- 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.
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.
..please see www.Spc.NOAA.Gov for graphic product...
Latitude...Lon 40889935 42750195 43790309 44510273 44330176 43360023
42209894 41009804 40889935