SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1248 AM CDT Sun Aug 17 2025
Valid 171200Z - 181200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS LATE THIS
AFTERNOON INTO TONIGHT ACROSS PARTS OF THE NORTHERN
ROCKIES...PORTIONS OF THE MIDDLE MISSOURI VALLEY AND ADJACENT GREAT
PLAINS...AND PARTS OF THE NORTHERN MID ATLANTIC...
...SUMMARY...
One or two clusters of thunderstorms may develop and gradually pose
a risk for strong to severe wind gusts across parts of the northern
Rockies into middle Missouri Valley late today into tonight. Strong
thunderstorm development accompanied by potentially damaging wind
gusts is also possible across parts of the northern Mid Atlantic
late this afternoon and evening.
...Discussion...
Large-scale mid-level troughing is forecast to undergo further
amplification across southeastern Quebec through portions of
northern New England and the Canadian Maritimes during this period.
As this occurs, a notable cold front is forecast to advance
southward across much of the Northeast, Mid Atlantic and Ohio Valley
by 12Z Monday, beneath the northeastern periphery of large-scale
mid/upper ridging encompassing much of the interior U.S and southern
Canadian Provinces.
The western flank of the intrusion of cooler/drier boundary layer
air is likely to stall across the lower Ohio Valley into Upper
Midwest vicinity, but a more modest southward cool surge is also
forecast across the northern high plains, in the wake of weak
mid-level troughing slowly migrating within the larger-scale ridging
across and east of the Dakotas/Nebraska vicinity.
A number of other smaller-scale perturbations are forecast to
progress through modest/weak cyclonic to anticyclonic flow inland of
the Pacific coast through the Rockies, downstream of a significant,
amplifying large-scale trough offshore of the British Columbia and
Pacific Northwest coast.
...Parts of the mid Missouri Valley into Midwest...
Models indicate unsaturated lower/mid-tropospheric thermodynamic
profiles with steep lapse rates, aided by continuing eastward
advection of elevated mixed-layer air, will be maintained across
much of the South Dakota and Nebraska vicinity through this period.
Beneath this regime, seasonably high boundary-layer moisture content
will continue to contribute to the development of moderate to large
potential instability with insolation, along and south of an
initially stalled frontal zone across southern Minnesota/southern
Iowa through southern South Dakota.
Residual convection with a possible MCV may be in the process of
spreading well to the cool side of this boundary, across and
northeast of the eastern Dakotas at 12Z this morning. There appears
at least some potential for renewed convective development along the
southern flank of this activity, in closer proximity to the frontal
zone and destabilizing boundary-layer (roughly focused near the
Minnesota/Iowa border), aided by forcing for ascent associated with
warm advection during the day. Otherwise, stronger thunderstorm
development may await forcing associated with a perturbation within
the southern periphery of the weak mid-level troughing forecast to
overspread the (roughly) South Dakota/Nebraska border vicinity by
early this evening.
There is considerable spread evident among the models concerning
these and subsequent developments, with the NAM depicting the
evolution of a notable convective perturbation. Other output is
less pronounced, but, despite generally weak wind fields and shear,
forecast thermodynamic profiles appear potentially conducive to the
evolution of an upscale growing and organizing cluster with the
potential to produce strong to severe wind gusts this evening and
overnight.
...Montana...
Downstream of the northeastern Pacific mid-level trough, at least
broadly difluent upper flow, perhaps enhanced ahead of a subtle
mid-level wave crossing the northern Rockies, may aid the
development of thunderstorm activity off the mountains of south
central into southeastern Montana late this afternoon and evening.
Forecast soundings east of the higher terrain appear to include
sufficient shear and favorable thermodynamic profiles for the
evolution of a small east-southeastward propagating cluster with
potential to produce severe wind and hail.
...Northern Mid Atlantic...
Seasonably high moisture content and boundary-layer heating focused
along a pre-frontal surface trough may contribute to an environment
conducive to a few strong downbursts, aided by heavy precipitation
loading. It is possible that forcing for ascent associated with a
remnant convectively generated perturbation may contribute to a
clustering of thunderstorm activity along this corridor, from parts
of northern Virginia through southeastern Pennsylvania and New
Jersey, late this afternoon or evening.
..Kerr/Lyons.. 08/17/2025
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