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Thursday, June 10, 2021

SPC Jun 10, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0756 AM CDT Thu Jun 10 2021 Valid 101300Z - 111200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM MUCH OF NORTH DAKOTA AND EXTREME EASTERN MONTANA TO MUCH OF NEBRASKA... ...SUMMARY... Scattered severe storms are likely late this afternoon and evening across much of the northern and central Plains. All severe hazards are possible, including very large hail, gusts above 70 mph, and tornadoes. ...Synopsis... In mid/upper levels, longwave troughing will remain over the northeastern Pacific. In the downstream southwesterlies, an intense shortwave trough -- now evident in moisture-channel imagery over the Sierra Nevada and northwestern NV -- will eject to a position from western MT to northern UT by 00Z. Though deamplifying with time, this still will be a potent shortwave trough tonight as it assumes more negative tilt, reaching the western Dakotas by 12Z tomorrow. As associated height falls occur, downstream ridging will be shunted eastward across the Upper Midwest to the upper Mississippi Valley, and southeastward over the central Plains. A weaker shortwave trough, that has penetrated the mean ridge, will pivot south-southeastward from northern ON across the upper Great Lakes. Farther southeast, a broad area of modest cyclonic flow aloft is related to a large but diffuse/slow-moving trough over the eastern CONUS. This feature will support widespread potential for isolated to scattered general thunderstorms, and isolated/marginal severe (mainly damaging gusts) cannot be ruled out away from better-focused potential apparent in existing outlook areas. At the surface, 11Z analysis showed a wavy stationary to warm frontal zone from the NYC area across PA, Lake Erie, central Lake Michigan, western Upper MI, and northern WI, to an intersection with a stationary front over southeastern ND and northern SD. That, in turn, became a warm front arching roughly westward to a low over southeastern MT near MLS. The front then connected to another low over central ID, with a cold front from there south-southwestward over western UT. The MT low will move little through the day as the cold front crosses the northern Rockies, intensifies, and catches up -- and the warm front moves northward over western ND and northeastern MT. The cyclone then will strengthen and move northeastward over western/northern ND overnight, with the cold front reaching the eastern Dakotas, central NE, and eastern CO by 12Z. A dryline and lee trough will shift slightly eastward from current position, to a 00Z alignment from extreme eastern WY across eastern CO, the TX Panhandle, Permian Basin and Big Bend area. ...Northern/central Plains... Thunderstorms should develop initially this afternoon into early evening near the surface low and adjoining frontal segments over eastern MT and the western Dakotas. Supercells with all hazards (tornado, large to giant hail, severe gusts) are expected in the first few hours of the convective cycle. This should be followed by a transition to one or more organized convective complexes with a dominant destructive-wind hazard, with isolated severe hail and a marginal tornado threat. During and after that transition, very damaging gusts to hurricane force may be observed. The main adjustments for this outlook cycle are to expand the wind area for overnight potential, and to better distinguish the threats spatially for severe to significant hail (greater west) vs. wind (more probable east, still with overlap due to mode-transition uncertainties). Very steep midlevel lapse rates will overlie a corridor of moist/theta-e advection and diurnal heating south of the warm front and east of the low/cold front. This will contribute to MLCAPE strengthening into the 2000-3000 J/kg range (locally higher), amidst intensifying deep shear (effective-shear magnitudes reaching 35-50 kt). Low-level shear should be maximized along the warm front and east of the surface low, but will be favorable for supercells over a broad area in the western part of the outlook. Farther south across western portions of SD/NE and perhaps the northeastern corner area of CO, supercells still should form late this afternoon into early evening near the dryline/lee trough, offering significant hail and severe gusts. However this activity may evolve upscale more quickly to a QLCS configuration with a dominant wind threat. Deep, well-mixed subcloud layers will support maintenance of both severe hail and gusts to the surface during earlier, relatively discrete stages, but also, foster quick cold-pool aggregations and rapid expansion. Forced ascent of a foregoing boundary layer containing 60s to near 70 F surface dew points, with MLCAPE commonly remaining above 2000 J/kg even well after dark, will help to drive the severe-wind threat southeastward over much of NE tonight. As such, wind probabilities supporting an Enhanced category have been expanded in that direction for the nocturnal threat. As the convective-wind scenario in particular becomes better focused within the broad "Enhanced" area now valid, mesoscale trends and 12Z and later model guidance may inform a stronger corridor of probabilities meeting "Moderate Risk" thresholds. ...Northern WI/Upper MI vicinity... Widely scattered thunderstorms -- some in clusters -- may develop today in a weakly capped air mass characterized by favorably rich moisture, but weak deep shear. The lake breeze, localized convergence zones and outflow boundaries should be the primary foci. The combination of large-scale lift ahead of the ON shortwave trough with boundary layer heating will steepen low/middle-level lapse rates favorably, contributing to MLCAPE in the 1500-2500 J/kg range south of the Superior lake-breeze front. ...AR/LA area... Scattered thunderstorms should form throughout the day from an existing convective area over west-central MS northwestward across parts of AR and northeastern LA. Locally damaging downbursts of water-loaded cores will be the main concern from a severe-weather perspective. An outflow/differential-heating zone related to prior overnight convection -- and oriented with a substantial component parallel to the northwesterly midlevel flow vectors, should provide most of the early focus for this development. Diurnal heating of a very moist boundary layer -- with surface dew points in the 70s F and PW near 2 inches -- will foster peak MLCAPE in the 2500-3500 J/kg range, in a very deep troposphere. Weak vertical shear will limit organization to pulse characteristics with multicell modes. ...West TX dryline... A few late-afternoon thunderstorms again are possible over higher terrain of the Davis Mountains/Big Bend area, and more conditionally, the adjoining segment of the dryline, offering locally strong-severe gusts and hail. Strong capping will make convective development more improbable with northward and eastward extent. The lack of more-robust deep-layer winds (and of stronger vertical shear) should keep convection multicellular in character and slow-moving, remaining close to its genesis area. Hot, deep, well-mixed boundary layers are expected, with DCAPE likely exceeding CAPE values by most measures, in support of marginal gust/hail potential. ..Edwards/Kerr.. 06/10/2021 Read more LIVE: spc.noaa.gov
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