of stronger westerlies across the northern mid-latitude
Pacific is trending more zonal, but remains amplified downstream,
near and north of the Canadian/U.S. border. This includes
large-scale mid-level troughing inland of the British Columbia and
Pacific Northwest coast through the Canadian Prairie provinces and
adjacent portions of the U.S. Great Plains, and downstream ridging
across the Upper Midwest and adjacent Great Lakes region into the
Hudson Bay vicinity.
Within the large-scale troughing, a deep embedded mid-level low is
forecast slowly shift northward into eastern portions of the
Canadian Northwest Territories during this period. As this occurs,
a significant short wave impulse is forecast to dig across and east
of the Montana Rockies by late tonight, and the westerlies may
gradually begin to split. This is a source of model spread and the
associated influence on downstream flow remains at least somewhat
unclear. However, a preceding short wave trough is currently in the
process of pivoting northeast of the Saskatchewan/North Dakota
Meanwhile, in lower latitudes, a subtropical high, centered along a
major axis from the Southwest into the southern Great Plains, will
remain prominent through today and tonight, with subtropical ridging
centered over the western Atlantic also influential along the south
Atlantic coast. Weak troughing likely will linger between the
ridging near/east of the middle and lower Mississippi Valley.
In association with this regime, seasonably high moisture content
air will remain focused either side of the weak southern trough
axis, and south of a remnant surface frontal zone near southern New
England and northern Mid Atlantic coastal areas into the Ohio
Valley. However, boundary layer dew points continue to increase
within lee surface troughing across the Great Plains, beneath steep
mid-level lapse rates associated with a plume of warm and capping
elevated mixed-layer air.
...Eastern Dakotas/northern Minnesota...
In association with the lead short wave impulse migrating north of
the international border area, a modest low within the lee surface
troughing may migrate from the central Dakotas through the Red River
Valley vicinity by early this evening, before continuing or
redeveloping northward through eastern Manitoba/northwestern Ontario
tonight. Within the warm sector of this low, southward along a
trailing wind shift/weak cold front, it appears that boundary layer
moistening will contribute to large CAPE in excess of 3000 J/kg.
To varying degrees, latest model output continue to suggest that
low-level forcing, beneath difluent mid/upper flow, will be
sufficient to overcome mid-level inhibition and allow for the
initiation of storms near/east of the Red River Valley. Although
mid/upper flow probably will remain fairly weak across this region,
it appears that a 30+ kt south-southwesterly 850 mb jet may
contribute to sufficient shear to support an upscale growing,
organized cluster of storms by this evening. If this occurs,
initial storms may pose a severe hail/isolated tornado threat,
before strong wind gusts become the primary threat, until convection
weakens in waning boundary layer instability overnight.
...Southeastern Montana into Dakotas...
Boundary-layer moisture will remain maximized (though more modest
than across the eastern Dakotas) along the wind shift/weak cold
front trailing southwestward/westward near the North and South
Dakota state border area into the higher terrain of southern
Montana. This is expected to contribute to sufficient CAPE to
support vigorous high based thunderstorms within the post-frontal
upslope flow regime across southeastern Montana by early this
evening. It appears that this will be aided by substantive
mid-level cooling and forcing for ascent associated with the
approaching troughing, and deep-layer shear, due to pronounced
veering of winds with height beneath 40-50 kt cyclonic 500 mb flow,
may become quite strong.
It seems increasingly probable that thermodynamic profiles,
including a relatively dry sub-cloud layer with steep lapse rates,
coupled with the strong shear, will become conducive to the
evolution of an organized convective system capable of producing a
swath of severe wind gusts. This may commence across parts of
southeastern Montana by early evening, then continue into and
through the overnight hours, aided by inflow of increasingly moist
and potentially unstable air with eastward extent, roughly near and
north of the North and South Dakota state border area.
...Parts of central Great Plains...
The lee surface trough likely will be the primary focus for any
thunderstorm development late this afternoon and evening. Mid-level
inhibition still appears likely to result in sparse storm coverage,
but models continue to suggest a weakness within the capping will
allow for a couple of storms, particularly across parts of central
Nebraska into northwest Kansas. Shear beneath modest westerly to
northwesterly mid-level flow may be sufficient to enhance activity,
contributing to a risk for locally severe wind and hail.
...Southeastern Great Plains into lower Mississippi Valley..
Given the moist environment characterized by moderate to strong
potential instability, it is possible that inhibition may be weak
enough to allow for thunderstorm redevelopment this afternoon along
the eastern edge of the plume of warmer and more strongly capping
elevated mixed-layer air. Beneath weak to modest northwesterly
mid-level flow, if storms are able to initiate, this environment
probably will be conducive to strong storms which could produce
locally severe wind and hail.
Additional thunderstorm development is possible overnight, aided by
a zone of enhanced mid-level warm advection.
NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 2000Z