Severe Storm Outlook Narrative (AC)
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Issued by NWS
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501 ACUS01 KWNS 230603 SWODY1 SPC AC 230602 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0102 AM CDT Sun Jun 23 2024 Valid 231200Z - 241200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF NEW ENGLAND... ...SUMMARY... Scattered damaging winds and a few tornadoes are likely across parts of the Northeast into the OH/TN valleys, this afternoon and evening. Isolated severe storms are also possible over parts of the central and northern Plains. ...Synopsis... Mostly zonal mid-level flow is forecast to remain over the northern third of the US as a shortwave trough moves east across parts of WA and the southern Canadian Rockies. To the east, a low-amplitude shortwave trough embedded within broader zonal flow aloft over the Great Lakes and southern Ontario is expected to consolidate and strengthen as flow aloft increases over much of New England. A secondary trough over the upper MS River Valley is also expected to amplify as zonal flow aloft becomes southwesterly over the Northeast. Height falls and increasing DPVA will aid in strong surface cyclogenesis over the lower St Lawrence Valley. To the east of the deepening surface cyclone, a warm front will extend eastward across New England to the Atlantic coast, while a cold front and surface trough trails toward the southwest into the OH valley. ...New England to the Mid Atlantic... At the start of the forecast period, scattered thunderstorms are likely to be ongoing near the warm front from southern Quebec southeastward to southern New England. Insentropic ascent along the front from a 30-40 kt LLJ will continue to support scattered storms near the frontal zone through the morning hours as rich boundary-layer moisture (dewpoints in the upper 60s to low 70s F) are advected northward. In the wake of the early morning convection, filtered solar heating will allow for moderate destabilization over much of New England. Most guidance shows 1500-2000 J/kg of MLCAPE developing by early to mid afternoon despite only modest low and mid-level lapse rates. More than sufficient for strong updrafts, scattered storm development is expected first near the warm front and subsequently along the trailing surface trough. Deep-layer shear profiles of 45-60 kt support both supercells and short line segments. With a favorable parameter space for organized storms, damaging winds, isolated hail and a few tornadoes are possible. Confidence in a corridor of locally greater severe risk remains highest across portions of southern ME, VT, NH and west-central MA. Here, area model soundings show large curved hodographs with ESRH of 250-350 m2/s2 and STP values of 1.5-3, near the climatological maximum for the region. With the potential for a mix of supercells and line segments, the risk for tornadoes appears somewhat higher. Eventual upscale growth into a linear cluster may also support a locally greater risk of damaging winds toward the coast later this evening. Additional storms are possible as the cold front moves through later this evening, though multiple rounds of prior convection may limit the severity of any subsequent convection later this evening. Across the Mid Atlantic, more isolated storm coverage is expected with southward extent. Displaced from the main lobe of ascent from the passing shortwave trough, convection may take longer to organize owing to the warmer mid-level temperatures and weaker synoptic forcing. Still, deep-layer shear and buoyancy are favorable for organized multi-cell clusters and a few supercells. Damaging winds, and occasional hail are the most likely threats, though a tornado or two will remain possible with any sustained supercell. ...Upper OH to the Mid TN Valleys... Ahead of the trailing cold front, scattered thunderstorm development is expected during the afternoon. Despite veered surface flow west of the surface trough over the Mid Atlantic, rich boundary-layer moisture will support a broad plume of moderate instability. Several bands of thunderstorms should track east southeast along the southern periphery of the enhanced flow aloft. Sufficient for some storm organization, 20-30 kt of effective shear should support a few clusters capable of damaging gusts and isolated hail. ...Northern Plains... Accompanying the wave over southern Canada and the Northwest, a deepening surface cyclone with a cold front trailing southward into MT will serve as a focus for isolated storm development this afternoon and evening. Low-level southeasterly flow will transport a confined plume of modest low-level moisture into parts of east MT, resulting in a corridor of MLCAPE from 500-1500 J/kg. Deep-layer shear will be favorable for organized convection capable of isolated damaging winds and hail, but large-scale ascent attendant to the shortwave trough may tend to remain north of the international border. Development of a severe storm or two across east MT is possible by early evening, which would then potentially spread into northwest ND later into the evening and overnight hours. ...Central Plains... Very warm temperatures are expected along a lee trough across the central Plains this afternoon. With temperatures near 100 F, scattered high-based thunderstorms are likely over parts of eastern CO, southwest NE and western KS. Deep inverted-v profiles will support a risk for severe gusts with strong downdrafts. Located on the southern periphery of the stronger upper-level flow, these storms may persist into early evening despite the lack of stronger forcing for ascent. ..Lyons/Wendt.. 06/23/2024 $$