U.S. Severe Weather Forecast (Convective Outlook)

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000 
acus01 kwns 251954 
swody1 
Storm Prediction Center ac 251952 


Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0252 PM CDT sun Aug 25 2019 


Valid 252000z - 261200z 


..there is a slight risk of severe thunderstorms over a the central 
Dakotas into extreme northern Nebraska... 


... 
Thunderstorms with isolated large hail and a few damaging gusts are 
expected mainly over the central Dakotas this afternoon/evening into 
northern Nebraska overnight. 


... 


Primary change to this outlook has been to continue to trim the 
eastern bound of the slight across ND primarily due to thermodynamic 
concerns related to the widespread stratus that has been slow to 
erode. Have also dropped the marginal risk area over the lower MS valley 
region, as much of the airmass is in the process of being overturned 
by numerous multicell storms. 


Otherwise, expect storms to develop along the pre-frontal trough 
over the Dakotas this afternoon. Here, the atmosphere has become 
moderately unstable (1500-2000 j/kg mlcape), and vertical wind 
profiles with 30-40 kt effective bulk shear should support a few 
supercell structures, but eventually evolving into multicell line 
segments. Reference sels mesoscale discussion 1857 for more details. 


.Dial.. 08/25/2019 


Previous discussion... /issued 1124 am CDT sun Aug 25 2019/ 


..Dakotas to NE this afternoon into tonight... 
A northern stream shortwave trough will amplify over the northern 
High Plains through tonight. An initial surface trough/cold front 
across the western Dakotas will continue eastward into the eastern 
Dakotas and southeastward into NE by the end of the period. A 
narrow zone of surface heating along the surface trough and weak 
large-scale ascent will support thunderstorm development this 
afternoon/evening across the central Dakotas, and storms will 
subsequently spread eastward and southeastward tonight. Steep 
midlevel lapse rates will combine with the surface heating and 
boundary-layer dewpoints in the mid 60s to result in MLCAPE of 
1500-2500 j/kg immediately east of the surface trough this 
afternoon. 


Deep-layer vertical shear with straight hodographs will initially 
favor supercells capable of producing large hail, though upscale 
growth into clusters/line segments with damaging winds appears 
probable this evening before the convection begins to spread east of 
the stronger buoyancy. The southern flank of the frontal convection 
is likely to maintain intensity the longest, potentially developing 
southward into NE in conjunction with the surface triple point and 
warm advection on the nose of 30-40 kt nocturnal low-level jet. 


Other more isolated storms will be possible later this evening from 
the big horns into southwestern SD, in the immediate Post-frontal 
upslope flow regime. Deep-layer vertical shear will be favorable 
for supercells, though limited moisture/buoyancy should tend to 
temper the hail/wind threat somewhat. 


..OK/AR/MO this afternoon... 
A convectively enhanced midlevel trough is moving eastward over 
eastern Kansas and northeastern OK as of mid morning. Vertical shear 
will be strongest to the west of the trough in conjunction with the 
stronger northwesterly midlevel flow. However, storm initiation 
beyond ongoing convection in the warm advection zone near I-35 along 
the Kansas/OK border is unlikely given the expected onset of subsidence 
in the wake of the midlevel trough. Some destabilization is 
expected south through east of the convective outflow, though clouds 
are relatively widespread and will likely continue to slow surface 
heating. Overall, it appears that a marginal threat for damaging 
wind is the primary concern with multicell clusters along the 
outflow boundary this afternoon from southern MO into northwestern 
Arkansas and southeastern OK. 


$$ 

Mesoscale Discussion


000 
acus11 kwns 251951 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 251950 
ndz000-sdz000-252145- 


Mesoscale discussion 1857 
National Weather Service Storm Prediction Center Norman OK 
0250 PM CDT sun Aug 25 2019 


Areas affected...portions of North Dakota and South Dakota 


Concerning...severe potential...watch likely 


Valid 251950z - 252145z 


Probability of watch issuance...80 percent 


Summary...observational trends and model guidance suggest that a 
Severe Thunderstorm Watch will be needed by around 21z or so. 


Discussion...latest objective analyses and satellite imagery 
suggests a destabilizing atmosphere especially across western 
portions of the discussion area, with insolation now yielding 
towering cumulus near/east of isn through west of bis. Warming 
surface temperatures (into the 80s f) was contributing to MUCAPE 
values around 2000-3000 j/kg amidst steep (7-8c/km) mid-level lapse 
rates and modest (30-40 kt) deep shear. Meanwhile, a weak mid-level 
shortwave trough over eastern Montana was approaching the region 
from the west, which should provide additional ascent/cooling aloft 
and further aid in convective initiation. The region also resides 
beneath the exit region of a gradually strengthening mid-level jet 
that should impinge on the region through tonight. 


Latest thinking is that storms will continue to increase in coverage 
and intensity over the course of the afternoon and will likely grow 
upscale into a mix of cells and clusters as cold pools mature amidst 
modest low-level shear. Severe hail and damaging wind gusts will be 
the most probable threats with this activity. The threat will be 
widespread enough to necessitate a ww issuance, and this will be 
coordinated with affected wfos during the 20-21z window. 


.Cook/Thompson.. 08/25/2019 


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


Attention...WFO...fgf...abr...bis...unr... 


Latitude...Lon 48930223 49150150 49080000 48469939 46859919 44819915 
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