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Archives for the SPC Convective Outlook are updated daily (approximately) with a live map at the beginning of each article. Follow the link at the end of the article to check for current updates on the NOAA/NWS Storm Prediction Center website. Also, see Archives for Chicago's hourly weather data on CARDINAL NEWS Magazine.


Tuesday, September 19, 2023

SPC Sep 19, 2023 1300 UTC Day 1 Convective Outlook

LIVE MAP (ABOVE) ... SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0744 AM CDT Tue Sep 19 2023 Valid 191300Z - 201200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF THE SOUTHERN PLAINS... ...SUMMARY... Severe thunderstorm gusts and large hail are possible today and tonight across portions of the southern Plains. ...Synopsis... In mid/upper levels, amplification of the pattern is forecast over parts of the northwestern and north-central CONUS later today into day-2, related substantially to a strong shortwave trough and embedded 500-mb cyclone now over the AK Panhandle and adjacent coastal waters. This trough should dig southeastward to southwestern AB, southern BC and WA by 12Z tomorrow, while a downstream ridge builds across parts of the upper Mississippi Valley into ON. As this occurs, initially zonal but increasingly difluent, westerly to northwesterly flow will prevail downstream to the trough's southeast, over the central/southern parts of the Rockies and Plains. Within the increasing difluence, a shortwave trough now over parts of western KS to the TX Panhandle will shift eastward to eastern portions of KS/OK by 00Z. This feature may decelerate in the difluent regime tonight over the Ozarks to Arklatex. Following that, just upstream, a smaller perturbation is evident in moisture- channel imagery over southeastern CO and northeastern NM. This trough should track eastward to parts of western/central KS and northern/central OK through 00Z, then weaken eastward in favor of convectively driven vorticity processes, and perhaps merge with the lead wave late in the period. At the surface, 11Z analysis showed a cold front over south-central FL, becoming quasistationary over the north-central Gulf, and a warm front northwestward across southeast to north-central TX. This boundary is expected to move slowly northward/northeastward over the east TX/eastern OK region today, while a dryline develops across western KS, western OK, northwest TX, to the Big Bend area. A pseudo-warm-frontal zone of relatively backed near-surface flow may develop over parts of OK, in the western/modifying part of outflow air and behind morning convection, encouraged by isallobaric forcing related to weak cyclogenesis along the dryline. There is uncertainty on this process -- depending on mesobeta-scale thermodynamic recovery and kinematic response behind morning convection. ...Southern Plains... Widely scattered to scattered showers and thunderstorms are ongoing across portions of northwest Texas, northward and northeastward over parts of OK, in a zone of large-scale ascent (warm advection and DCVA preceding the leading mid/upper perturbation. This activity should remain largely nonsevere as it shifts eastward through the remainder of the morning, though an isolated, marginally severe instance of hail cannot be ruled out. Convective and severe potential should ramp up this afternoon as the boundary layer diurnally/diabatically destabilizes south of the morning clouds/convection (across parts of southern OK and north TX), southwest of it atop a strongly heated/mixed boundary layer near the dryline, and perhaps behind it over western/central OK late this afternoon. The latter potential is more uncertain and conditional, given time needed for both direct heating/ destabilization and warm advection in the recovering low-level air mass immediately preceding the second mid/upper-level perturbation. Where convection can develop over northwest TX and OK, especially near any dryline/outflow intersection, supercells are possible amid strong veering of winds with height in low/middle levels, yielding well-curved and favorably enlarged hodographs. Development appears best focused near the dryline over the southwestern OK/northwest TX area, with sustained heating and MLCINH erosion likely, but less low-level shear. Conversely, coverage is more uncertain in and north of the backed-flow regime, though hodographs will be more favorable. As such, too much mesoscale uncertainty lingers to assign a more-specific, focused area of higher unconditional probabilities at this time. Forecast soundings near the trailing dryline depict small 0-1-km SRH and shear due to uniformity of winds in that layer, however, 200-400 J/kg effective SRH and 40-45-kt effective-shear magnitudes should be common (with larger values northward). A corridor of mid 50s to lower 60s F surface dewpoints -- even after diurnal mixing -- should combine with steep low/middle-level lapse rates to yield 1500-2500 J/kg MLCAPE, in a northward-narrowing moist-sector corridor. In addition to severe downdrafts encouraged by the deep mixed layer, isolated significant (2+ inch) hail appears possible given the supercell-favoring shear, the shape/depth of the buoyant profile, sounding analogs, and peak size given by 2D hail models applied to progged soundings. Some evening/nocturnal clustering of convection also is possible, locally boosting wind potential for a couple hours. However, given aforementioned mesoscale uncertainties, great spread understandably appears among convection-allowing guidance on where and how intensely any such wind threat will organize. ..Edwards.. 09/19/2023 Read more CHECK UPDATE ZOOM GRAPHIC
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