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Sunday, September 12, 2021

SPC Sep 12, 2021 0600 UTC Day 2 Convective Outlook

SPC 0600Z Day 2 Outlook Day 2 Convective Outlook NWS Storm Prediction Center Norman OK 1256 AM CDT Sun Sep 12 2021 Valid 131200Z - 141200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE CENTRAL ROCKIES TO NORTH-CENTRAL WISCONSIN...AND THE MID ATLANTIC FROM LAKE ERIE TO NEW JERSEY.... ...SUMMARY... Isolated strong to severe storms are possible from the central Rockies across the northern Plains and into central Wisconsin. A few strong to severe storms may also occur in the Mid-Atlantic from near Lake Erie to New Jersey. ...Synopsis... A stationary front is expected to be in place from central South Dakota across the Midwest and Great Lakes and into the Mid-Atlantic Monday morning. Meanwhile, a weak mid-level shortwave trough is forecast to move from central Wyoming at 12Z Monday to the Upper Midwest by 12Z Tuesday. As this trough moves across the Plains, surface cyclogenesis is expected with a low pressure center expected to consolidate near the Nebraska/Kansas border Monday afternoon and move to northern Wisconsin by 12Z Tuesday. Surface flow is expected to strengthen in response to the cyclogenesis and tightening pressure gradient. As it does, the stationary front is forecast to tighten and start to lift north as a warm front across the Midwest on Monday afternoon. In addition, a cold front is expected to move southeastward across South Dakota during the day. This cold front, and the eastward extending warm front are expected to serve as a focus for thunderstorm activity during the day Monday and into Monday night with the chance for some strong to severe storms. ...Central Rockies into the Central Plains... Ascent, associated with the passing mid-level shortwave trough, is expected to be sufficient for storm development across South Dakota and possibly Nebraska before 18Z Monday. The timing of this shortwave may be the primary hindrance to greater severe weather chances across portions of South Dakota. Storms are expected to develop before greater instability develops and these early storms may stabilize the warm sector too much to support more robust severe weather later in the day across South Dakota and Nebraska. However, the environment still looks quite favorable across far southern South Dakota into Nebraska with sufficient instability and shear for organized updrafts, including supercells. Therefore, there is an area, primarily across north-central Nebraska, where the environment supports higher probabilities, but questions regarding storm coverage remain too high to add these probabilities at this time. ...Southern Minnesota and Central Wisconsin... After 00Z Tuesday, the low-level jet is forecast to strengthen substantially as a mid-latitude cyclone becomes more mature and moves into the Upper Midwest. As a result, isentropic ascent is forecast to increase near the warm front with elevated thunderstorms expected through the overnight hours. 850mb dewpoints are only forecast to be around 10C and mid-level lapse rates are not expected to be particularly steep, thus MUCAPE will likely be around 1500 to 2000 J/kg. Moderate mid-level flow should provide ample (45 to 50 knots) of shear for some storm organization, but it is uncertain whether the thermodynamic environment will be favorable enough to support severe convection. Large hail with elevated storms would likely be the primary threat, but some guidance does show a cluster moving eastward along the warm front which could pose an isolated damaging wind threat during the overnight hours Monday night into early Tuesday morning. ...Mid Atlantic... Guidance is starting to come into better agreement regarding storm development across southern New York and northern Pennsylvania into New Jersey Monday evening into the overnight hours. A weak shortwave trough is forecast to move east-southeast through the region after 00Z which may provide sufficient ascent for storm activity along the front. There still remains some uncertainty regarding the amount of storm coverage with the 12Z ECMWF continuing to indicate a more significant convective signal with more muted precipitation signals from other global guidance and some of the CAMs. However, if storms develop in this region, the environment would be favorable for organized convection in a region with moderate to strong instability and moderate effective shear. ...Texas Gulf Coast... The National Hurricane Center continues to forecast a high likelihood of a tropical disturbance developing in the western Gulf and moving near the Texas Coast during the Monday/Tuesday period. Most of the latest forecast tracks keep the environment with any tornado threat over water during the Day 2 period. ..Bentley/Gleason.. 09/12/2021 Read more LIVE: spc.noaa.gov
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