acus01 kwns 191932
Storm Prediction Center ac 191930
Day 1 convective outlook
National Weather Service Storm Prediction Center Norman OK
0230 PM CDT Wednesday Jun 19 2013
Valid 192000z - 201200z
..there is a slight risk of severe thunderstorms late this afternoon into tonight for
portions of central/northern Montana into northwestern North
..there is a slight risk of severe thunderstorms late this afternoon and evening across
parts of the Southern Plains...
..20z outlook update...Plains...
probabilistic and categorical severe outlook lines have been
adjusted across parts of the Southern Plains to account for the
ongoing storm cluster and associated convective outflow boundary
advancing southward through parts of northwest Texas and the Texas
South Plains. Storms may still intensify during the next few hours
in a narrow corridor of stronger heating ahead of it...into portions
of the Edwards Plateau. Otherwise...the outflow boundary trailing
to the west/northwest...into parts of eastern New Mexico may remain
a focus for a relatively higher concentration of new...
strong/severe storm development late this afternoon and evening.
Storm development through the remainder of the plains...northward
across eastern Colorado/western Kansas through the Black Hills
area...while not out of the question...and with at least some risk
for severe weather...may remain somewhat sparse in coverage...as
very warm elevated mixed layer air slowly advects east of The
Rockies through much of the plains.
Previous discussion... /issued 1130 am CDT Wednesday Jun 19 2013/
..Mt/northwest ND through tonight...
A deep closed low over the Pacific northwest will progress inland as a
pronounced shortwave trough over Nevada rotates northeastward to Idaho by this
evening and Montana overnight. At the surface...strong cyclogenesis is
expected in the Lee of the northern rockies mountains across S central Montana
and northern Wyoming...in advance of the ejecting shortwave trough. The
pressure falls across Montana and the rest of the northern High Plains will
induce strong southeasterly low-level flow into Montana through tonight...in
advance of the surface low and a cold front which will cross Wyoming/Montana
in association with the midlevel trough. Regional 12z soundings
revealed rather modest boundary layer moisture from Nebraska to Montana with
100 mb mean mixing ratios of 8-10 g/kg...which translates to
dewpoints mostly in the middle-upper 50s at about 2500 feet mean sea level. The
only likely increase in moisture today into Montana will be through
evapotranspiration...given similar moisture observations upstream
across the central/northern High Plains. Thus...moderate instability
/MLCAPE of 1500-2500 j per kg/ is expected by middle-late afternoon as
surface temperatures warm into the 85-90 f range across central/eastern
Montana...beneath 8-9 c/km midlevel lapse rates.
Thunderstorm development is first expected by early-middle afternoon
over the higher terrain of SW/S central Montana...and this convection
will spread north-northeastward toward north central Montana this evening. Supercells with
large hail will be possible initially...though upscale growth into
clusters/line segments appears likely this evening. The upscale
growth will be driven by storm/outflow interactions in an
environment with rather modest deep-layer shear in the warm sector
/effective bulk shear of 25-30 knots due to backed flow aloft/...as
well as temperature-dewpoint spreads near 30 f and strong linear
forcing for ascent along the surging cold front. The greater risk
for more persistent supercells...with an accompanying risk for more
significant hail and perhaps a couple of tornadoes...will be along
and on the immediate cool side of the baroclinic zone across central
Montana /from about hvr-lwt-gtf-ctb to ctb/...where low-level shear and
moisture will be maximized. Otherwise...expect convection to expand
eastward overnight along the cold front into eastern Montana...with the potential
for isolated damaging winds/hail.
..central/southern High Plains through tonight...
An mesoscale convective system continues to move southeastward across the southeast Texas Panhandle into northwest Texas
as of 16z. There will be some potential for re-invigoration of the
storms along the leading edge of the cold pool this afternoon as the
boundary layer destabilizes. Also...isolated thunderstorm
development will be possible farther west along the outflow boundary
into west central Texas/east central nm. Farther north...the focus for storm
initiation is uncertain given the tendency for slow height rises
across the High Plains. Surface heating/mixing along the extensive
Lee trough could support isolated thunderstorm development for a few
hours late this afternoon/evening from eastern Colorado/western Kansas southward into NE
nm/northwest Texas Panhandle in the zone of increasing low-level moisture...to
the west-northwest of the morning Texas Panhandle mesoscale convective system. This zone will have
sufficient instability and vertical shear for supercells...with a
risk for at least isolated large hail and damaging winds. Farther
north...the focus for storm development is less clear...other than the
orographic influence of the Black Hills.
acus11 kwns 192257
sels mesoscale discussion
Storm Prediction Center mesoscale discussion 192257
Mesoscale discussion 1133
National Weather Service Storm Prediction Center Norman OK
0557 PM CDT Wednesday Jun 19 2013
Areas affected...west-central and scntrl Montana
Concerning...Severe Thunderstorm Watch 328...
Valid 192257z - 200030z
The severe weather threat for Severe Thunderstorm Watch 328
Summary...the severe threat is expected to increase over the next
few hours across west-central and scntrl Montana as the storms expand in
coverage. Large hail and wind damage will be possible with the more
intense storms. A brief tornado also can not be ruled out.
Discussion...the latest surface analysis shows a 993 mb low just to the
south of Billings. Upslope flow exists to the north of the low
across the eastern half of Montana with a cold front located in west-central Montana. To
the east of the cold front...MLCAPE values are estimated in the 1000
to 2000 j/kg range with regional WSR-88D vwps showing 40 to 50 knots of
deep layer shear. In addition...a plume of steep middle-level lapse
rates extends northward from northwest Wyoming into west-central Montana. This environment should
support supercells and large hail will be possible in the stronger
updrafts. A wind damage threat should also exist with supercells and
may increase if a west to East Line can organize as is suggested by
several short-term model forecasts. The stronger supercells may also
have an isolated tornado threat.
..please see www.Spc.NOAA.Gov for graphic product...
Latitude...Lon 45750760 45240865 45350955 46091161 46981249 48101288
48591134 48140944 46510756 45750760