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Archives for the SPC Convective Outlook are updated daily (approximately) with a live map at the beginning of each article. Follow the link at the end of the article to check for current updates on the NOAA/NWS Storm Prediction Center website. Also, see Archives for Chicago's hourly weather data on CARDINAL NEWS Magazine.

Friday, September 1, 2023

SPC Sep 1, 2023 1300 UTC Day 1 Convective Outlook

LIVE MAP (ABOVE) ... SPC 1300Z Day 1 Outlook Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 0749 AM CDT Fri Sep 01 2023 Valid 011300Z - 021200Z ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM SOUTHERN ARIZONA AND SOUTHEASTERN CALIFORNIA TO SOUTHERN IDAHO... ...SUMMARY... Isolated severe gusts and hail will be possible today into this evening, from southern Arizona and southeastern California to southern Idaho. ...Synopsis... A blocky synoptic pattern will persist across the CONUS in mid/upper levels, anchored by a high over the south-central Great Plains, and split flow around a cyclone over the West Coast States. Through the end of the period, the associated 500-mb low should retrograde south-southwestward from its initial position near SLE to near Cape Mendocino, CA. This will help to maintain a belt of enhanced, seasonally strong mid/upper-level winds -- with substantially southerly component -- across much of the Great Basin and western parts of both the Sonoran Desert and Colorado Plateau, northward past the Wasatch Range and upper Snake River Valley. At the surface, 11Z analysis showed a cold front offshore from SC, becoming quasistationary to warm over central GA to a low over west-central AL, then cold again across southwestern AL and the northwestern Gulf to the TX coastal bend. The boundary should move little through the remainder of the period, and will keep optimal Gulf moisture shunted well south of the midlatitude westerlies, east of the northern Rockies. ...AZ/southeastern CA to eastern Great Basin... A broad plume of persistent low-level convergence and high-PW air, of low-latitude marine origins, will combine with more-local evapotranspiration from moist ground to support both: 1. Potential continuation and eventual diurnal re-intensification over parts of UT, of ongoing convection moving northward out of AZ, and... 2. Scattered afternoon and evening thunderstorms developing in already weak, diabatically and orographically removed MLCINH across the rest of the outlook area. Quite often, occurrence of extensive thunderstorms over AZ one day, with cloud cover lingering into morning the next, significantly suppresses potential through convective stabilization and insufficient airmass modification the second day. However, in this somewhat atypical regime, the persistent plume of strong deep-layer southerlies will support enough warm advection and moisture transport to move the convectively processed boundary-layer air northward and, to some extent, modify it, such that enough destabilization to support more development is probable by mid/late afternoon. That activity, in turn, should move roughly northward. In many areas, surface heating will not be as long nor strong as yesterday, but still enough to build a well-mixed subcloud layer, supporting strong to locally severe gusts. With enough mid/upper flow and deep shear to support both organized multicells and at least transient supercells (effective-shear magnitudes of 25-40 kt), isolated, marginally severe hail is possible again today as well. ..Edwards/Leitman.. 09/01/2023 Read more CHECK UPDATE ZOOM GRAPHIC