acus02 kwns 191732
Storm Prediction Center ac 191730
Day 2 convective outlook
National Weather Service Storm Prediction Center Norman OK
1230 PM CDT Wednesday Jun 19 2013
Valid 201200z - 211200z
..there is a slight risk of severe thunderstorms Thursday and Thursday night
across parts of the northern plains into upper Mississippi Valley...
The eastward progression of a broad closed low...now near the
Pacific northwest coast...appears likely to remain slow through this
forecast period. Variability exists among the model data... but
mostly in response to a significant impulse pivoting around its
eastern and northern periphery...the circulation center is expected
to only reach somewhere between the Columbia Plateau and the
northern rockies by late Thursday night.
Downstream...large-scale upper ridging is expected to persist across
eastern portions of the northern plains into the upper Mississippi
Valley. But this feature may be modulated by smaller scale
perturbations emanating from cyclonic flow to the south of the
closed low...across the Great Basin and north central rockies
through the crest of the ridge across the upper Midwest.
Farther east...weak middle/upper troughing probably will linger across
much of the southeast...to the south of relatively strong ...
generally confluent middle/upper flow regime across eastern Canada and
adjacent areas of the upper Great Lakes into New England.
..northern plains into upper Mississippi Valley...
Downstream of the northwestern U.S. Closed low...very warm and
deeply mixed boundary layer air emerging from the intermountain west
and rockies region is forecast to advect east of the higher
terrain...through much of the Great Plains by late Thursday
afternoon. This will occur above lower level moistening on modest
southerly flow ahead of a surface trough developing to the Lee of
the northern and central rockies...and contribute to significant
destabilization. Steep middle-level lapse rates...coupled with
boundary layer moisture characterized by middle/upper 60s surface dew
points...are expected to contribute to a sizable area of moderate to
large cape /in excess of 2000 j per kg/.
Thermodynamic profiles across the region will certainly be
supportive of severe storm potential...including the risk for very
large hail...at least localized strong and damaging convective wind
gusts...perhaps a few tornadoes. The environment probably will be
conducive to the development supercells and the upscale growth of
one or more sustained...organized storm clusters.
However...considerable uncertainty remains concerning the location
and details of convective development and evolution Thursday and
Thursday night. This is largely due to model variability concerning
the smaller scale features...and their impact on the flow fields and
shear across the region...which may be characterized by weaknesses
at various levels compared to days with more clear-cut severe
It currently seems possible that forcing associated with a weak
impulse progressing into/through broader scale anticyclonic
flow...around the northern periphery of the more strongly capping
elevated mixed layer air...will contribute to the initiation of
discrete storms by late Thursday afternoon across parts of the
central and eastern Dakotas. A subsequent increase in storm
development...gradual consolidation and upscale growth into a large
mesoscale convective system then appears possible eastward toward
the upper Mississippi Valley during the evening hours. This may be
aided by a nocturnally strengthening southerly 850 mb
jet...northeast of the middle Missouri Valley through portions of
..southern High Plains into parts of the Central Plains...
Forcing for convective development near the Lee surface trough
remains unclear. However...strong surface heating and deep boundary
layer mixing may overcome inhibition and allow for the initiation of
scattered storms off the higher terrain and south/ east of the
southeast Colorado thermal low. If this occurs...thermodynamic
profiles will be conducive to strong convective surface
gusts...perhaps some hail. Strong wind gust potential could persist
with lingering convection after dark... aided by low-level jet
intensification across the Texas/Oklahoma Panhandle region into