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Wednesday, August 18, 2021

SPC Aug 18, 2021 1300 UTC Day 1 Convective Outlook

SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0753 AM CDT Wed Aug 18 2021 Valid 181300Z - 191200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS IN PARTS OF THE INLAND MID-ATLANTIC AND CENTRAL APPALACHIANS REGION... ...SUMMARY... Tornado potential exists today in parts of the inland Mid-Atlantic and central Appalachians region, over the eastern sector of Tropical Depression Fred. ...Synopsis... The main mid/upper-level features for this period will be the trough related to the remnants of Fred, as well as a large, pronounced cold-core cyclone that has developed over the interior Northwest. The latter's 500-mb low -- initially located over the PUW/LWS area -- is forecast to move/redevelop southeastward to the northeastern corner of NV by 12Z tomorrow. By that time, associated cyclonic flow will cover most of the western CONUS, with mid/upper-level trough from the northern Rockies, through the low, to the lower Colorado River Valley. At the surface, 11Z analysis showed a wavy cold front from southeastern MB to a low near DIK, then roughly southwestward across central WY, northwestern UT, and eastern/southern NV. The low should remain over western ND most of the day, then move northeastward along the baroclinic zone to northeastern ND or southeastern MB by 12Z tomorrow. In the meantime, the cold front will move southeastward to the western Dakotas, southeastern WY, and central/southwestern UT by 00Z, then overnight to the central Dakotas, while decelerating over southern WY and UT. ...Interior Mid-Atlantic/central Appalachians... As noted in WPC advisories, Tropical Depression Fred -- initially near the OH/WV border -- is expected to proceed northeastward over the central Appalachians region through the day, while continuing a gradual weakening trend. Nonetheless, a threat for tornadoes and damaging/isolated severe thunderstorm gusts will develop the next few hours into midday (see SPC mesoscale discussion 1550), and shift northeastward over the inland Mid-Atlantic today, before weakening overnight. Despite the weak surface wind speeds over most of the area, a corridor of enhanced gradient flow near the top of the boundary layer (i.e., around 850 mb) will help to maintain favorable low-level hodographs and shear vectors in the eastern semicircle of the remnant circulation. This flow field will overlap very rich low-level moisture (surface dewpoints commonly in the 70s F). Theta-e advection, moisture transport and pockets of at least subtle diabatic/diurnal heating will contribute enough destabilization to offset weak midlevel lapse rates for MLCAPE 500-1500 J/kg (highest in southern/eastern areas of the outlook). Meanwhile, effective SRH 200-400 J/kg should shift northeastward in step with the translation of Fred, most probably superimposed with favorable buoyancy in and near the "slight risk" outlook area. Tornado potential is more conditional/marginal with southward extent into greater instability but weaker shear, and vice versa to the north and northeast. ...Intermountain West... Widely scattered to scattered thunderstorms are expected to cross the outlook area episodically through the period, peaking during afternoon into early overnight hours. Isolated severe gusts and hail will be possible. Height falls, strengthening flow aloft/deep shear, and increasing large-scale ascent preceding the mid/upper cyclone are forecast over western/northern UT through the day, and in the evening farther east into more of southwestern WY and central UT, in support of storm organization. Forecast soundings indicate as much as 40-45-kt effective-shear magnitudes may develop over the SLC area, near the northwestern fringes of the favorable buoyancy, with largely unidirectional deep-layer flow. Frontal lift and diurnal heating/ reduction of MLCINH in the warm sector will contribute to development, as will upslope sides of elevated terrain, and lift on outflow/differential/heating boundaries from prior convection. Favorable low-level moisture will characterize the warm sector, with PW of 0.75-1.2 inches already commonly noted via GPS sensors. The ongoing presence of cloud cover and precip over much of the area remains a complicating factor, likely to delay substantial heating for several hours. A mesoscale area of greater severe threat may develop today within the broader outlook, but potential appears too conditional for such a delineation at this time. ...Mid-South/Tennessee Valley... Scattered to locally numerous multicellular thunderstorms are expected to develop and move generally northeastward across the outlook area from midday into parts of this evening, offering locally damaging/isolated severe gusts. A weak but readily apparent southern-stream perturbation -- in moisture-channel imagery over south-central/southeastern OK -- is expected to eject northeastward across AR toward the lower Ohio Valley through the period. This feature should provide both large-scale lift and slightly enhanced midlevel winds/deep shear on the mesoscale to its east and southeast, supporting some convective organization and perhaps relatively dense coverage today. Rich low-level moisture -- with surface dew points commonly in the upper 60s to mid 70s F -- and diurnal heating will overcome weak MLCINH in support of 1500-2000 J/kg MLCAPE, suitable for both: 1. Intensification of holdover convection from this morning, and 2. Formation anew along outflow/differential-heating boundaries. Upscale growth/clustering is possible, with potential for concentration of strong to marginally severe thunderstorms on the mesobeta and smaller scales. ..Edwards/Kerr.. 08/18/2021 Read more LIVE: spc.noaa.gov
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