SPC Aug 4, 2021 1300 UTC Day 1 Convective Outlook

1 day 7 hours ago
SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0750 AM CDT Wed Aug 04 2021 Valid 041300Z - 051200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE NORTHERN ROCKIES AND COASTAL NORTH CAROLINA... ...SUMMARY... Isolated strong to severe gusts may accompany afternoon storms over parts of the northern Rockies. A marginal tornado threat exists over coastal North Carolina mainly this morning. ...Synopsis... In mid/upper levels, a persistent pattern of western mean ridge and eastern trough will persist through this period, though it will begin to deamplify tonight, then break down more fully from day-2 onward, as discussed in those outlooks. A weak shortwave trough -- evident in moisture-channel imagery over the central/eastern Carolinas -- should eject northeastward along and offshore from the Mid-Atlantic Coast through the period. Upstream, a long but weak trough now over the High Plains will move slowly southeastward, supporting a corridor of general thunderstorm potential today before reaching eastern NE, central KS and northwest TX by 12Z tomorrow. Meanwhile, a compact but well-defined trough -- initially over northern WA and southern BC -- will move through the northern Rockies mean-ridge position today, weakening by the time it reaches north-central MT and southeastern SK around 12Z. At the surface, the 11Z analysis showed a low over coastal waters near ILM, with warm front northeastward along the Outer Banks past HSE, and cold front southwestward to between SAV-JAX, becoming quasistationary westward near the northern Gulf Coast to south- central TX. The low is expected to travel northeastward along the front and over the Pamlico Sound area, before moving to the Atlantic off the Hampton Roads region this afternoon and evening. The trailing frontal zone will move eastward off NC while remaining nearly stationary southwestward to another low over the northeastern Gulf, and should move southward off the AL/MS/LA coastline behind the Gulf low. ...NC Outer Banks area... Widely scattered thunderstorms -- mostly embedded in a broader area of precip -- will move northeastward to north-northeastward across the outlook area and adjoining waters today. Potential for a brief tornado exists through midday, but the threat is very isolated and conditional. Though the bulk of sustained, surface-based convection and related severe potential will remain offshore, the western fringe in the frontal zone may brush portions of the Outer Banks region through the remainder of the morning. The warm sector just offshore will be characterized mid-70s F surface dew points, slightly higher temperatures, MLCAPE in the 1000-1500 J/kg range, and somewhat enlarged low-level hodographs with 150-200 J/kg effective SRH possible. That parameter space will weaken substantially with inland extent, deeper into the precip plume and west of the front. By afternoon, with the low passing north of the area, veering low-level flow and a more-offshore shift of the convective axis will reduce the already marginal threat. ...Northern Rockies... Isolated to widely scattered, very high-based thunderstorms are possible this afternoon, posing the threat of a few strong to briefly severe gusts before moving eastward onto lower elevations and weakening this evening. Activity should develop as deep-layer lapse rates steepen to some extent aloft, with the approach of the trough and related DCVA, but especially in low/middle levels with heating of higher terrain. Sufficient low-level moisture will be available to support pockets of MLCAPE around 500 J/kg, with deep and well-mixed subcloud layers fostering downdraft acceleration and potentially damaging local gusts. ..Edwards/Leitman.. 08/04/2021 Read more

SPC Aug 3, 2021 1300 UTC Day 1 Convective Outlook

2 days 7 hours ago
SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0754 AM CDT Tue Aug 03 2021 Valid 031300Z - 041200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER SOME PARTS OF CENTRAL/EASTERN OREGON AND WASHINGTON... ...SUMMARY... Isolated strong to severe thunderstorm gusts will be possible this afternoon and early evening over over some parts of central/eastern Oregon and Washington. ...Synopsis... The CONUS portion of the large-scale upper-air pattern -- western ridge/eastern trough -- will change little through the period. A 500-mb high will remain near LAS, with ridge northward to the northern Rockies. A compact vorticity lobe and associated shortwave trough are apparent in moisture-channel imagery about 150-155 nm offshore from WA/OR, with an embedded circulation center roughly west of AST. This perturbation should move east-northeastward through the period, crossing the Olympic Peninsula around 00Z, and reaching northeastern WA/southeastern BC around 12Z tomorrow. Meanwhile, a series of shortwaves will traverse a broad cyclonic- flow field covering most of the nation east of the Rockies. At the surface, a wavy, stationary to slow-moving cold front was drawn at 11Z from just off the coast of the Carolinas to southern GA/northwestern FL, westward over southern LA and south-central TX. A weak frontal-wave low is progged to develop today over the southeastern GA area and ripple northeastward, with some deepening possible tonight near coastal northeastern SC and southeastern NC. Farther northwest, a weak surface low initially drawn between DLS-PDT may move slowly north-northeastward along a quasistationary trough extending from central OR to eastern WA. ...Interior Pacific Northwest... Isolated to widely scattered thunderstorms are possible in two main areas: 1. Eastern OR, perhaps extending into the Hells Canyon area before convection dissipates this evening. Strong diurnal heating/mixing will strip away MLCINH and reduce dew points somewhat, while enough moisture still remains in the profile to support high-based thunder potential. MLCAPE may reach the 300-800 J/kg range, with around 30-40 kt peak effective-shear magnitudes. This area appears to have the best-organized threat for strong/locally severe convective gusts. 2. Central/northeastern WA ahead of the shortwave trough, where DCVA preceding the trough will begin to overlap an area of favorable moisture and residual, diurnally driven warmth in the boundary layer. Those factors may provide enough low-level theta-e to support orographically aided thunderstorm formation, along with some potential for development near the surface trough. A strongly mixed boundary layer in the lower elevations will support isolated downburst potential. Convective coverage is more uncertain than farther south, and may be quite isolated, but will refrain from removing this area entirely for now, pending mesoscale trends and additional numerical guidance based on 12Z and later observational input. A relative minimum in convective potential should exist between the two regimes, south of the greater large-scale ascent preceding the shortwave trough, and northwest of the surface low where boundary-layer flow and convergence are weak. ...Southeastern CONUS... Isolated to scattered thunderstorms will be possible in a broad swath from south TX to FL and parts of the Carolinas, both south of the front and where sufficient residual moisture lingers to its north. The greatest convective coverage may be over the northern Gulf, central/northern FL, and the eastern Carolinas to adjoining Atlantic. While an isolated damaging gust from a water-loaded downdraft may occur almost anywhere in these areas, organized severe potential appears minuscule, but for one conditional, late concern. During the last few hours of the period, thunderstorms should increase in coverage and intensity near the central/southern NC coastline (including southern Outer Banks), as low-level lift strengthens near and southeast of the low. At this time, the most favorable CAPE/shear parameter space is expected to remain offshore, but may brush the coast as well. This would be dependent on mesobeta-scale precip processes, specifically whether too much precip does not cover the coastal areas and prevent favorable destabilization. Severe potential appears too conditional for a categorical outlook area, but will be assessed further for potential to develop inland. ..Edwards/Leitman.. 08/03/2021 Read more

SPC Jul 29, 2021 1300 UTC Day 1 Convective Outlook

1 week ago
SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0752 AM CDT Thu Jul 29 2021 Valid 291300Z - 301200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF CENTRAL/EASTERN PA...NORTHERN MD...DE...AND NJ... ...SUMMARY... Damaging thunderstorm winds and a tornado or two will be possible today over parts of the Ohio Valley, central Appalachians and Mid-Atlantic. ...Synopsis... The CONUS portion of the large-scale mid/upper pattern will change little through the period, anchored by a stout anticyclone whose core region of 594+ dm 500-mb heights will extend from the central Rockies to the Mid-South. To its northeast, a belt of northwest flow aloft will extend from SK/MB across the upper Great Lakes, becoming cyclonically curved through a trough over the Northeast. The most important feature embedded in that flow, for this forecast, is a shortwave trough initially located over portions of southern ON to northwestern OH and IN. Embedded MCVs -- related to the prior/overnight convective complex in the upper Great Lakes region -- were apparent east of southern Lake Huron, and over northwestern OH. This shortwave trough will pivot southeastward to eastward over the Mid-Atlantic region and southern New England today, moving offshore mid/late afternoon. At the surface, a low influenced by the aforementioned shortwave trough was analyzed at 11Z near APN, somewhat ahead of a cold front extending from eastern Upper MI across southwestern WI, northern IA, western NE and southeastern WY. The low and front should link today, with the low crossing Lake Ontario by 00Z and reaching near PWM by 12Z. The trailing cold front should reach northern OH, central portions of IL/IN, northern MO, southern/southwestern NE and south- central WY by 00Z. During the day, a warm front initially drawn across NJ/PA will move northeastward, its western segment near the low reaching southeastern NY and coastal southern New England late this afternoon into this evening. ...Mid-Atlantic and vicinity... Isolated to scattered thunderstorms should develop ahead of the surface cold front and along or south of the warm front today, with damaging gusts as the main threat, and a tornado or two possible. The greatest potential concentration of both convection and severe threat should be in a zone of favorably destabilized air ahead of the MCV/shortwave trough. The latter will provide large-scale ascent to boost ambient midlevel lapse rates somewhat, while also enhancing vertical shear to its southeast over parts of the Mid- Atlantic. Low-level shear should be favorable near the warm front, where surface winds will be relatively backed; however, buoyancy will be less than farther south amidst greater cloud cover and related slowing of diurnal heating. Rich moisture will be present over the area east of the Appalachians, with warm-sector surface dew points commonly in the upper 60s to mid 70s F. The associated high theta-e and diabatic heating will help to offset the modest lapse rates aloft, enough to support peak preconvective MLCAPE in the 1000-1500 J/kg range (briefly approaching 2000 J/kg locally). Greatest buoyancy should reside across the Chesapeake Bay/Delmarva and Delaware Valley regions eastward over NJ, with some extension across small parts of southeastern NY into southern New England possible prior to convective passage. Thunderstorms should concentrate most densely in and near the 30% wind area, shifted slightly northward to be closer to the low-level convergence max associated with the low, and under somewhat stronger deep-layer flow/shear. Forecast soundings indicate 35-45-kt effective-shear magnitudes in this area, generally decreasing southward. ...OH Valley to MO/southeast NE... Widely scattered to scattered thunderstorms are expected to develop along the surface cold front and prefrontal outflow boundaries, mainly over the OH Valley region, but potentially westward across northern MO/southeastern NE area. Damaging, isolated severe gusts and sporadic large hail are possible. The low levels south of the boundaries will remain very moist, with strong diurnal heating and a deep troposphere helping to drive MLCAPE into the 3000-4500 J/kg range over the corridor from NE to western IL, and 2000-3000 J/kg across portions of IN/OH that can recover in time from the ongoing clouds/precip. As buoyancy will increase westward, mid/upper-level flow and deep shear will decrease, though the latter still should be sufficient to aid some organized multicell to transient supercell structures, particularly over the OH Valley region. Severe potential, in reality, will not be uniformly distributed across the lengthy 15%/slight area, with upscale clustering and related concentration of wind potential in particular being possible on the mesoscale. ...Central High Plains... Isolated to widely scattered thunderstorms are expected to form this afternoon, mainly on higher terrain from the southern Bighorns into the Laramie Range and vicinity, and more conditionally, east- southeastward near the front. Isolated severe gusts/hail are possible from this activity as it moves northeastward to eastward late this afternoon and evening, before dissipating in a nocturnally stabilizing boundary layer. Overall coverage and longevity generally should diminish westward from MO into NE, hence the narrowing outlook area. A substantial easterly component of low-level winds will become established behind the front, enabling upslope lift, westward moisture/theta-e advection and greater storm-relative boundary-layer flow. This also will contribute to substantial veering of flow with height to augment deep shear, despite the lack of stronger deep- layer wind speeds near the mid/upper anticyclone. Insolation of higher terrain will weaken MLCINH as low-level moisture increases, contributing to steepening low-level lapse rates and strengthening buoyancy, with MLCAPE reaching 1000-2000 J/kg in a corridor of roughly front-parallel instability extending from east-central/ southeastern WY across the NE Panhandle and southwestern NE. Meanwhile, 35-40 kt effective-shear magnitudes are possible, supporting occasionally organized convective structures. Clustering and cold-pool forcing also may occur on the mesobeta scale for a few hours, locally augmenting the wind potential, though spatial uncertainties for such processes are too great to focus larger unconditional probabilities at this time. ..Edwards/Gleason.. 07/29/2021 Read more

SPC Jul 27, 2021 1300 UTC Day 1 Convective Outlook

1 week 2 days ago
SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0755 AM CDT Tue Jul 27 2021 Valid 271300Z - 281200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM WESTERN NEW YORK ACROSS PARTS OF NEW ENGLAND...AND OVER PORTIONS OF THE UPPER MISSISSIPPI VALLEY REGION... ...SUMMARY... Severe thunderstorms are possible today from western New York across parts of New England, and mainly tonight over portions of the upper Mississippi Valley region. ...Synopsis... In mid/upper levels, slight amplification is expected in the western-ridge/eastern-trough pattern that will continue through the period. The closed 500-mb anticyclone over the central/southern Rockies and adjoining Plains will expand in size and intensify gradually, with the high strengthening to around 597 dm overnight over the central High Plains. Meanwhile, height falls should occur over much of the Northeast, both with gradual amplification of the slow-moving synoptic trough, and assorted vorticity maxima and shortwaves pivoting through the associated cyclonic-flow field. The most pertinent such perturbation for this forecast appears to be an MCV -- now apparent in composited radar imagery crossing from northern/central Lower MI to Lake Huron. This feature, preceded by a field of large-scale lift and somewhat enhanced deep shear, should move east-southeastward across southern ON through 18Z, and NY/New England through this evening. At the surface, 11Z analysis showed a cold front from northern ME across southern ON to a weak frontal-wave low over northern Lower MI -- which also may be a surface manifestation of an MCV nearby. The cold front should move slowly southeastward through the period into northern NY and more of northern New England. An outflow boundary was drawn across central Lower MI, southern portions of Lake Michigan and WI, to central MN. This boundary should move southward more over southern WI and Lower MI before stalling later today. ...NY/New England... Surface-based thunderstorms are expected to develop by midday over parts of western NY and/or adjoining ON, along and south of the front. Activity should increase in coverage and move mostly eastward across central/eastern portions of NY and south-central New England through the afternoon, its movement aligned with an instability gradient and boundary-layer moist axis. Damaging to locally severe wind gusts will be the main concern. Confidence has increased that enough large-scale lift (related to the MCV) and diurnal low-level destabilization will occur to support maintenance of the threat eastward into New England. The 15% wind area has been extended accordingly. Surface dewpoints generally in the low/mid 60s F will be common in the preconvective environment across the region, with minimal MLCINH. Accordingly modified RAOBs and forecast soundings suggest that -- despite weak midlevel lapse rates -- MLCAPE around 500-1000 J/kg will develop (locally/briefly higher), atop a reasonably well-mixed boundary layer with steep low-level lapse rates. A roughly unidirectional deep-layer wind profile is expected, with effective-shear magnitudes in the 30-40-kt range. Given the weak capping and progressive regime of ascent preceding the MCV, some cold-pool aggregation may occur in upscale growth of convection, enabling forward propagation to impinge on the destabilized boundary layer, potentially as far east as the MA and perhaps NH coastlines given the westerly flow that can advect favorable boundary-layer theta-e that far. ...Upper Mississippi Valley region... Isolated severe gusts/hail may be noted from a cluster of thunderstorms that developed over the cold pool, and has been moving southeastward over west-central WI. See SPC mesoscale discussion 1363 for near-term details. Severe hail and gusts will be expected from any thunderstorms that can develop from late this afternoon through tonight over this corridor. The MCS that crossed WI last night produced a pronounced outflow boundary whose backing cold pool since has been reinforced by rounds of elevated convection to its north, including the ongoing activity. The associated baroclinic zone -- both at the surface and elevated over the cold pool to the boundary's north -- will be the main focus for additional convective potential through the period. Greater certainty exists in evening convective development in and near the somewhat-expanded eastern parts of the outlook area, where the boundary will be stronger and CINH weaker. A preponderance of numerical guidance -- from synoptic models to assorted CAMs, support this general concept, even if differing in timing/location details on the mesoscale as should be expected. Farther west, the outlook becomes more conditional -- with respect to both coverage and timing of storms -- in a volatile near-boundary environment that will favor severe with any sustained convection that can develop. The southern MN part of the outlook in particular is more of a "boom or bust" scenario with convective-development/maintenance uncertainties in a thermodynamically very favorable parameter space (e.g., 70s F surface dew points beneath steep midlevel lapse rates, 3500-5000 J/kg MLCAPE, and 30-40-kt effective-shear magnitudes supporting organized potential). Additional development is possible late tonight into early tomorrow morning over parts of west-central/ southern NM as well, with coverage/intensity uncertainties. ..Edwards/Gleason.. 07/27/2021 Read more

SPC MD 1363

1 week 2 days ago
MD 1363 CONCERNING SEVERE POTENTIAL...WATCH UNLIKELY FOR PORTIONS OF FAR SOUTHEASTERN MN...FAR NORTHEASTERN IA...AND SOUTHWESTERN WI
Mesoscale Discussion 1363 NWS Storm Prediction Center Norman OK 0748 AM CDT Tue Jul 27 2021 Areas affected...Portions of far southeastern MN...far northeastern IA...and southwestern WI Concerning...Severe potential...Watch unlikely Valid 271248Z - 271445Z Probability of Watch Issuance...5 percent SUMMARY...A small cluster of storms may pose an isolated hail/wind threat in the short term, but watch issuance is not expected at this time. DISCUSSION...Convection associated with a modest west-southwesterly low-level jet has persisted over the past several hours across western WI. This activity has largely remained elevated while posing mainly an isolated threat for small hail. Recently these storms have congealed into small cluster while moving south-southeastward. Recent imagery from the KARX radar shows strong inbound velocities (40-50+ kt) moving across west-central WI, and isolated severe/damaging wind gusts appear possible in the short term as this activity spreads into parts of southwestern WI. Marginally severe hail may also occur with any embedded supercell. However, instability rapidly decreases with southward extent across this region. Current expectations are for these storms to weaken over the next couple of hours as they encounter a less favorable thermodynamic environment and as the low-level jet shifts eastward across the Great Lakes. ..Gleason/Edwards.. 07/27/2021 ...Please see www.spc.noaa.gov for graphic product... ATTN...WFO...GRB...MKX...ARX... LAT...LON 43629174 43979173 44279130 44429046 44368998 43848964 43318960 42939013 43009107 43629174 Read more

SPC Jul 23, 2021 1300 UTC Day 1 Convective Outlook

1 week 6 days ago
SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0743 AM CDT Fri Jul 23 2021 Valid 231300Z - 241200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATER THIS AFTERNOON INTO EARLY TONIGHT FROM ND INTO MN AND NORTHWESTERN WI... ...SUMMARY... Severe thunderstorms with damaging winds and large hail are possible later this afternoon into early tonight from North Dakota eastward across northern Minnesota into northwestern Wisconsin. ...Northern Plains/upper MS Valley this afternoon into tonight... A midlevel shortwave trough over SK this morning will crest the larger-scale ridge and begin to move east-southeastward over MB/ON through tonight, with height falls extending southward across ND/MN. This midlevel trough will be accompanied by a surface cold front that will move southeastward across the Dakotas and northwest MN this afternoon into tonight. Some weak elevated convection persists this morning from central SD into northwest MN, in association with a diffuse lead shortwave trough. In the wake of this weak convection/cloud band, surface heating will again drive relatively deep mixing across the Dakotas ahead of the front, where temperatures should warm into the 90s. Modest low-level moisture and the strong surface heating will result in inverted-v profiles with MLCAPE of 1000-2000 J/kg ahead of the front, and should support at least scattered thunderstorm development by late afternoon along the front and/or a weak pre-frontal trough from ND into northwest MN, and potentially into SD. Though low-level flow should remain rather weak, effective bulk shear of 30-40 kt and straight hodographs will favor a mix of organized clusters and/or splitting supercells capable of producing damaging winds and large hail. The storms are expected to persist into central MN and northwestern WI before weakening by about 06z. ...Northwest NM into northern AZ this afternoon/evening... Widespread convection occurred overnight across AZ, with a broad area of lingering rain still ongoing from Phoenix to Tuscon. This will limit surface heating (keeping temperatures well below normal) and reduce buoyancy today near and southwest of the Rim. However, some clearing/destabilization will occur from NM into northeast AZ by afternoon, when another round of convection is expected to form and spread west-southwestward into tonight in advance of a midlevel trough moving westward over NM. Though lapse rates have been reduced by the recent convection, an increase in east-northeasterly vertical shear and MLCAPE up to 1000 J/kg over the higher terrain could support a few organized clusters capable of producing isolated severe gusts and marginally severe hail this afternoon into early tonight. ..Thompson/Gleason.. 07/23/2021 Read more

SPC Jul 22, 2021 1300 UTC Day 1 Convective Outlook

2 weeks ago
SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0744 AM CDT Thu Jul 22 2021 Valid 221300Z - 231200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE THIS AFTERNOON INTO EARLY TONIGHT FROM PARTS OF CENTRAL INTO NORTHEAST MT... ...SUMMARY... Thunderstorms capable of severe wind gusts are possible late this afternoon into early tonight from parts of central into northeast Montana. ...Northern High Plains this afternoon into early tonight... Around the northern periphery of the persistent midlevel ridge over the Rockies, and shortwave trough will move over southern AB/SK and glance northern MT. The midlevel trough will be accompanied by a cold front across MT this evening into tonight, and preceded by a lee trough across the northern High Plains. Low-level moisture is a bit limited across the northern Plains with a shallow boundary layer of low-mid 60s dewpoints this morning. The shallow moisture will be redistributed vertically by deep mixing, resulting in largely inverted-v profiles this afternoon across MT and the Dakotas to the south of a weak stalled front that now stretches from northeast MT across northern ND. North of the stalled boundary, moisture will be less prone to mixing, but a little cooler surface temperatures will help maintain some convective inhibition. Two general scenarios are apparent for storm initiation today: 1) with deep heating/mixing along the lee trough this afternoon close to the MT/ND/SD borders, and 2) in the zone of deep mixing and increasing large-scale ascent in closer proximity to the cold front and midlevel height falls across central MT this evening. The former scenario is a bit uncertain in terms of storm coverage, with an environment favoring strong outflow winds along the lee trough, and a somewhat greater threat for large hail with larger buoyancy and stronger vertical shear north of the stalled front (closer to the international border). Storm initiation appears more probable in the latter scenario across central MT, with storms spreading into northeast MT early tonight. Here, damaging winds will be possible with the steep lapse rates and at least some potential for cluster development. Isolated large hail may also occur, primarily with any splitting/supercell storms. ...AZ this evening into early tonight... Another day of scattered thunderstorms is expected, with the more concentrated convection expected to move generally westward this evening into tonight from eastern into central/south central AZ. East and northeasterly mid-upper flow around the Four Corners high will continue to steer convection from the higher terrain toward the lower deserts, while thermodynamic profiles will favor isolated severe outflow winds given typical inverted-v profiles with DCAPE of 1000-1500 J/kg. ..Thompson/Smith.. 07/22/2021 Read more
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