U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 221300 
Storm Prediction Center ac 221259 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0759 am CDT Thu Jun 22 2017 

Valid 221300z - 231200z 

..there is a slight risk of severe thunderstorms over parts of the 
Mississippi Delta...Tennessee Valley and central Gulf Coast 

..there is a slight risk of severe thunderstorms over parts of the 
central and southern High Plains and Colorado Front Range... 

..there is a slight risk of severe thunderstorms from parts of Iowa 
to parts of lower Michigan... 

..there is a marginal risk of severe thunderstorms surrounding the 
slight risk in the south... 

..there is a marginal risk of severe thunderstorms surrounding and 
connecting the other two slight risks... 

A few tornadoes and isolated damaging gusts are possible over a 
broad area east and northeast of the center of inland Tropical Storm 
Cindy. Elsewhere, severe thunderstorms with damaging winds and 
large hail are possible from the south-Central High plains to 
portions of the Great Lakes. 

The mid/upper-level pattern will remain dominated by three primary 
synoptic to mesoalpha-scale features: 
1. Tc Cindy and its accompanying upper-air perturbation; 
2. An intense ridge that will remain entrenched over the 
southwestern U.S.; 
3. A belt of relatively fast northern-stream flow from the Pacific 
northwest across the northern plains and Great Lakes, to New 
England. The northern plains and upper Midwest segment of this 
pattern will become more cyclonic with time, as a cyclone now over 
mb and trailing positively-tilted trough move southeastward. By 
00z, the trough should extend from the southern section of mb/on 
border across ND to southwestern Montana. By 12z the mid/upper trough 
should reach the Minnesota arrowhead and extend to southwestern Minnesota then 
westward over South Dakota. 

At the surface, the 11z analysis showed a weak frontal-wave low over 
north-central NE, with a cold front southwestward across 
southwestern NE and south-central Colorado. The frontal zone extended 
from the low across southern nm -- north of an area of convective 
outflow with a southern boundary arching across northeastern Iowa. 
The front then was analyzed over central lower Michigan and Lake Erie. 
That boundary will sag southward as a cold front today across WI, Iowa 
and eastern NE, as well as the south-Central High plains, as the 
northern-stream trough moves southeastward. By 00z the front should 
extend from northern lower Michigan to central IA, 
northwestern/west-central Kansas and northeastern nm. By the end of the 
period, the front should extend from Southern Lower Michigan across 
southwestern MO, southwestern OK and east-central nm. 

..southern states: Tropical Storm Cindy... 
T.S. Cindy is forecast to progress farther inland/northeastward 
through the period toward an eventual merger with the aforementioned 
frontal zone day-2, experiencing a general weakening of surface 
winds and filling of central pressure, as forecast by NHC. However, 
as often is the case with inland-penetrating tropical and 
subtropical systems, low-level flow above the surface will be slower 
to weaken. This will lead to maintenance (and perhaps areas of 
mesobeta-scale strengthening) of low-level shear/hodographs in the 
favorable northeastern and eastern sector through the day. In 
addition, this system's dramatically asymmetric structure will 
continue to spread a rather large area of at least marginally 
favorable shear across the south. 

Last night's 00z 500-mb analysis showed several pockets of 
distinctive, relatively warm and cold (and dry and moist) air 
wrapping into this system, revealing complex internal baroclinic 
influences revolving around the circulation that will contribute to 
moving swaths of relative clearing today. With low/middle-level 
lapse rates nearly moist adiabatic, only a few deg f of surface 
heating will be needed to boost the magnitude and depth of buoyancy 
such that MLCAPE 200-500 j/kg should be fairly common, with patches 
of 500-1000 j/kg values this afternoon, amidst effective srh 200-600 
j/kg. Any sustained discrete or semi-discrete cells moving through 
such a favorable environment will pose a tornado risk, though 
coverage of those may be isolated overall. Also, differential- 
heating boundaries and convergence bands will be corridors to 
monitor for more localized augmentation of tornado potential within 
this broad-brushed outlook area. The tornado potential should peak 
mid-late afternoon and generally ramp down overnight (but not go 
away altogether), amidst weaker boundary-layer instability, lower 
convective coverage, and more nebulous foci. 

Isolated damaging/nontornadic wind gusts also are possible from the 
deeper cells even well away from center, especially where areas of 
diabatic heating create shallow but well-mixed subcloud layers that 
augment downward momentum Transfer out of the layer of faster winds 
just above surface. For near-term coverage of tornado potential, 
see Storm Prediction Center watch 363 and related mesoscale discussions. For forecast 
of Cindy's path/intensity and tropical-related watches/warnings, 
refer to NHC advisories. 

..central/southern High Plains, Colorado Front Range... 
Widely scattered thunderstorms are expected to develop near the 
front this afternoon, in an environment characterized by a hot and 
well-mixed warm-sector boundary layer supporting potential for large 
hail and damaging gusts generated aloft to reach the surface. 
Concerns exist over storm coverage, and some areas of this outlook 
may not experience any. The threat itself is probably more locally 
clustered than the outlook area indicates; however it is too soon to 
narrow that down until mesoscale destabilization/lift trends near 
the frontal zone become more readily apparent. Though low-level 
winds will be weak, strong directional shear will contribute to 
30-40 kt effective-shear magnitudes amidst MLCAPE 1000-1500 j/kg, 
supporting a few clusters of organized and probably outflow-dominant 

Post-frontal flow is expected to veer to a substantially easterly 
component by this afternoon, setting up an upslope regime into the 
Front Range region, beneath difluent northwest flow aloft. This 
will contribute to moist advection, increasing deep speed/bulk 
shear, and favorable storm-relative low-level wind vectors for 
supercells moving either leftward or rightward off a long, nearly 
straight hodograph. Large hail and isolated damaging gusts each 
will be possible, given the deep/well-mixed subcloud layers likely 
with 40s f surface dew points. A conditional threat for isolated 
significant/2+ inch hail may develop in the I-25/I-76/I-70 areas 
near and downshear from the den/fcl corridor, though uncertainties 
remain related to both optimal cloud-water content and maintenance 
of discrete storm modes most suitable for it. Mesoscale trends will 
be monitored for enough coverage of this threat to introduce an 
unconditional significant-hail area in a later outlook. Convection 
will offer some wind/hail threat eastward across the High Plains of 
eastern Colorado before diminishing. 

..portions of the corn belt to Great Lakes... 
Multiple episodes of scattered thunderstorms are possible through 
the period, with the greatest cumulative concentration expected 
across the 15% wind/hail corridor representing the categorical 
slight risk. This includes ongoing convection that will move 
eastward from northern Iowa across at least parts of southern WI and 
perhaps northern IL, offering a threat for sporadic large hail. 
Additional thunderstorms currently north of the surface low and 
frontal zone across central/eastern SD, as well as in southern Minnesota 
near the front and north of the outflow boundary, will pose a 
marginal risk of isolated large hail as well as damaging gusts in 
the next few hours, the South Dakota/Minnesota activity, based on modifications to 
the oax sounding and rap forecast soundings, resides in an 
environment of steep midlevel lapse rates and elevated moistening, 
atop a dry layer between 750-900 mb that may support evaporative 
downdraft-parcel accelerations through the near-surface stable 

Additionally, surface-based thunderstorms are expected to develop 
this afternoon along the front and outflow boundary, as well as in a 
low-level warm advection/convergence zone farther east across parts 
of Lake Michigan and lower Michigan. This activity should occur mainly 
south of the southern rim of optimally favorable mid/upper level 
flow. Still, sufficient deep shear will exist for organized 
multicellular convection or transient/marginal supercells, with hail 
and damaging gusts possible. Diabatic surface heating and dew 
points 60s f should contribute to MLCAPE 1500-2500 j/kg amidst 30-40 
kt effective-shear magnitudes. 

.Edwards/picca.. 06/22/2017 


Mesoscale Discussion

acus11 kwns 220812 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 220811 

Mesoscale discussion 1119 
National Weather Service Storm Prediction Center Norman OK 
0311 am CDT Thu Jun 22 2017 

Areas affected...portions of the northern plains 

Concerning...severe potential...watch unlikely 

Valid 220811z - 220915z 

Probability of watch issuance...20 percent 

Summary...a few intensifying thunderstorms early this morning may be 
capable of isolated instances of large hail, and perhaps a few 
stronger gusts. However, the threat is currently expected to remain 
limited enough to preclude watch issuance. 

Discussion...on the southern fringe of a mid/upper jet maximum, 
isolated/widely scattered thunderstorms have developed early this 
morning. Recent Aberdeen, South Dakota (kabr) vwp data exhibit westerly 
mid-level flow around 50-60kt, with resultant deep-layer shear 
sufficient for updraft organization/rotation. Therefore, as these 
cells move east into more substantial mid-level buoyancy (e.G., 
MUCAPE around 1000-2000 j/kg), they may be capable of isolated large 
hail reports. Moreover, despite being rooted above the surface 
(likely near 700mb), pockets of sufficient low-level temp/dew point 
spreads may support occasional strong, gusty surface winds. This 
threat will be further enhanced by small bowing segments resulting 
from somewhat Uni-directional flow in the convective layer. 
Nonetheless, any severe threat is currently expected to remain 
spatially limited, likely precluding overnight watch issuance. 

.Picca/Edwards.. 06/22/2017 

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


Latitude...Lon 44000089 44280097 44770004 45259662 45249473 45069418 
44459395 44009406 43729504 43459636 43289751 43299912 
43370028 43880088 44000089