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Sunday, June 6, 2021

SPC Jun 6, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0757 AM CDT Sun Jun 06 2021 Valid 061300Z - 071200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF THE SOUTHERN PLAINS AND UPPER MISSISSIPPI VALLEY REGION... ...SUMMARY... Strong to locally severe winds and hail will be possible across portions of the southern Plains and upper Mississippi Valley region. ...Synopsis... On the synoptic scale, these primary mid/upper-level features will influence the convective forecast for this period: 1. Persistent troughing off the Pacific Northwest Coast, within which an embedded, compact 500-mb cyclone now is centered about 400 nm west of Cape Flattery. The associated low and shortwave trough will move ashore between 00-06Z, supporting height falls across much of the Northwest and northern Rockies. Meanwhile, a downstream perturbation -- now over the northern Rockies -- will eject east-northeastward across the southern Prairie Provinces and deamplify somewhat. 2. A broad, weak, closed cyclone -- apparent in moisture-channel imagery covering much of TX/OK/KS and centered approximately over the Red River between DFW-ADM. This feature will broaden further and become an open-wave trough that drifts north-northeastward. The axis of its cyclonic flow should be near an MKC-TYR-BRO line by the end of the period. At the surface, a wavy cold front was drawn at 11Z across southern MB, eastern ND and southwestern SD, with attached lows over southern MB and near MBG. The cold front is expected to move eastward through the day, reaching northeastern/central MN, south-central SD, the NE Panhandle and southeastern WY by 00Z. A dryline, initially from southwestern CO southward across eastern NM to the Big Bend region of west TX - should mix eastward today to near the KS/CO border, the Caprock in northwest TX, and the lowest part of the Pecos Valley. ...Southern Plains... Isolated to scattered thunderstorms -- some in clusters or bands -- may offer strong/severe gusts and hail from this afternoon through overnight within this broad, nebulously focused risk area. Areas of favorable boundary-layer moisture and diurnal heating will support this convective/severe potential, particularly in the southwestern quadrant of what now is the closed cyclone, from eastern NM across north TX and southwestern OK. Mid/upper winds and related deep-layer speed shear will be weakest closer to the low over OK, where colder air aloft will support steeper lapse rates, and possibly greater convective coverage. Meanwhile, though weaker in terms of large-scale ascent, greater overall bulk shear (with very strong directional shear) will be possible from eastern NM to central TX. Activity may develop on the higher terrain of NM, along the eastward-shifting dryline, differential-heating/outflow boundaries over OK and north TX, and/or just in the free warm sector with convective temperatures easily reached via insolation. MLCAPE to near 2000 J/kg should become common over the outlook area east of the dryline by midday, with some areas potentially reaching 3000 J/kg. Weak deep-layer winds could be a limiting factor. Effective- shear magnitudes generally will peak in the 35-40-kt range, thanks mostly to strong veering of modest flow with height. Convective coverage is a major uncertainty, especially after 00Z when great spread is apparent in guidance. Synoptic and convection-allowing guidance has been inconsistent -- even from run to run in some of the same progs, regarding potential for a forward-propagating thunderstorm cluster or bowing complex to evolve at some point this upcoming evening or tonight, somewhere in the north-central/northwest TX to southern OK area. Given that a nocturnal LLJ should develop to support any such complex overnight, the inconsistencies appear related to the subtlety of the low-level forcing needed to tip the convective balance upscale into organized cold-pool growth -- in an ambient regime characterized by weak MLCINH (but also subtle overall lift). If mesoscale trends and additional numerical guidance (informed by 12Z upper-air data) become better-focused in that regard, larger unconditional severe probabilities nay need to be added within mainly eastern portions of the ongoing 5%/marginal area. For now, too much mesoscale uncertainty remains to do so. ...Upper Mississippi Valley... Isolated to widely scattered thunderstorms are possible near the front late this afternoon into evening, offering hail/gusts near severe limits where cells can last long enough to mature. The environment will be conditionally favorable for isolated severe even somewhat south of the marginal area. However, uncertainty regarding strength of lift, convective initiation and storm maintenance increases with southwestward extent down the frontal zone from the Boundary Waters vicinity. The most favorable frontal forcing and deep-layer/large-scale support ahead the ejecting mid/upper-level shortwave trough will be north of the international border in northwestern ON. Strong diurnal heating of the prefrontal boundary layer will combine with surface dew points in the upper 50s to low 60s (somewhat diurnally reduced by mixing), to build MLCAPE into the 1000-1500 J/kg in a narrow corridor parallel to the front. This swath of destabilization will shrink in width and magnitude this evening, quickly reducing severe potential with any remaining convection. Weaknesses in mid/upper-level winds (i.e., in the 500-300-mb layer) will limit deep shear, though a well-mixed subcloud layer will encourage strong/locally severe downdraft potential and maintenance of hail to the surface. ...Lower Delta region... Scattered to numerous thunderstorms are expected to move northward to northeastward across the region today, posing mainly a heavy-rain threat per WPC guidance. However, locally enlarged low-level hodographs (such as observed in the 12Z LIX RAOB, with 200-300 J/kg effective SRH and 1000-2000 J/kg MLCAPE) will be enabled by a broad LLJ southeast of the mid/upper trough. This will occur in a very moist and convectively uninhibited boundary layer, with PW commonly near 2 inches and low LCL. Modest cyclonic horizontal shear already has been noted in radar velocity presentations from a few cells in this region, and a tornado cannot be ruled out from more sustained/ intense storm-scale circulations. ..Edwards/Kerr.. 06/06/2021 Read more LIVE: spc.noaa.gov
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