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Monday, June 21, 2021

SPC Jun 21, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0752 AM CDT Mon Jun 21 2021 Valid 211300Z - 221200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE UPPER OHIO VALLEY AND MID-ATLANTIC INTO NORTHERN NEW ENGLAND... ...SUMMARY... Isolated to scattered severe thunderstorms are expected from the upper Ohio Valley and Mid-Atlantic into northern New England, offering damaging gusts, sporadic severe hail, and perhaps a couple of tornadoes. ...Synopsis... A high-amplitude mid/upper-level pattern is in place and will continue through the period -- featuring: * A longwave trough from north of Hudson Bay to the northern Gulf of Mexico, * Ridging from northern MX northwestward to WA/ID, and * A quasistationary to northward-drifting cyclone off the CA coastline. Numerous shortwave troughs and related vorticity maxima (some convectively generated/enhanced) will pivot through the cyclonic- flow field from the northern/central Rockies to the mid/upper Mississippi Valley and northeastern CONUS. The most prominent is a perturbation now apparent in moisture-channel imagery from Hudson Bay across northwestern ON to northern MN. This feature will pivot across the upper Great Lakes through this evening, before turning northeast and weakening late tonight. At the surface, 11Z analysis showed a strong/992-mb surface low near PLN, with cold front southward over northeastern IN then southwestward across southern IL, southern MO, southwestern OK, the TX South Plains, and central NM. As the low ejects northeastward over western QC by 00Z, the front should reach western portions of NY/PA/WV, southwestward to mid TN, northern MS, and central/ southwest TX. By 12Z, the front should reach ME, NJ, the Blue Ridge region of VA/NC, to northern AL, south-central TX, and Chihuahua. ...Northeast... Remnants of small MCSs are noted this morning over northwestern PA and northern NY, each trailing outflow boundaries extending over western NY and north-central OH. The MCV from the trailing area may aid convective potential over western/northern NY later this morning. Otherwise, scattered thunderstorms are expected to develop from late morning through early afternoon across western parts of the outlook area, along and ahead of the surface cold front. Initial severe potential should arise from the Lake Erie vicinity across portions of OH, western PA and western WV, as the boundary layer destabilizes diabatically (and in some areas near the Great Lakes, recovers behind morning convection). Activity should move eastward to northeastward through the rest of the afternoon. Most of the convection should evolve into broken to solid lines, though early-stage supercells are possible given the favorable wind profiles. Damaging gusts (including some potentially exceeding 50-kt severe limits) will be the main concern, though isolated severe hail may be observed in any supercells, along with a tornado or two. An isolated, localized threat for QLCS tornadoes also may develop where any line of convection encounters higher SRH -- mainly near boundaries left behind by the morning activity. Rich moisture and weak MLCINH will support early development and afternoon-long maintenance of convection across the outlook area. Lower-elevation surface dew points should be commonly in the mid/upper 60s F, with 70s east of the mountains in the lowlands of the Potomac/Chesapeake Bay/Delaware Valley areas. Diurnal heating will be more sporadic/fragmented in northern parts of the outlook area due to passing areas of cloud cover from earlier/upshear convection. Still, a broad prefrontal corridor of 1000-1500 J/kg MLCAPE should develop, locally near 2000 J/kg. Tightening height gradients aloft -- both related to general/regional synoptic height falls and the upper Great Lakes shortwave trough -- will strengthen mid/upper winds over the region. However, the strongest flow aloft should remain behind the front, and mid/upper winds will carry a substantial front-parallel component. Forecast soundings suggest preconvective effective-shear magnitudes generally in the 35-45-kt range over NY/PA/northeastern OH, and 25-35 kt southward from there. ...KY/TN to central/south TX... Widely scattered to numerous thunderstorms will form on both sides of the front today. Locally damaging/isolated severe gusts are possible from frontal and warm-sector convection, primarily this afternoon into evening. Severe concern will be widely dispersed and primarily along/ahead of the front, though a rogue hail-producing cell cannot be ruled out behind the front across the southern Plains. A general increase in diurnal heating and low/midlevel lapse rates, but decrease in vertical shear and frontal slope, will be noted with southwestward extent along/ahead of the front, from the northern/central Cumberland Plateau southwestward to south TX. Rich low-level moisture will support MLCAPE in the 1500-2500 J/kg range from the Mid-South northeastward, and 2000-3000 J/kg farther southwest where midlevel lapse rates are greater in closer proximity to the EML. Water-loaded downdrafts and mesobeta-scale cold-pool surges (with low predictability at this range) will pose the greatest concern for damaging wind. ...Interior OR, extreme northern CA... Late this afternoon into evening, isolated to widely scattered high-based thunderstorms are expected to develop across areas near the OR/CA line and move northward to northwestward, with the threat for isolated severe gusts. This potential will grow in step with increasing low-level moisture through the afternoon, as the upper ridge moves east of the area and strongly difluent flow sets up northeast of the Pacific cyclone. Moist advection should offset the effects of antecedent vertical mixing processes more with time, increasing PW to between 0.75 and 1 inch tonight. Weak deep shear will limit overall organization, but gust potential will be aided by a deep, well-mixed subcloud layer supporting downdraft acceleration. See the SPC day-1 fire-weather outlook for concerns regarding dry thunderstorms over this region. ..Edwards/Broyles.. 06/21/2021 Read more LIVE: spc.noaa.gov
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