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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.

Monday, September 25, 2023

SPC Sep 25, 2023 0600 UTC Day 1 Convective Outlook

LIVE MAP (ABOVE) ... SPC 1200Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1234 AM CDT Mon Sep 25 2023 Valid 251200Z - 261200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS PORTIONS OF SOUTHERN TEXAS AND CENTRAL FLORIDA... ...SUMMARY... Isolated strong to severe thunderstorms will be possible across parts of Texas and Florida today. ...Synopsis... Strong to severe thunderstorm potential is expected to remain confined to southern portions of Texas and the Florida Peninsula for today. An occluding low over the upper MS River Valley will continue to weaken as an attendant mid-level impulse shifts east through the day. In its wake, upper-level ridging will become established across the Intermountain West. This synoptic regime will favor a gradually building surface high over the Plains (as evidenced by steady pressure rises over the past 12 hours). As a result, the diffuse cold front draped across the southern Plains will continue to migrate south through the day, and will be a focus for thunderstorm development later this afternoon and evening. ...Texas... Latest forecast guidance shows reasonably good agreement in the southward progression of the cold front across southern TX today. This boundary may be convectively reinforced by ongoing thunderstorm clusters across eastern TX at the start of the period. This early-morning convection may re-intensify and pose a severe wind threat for the TX coastal plain during the afternoon hours amid diurnal warming of a moist air mass downstream. To the west, closer to the Edwards Plateau and the Big Bend region, thunderstorm develop along the cold front is expected by mid/late afternoon. MLCAPE values are forecast to reach near 2000 J/kg as temperatures warm into the low to mid 90s. Although mid-level winds will be somewhat modest (20-30 knots), southeasterly low-level flow will support elongated hodographs with effective bulk shear values near 30-40 knots. Such wind profiles and weak ascent along the front may support initially discrete modes, including the potential for a supercell or two with an attendant large-hail threat. However, deep-layer shear and storm-motion vectors largely along the frontal boundary may favor a tendency for storm clustering through the late afternoon/evening. Thunderstorm development off the higher terrain of the Davis and Serranias del Burro mountains may also support discrete supercells, but confidence in storm coverage via orographic ascent is limited. ...Florida... A weak tropical low north of the Yucatan Peninsula is establishing a plume of rich moisture into FL (as sampled by 00z soundings). Moderate heating of this air mass through the day should support a modestly buoyant, but uncapped environment (MLCAPE upwards of 1000 J/kg) by mid afternoon over central FL. Glancing ascent from the passing mid-level trough to the north will support scattered to widespread thunderstorms, and may sufficiently strengthen mid-level flow to support effective bulk shear values between 20-25 knots. Although the kinematic environment will be meager, a few instances of hail approaching severe criteria are possible. Additionally, steepening low-level lapse rates and PWAT values approaching 2 inches will be conducive for wet downbursts capable of strong to severe winds. ..Moore/Leitman.. 09/25/2023 Read more CHECK UPDATE ZOOM GRAPHIC

Saturday, September 23, 2023

SPC Sep 23, 2023 0600 UTC Day 1 Convective Outlook

LIVE MAP (ABOVE) ... SPC 1200Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1257 AM CDT Sat Sep 23 2023 Valid 231200Z - 241200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM THE LOWER MISSOURI VALLEY INTO NORTHEAST OKLAHOMA... ...SUMMARY... Thunderstorms, associated with large to very large hail, wind damage, and tornadoes, will be possible today across eastern parts of the Great Plains, and Mississippi Valley. A couple of tornadoes are also possible in parts of the Mid-Atlantic. ...Eastern Great Plains/Mississippi Valley... Early-morning water-vapor imagery depicts a well-defined upper low over northern WY, advancing east toward the Black Hills, in line with latest model guidance. 60kt 500mb speed max is forecast to translate across NE to near FSD by 18z, then into northern MN during the overnight hours. This evolution will encourage a surface low to lift into northeast NE by 18z, with some deepening expected during the latter half of the period as the low occludes over southeastern ND by 24/12z. A north-south cluster of strong convection has evolved ahead of the upper trough over the High Plains from western NE into southwestern SD. This activity may continue through sunrise as it propagates east, aided by large-scale ascent ahead of the trough. Additionally, latest model guidance suggests LLJ will strengthen ahead of this activity into western IA by mid day such that renewed development is likely. NAM aggressively strengthens the LLJ in excess of 60kt over south-central MN by late afternoon. Wind profiles and low-level shear will strengthen ahead of the deepening surface low. This supports the potential for supercells across the upper MS Valley, especially near/south of the northward-advancing warm front. Forecast soundings exhibit strong shear with substantial sfc-3km SRH, though lapse rates will remain weak. Aside from the threat for damaging winds with this pre-frontal activity, it appears environmental conditions also support a tornado risk. In addition to supercell threat, large-scale forcing may encourage some clustering and potential line segments. Severe risk will spread into central MN by late evening as the low occludes over the upper Red River (of the North) Valley. Farther south, high-level difluent flow will be noted across the lower MO Valley into eastern OK. 500mb flow is expected to strengthen south of I-70 to near the TX/OK Red River as weak height falls will be noted at these lower latitudes. Surface front will advance into eastern KS, arcing across central OK into northwest TX by late afternoon. Strong boundary-layer heating will be noted ahead of this wind shift, with surface temperatures forecast to rise into the mid 90s to near 100F from portions of western OK into northwest TX. As a result, convective temperatures will be breached as CINH is reduced. Scattered thunderstorms should readily develop along the wind shift, initially over eastern KS, then southwest along the boundary at least into central OK. Forecast soundings exhibit seasonally steep lapse rates with high PW values (approaching 2 inches). Wind profiles strongly favor organized convection and initial activity should be supercellular in nature. With time, one or more MCSs may ultimately evolve over the ENH Risk area, with southward propagation expected across eastern OK/western AR late in the period. Hail should be common with the initial activity, some of it likely exceeding 2 inches in diameter. While a few tornadoes may be noted with the supercells, storm mode may ultimately lead to a complex that could produce damaging winds as convection spreads southeast during the overnight hours. ...TS Ophelia... Ophelia will soon move onshore along the NC Coast, then advance inland before arcing northeast toward the Delmarva late in the period. Strong low-level shear will develop across the southern Middle Atlantic east of the low track. Convection that develops across this region will be sufficiently sheared for supercells and possibly a few tornadoes, in addition to gusty winds. ..Darrow/Moore.. 09/23/2023 Read more CHECK UPDATE ZOOM GRAPHIC

Friday, September 22, 2023

SPC Sep 22, 2023 0600 UTC Day 1 Convective Outlook

LIVE MAP (ABOVE) ... SPC 1200Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1246 AM CDT Fri Sep 22 2023 Valid 221200Z - 231200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF THE NORTHERN AND CENTRAL HIGH PLAINS...AND OVER THE SOUTHERN COASTAL MIDDLE ATLANTIC... ...SUMMARY... Severe thunderstorms are possible across parts of the central and northern High Plains, mainly this afternoon and this evening. A few tornadoes are also possible along the Mid-Atlantic Coast early Saturday morning in association with the approaching potential tropical system. ...Northern/Central High Plains... Strong upper low that has settled into the northern Great Basin will advance into western WY by late afternoon as a 70kt 500mb speed max rotates through the base of the trough to near WY/UT/CO border by 23/00z. Seasonally cold mid-level temperatures (-20C at 500mb beneath the upper low) will spread east in association with this feature, and the exit region of aforementioned jet will overspread the High Plains of eastern WY by early afternoon. Latest model guidance suggests boundary-layer flow will remain decidedly easterly across NE and the Dakotas into eastern WY. As a result, strongest boundary-layer heating will be noted across the higher plateau region of WY immediately ahead of the approaching trough. Forecast soundings suggest convective temperatures will be breached by 19z, and thunderstorms should develop shortly thereafter. Soundings across eastern WY exhibit substantial SBCAPE with steep lapse rates through 8km. NAM 21z forecast sounding for LUS strongly favors supercells with 60kt surface-6km bulk shear. Very large hail may accompany some of this activity as it spreads toward the southern Black Hills region/northwestern NE. Additionally, relatively moist boundary-layer conditions and low LFCs suggest some tornado threat. It's not clear how far this activity will spread east before weakening, as the updrafts will likely decouple from the boundary layer due to much cooler surface temperatures and weaker low-level lapse rates. For this reason have focused higher severe probabilities across the High Plains where stronger surface heating is expected. ...Potential Tropical Cyclone Sixteen... PTC 16, currently located well southeast of the Carolina Coast, is forecast to move north-northwest over the next 24hr, likely intensifying as it approaches the NC Coast early Saturday. As this system intensifies, low-level shear will increase markedly across the southern Middle Atlantic Coast. Probabilities for supercells will increase late, especially after 06Z, along with some risk for tornadoes, primarily east of the low track. Reference for more information on this developing system. ..Darrow/Moore.. 09/22/2023 Read more CHECK UPDATE ZOOM GRAPHIC

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