U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 272001 
Storm Prediction Center ac 271959 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0259 PM CDT sun may 27 2018 

Valid 272000z - 281200z 

..there is an enhanced risk of severe thunderstorms across parts of 
southwest Nebraska...far eastern Colorado...western Kansas and the 
Oklahoma Panhandle... 

..there is a slight risk of severe thunderstorms across parts of 
the High Plains and northern rockies... 

Severe storms with large hail and wind damage are possible late this 
afternoon into this evening across parts of the High Plains. A few 
tornadoes may occur across the Florida Peninsula. 

Several changes have been made to the outlook for this issuance. The 
first change to the outlook is to add an enhanced risk area to parts 
of far eastern Colorado, southwest Nebraska, western Kansas and the 
Oklahoma Panhandle. Confidence has increased that storms will be 
numerous enough to warrant adding a narrow corridor for 30 percent 
hail coverage where the storms are expected to interact with strong 
instability late this afternoon. The second change to the outlook is 
to expand the slight risk area westward across west-central Wyoming 
where strong to severe storms are ongoing. The third change to the 
outlook is to expand the marginal risk area across lower Michigan 
northward to a cold front where several severe storms are also 

.Broyles.. 05/27/2018 

Previous discussion... /issued 1125 am CDT sun may 27 2018/ 

A large upper low is positioned over Nevada today, with multiple 
shortwave troughs rotating around the trough into the central 
rockies. Models are very consistent in the development of a band of 
thunderstorms this afternoon across southern Wyoming arcing into 
northeast Colorado. These storms are expected to spread northward through 
the evening into parts of western South Dakota and western NE. Easterly 
low-level winds and favorable thermodynamics may support a few 
supercells in this regime, but it appears likely that congealing 
outflows will lead to multiple bowing line segments across the risk 
area. Damaging winds and hail are the main threats, with isolated 
tornadoes possible early in the convective evolution. 

..West Texas into eastern Colorado/western Kansas... 
Mid level height falls are spreading into the Central High plains, 
where strong heating and rapid destabilization is occurring. This 
will lead to scattered thunderstorms throughout the region later 
today. Deep boundary-layer mixing and steep low-level lapse rates 
will promote strong outflows in the storms, with damaging winds and 
hail being the main threats. 

..FL and coastal GA/SC... 
Relatively strong low-level winds are present today across most of 
the Florida Peninsula, although veering/weakening is occurring across 
southwest Florida behind Alberto. Visible satellite shows broken clouds 
across the region, providing some potential for daytime 
heating/destabilization. This may lead to a few rotating cells 
later this afternoon with a risk of isolated tornadoes. Based on 
12z model solutions, am extending low tornado probabilities into 
coastal sections of Georgia/SC for late tonight. 

Visible satellite imagery shows a pocket of strong heating occurring 
over parts of southern PA into northern Virginia. Meanwhile, a back-door 
cold front is surging southward into the region. Given the 
moist/unstable air mass in this region, have added a marginal risk for 
afternoon thunderstorms spreading east-southeastward as an weak 
upper trough grazes the area. Locally gusty winds and hail are 
possible with the strongest cells. 

..eastern lower Michigan... 
12z cam solutions are consistent that a cluster of thunderstorms 
will form this afternoon over southeast lower Michigan ahead of a weak 
shortwave trough and in region of strong heating. Forecast 
soundings show considerable cape and sufficient vertical shear for 
hail and gusty winds in the strongest cells. 


Mesoscale Discussion

acus11 kwns 272306 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 272306 

Mesoscale discussion 0546 
National Weather Service Storm Prediction Center Norman OK 
0606 PM CDT sun may 27 2018 

Areas affected...western and central South Dakota...northeastern 

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

Valid 272306z - 280030z 

Probability of watch issuance...95 percent 

Summary...ongoing convection northern/eastern Wyoming will move into 
the area with possible new development ahead of that activity. A 
severe wind threat will be possible with any storms. An isolated 
hail threat is also possible. Severe Thunderstorm Watch issuance is 
expected within the next hour. 

Discussion...ongoing convection to the south and southwest of the 
discussion area is expected to move into northeastern Wyoming and 
western South Dakota within the next few hours. New development is 
also possible ahead of the ongoing storms with surface analysis 
showing a front between Chadron, NE and Alliance, NE. Rap analysis 
shows MUCAPE values from 2000-4000 j/kg along with 30-35 kts of 
effective bulk shear mainly in western South Dakota and diminishing 
values to the west. Storms that develop in or move into the area 
will pose a damaging wind threat with the likely storm Mode being 
linear. However, an isolated hail threat will also exist. 

.Wendt/Gleason/grams.. 05/27/2018 

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


Latitude...Lon 43190448 43720592 44490612 45160448 45680312 45770132 
45589962 45209896 44399898 43599936 43099975 43100069 
43110139 43040316 43110391 43190448