U.S. Severe Weather Forecast (Convective Outlook)

Day Three

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Storm Prediction Center ac 272036 

Day 1 convective outlook resent 3 
National Weather Service Storm Prediction Center Norman OK 
0336 PM CDT Tue Jun 27 2017 

Valid 272000z - 281200z 

..there is an enhanced risk of severe thunderstorms across portions 
of Nebraska and southern South Dakota... 

..there is a slight risk of severe thunderstorms across the 
north-Central Plains... 

..there is a marginal risk of severe thunderstorms from the 
north-Central High plains to the upper Midwest... 

..there is a marginal risk of severe thunderstorms across the 
northeast states/New England... 

..there is a marginal risk of severe thunderstorms across the 
Florida Peninsula... 

Severe thunderstorms capable of large hail, damaging winds and a 
tornado or two are expected over much of Nebraska and South Dakota 
late this afternoon and evening, with the potential for significant 
wind gusts. Other strong to severe thunderstorms will be possible 
across parts of the northeast states as well as the Florida 

..Wyoming/Colorado High Plains eastward to southwestern Minnesota 
and Iowa... 
recent satellite/radar observations and objective analyses indicate 
convection developing on the western edge of an increasingly 
unstable airmass advancing northward in portions of the 
central/northern plains. Observations and high-res guidance 
continue to indicate that storms will gradually increase in 
intensity and coverage, with one or two forward-propagating linear 
segments eventually developing across western portions of South 
Dakota and Nebraska. Widespread damaging wind gusts and hail are 
likely with this activity, with perhaps an isolated tornado threat 
also evolving especially within the enhanced risk area as low-level 
shear profiles increase attendant to a strengthening low-level jet 
near/after dark. Portions of the slight risk have been expanded 
into western Iowa and southwestern Minnesota as recent 
high-resolution guidance suggest that at least some lingering severe 
threat will exist in that region after sunset as convection 
develops/migrates into that area. 

A small part of northeastern Kansas has been added to marginal 
wind/hail probabilities as a result of a localized, persistent 
cluster of storms in that area. 

..portions of the northeast... 
Scattered thunderstorms are ongoing across the marginal risk area, 
with an isolated threat of hail, wind damage, and perhaps a tornado. 
This threat should persist for the remainder of the afternoon before 
waning some after sunset. 

.Cook/Peters.. 06/27/2017 

Previous discussion... /issued 1141 am CDT Tue Jun 27 2017/ 

..central/northern High Plains to middle MO valley... 
A shortwave trough and related mid/high-level speed Max are readily 
evident across the eastern Idaho/southwest Montana/western Wyoming vicinity and 
adjacent Great Basin per water vapor imagery and 12z upper-air 
analysis. This eastward-progressive shortwave trough will reach the 
Dakotas and middle MO valley by this evening amid a steadily 
strengthening belt of mid-level westerlies. 

An initial increase in widely scattered strong thunderstorm 
development should occur this afternoon across interior Wyoming (and far 
southern mt), where modest moisture/buoyancy will support some 
potential for severe hail and severe-caliber wind gusts. As forcing 
for ascent spreads eastward, additional development should occur by 
late afternoon in vicinity of the north/south-oriented Lee trough, 
particularly across eastern Wyoming/NE Panhandle and Black Hills 
vicinity. Large hail and damaging gusts will be possible for the 
first few hours, although very steep lapse rates and a relatively 
well-mixed environment should result in an upscale-growing and 
eastward-accelerating complex primarily across NE this evening. As 
mentioned in the prior outlook discussion, localized hurricane-force 
thunderstorm gusts are possible from the most intense surges of 
convection this evening. 

..northeast states/New England... 
Aided by a northeast-moving shortwave trough, scattered low-topped 
thunderstorms will continue to increase and spread 
east/northeastward across the region this afternoon. Relatively 
steep lapse rates, accentuated by cool mid-level temperatures (near 
-20c at 500 mb) will support stronger low-topped storms capable of 
hail, with a few low-topped supercells possible across southern New 
England where deep-layer shear will be somewhat stronger. For 
additional short-term information, see mesoscale discussion 1160. 
Convection should weaken as it moves eastward over either land-based 
evening diabatic cooling and/or the Atlantic marine layer. 

Ample insolation and a moist environment will lead to moderate 
buoyancy this afternoon across the Florida Peninsula near and south of a 
stalled front. A few thunderstorms could produce strong/locally 
severe-caliber downdrafts this afternoon within a weak deep-layer 
shear environment. 


Mesoscale Discussion

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sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 280002 

Mesoscale discussion 1165 
National Weather Service Storm Prediction Center Norman OK 
0702 PM CDT Tue Jun 27 2017 

Areas affected...central South Dakota into portions of northeast and southeast 
South Dakota 

Concerning...severe potential...Severe Thunderstorm Watch likely 

Valid 280002z - 280130z 

Probability of watch issuance...80 percent 

Summary...the severe weather threat will move into the area from the 
west-southwest during the evening. Severe gusts 50-65 kt are 
possible with the strongest gusts near the MO river. 

Discussion...radar mosaic shows a band of storms over southwest South Dakota 
and into the NE sandhills moving east-northeast at 45-50 kt. The 
discussion area is downstream of an approaching mid- to upper-level 
shortwave trough moving across Wyoming and into western South Dakota and the NE 
Panhandle later this evening. A warm boundary layer with 
temperatures ranging from around 90 degrees f near the MO river to 
around 80 f from abr south through Hon. A moist axis extends from 
south to north across central NE into central South Dakota with surface 
dewpoints ranging from the upper 50s to the lower 60s. 

An intensification of a southerly low level jet is forecast this evening with 
the ongoing band likely interacting favorably on the nose of the 
low level jet. Yet, forecast soundings indicate the capping inversion is more 
pronounced with east extent which may act as a limiting factor. 
Anticipating the thunderstorm band to continue east across South Dakota this 
evening. Scattered severe gusts are possible with the stronger 
cores/downdrafts in the band. The severe threat will likely move 
out of the east side of severe watch 372 by 02-03z. 

.Smith/Hart.. 06/28/2017 

..please see www.Spc.NOAA.Gov for graphic product... 


Latitude...Lon 45030119 45559979 45589808 42839800 43059858 43099952 
43960099 45030119