Severe Storm Outlook Narrative (AC)
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000 ACUS11 KWNS 222058 SWOMCD SPC MCD 222057 MNZ000-IAZ000-NEZ000-SDZ000-222300- Mesoscale Discussion 1667 NWS Storm Prediction Center Norman OK 0357 PM CDT Fri Sep 22 2017 Areas affected...Portions of Nebraska northeastward to far southwest Minnesota Concerning...Severe potential...Watch unlikely Valid 222057Z - 222300Z Probability of Watch Issuance...20 percent SUMMARY...Thunderstorms are expected to gradually develop from central Nebraska northward into South Dakota by early evening. Isolated storms may be capable of large hail and damaging winds. While a watch is not currently anticipated, organizational trends will be monitored for a potential uptick in the overall severe threat. DISCUSSION...Visible satellite data illustrate developing cumulus along a cold front from central Nebraska northeastward into South Dakota this afternoon. Ahead of this front, temperatures have risen into the 90s, owing to strong surface heating. While deep mixing has lowered dew points into the 50s across parts of Nebraska, boundary-layer moisture remains greater farther north and east. In tandem with steep mid/level lapse rates, these boundary-layer conditions are yielding around 2000-3000 J/kg of MLCAPE. Forcing for ascent is not particularly strong across the region and relatively dry 850-700mb conditions may stunt nascent updrafts initially; however, broad/weak ascent and further heating will likely be sufficient for isolated/widely scattered convective initiation. Southerly surface winds around 15-20 kt, veering to southwesterly mid/upper flow around 40-50 kt, should offer adequate effective shear for organization/rotation in the most robust cores, with an attendant large hail and damaging wind threat. Additionally, while large surface T/Td spreads will likely limit the tornado threat, favorable low-level hodographs suggest a tornado may be possible, primarily across far eastern South Dakota and far southwestern Minnesota later this evening. Although watch issuance is not currently expected, one may be considered if greater storm organization/coverage is observed. ..Picca/Hart.. 09/22/2017 ...Please see www.spc.noaa.gov for graphic product... ATTN...WFO...MPX...FSD...OAX...ABR...GID...LBF... LAT...LON 41480022 42399925 43339801 44879765 45189707 45159624 44509574 43309597 41709720 40959843 40759907 40729974 40960020 41160031 41480022

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