U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 151301 
Storm Prediction Center ac 151259 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0759 am CDT Wed Aug 15 2018 

Valid 151300z - 161200z 

..there is a slight risk of severe thunderstorms over parts of 
Nebraska and southern South Dakota... 

The main severe-storm threat area is over parts of the north-Central 
Plains region this afternoon and evening, for large hail and severe 

The mid/upper-level pattern is transitioning toward a 
more-progressive regime over much of the conus, though the primary 
belt of midlatitude westerlies is forecast to remain to the north 
over Canada. A leading trough -- currently over New England and the 
coastal mid-Atlantic, is forecast to eject across the Atlantic and 
Canadian Maritimes, while weakening. This will occur as an upstream 
perturbation -- now evident in moisture-channel imagery over 
northern on -- shifts east-southeast and amplifies somewhat. The 
latter trough is forecast to reach northern New England between 
06-12z overnight, as a well-defined but lower-amplitude shortwave 
feature than the one now over New England. 

Farther west, a broad, weakening perturbation with several embedded 
vorticity lobes is apparent from Iowa across western MO to near the 
OK/Arkansas border. The associated 500-mb trough should move to an axis 
from Southern Lake Michigan to eastern Arkansas by 00z, then decelerate 
and deamplify across lower mi, the in/Illinois border area and western Kentucky 
overnight. A pronounced, upstream trough -- currently located over 
eastern Montana and eastern Wyoming -- is expected to move eastward across the 
northern plains through the period. Perhaps aided by convective 
vorticity generation/augmentation, this feature is progged to 
develop a closed 500-mb low this evening and overnight over southern 
SD, reaching the mhe-fsd vicinity by 12z. 

At the surface, 11z analysis showed a cold front from southern qc 
across Southern Lake Huron to a frontal-wave low over the lse area, 
then southwestward across western Kansas. The central U.S. Portion of 
the cold front will merge with a prefrontal trough and weak low, 
associated with the weakening/departing mid/upper wave, through 
tonight. Meanwhile the eastern frontal segment will move 
southeastward across the St. Lawrence Valley, parts of Maine and 
northeastern New York by the end of the period. A dryline/mixing boundary 
should develop today over eastern Colorado/Wyoming and move eastward this 
afternoon onto parts of the Central High plains as well. 

..northern/Central Plains... 
Scattered thunderstorms are expected to form over the western parts 
of the outlook area through this afternoon -- including evolution of 
ongoing activity over the Black Hills vicinity into a destabilizing 
boundary layer farther east and southeast. Damaging gusts and large 
hail are expected from the most intense storms, some of which will 
be supercells capable of significant/2-inch or larger hail 
production, given the forecast lapse rates and shear. This process 
will be aided by steepened lapse rates from a combination of 
large-scale cooling/ascent in midlevels (ahead of the shortwave 
trough) with diabatic surface heating. 

Modified raobs and forecast soundings suggest surface dew points 
upper 50s to lower 60s f will support MLCAPE around 1500 j/kg, 
amidst sufficient veering of flow with height to contribute to 
40-50-kt effective-shear magnitudes. Any relatively 
sustained/discrete cells may leave a swath of hail, in addition to 
isolated damaging gusts, while wind potential will be greater with 
any upscale clustering/cold-pool aggregation. Activity overall 
should diminish after about 04z as the foregoing boundary layer 
diabatically stabilizes. 

..northeastern nm/southeastern Colorado southeastward... 
A relative gap in severe potential exists where drying and mixing 
will reduce cape substantially along and behind the boundary over 
northern/eastern co, with only weak lift. However, widely scattered 
thunderstorms may develop over the Sangre de Cristos, and nearby 
foothills/mesas this afternoon and move southeast into a favorably 
well-mixed environment for isolated severe-gust potential. Storms 
should develop and intensify largely where the air mass can 
destabilize diabatically, with heating of higher terrain behind 
(north and northwest of the ongoing cloud/precip area in 
northeastern nm. 

..Ohio Valley to eastern OK... 
Scattered thunderstorms will affect this corridor through the 
evening hours. Though most of the convection will be nonsevere, 
offering only localized potential for minor wind damage, isolated 
severe gusts may occur in the most intense, water-loaded cores. 

The already marginal severe potential will be very 
patchy/discontinuous through the outlook area, thanks to 
aftereffects of morning convection, related outflow and zones of 
differential heating that also may focus lift and low-level shear 
locally. Given the mesoscale uncertainties still involved, the 
outlook remains rather broadbrushed at this time. Rich low-level 
moisture largely is in place and will remain so, with surface dew 
points upper 60s to low 70s and precipitable water commonly 1.75-2 inches. Any 
areas of sustained insolation will contribute to preconvective 
MLCAPE 1000-2000 j/kg over the lower Ohio Valley region, to over 
3000 j/kg in eastern OK. Weak deep shear will limit overall severe 
potential, though pockets of enhanced low-level shear may develop 
around mesoscale boundaries and beneath a 30-40-kt southwesterly 
low level jet. 

..northern New England and northern New York... 
Widely scattered to scattered thunderstorms are expected to develop 
in multiple episodes today into this evening over the outlook area, 
and moving into the region from adjoining parts of Canada. Isolated 
marginally severe gusts are possible. 

Activity should occur along the front, as well as a prefrontal 
surface trough and perhaps boundaries left behind by morning 
clouds/precip. Surface dew points generally in the 60s f should 
combine with steep boundary-layer lapse rates to yield little or no 
cinh, with temps mid/upper 70s f and higher, based on forecast 
soundings. MLCAPE commonly should reach the 1500-2000 j/kg range 
(locally higher). Deep-layer flow should be nearly unidirectional/ 
westerly, which will limit convergence, through the aforementioned 
weak capping should permit storms to form along even subtle 
boundaries. Although low-level winds and shear will be weak, mid/ 
upper flow should strengthen with time this afternoon into evening, 
from west to east across the outlook area, as the shortwave trough 
approaches. This will aid in multicellular storm organization 
through increased cloud-layer shear. 

.Edwards/Peters.. 08/15/2018 


Mesoscale Discussion

acus11 kwns 151403 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 151403 

Mesoscale discussion 1293 
National Weather Service Storm Prediction Center Norman OK 
0903 am CDT Wed Aug 15 2018 

Areas affected...southwestern South Dakota...extreme northwest 

Concerning...severe potential...watch unlikely 

Valid 151403z - 151630z 

Probability of watch issuance...20 percent 

Summary...elevated thunderstorms capable of producing mainly severe 
hail are expected to persist along/north of the South 
Dakota/Nebraska border for at least a few more hours. A ww issuance 
is not currently anticipated. 

Discussion...elevated thunderstorms are currently ongoing across 
Fall River County, to Mellette County South Dakota. These storms 
have shown signs of organization/persistence, with up to 1.0-1.5 
inch diameter mrms-mesh occasionally noted with some of the stronger 

An eastward propagating 500 mb shortwave trough and associated vort 
Max, currently located across northeast Wyoming, has provided deep 
layer ascent to initiate this convection. Storms are currently 
rooted above a stable boundary layer, most likely above 850 mb (as 
indicated by 1300 UTC pfc rap guidance). With up to around 500 j/kg 
MUCAPE, 30 knots of bulk effective shear, and continued deep-layer 
ascent associated with the vort Max present across the discussion 
area, storms may persist for at least a few more hours, bearing a 
potential for at least marginally severe hail. Given the isolated 
and marginal extent of the severe hail threat, a ww issuance is not 
expected at this time. 

.Squitieri/Hart.. 08/15/2018 

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


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