Severe Storm Outlook Narrative (AC)
Issued by NWS
Issued by NWS
045 ACUS02 KWNS 141732 SWODY2 SPC AC 141730 Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1230 PM CDT Sun Jul 14 2024 Valid 151200Z - 161200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FOR PARTS OF THE MIDWEST INTO THE OHIO VALLEY... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR PORTIONS OF THE CENTRAL HIGH PLAINS... ...SUMMARY... Strong to severe thunderstorms are likely from the Upper Midwest into the Mid Mississippi Valley and Upper Great Lakes region tomorrow (Monday). A focused corridor of damaging winds is possible. Isolated severe thunderstorms are also possible across the central Plains as well as south-central/southeast Arizona. ...Synopsis... A complex severe weather scenario is possible across portions of the Midwest tomorrow (Monday). Upper ridging will prevail across the western CONUS as broad cyclonic flow remains in place, centered over the Upper MS Valley. Multiple mid-level impulses will pivot around the cyclonic flow from the Midwest into the Northeast, with a surface lee trough and associated cold front poised to sweep southeastward into the Midwest and OH Valley regions. Multiple areas/rounds of thunderstorm development are expected from the Midwest to the Northeast, with preceding storms exhibiting potential influence on the evolution of subsequent convective complexes. Nonetheless, the latest high-resolution guidance consensus, general synoptic pattern, and regional climatology, all suggest that at least one focused round of organized severe thunderstorms should occur across the Midwest at some point from the late morning to late afternoon hours, perhaps accompanied by a focused corridor of strong to severe wind gusts. Meanwhile, surface high pressure will dominate the Intermountain west, with monsoonal mid-level moisture pivoting around the upper ridge, fostering multiple areas of high-based strong to isolated severe thunderstorms from the Desert Southwest to the central High Plains. ...Midwest into the OH Valley... Multiple convective scenarios may unfold across portions of the Midwest tomorrow, including the possibility of a focused corridor or of severe gusts produced by a bow-echo MCS. By late morning/early afternoon, a pronounced mid-level impulse will quickly traverse the region, overspreading a generally west-to-east oriented surface baroclinic zone. Along this boundary, upper 70s F surface dewpoints will be in place, that in tandem with surface heating, will boost MLCAPE to at least 3500-4500 J/kg. Given strong mid-level westerly flow accompanying the impulse, appreciable low-level shear and modest deep-layer shear is expected, characterized by enlarged and curved low-level hodographs, but short hodographs aloft. This environment climatologically favors the development of a progressive bow-echo capable of producing a focused corridor of severe gusts (perhaps exceeding 75 mph in spots). Furthermore, some HREF members also depict possible bow echo development. However, there are some uncertainties regarding how the bow echo could form, with some members showing a bow echo approaching from MN/IA from Day 1 convection, or from new convective development over central IA. Since multiple rounds of preceding convection may also influence the position of the baroclinic zone and associated gradient of strong to extreme buoyancy, there is some uncertainty as to exactly where a bow echo would materialize and track. A Category 3/Enhanced risk has been introduced in the general region where a bow echo may form, and these highlights may need some adjustments during the Day 1 Outlook to account for baroclinic zone placement, or impacts from preceding convection. 5 percent tornado probabilities have also been added where a bow echo would most likely reach mature stage, where line-end and leading-line/system-scale mesovortices may foster leading-line QLCS tornado potential. However, even in the event that a bow echo does not materialize, a surface cold front will approach the Midwest tomorrow, contributing to linear convective initiation amid strong/extreme buoyancy, where severe gusts will still be of concern. ...Portions of the Northeast... The first in a series of mid-level impulses traversing the cyclonic flow aloft will approach the northern Appalachians by late morning/early afternoon, contributing to increasing thunderstorm coverage during the day. Surface dewpoints exceeding 70 F in several areas, along with modest mid-level lapse rates (perhaps approaching 7 C/km in spots), will result in widespread 1500-2500 J/kg MLCAPE. Stronger flow aloft will likely remain west into the OH Valley though, so vertical shear should be modest at best (i.e. bulk effective shear values around 25 kts). Pulse cellular and occasional multicell complexes are possible, accompanied by mainly a damaging gust threat. Storms should gradually weaken around or after sunset. ...Portions of the Central High Plains... Upslope low-level flow by late afternoon/early evening, in tandem with peak afternoon heating, will encourage at least scattered thunderstorm development along the lee of the Rocky Mountains. 30-40 kts of westerly 500 mb flow pivoting around the north side an anticyclone will assist in thunderstorms quickly moving into the central High Plains. Here, thunderstorms will be preceded by a well mixed boundary layer extending up to 700 mb. Surface to 500 mb lapse rates should be in the 8.5-9.5 C/km range, which favors deep-layer convective mixing and associated potential for severe gusts. A Category 2/Slight Risk has been added to portions of the central High Plains where thunderstorm clustering is expected to be greatest (based on the latest high-resolution guidance consensus), and where cold-pool mergers are most likely to occur. ...Portions of southern into central Arizona... Thunderstorms are expected to develop along the Mogollon Rim by afternoon peak heating in central AZ. Easterly mid-level flow pivoting around the south side of an anticyclone may assist the storms in moving off of the higher terrain, amid a well-mixed boundary layer. Surface temperatures approaching 100 F ahead of the storms, beneath the monsoonal mid-level moisture, will support MLCAPE exceeding 1000 J/kg in spots. Modest elongation of the hodographs may result in effective bulk shear values approaching 30 kts, which may support sustained multicellular clusters by late afternoon into early evening, accompanied by an isolated severe gust threat. ..Squitieri.. 07/14/2024 $$