National Weather Service
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Severe Storm Outlook Narrative (AC)
acus01 kwns 191247
spc ac 191245
day 1 convective outlook
nws storm prediction center norman ok
0745 am cdt wed jun 19 2013
valid 191300z - 201200z
... there is a slgt risk of svr tstms across central mt ewd to nwrn
nd for this afternoon into tonight...
... there is a slgt risk of svr tstms across portions of sern co swd
to the tx south plains and nern nm this afternoon into this
a strong ... negatively tilted mid - level trough will pivot around the
ern semicircle of a deep cyclone that will only slowly advance ewd
across the pacific nw to the nrn 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 nrn rockies through the day before entering srn parts
of british columbia ... alberta ... and saskatchewan early thu morning.
as this occurs ... a sfc lee trough will sharpen over the high plains.
this lee trough will arc nwwd into central and south central mt 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 sfc low that will develop nwd/nwwd along the
front as deep - layer forcing for ascent increases downstream of the
... central mt ewd to nwrn nd for this afternoon into tonight...
a corridor of rich low - level moisture will continue to arc nwwd/wwd
from the lower plains into central montana ... as cyclogenesis
maintains an ely flow component n of the arcing lee trough. to the
n/ne of the lee trough ... sfc 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 raobs and
forecast soundings suggest that sfc temperatures reaching the 80s
amidst dewpoints in the mid and upper 50s will yield mlcape values
reaching 2000 - 3000 j/kg this afternoon. very steep mid - level lapse
rates -- e. g. around 8. 5 c/km in the 700 - 500 - mb layer per tfx and
ggw 12z raobs -- will support large normalized cape ... as well.
thunderstorms will likely form over the mountains of sw/s - cntrl mt
with the approach of mid - level height falls downstream of the
shortwave trough ... deepening orographic circulations ... and
increasing low - level ascent accompanying the sharpening lee trough.
strengthening sly mid - 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 kt. 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
wnw/ene - oriented squall line spreading ewd/nwd during the evening
and into the overnight hours across parts of n - cntrl and ern mt and
into nwrn nd. svr 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 mid - level sly/s ... yielding effective
storm - relative helicity values of around 100 - 200 m2/s2 this
afternoon. despite relatively high lcl/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 esely current at
850 mb and modestly lowering lcl/s.
... elsewhere across the high plains and across the lower elevations
of nw tx for today into this evening...
svr potential will be largely conditional along the entire extent of
the lee trough ... and overall forecast confidence is low regarding
afternoon thunderstorm development/evolution. mid - level warming
accompanying height rises will contribute to an increasingly hostile
thermodynamic environment for storm initiation ... thus breeding large
uncertainty regarding the total svr threat. regardless ... isolated
storm development cannot be ruled out anywhere along the lee trough
from sern mt swd to swrn tx and ern nm owing to deepening
boundary - layer circulations augmenting increasing low - level ascent
along the trough. sufficient mid - 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 -- e. 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 sern co swd into the tx s plains/nern nm.
ascent accompanying an outflow boundary being laid out by ongoing
tx - panhandle convection may bolster low - level ascent near the lee
trough sufficiently for somewhat greater thunderstorm/svr 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 sewd on the immediate n side of a
composite outflow - diffuse - synoptic boundary and could pose an
isolated svr wind/hail risk across parts of nwrn tx into this
... southeast states for this afternoon...
a weak sfc front will remain draped ene/wsw across parts of the sern
states beneath a plume of rich deep moisture. modified 12z raobs 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
mid - level shortwave trough will offer sufficient mid - level flow/deep
shear for a few multicell clusters with dmgg wind gusts possible.
downdraft strength will be aided by precip loading owing to pw
values from 1. 75 to 2. 00 inches.
.. cohen/mead.. 06/19/2013