U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 171248 
Storm Prediction Center ac 171246 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0746 am CDT Tue Jul 17 2018 

Valid 171300z - 181200z 

..there is a slight risk of severe thunderstorms Central High 
plains and the northeast... 

Scattered severe storms are possible across parts of the Central 
High plains during the late afternoon and evening. Strong wind gusts 
producing scattered damaging winds are possible over the northeast 
through early evening. 

..Central High plains... 
In the wake of an mesoscale convective system over central KS, southeasterly surface winds 
will maintain an upslope flow component towards the central/southern 
rockies. A shortwave trough over the northern rockies will shift 
east across the northern High Plains. 30-40 kt 500-mb westerlies 
attendant to this feature will overspread much of Wyoming with mid to 
late afternoon storm initiation expected off the higher terrain. 
While deep-layer shear will be stronger relative to Monday, 
deleterious effects of the ongoing mesoscale convective system on boundary-layer moisture 
should result in a more spatially confined plume of MLCAPE ranging 
from 1000-2000 j/kg. Speed shear through the cloud-bearing layer 
should be sufficient for a supercells with large hail and isolated 
severe gusts the primary hazards. Some upscale growth may occur in 
the evening, but guidance differs greatly on the degree of low-level 
warm advection. 

Scattered storms are ongoing within the warm conveyor ahead of a 
cold front that stretches from the Saint Lawrence valley southwest 
into the upper Ohio Valley. Within a weak mid-level lapse rate 
environment, surface heating will be critical to strong storm 
potential. Early-day visible satellite imagery suggests this will be 
pronounced ahead of the ongoing convection. With pervasive low to 
mid 70s surface dew points, 1000-2000 j/kg MLCAPE should be common. 
While effective shear will remain modest around 20-30 kt, numerous 
storms should Foster several multicell clusters by early to mid 
afternoon. Scattered downed trees will be the primary hazard from 
strong wind gusts. 

Ongoing mesoscale convective system across central Kansas has yielded an mesoscale convective vortex along the 
north-central Kansas/south-central NE border with outflow continuing to 
push southeast in southwest Kansas. Cam and non-cam guidance have poorly 
handled the spatiotemporal aspects of this mesoscale convective system. They also widely 
vary in the shear/instability environment and with simulations of 
renewed storm development this afternoon/evening. With a hot air 
mass becoming prevalent south/southwest of the outflow, and a 
moderately buoyant air mass to its southeast, it appears plausible 
that a couple supercells and multicell clusters may develop later 
today. Low confidence in when/where and how that may evolve will 
necessitate deferral on a categorical upgrade. 

.Grams/Mosier.. 07/17/2018 


Mesoscale Discussion

acus11 kwns 171435 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 171435 

Mesoscale discussion 1064 
National Weather Service Storm Prediction Center Norman OK 
0935 am CDT Tue Jul 17 2018 

Areas affected...portions of New England and southeast New York 

Concerning...severe potential...watch likely 

Valid 171435z - 171630z 

Probability of watch issuance...80 percent 

Summary...bands of thunderstorms are expected to intensify through 
this afternoon, resulting in an increasing threat for damaging wind 
gusts. A watch will likely be issued within the next 1-2 hours. 

Discussion...regional radar data illustrate a broken band of 
thunderstorms extending from southern New Hampshire southwest 
towards the Hudson Valley late this morning. This cluster has 
persisted along the southeastern fringe of a subtle shortwave 
impulse (embedded within larger-scale cyclonic flow) over New 
England. Ahead of these storms, visible satellite suggests gradual 
low-level destabilization, evidenced by a slow decrease in wave 
clouds and an increase in boundary-layer cumulus. Amidst a very 
moist air mass (e.G., Regional 12z soundings sampled pws around 
1.6-1.9"), further surface heating should promote MLCAPE values 
around 1000-2000 j/kg through this afternoon. In turn, thunderstorms 
are expected to further intensify over the next several hours. 

Relatively Uni-directional deep-layer wind profiles, combined with 
30-40 kt of mid-level flow, will encourage lines of storms capable 
of damaging winds, especially this afternoon. Considering this 
growing severe potential, watch issuance will likely be needed. 

.Picca/guyer.. 07/17/2018 

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


Latitude...Lon 41937482 43367234 43687174 43797082 43677050 43447052 
42767075 41817113 41607208 41397363 41457445 41787488