000 ACUS01 KWNS 221300 SWODY1 SPC AC 221259 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0759 AM CDT Thu Aug 22 2019 Valid 221300Z - 231200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM OKLAHOMA TO PARTS OF THE OHIO VALLEY...MID-ATLANTIC AND NEW ENGLAND...AND OVER PORTIONS OF THE CENTRAL AND NORTHERN HIGH PLAINS... ...SUMMARY... Damaging gusts, with a few severe, are possible this afternoon and evening from Oklahoma to parts of the Ohio Valley, Mid-Atlantic and New England. Isolated severe hail and wind will also be possible over the central and northern High Plains. ...Synopsis... An amplified and progressive northern stream will characterize the mid/upper-level pattern trough the period, with sharply defined, synoptic-scale ridging moving eastward across the northern Plains, Upper Midwest and areas of Canada near western Hudson Bay. To the east, a well-defined cyclone near the James Bay coast of QC will progress eastward toward the northern Gulf of St. Lawrence. A trailing, positively tilted shortwave trough will deamplify as it moves eastward from the upper Great Lakes to New England, amidst broadly confluent flow. Upstream of the synoptic ridge, another substantial shortwave trough was readily apparent in moisture-channel imagery over the interior Pacific Northwest. This perturbation will decouple from an initially phased but more progressive vorticity banner over western Canada, then move slowly eastward across the northern Rockies. By the end of the period, the 500-mb trough should extend from north-central MT to northwestern UT. To its southeast, and south of the synoptic ridge, a weak shortwave trough is located over the central/southern High Plains, and should move slowly eastward across OK and northwest TX through the period. ...Northeastern CONUS to southern Plains... Isolated to scattered thunderstorms are expected to develop episodically throughout the period -- but maximized from afternoon into early evening -- offering sporadic damaging winds, isolated severe gusts, and in the western parts of the outlook area, isolated severe hail. Mid/upper winds will be maximized over the Northeast, while moisture is greatest across the South, and midlevel lapse rates maximized from the southern Plains to the Mid-South. A few loosely organized bands of convection may develop under the southern rim of somewhat stronger flow aloft, across the north-central Appalachians into the Mid-Atlantic coastal plain, as well as portions of southern/eastern New England where favorable low-level moisture will remain. One of those convective bands may involve an eastward shift of an area of strong thunderstorms ongoing over portions of OH/WV, later impinging upon areas of diabatically driven surface destabilization. Such potential will depend on the structure/survival of the related cold pool across the Appalachians. A limiting factor will be weak deep-layer lapse rates, keeping MLCAPE below 1000 J/kg in most areas from central VA northward/northeastward, with pockets below 500 J/kg. Buoyancy will increase southward over the Carolinas and southwestward across the southern Appalachians region, where heating and low-level lapse rates will be stronger. From this region westward to the southern Plains, prefrontal outflow/differential- heating boundaries will be the main foci for convective development and localized intensification. Although flow aloft and deep shear will be weak, limiting overall organization of the severe potential, relative maxima in convective coverage may occur over portions of OK and the Ozarks region. Subtle large-scale lift ahead of the southern High Plains shortwave trough will augment already steep midlevel lapse rates, while intense surface heating occurs ahead of the front and away from areas of persistent/antecedent cloud cover, weakening MLCINH to negligible levels. Well-mixed subcloud layers will support maintenance of hail and downdrafts to the surface. Localized concentrations of severe potential probably will develop within this lengthy outlook area, and a 15%/slight risk may be added as mesoscale trends warrant. However, uncertainties on that and smaller scales preclude greater than marginal unconditional severe probabilities being assigned at this time. ...Northern Rockies/Plains to central High Plains... Isolated to scattered thunderstorms are expected to develop across this area during the afternoon, and into evening with eastward extent. Isolated severe gusts/hail are the main concerns. Primary sources for lift will include the Pacific cold front and lee troughing over MT and northeastern WY, and upslope flow with heating of higher terrain along the western rim of the outlook in WY/CO. A relative concentration of convection is possible during about 00Z-06Z across portions of eastern MT and perhaps northeastern WY. During that time frame, some of the stronger low-level mass response and convergence (related to the approaching shortwave trough), and perhaps the leading fringe of mid/upper synoptic forcing, will impinge upon a narrow plume of relatively maximized buoyancy and boundary-layer moisture. Preconvective MLCAPE of 1000-2000 J/kg is possible this evening near an axis from RAP-GGW, amidst well-mixed subcloud layers supporting strong-severe gust potential. However, weak mid/upper-level winds and related minuscule values of effective shear, along with progs of quick transition to messy convective mode, suggest severe potential is fairly disorganized and/or short-lived. ..Edwards.. 08/22/2019 $$