U.S. Severe Weather Forecast (Convective Outlook)

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Storm Prediction Center ac 271641 


Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1141 am CDT Tue Jun 27 2017 


Valid 271630z - 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 
northern rockies/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 possibility that a few 
gusts may exceed hurricane force. Other strong to severe 
thunderstorms will be possible across parts of the northeast states 
as well as the Florida Peninsula. 


..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. 


.Guyer/Dean.. 06/27/2017 


$$ 

Mesoscale Discussion


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Storm Prediction Center mesoscale discussion 271604 
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Mesoscale discussion 1161 
National Weather Service Storm Prediction Center Norman OK 
1104 am CDT Tue Jun 27 2017 


Areas affected...portions of the Florida Peninsula 


Concerning...severe potential...watch unlikely 


Valid 271604z - 271730z 


Probability of watch issuance...5 percent 


Summary...thunderstorms will continue to develop/increase in 
coverage through the afternoon. Isolated damaging winds may occur 
with the strongest thunderstorms. Due to expected limited coverage, 
a watch is currently not anticipated. 


Discussion...thunderstorms will continue to develop this afternoon 
within a moist, tropical environment across the Florida Peninsula 
this afternoon. The overall large-scale environment will be 
characterized by most-unstable cape values on the order of 2500-3500 
j/kg and deep-layer shear values below 20 kt. This weak deep-layer 
shear should preclude widespread thunderstorm organization, despite 
the overall degree of instability. 


Despite the lack of overall thunderstorm organization, there will be 
potential for isolated, wet microbursts given precipitable water 
values on the order of 2 inches. 


.Marsh/guyer.. 06/27/2017 


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


Attention...WFO...mfl...mlb...key...tbw...jax...tae... 


Latitude...Lon 26158220 28468314 29908327 30238114 28258037 26547977 
25318016 25058122 25868194 26158220