acus01 kwns 191247
Storm Prediction Center ac 191245
Day 1 convective outlook
National Weather Service Storm Prediction Center Norman OK
0745 am CDT Wednesday Jun 19 2013
Valid 191300z - 201200z
..there is a slight risk of severe thunderstorms across central Montana eastward to northwestern
ND for this afternoon into tonight...
..there is a slight risk of severe thunderstorms across portions of southeastern Colorado southward
to the Texas South Plains and northestern nm this afternoon into this
A strong...negatively tilted middle-level trough will pivot around the
eastern semicircle of a deep cyclone that will only slowly advance eastward
across the Pacific northwest to the northern Great Basin through the period.
Morning water vapor imagery depicts this shortwave trough moving
through parts of the Great Basin...and this feature is forecast to
approach the northern rockies through the day before entering southern parts
of British Columbia...Alberta...and Saskatchewan early Thursday morning.
As this occurs...a surface Lee trough will sharpen over the High Plains.
This Lee trough will arc nwwd into central and south central Montana this
afternoon...where it will intersect a cold front attendant to the
shortwave trough. Cyclogenesis at this intersection point will give
way to a strengthening surface low that will develop northward/nwwd along the
front as deep-layer forcing for ascent increases downstream of the
..central Montana eastward to northwestern ND for this afternoon into tonight...
A corridor of rich low-level moisture will continue to arc nwwd/westward
from the Lower Plains into central Montana...as cyclogenesis
maintains an Ely flow component north of the arcing Lee trough. To the
north/NE of the Lee trough...surface dewpoints will mix out to some extent
during the afternoon owing to vertical mixing accompanying the
diabatically heating/deepening boundary layer...though a very
unstable air mass is still anticipated. Modified 12z radiosonde observations and
forecast soundings suggest that surface temperatures reaching the 80s
amidst dewpoints in the middle and upper 50s will yield MLCAPE values
reaching 2000-3000 j/kg this afternoon. Very steep middle-level lapse
rates -- east.G. Around 8.5 c/km in the 700-500-mb layer per tfx and
ggw 12z radiosonde observations -- will support large normalized cape...as well.
Thunderstorms will likely form over the mountains of SW/S-cntrl Montana
with the approach of middle-level height falls downstream of the
shortwave trough...deepening orographic circulations...and
increasing low-level ascent accompanying the sharpening Lee trough.
Strengthening southerly middle-level flow will allow convection to spread
over the High Plains of central Montana through the late
afternoon...while supporting effective bulk shear magnitudes
increasing to around 30-40 knots. Given some orthogonal component of
the deep shear vector to the initiating boundary/Lee
trough...discrete/semi-discrete supercells may initially be the
predominant convective Mode. And...with steep low/mid-level lapse
rates...very large hail and potentially significantly severe winds
will be possible with these storms. Strengthening deep-layer forcing
for ascent through the late afternoon and into the evening will
likely contribute to increasing storm coverage and perhaps a
west-northwest/ENE-oriented squall line spreading eastward/northward during the evening
and into the overnight hours across parts of north-central and eastern Montana and
into northwestern ND. Severe winds would likely become increasingly likely if
such a line materializes.
Regarding the tornado potential...at least modest hodograph
curvature/length will be supported by moderate low-level Ely/S
beneath strengthening middle-level sly/S...yielding effective
storm-relative helicity values of around 100-200 m2/s2 this
afternoon. Despite relatively high local/S and sizable
temperature-dewpoint spreads...a few tornadoes will be possible
owing to the magnitude of the low/mid-level lapse rates and
sufficient effective srh...especially with the initial supercell
modes across central Montana this afternoon. Some potential for
tornadoes may continue as storms assume more quasi-linear modes
through the evening and into the overnight hours...as low-level
shear increases in association with a strengthening east-southeasterly current at
850 mb and modestly lowering LCL/S.
..elsewhere across the High Plains and across the lower elevations
of northwest Texas for today into this evening...
severe potential will be largely conditional along the entire extent of
the Lee trough...and overall forecast confidence is low regarding
afternoon thunderstorm development/evolution. Middle-level warming
accompanying height rises will contribute to an increasingly hostile
thermodynamic environment for storm initiation...thus Breeding large
uncertainty regarding the total severe threat. Regardless...isolated
storm development cannot be ruled out anywhere along the Lee trough
from southeastern Montana southward to southwestern Texas and eastern nm owing to deepening
boundary-layer circulations augmenting increasing low-level ascent
along the trough. Sufficient middle-level flow/deep shear and
sufficiently steep low/mid-level lapse rates will exist for a
conditional supercell threat...with large hail and damaging winds
potentially accompanying any of this activity. Isolated elevated
heat sources near the Lee trough -- east.G. The Black Hills -- may be a
focus for deeper convective initiation...but uncertainty is rather
high for such development.
One area where relatively higher probabilities for storm development
may exist will be from southeastern Colorado southward into the Texas S plains/northestern nm.
Ascent accompanying an outflow boundary being laid out by ongoing
Texas-Panhandle convection may bolster low-level ascent near the Lee
trough sufficiently for somewhat greater thunderstorm/severe potential
this afternoon and evening. The slight-risk area has been largely
re-focused to account for this scenario. Also...the ongoing
convection may continue to track southeastward on the immediate north side of a
composite outflow-diffuse-synoptic boundary and could pose an
isolated severe wind/hail risk across parts of northwestern Texas into this
..southeast states for this afternoon...
A weak surface front will remain draped east-northeast/west-southwest across parts of the southeastern
states beneath a plume of rich deep moisture. Modified 12z radiosonde observations and
forecast soundings near and S of the front suggest that the air mass
will become moderately to strongly unstable -- MLCAPE values of
1500-2500 j/kg -- owing to insolation/diabatic heating amidst a
moist boundary layer. Frontal convergence and ascent along residual
outflow boundaries will likely serve as initiating mechanisms for
deep convection this afternoon. The glancing influence of a weak
middle-level shortwave trough will offer sufficient middle-level flow/deep
shear for a few multicell clusters with damaging wind gusts possible.
Downdraft strength will be aided by precipitation loading owing to precipitable water
values from 1.75 to 2.00 inches.
acus11 kwns 190221
sels mesoscale discussion
Storm Prediction Center mesoscale discussion 190221
Mesoscale discussion 1127
National Weather Service Storm Prediction Center Norman OK
0921 PM CDT Tuesday Jun 18 2013
Areas affected...western Montana
Concerning...Severe Thunderstorm Watch 326...
Valid 190221z - 190315z
The severe weather threat for Severe Thunderstorm Watch 326
Summary...severe threat is diminishing across western Montana.
Discussion...a few strong updrafts remain across Toole
County...otherwise thunderstorm activity is diminishing in intensity and
areal coverage. Aside from isolated hail or perhaps gusty winds it
appears severe threat is minimal across most of ww326.
..please see www.Spc.NOAA.Gov for graphic product...
Latitude...Lon 46181412 48961435 48951040 46181036 46181412