acus01 kwns 201252
Storm Prediction Center ac 201250
Day 1 convective outlook
National Weather Service Storm Prediction Center Norman OK
0750 am CDT Thursday Jun 20 2013
Valid 201300z - 211200z
..there is a slight risk of severe thunderstorms from parts of the northern and Central
Plains into the upper-MS valley and upper Midwest...
An initially elongated upper low stretching from just off the Washington/southern
British Columbia coast to the northern rockies will contract and gradually deepen over
the northern intermountain region as a series of vorticity maxima pivot
around the parent system. The most prominent vorticity lobe /in the
form of a negatively tilted shortwave trough/ over central/eastern Montana will
rotate northward into southern parts of sk/ab while another upstream
impulse/jet streak translates from the Pacific northwest coastal waters
into northern parts of California/Nevada and southeastern Oregon by 21/12z. In the wake of the
northern High Plains short-wave trough...midlevel heights will rebound
slightly across lower elevations of the northern/Central Plains...on the
northern periphery of an upper high centered over the Southern Plains.
In the low levels...ongoing storms across the northern plains into upper
MS valley will modulate the position of surface boundaries today.
In general...it appears that a Pacific front will advance eastward into
the central Dakotas by afternoon...stretching from an occluding low
over southern sk to a deepening Lee cyclone over the tri-state region of
co/neb/KS. A warm front /reinforced by convective outflow/ will lie
west-east from it/S intersection with the Pacific front into northern or central
parts of Minnesota and perhaps northern WI. Elsewhere...a Lee trough/dryline
will strengthen today from the Central High plains Lee low southward to
along the Texas-nm border.
..nrn plains into the upper-MS valley and upper Midwest today and
Multiple clusters of thunderstorms are ongoing as of 12z over the northern plains
into upper-MS valley. While isolated occurrences of hail and
damaging winds will remain possible with persisting storms this
morning...a potentially greater near-term severe threat may evolve
across parts of northestern South Dakota into west-central Minnesota. For additional information
on this area...see mesoscale discussion 1139.
In the wake of the morning storms...the combination of a
northeastward-advecting eml /characterized by 700-500-mb lapse rates of 8-9 c
per km/ atop a moistening boundary layer will contribute to moderate
to strong instability by afternoon with MLCAPE approaching
1500-3000+ j/kg. The highest probability of surface-based storm
development appears to be along the Pacific front and the western
extension of the composite outflow boundary/warm front by middle
afternoon where low-level convergence will align with the glancing
influence of the midlevel trough passing to the north. Here...the
southeastern fringe of middle and upper-level jet streaks attending the
short-wave trough will boost deep-layer shear...supporting an
environment favorable for supercells capable of large hail /some
significant/ and perhaps a few tornadoes.
Merging storm-scale outflows coupled with the intensification of a
nocturnal low-level wind maximum will promote upscale growth of
storms into a southeastward-propagating mesoscale convective system by this evening. Given the
degree of instability across the inflow air mass...potential will
exist for a longer-lived wind event with corridors of enhanced wind
damage possible across parts of Minnesota into tonight.
..cntrl High Plains this afternoon and evening...
Low-level upslope flow will be enhanced into the Cheyenne Ridge
along/just north of southeastward-settling Pacific front and to the north of
deepening Lee low. This low-level forcing will be offset by slight
height rises...which should limit the coverage of afternoon/evening
thunderstorm development to isolated to widely scattered. Given the
combination of steep lapse rates...MLCAPE up to 2000-3000 j/kg...and
35-45 knots of effective bulk shear...the setup will be favorable for
supercells with large hail being the primary risk.
..srn High Plains this afternoon and evening...
Isolated to widely scattered thunderstorms will once again be possible this
afternoon along the dryline/Lee dryline. Here...steep lapse rates
will align with modest amounts of boundary moisture and vertical
shear for some storm organization with a threat for locally damaging
winds and some hail through this evening.
..FL this afternoon...
12z soundings reveal a very moist boundary layer beneath seasonably
cool midlevel temperatures which should yield a moderately to
strongly unstable air mass by afternoon. Scattered to numerous
storms are expected to develop later today along evolving sea-breeze
circulations. An isolated severe storm or two is possible given the
degree of instability and high precipitable water environment.
acus11 kwns 201156
sels mesoscale discussion
Storm Prediction Center mesoscale discussion 201155
Mesoscale discussion 1139
National Weather Service Storm Prediction Center Norman OK
0655 am CDT Thursday Jun 20 2013
Areas affected...northestern South Dakota...west central Minnesota
Concerning...severe potential...watch possible
Valid 201155z - 201400z
Probability of watch issuance...40 percent
Summary...a corridor of severe hail and wind may develop over the
next few hours. If storms can persist and grow upscale...a watch may
be required later this morning.
Discussion...a rather vigorous cluster of storms has formed over
northestern South Dakota on the outflow boundary where strong mass convergence
exists...aided by a strong southerly low level jet. Radar indicates
initial storm is bowing eastward toward I-29...with upwind development
persisting. With cool temperature profiles aloft...downdraft
coolness is likely being aided by hail.
Gradual destabilization will occur after sunrise...and 850 mb
Theta-E is forecast to increase as well. All this suggests the
potential for these storms to continue to be a threat...at least on
a localized level. If storms can increase cold pool side and forward
speed...a watch might be required for both damaging winds and hail.
..please see www.Spc.NOAA.Gov for graphic product...
Latitude...Lon 45189795 45409739 45649723 45859734 46009714 45829641
45859549 45609513 44949498 44599509 44329564 44289600
44449669 44539749 45189795