acus02 kwns 180548
Storm Prediction Center ac 180546
Day 2 convective outlook
National Weather Service Storm Prediction Center Norman OK
1246 am CDT Tuesday Jun 18 2013
Valid 191200z - 201200z
..there is a slight risk of severe thunderstorms most of Montana into western ND Wednesday
afternoon to Wednesday night...
A vigorous shortwave impulse embedded within the basal portion of
the amplified western Continental U.S. Upper-level trough will take on a negative
tilt as it ejects northeastward from the northwestern Great Basin to the northern rockies
by early Thursday. This will induce weak height rises across much of the
Great Plains as a middle-level ridge continues to slowly slide eastward. At
the surface...a Lee cyclone should become centered over southeastern Montana Wednesday
evening with a cold front accelerating eastward across the northern High
Plains Wednesday night. Farther S...the dryline will mix east over the
central/southern High Plains.
..nrn rockies/High Plains...
Preceding days of low-level moist advection should yield
predominately 50s surface dew points across most of the Lee of the
northern rockies at peak heating Wednesday. With steep lapse rates throughout
the troposphere...air mass should become rather unstable with MLCAPE
likely reaching 1500-2500 j/kg to the east of the cold front. Middle-level
height falls and DCVA downstream of the approaching shortwave trough
will overspread southwestern Montana during the afternoon and early
evening...with isolated to scattered thunderstorm development likely
occurring over the higher terrain. Initially meridional
middle/upper-level wind profiles will favor north/northeasterly storm
motions with a few supercells primarily producing large hail /some
of which may be significant over central Montana along the periphery of
greater buoyancy/. During the evening/night...the low level jet will
strengthen and more pronounced veering of wind profiles with height
should exist with eastern extent. Setup should yield upscale growth and
may result in a severe wind-producing mesoscale convective system propagating east/northeastward across
northestern Montana. Given continued maintenance of large instability...enhanced
by advection of lower to middle 60s surface dew points...a few
significant severe wind gusts may occur as well.
Although moderate to strong buoyancy should exist across the bulk of
the plains...overall pattern still appears nebulous for warranting
slight risk probabilities at this time. With middle-level heights slowly rising
in response to the northwest shortwave trough and moderate 700 mb
winds becoming predominately southwesterly...the western periphery of the
persistent moisture plume should mix east of the central/southern rockies.
With a strengthening eml/capping inversion...any dryline-induced
convection should struggle to develop eastward and likely remain isolated
Farther east...convectively-parametrized guidance is highly consistent
that a convective cluster will be ongoing across parts of Nebraska/Kansas at
12z/Wed. This guidance suggests convection may persist through the
diurnal heating cycle with activity developing east or S...which would
probably yield an isolated severe wind/hail threat.
However...convection-allowing guidance is much less emphatic about
this occurring...Breeding substantial uncertainty in delineating a
slight risk. Wednesday night...a few elevated thunderstorm clusters may form along
the leading edge of the cap as the low level jet strengthens. Some of this
activity may pose a risk for marginally severe hail/wind.