U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 262000 
Storm Prediction Center ac 261958 

Day 1 convective outlook corr 1 
National Weather Service Storm Prediction Center Norman OK 
0258 PM CDT Sat may 26 2018 

Valid 262000z - 271200z 

..there is a slight risk of severe thunderstorms across parts of 
the northern plains and South Florida... 

Corrected to include thunder in eastern Iowa 

A severe threat with isolated large hail and wind damage will be 
possible across parts of the northern High Plains late this 
afternoon into the evening. A threat for tornadoes will also be 
possible in parts of South Florida late tonight. 

Two changes have been made to the outlook for this issuance. The 
first change is to add a 5 percent tornado and slight risk area to 
parts of South Florida. The hrrr suggests that rotating storms will 
move inland across South Florida after midnight. Rap forecast 
soundings increase low-level shear throughout the night. Forecast 
hodographs suggest that a tornado threat will be possible with 
semi-discrete rotating cells that move inland in the 06z to 12z 
timeframe. The second change is to move the thunder line westward to 
include eastern Iowa where thunderstorms are developing at this 

.Broyles.. 05/26/2018 

Previous discussion... /issued 1114 am CDT Sat may 26 2018/ 

A large upper low over California continues to drift northeastward, with 
strengthening mid level winds and large-scale ascent moving into the 
northern rockies. Strong heating is occurring over much of Idaho/Mt, 
with thunderstorms expected to form over the higher terrain by 
mid-afternoon. These initial storms will pose a threat of locally 
gusty winds and hail. During the evening, more intense and 
organized storms are expected to form over parts of central/eastern 
Mt, tracking into western ND tonight. The combination of a deep, 
mixed boundary-layer, sufficient cape, and relatively strong 
vertical shear profiles support the risk of a few high-based 
supercells capable of large hail and damaging winds. The activity 
may organize into bowing structures as they move east tonight. 

Low-level wind profiles around the northeast quadrant of Alberto are 
slowly strengthening, with cam guidance suggesting shear sufficient 
for rotating cells by later this afternoon along and off the coast 
of southwest Florida. Most model guidance keeps the bulk of the cellular 
convection and stronger wind fields offshore, but there is a marginal 
chance of an isolated tornado in vicinity of the southwest coast 
through tonight. 

No changes to forecast have been made to this area. Scattered 
thunderstorms are expected to form this afternoon over parts of WI 
and Michigan as an upper trough over Minnesota approaches the region. The 
strongest cells will be capable of gusty winds and hail, but 
coverage/organization is expected to be low. 

Clusters of strong/severe afternoon thunderstorms may form today 
over parts of northern MO and central Illinois. Strong instability is 
forecast for this area, with MLCAPE values over 3000 j/kg and steep 
low-level lapse rates. Shear will be relatively weak, promoting 
multicell storm structures with occasional gusty/damaging winds and 

..central New England... 
A weak surface front extends from northeast New York across parts of Vermont/New Hampshire 
today. Scattered thunderstorms are expected to form along this 
boundary later today, tracking toward the coast by evening. 
Instability is rather weak, but strong winds aloft, fast storm 
motions, and steep low-level lapse rates will promote a risk of 
gusty winds in the strongest cells. 


Mesoscale Discussion

acus11 kwns 262247 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 262246 

Mesoscale discussion 0533 
National Weather Service Storm Prediction Center Norman OK 
0546 PM CDT Sat may 26 2018 

Areas affected...eastern Montana...western North Dakota...and far 
northwest South Dakota 

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

Valid 262246z - 270045z 

Probability of watch issuance...95 percent 

Summary...thunderstorms are expected to develop across the plains of 
eastern Montana in the next few hours. These thunderstorms will pose 
a risk of large hail and severe thunderstorm winds. A Severe 
Thunderstorm Watch will be needed by 00z (6 PM mdt). 

Discussion...diffluent mid-level flow will continue to overspread 
portions of eastern Montana this evening as a short-wave trough 
rotates through the larger-scale flow associated with the closed low 
over Utah. In response to this, synoptic-scale ascent will act to 
weaken the remaining convective inhibition and allow thunderstorms 
to develop across the plains of eastern Montana. The airmass 
available to these thunderstorms is characterized by most-unstable 
cape values up to 4000 j/kg (although 2000 j/kg will be more common) 
and deep-layer shear values of 30-40 knots. This environment will be 
more than sufficient to support thunderstorm organization and 

Somewhat large temperature-dewpoint spreads (on the order of 30-40 
f) will yield a potential for strong, cold outflows with additional 
thunderstorm development along the outflows. Eventually these 
outflows should either grow upscale enough to be considered a 
mesoscale convective system (mcs), or congeal into one. Given the 
deep-layer shear and instability axis, this mesoscale convective system should move eastward 
with time, persisting through the evening (past sunset). With time a 
slight turn to the east-southeast may occur over the Dakotas as the 
better instability will remain across western and central South 

A Severe Thunderstorm Watch will be issued by 00z (6 PM mdt) to 
account for the initial thunderstorm threats of large hail and 
strong, gusty outflows. 

.Marsh/grams.. 05/26/2018 

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


Latitude...Lon 46390673 46950758 48200750 48820654 48500451 47560210 
46300162 45290243 45450363 45900516 46390673