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Thursday, August 12, 2021

SPC Aug 12, 2021 0600 UTC Day 1 Convective Outlook

SPC 1200Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1243 AM CDT Thu Aug 12 2021 Valid 121200Z - 131200Z ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE NORTHEAST AND FROM NORTHEAST KANSAS INTO NORTH-CENTRAL ILLINOIS... ...SUMMARY... Thunderstorms associated with damaging wind gusts will be possible today in parts of the Northeast, and from the central Plains to the southwestern Great Lakes. ...Northeast... A broad cyclonic flow pattern will be in place today across the north-central states and Great Lakes region. At the surface, a trough will move eastward into the lower Great Lakes today. Ahead of the trough, a moist airmass will remain over the Northeast where surface dewpoints should be mostly in the upper 60s and lower 70s F. The relatively high surface dewpoints will contribute to moderate instability by afternoon across much of the region. Thunderstorms are forecast to develop near the surface trough in the vicinity of Lake Erie and Lake Huron this morning with this convection moving eastward into the central Appalachians in the afternoon. The new model data suggests that a line segment will form during the early afternoon across western and northern New York, with this line moving east-southeastward into central New York and western New England during the late afternoon. RAP forecast soundings at 21Z from central New York into Vermont have veered low-level winds to the southwest with speed shear in the mid-levels. This combined with steep low-level lapse rates will aid downdraft acceleration, making damaging winds possible with the stronger parts of the line. The wind-damage threat may continue into the early evening as the line moves across southeast New York and southern New England. ...Central Plains/Mid to Upper Mississippi Valley/Southwestern Great Lakes... Northwest mid-level flow will be in place today from the northern Plains into the upper Midwest as a cold front advances southeastward into the central Plains and mid to upper Mississippi Valley. Ahead of the front, a very moist airmass will exist with surface dewpoints generally in the lower to mid 70s F. As surface temperatures warm, strong instability will develop ahead of the front from eastern Kansas to north-central Illinois. As low-level convergence increases along the front during the afternoon, convective initiation will become possible along parts of the front. Isolated to scattered thunderstorms are expected to form and move east-southeastward across parts of northeast Kansas, northern Missouri and north-central Illinois. RAP forecast soundings suggest that deep-layer shear will remain on the weak side to the southeast of the front, mostly in the 20 to 30 kt range. Therefore, multicell will be the favored storm mode. Steep low-level lapse rates will make damaging wind gusts possible along any organized line segment. The threat could continue into the early to mid evening as new cells develop near the front due to strengthening low-level flow. ..Broyles/Lyons.. 08/12/2021 Read more LIVE: spc.noaa.gov
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