U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 010559 
Storm Prediction Center ac 010558 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1258 am CDT Sat Aug 01 2015 

Valid 011200z - 021200z 

..there is a slight risk of severe thunderstorms from the middle MO valley into 
parts of the upper MS valley region... 

..there is a marginal risk of severe thunderstorms surrounding the slight risk from 
central Nebraska/South Dakota to WI...northern MO...and western and northern Illinois... 

..there is a marginal risk of severe thunderstorms across northern New England... 

isolated to scattered severe storms capable of hail...damaging 
winds...and a tornado threat should develop late this afternoon in 
southeast South Dakota. Additional strong to severe storms are 
expected to increase in coverage across Iowa and into the upper 
Mississippi Valley this evening with a continued potential for all 
severe hazards...though damaging winds may become the more prominent 
threat. A few severe storms with hail and strong wind gusts may also 
occur over northern New England this afternoon. 

An amplified upper pattern will prevail through today and tonight... 
with a ridge in the west and a broad longwave trough in the eastern 
U.S. Which is anchored by a deep closed low located over Hudson Bay. 
This closed low should begin to shift southward late in the forecast 
period. A strong cyclonic flow regime will extend from central 
Canada and the north-central U.S. Through the Great Lakes and Ohio 
Valley to the northeast states. A lead shortwave trough will track 
eastward across northern New England during peak heating this 
afternoon. Meanwhile...farther upstream a weak midlevel shortwave 
trough...currently located across the northern U.S. Rockies per 
moisture-channel imagery...is forecast to reach central and eastern 
South Dakota within the northwesterly flow aloft by late this afternoon and then 
proceed through the upper MS valley this evening and Saturday night. 
Strongest height falls are expected across the upper Midwest after 
02/00z as a separate shortwave trough moves into this region from 
western Canada...and the Hudson Bay low begins to shift southward. 

At the surface...a boundary...currently extending from central MO 
nwwd through southeast to northwest Nebraska...will move to the northeast 
as a warm front today...advancing through the middle MO to upper MS 
valleys. Meanwhile...a pre-frontal trough/dry line will move east 
across the northern plains with a cold front advancing across the 
Dakotas Saturday night. 

..mid MO valley to upper MS valley... 
A few showers and thunderstorms may be ongoing near the warm front in 
southern South Dakota and Nebraska at the start of day 1...with some of this 
activity potentially lingering through the morning to early 
afternoon given weak low-level warm air advection. Poleward moisture return is 
expected across the warm sector today as the warm front moves into 
the middle MO valley this afternoon. This combined with strong 
diabatic heating beneath an eastward-extending plume of steep midlevel 
lapse rates will result in very strong instability /MUCAPE 2000-3000 
j per kg/ by afternoon. Models suggest new storm initiation should 
occur across southeast South Dakota...near the intersection of the warm front 
and pre-frontal trough/dry line...as forcing for ascent attendant to 
the northern rockies trough reaches this region late this afternoon. 
A strengthening southwesterly low level jet across northestern Nebraska into southwestern Minnesota and winds 
veering and strengthening with height will be more than sufficient 
for organized storms including supercells. Early storm evolution 
should support all severe hazards. There is some indication for 
very large hail to occur across the western half of the slight risk 
area...and if confidence is greater for this threat in later 
outlooks...then this severe probability would need to be introduced 
at that time. 

Given stronger height falls expected this evening through tonight... 
further strengthening of the southwesterly low level jet is expected through tonight 
with this jet veering with time across the upper MS valley. Upscale 
growth into one or more mesoscale convective system/S is possible with one mesoscale convective system developing 
S/southeastward through Iowa...while additional storms and eventual mesoscale convective system 
development occurs across southern and central Minnesota into WI. Although 
damaging winds and hail would be the primary threats after 
dark...vertically veering winds and enhanced low-level shear in 
vicinity of the low level jet suggest a tornado threat could persist into the 
early evening across northern Iowa and the southern half of Minnesota. 

..northern New England... 
A cold front is expected to move across this region from late 
morning through the afternoon...with the pre-frontal environment 
expected to have destabilized given some poleward moisture return 
/pw values of 1-1.25 inches/ combined with surface heating and 
modest midlevel lapse rates around 6.5 c/km. Deep-layer 
unidirectional west/southwesterly winds increasing with height will result in 
long...straight hodographs...with effective bulk shear of 30-40 knots. 
This suggests muliticells and splitting supercells will be possible. 
Overall severe-weather coverage should be relatively low given a 
marginally unstable warm sector...with isolated hail and strong wind 
gusts being the main severe risk. 

.Peters/Marsh.. 08/01/2015 

Mesoscale Discussion

acus11 kwns 010927 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 010927 

Mesoscale discussion 1573 
National Weather Service Storm Prediction Center Norman OK 
0427 am CDT Sat Aug 01 2015 

Areas affected...portions of southwestern South Dakota into central Nebraska 

Concerning...severe potential...watch unlikely 

Valid 010927z - 011200z 

Probability of watch issuance...5 percent 

Summary...a risk for isolated severe hail will accompany thunderstorms evolving 
from parts of southwestern South Dakota into central Nebraska through the early-morning 
hours. However...ww issuance is not anticipated. 

Discussion...the nocturnally strengthened boundary-layer wind maximum 
and related increase in isentropic ascent across a northwest/se-oriented 
baroclinic zone analyzed from the northern plains to the lower MO valley 
is fostering an upswing in thunderstorm activity from the Black Hills region 
into central Nebraska. Earlier 00z radiosonde observations at lbf and rap sampled moderately 
steep middle-level lapse rates -- east.G. Around 7.0-7.5 c/km in the 700 mb-500 mb 
layer -- providing sufficient elevated instability for strong 
updrafts. The presence of around 30-40-kt middle-level nwlys sampled by 
area vwps is enhancing deep shear sufficiently for 
sustained/organized cells offering some severe-hail potential. Similar 
potential will likely continue into the early-morning hours. 
However...moisture-channel imagery suggests that the region is 
devoid of any salient middle/upper-level perturbations strongly 
enhancing deep ascent. This should result in no more than an 
isolated severe-thunderstorm risk. Furthermore...the decoupled nature of the 
planetary boundary layer should result in about none severe-wind risk. 

.Cohen/Edwards.. 08/01/2015 

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


Latitude...Lon 40889935 42750195 43790309 44510273 44330176 43360023 
42209894 41009804 40889935