U.S. Severe Weather Forecast (Convective Outlook)

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Storm Prediction Center ac 231255 


Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0655 am CST Thu Feb 23 2017 


Valid 231300z - 241200z 


..there is a slight risk of severe thunderstorms late afternoon to 
early evening over part of north-central Kansas... 


..there is a marginal risk of severe thunderstorms elsewhere from 
northern Kansas and southern Nebraska this afternoon/evening to 
eastern Iowa overnight... 


... 
Thunderstorms may produce isolated severe hail across portions of 
southern Nebraska, northern Kansas, northern Missouri, and Iowa from 
this afternoon into tonight. 


... 
The upper-air pattern will feature a broad area of cyclonic flow 
forecast to shift eastward from the western to central U.S. Through 
the period. Within that regime, a strong shortwave trough and 
intermittently closed mid/upper low were noted over the Great Basin, 
and will move eastward across the central rockies through the 
period. By 12z, the primary 500-mb low and vorticity Max should 
reach central/eastern NE, with shortwave trough southwestward across 
western Kansas and eastern nm. In the southern stream, a cyclone now 
centered near coastal southeastern Florida should move eastward over the 
northern Bahamas, then weaken and eject northeastward across open 
Atlantic waters. 


At the surface, a cold front extended from lower Michigan southwestward 
across central Illinois and central MO, becoming quasistationary over the 
Kansas/OK border region, then arching northwestward through an elongated 
area of low pressure from southeastern through central/northwestern 
Colorado. The western segment of the front should move northward through 
the period as a warm front across Kansas and western/central MO. 
Through mid-afternoon, the Colorado low should consolidate/re-develop 
southeastward along the warm front into southwestern KS, before 
strengthening and shifting northeastward across south-central Kansas by 
00z. A cold front behind that low will move southeastward through 
the southern High Plains and most of OK overnight. 


Around 00z, a sharply defined surface ridge still will extend from 
the Carolinas southwestward across southern al, the Florida Panhandle and 
the northern/central Gulf, with offshore flow over the northeastern 
Gulf in the wake of the Florida/Bahamas low. Associated anticyclonic 
trajectories and immaturity of Gulf air-mass modification will 
continue to hamper moisture return in the plains surface warm sector 
through this period. Moisture will increase farther east across the 
MS valley late in the period and into day 2, when the deep-layer 
wave ejects northeastward and severe potential increases markedly 
across parts of the Midwest. 


..Central Plains to eastern Iowa... 
It still appears that eml-related capping and a lack of more robust 
boundary-layer moisture will preclude diurnal, surface-based 
thunderstorms on the surface warm front or a dryline/trough 
extending southward from the low. Instead, widely scattered to 
scattered thunderstorms are expected to develop poleward of the 
surface low and warm front from late afternoon into evening, moving 
northeastward across the outlook area with the potential for 
isolated large hail. As the surface low and mid/upper-level wave 
advance eastward this evening and overnight, additional convection 
should form north of the surface warm front over northern MO and IA, 
likewise moving northeastward and offering the risk of isolated 
severe hail. 


Afternoon development over parts of northern Kansas and southern NE will 
be related to a spatial juxtaposition of: 
1. Frontogenetic/cyclogenetic forcing above the surface, 
2. Large-scale DCVA/cooling aloft ahead of the mid/upper shortwave 
trough, 
3. The western fringe of at least marginally supportive low-level 
moisture mainly atop the boundary layer, 
4. Supplementary lift beneath a coupled upper-jet structure, 
characterized by the left-exit region of a cyclonically curved 
member over the southwest and Southern Plains, and the 
right-entrance region of a jet Max over the upper MS valley and 
Great Lakes. 


By 21-03z, a narrow corridor of 6-8 degree c 850-mb dew points 
should appear north of the surface front and northeast of the low, 
expanding both laterally in north-south width and extending westward 
to the north of the surface low. This should suffice for storms to 
form amidst strong deep-layer lift, and in a favorable kinematic 
environment with strengthening deep shear. This regime will shift 
eastward through the evening and overnight, amidst increasing 
elevated moisture return, a widening moist plume above the surface, 
and overlying steep preconvective lapse rates, all in support of 
hail potential. 


Forecast soundings from several progs reasonably show 55-65-kt 
effective-shear magnitudes, and in a small area of north-central KS, 
a brief spatiotemporal "sweet spot" of 500-800 j/kg MUCAPE around 
00z. These conditions may support an isolated, fairly long-lived, 
severe supercell or two, perhaps even with damaging-wind risk in 
addition to a hail swath. A storm in that environment briefly may 
become nearly surface-based, considering 
1. Effective-inflow parcels near the surface in some forecast 
soundings, and 
2. The potential for internal storm dynamic processes to force 
stronger/deeper convective-scale lift than enabled by parcel theory 
alone. 
That early in the convective period, capping will limit storm 
coverage enough that any such supercell(s) should be isolated in 
nature. For now, the low-probability but potentially higher-yield 
mesoscale threat still is not certain, but hail probabilities have 
been bumped up a notch, with marginal wind probabilities introduced 
to account for that conditional threat. 


.Edwards.. 02/23/2017 


$$ 

Mesoscale Discussion


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Storm Prediction Center mesoscale discussion 231231 
nez000-sdz000-wyz000-231700- 


Mesoscale discussion 0199 
National Weather Service Storm Prediction Center Norman OK 
0631 am CST Thu Feb 23 2017 


Areas affected...portions of western and northern Nebraska...and 
extreme south-central South Dakota 


Concerning...heavy snow 


Valid 231231z - 231700z 


Summary...heavy snowfall rates around 1 inch/hour are expected 
through the morning from part of north-central NE (in vicinity of 
Keya Paha County through Cherry and Sheridan counties, and perhaps 
into western parts of the NE panhandle). This snowfall rates should 
also develop into extreme south-central South Dakota (primarily from Bennett 
to Tripp counties). 


Discussion...trends in mosaic radar imagery indicated mesoscale 
banding developed around 09z, and has persisted through 12z so far 
across mainly northern Cherry County to Sheridan County. Surface 
observations show this precipitation is in the form of snow, with 
this region located north of the wet-bulb 0c line which at 12z 
extended from northeast to southwest NE. Northeasterly surface 
winds are expected to increase some this morning into the afternoon, 
as an area of low pressure forms across eastern Colorado and deepens, 
tracking into west/southwest Kansas this afternoon. This indicates 
cooler temperatures should be maintained across the discussion area 
today to sustain the precipitation type as snow. 


Trends in mosaic radar imagery indicated some increase in intensity 
within the snow bands. This trend should persist through this 
morning, as height falls/DPVA spread into western and central NE, 
with the approach of the Great Basin trough, further enhancing 
midlevel frontogenesis within the 600-700-mb layer. In addition, 
11z rap forecast soundings indicated the upward vertical motion in 
the 600-700-mb layer will occur within the favorable dendritic 
growth zone this morning, supporting the heavier snowfall rates. 


.Peters.. 02/23/2017 


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


Attention...WFO...lbf...unr...cys... 


Latitude...Lon 43239975 42779971 42350100 42020226 41670354 41760409 
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