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SPC May 20, 2024 0600 UTC Day 1 Convective Outlook

1 year 3 months ago

SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1257 AM CDT Mon May 20 2024 Valid 201200Z - 211200Z ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS NORTHEAST COLORADO...SOUTHWEST NEBRASKA...AND FAR NORTHWEST KANSAS... ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR PORTIONS OF NORTHEAST ILLINOIS INTO SOUTHEAST WISCONSIN AND NORTHWEST INDIANA... ...SUMMARY... Severe thunderstorms are expected across parts of the central Plains this evening into the overnight hours. Other severe storms are possible from the Middle Mississippi Valley to Lower Michigan through early evening. ...Synopsis... The large-scale pattern on Monday will feature a positively tilted mid-level ridge across the eastern CONUS with a trough in the west. A shortwave trough currently across northeast Kansas early this morning will progress through the Midwest and into the Great Lakes by tonight. In addition, a closed upper low across the eastern Pacific will phase with the upper-level trough across the western CONUS and accelerate toward the Plains by the end of the period. A lee cyclone will deepen across the southern High Plains through the period as mid-level troughing amplifies across the western CONUS and mid-level flow strengthens across the Southwest. To the north of this surface low, a stationary front will become better defined from near the Palmer Divide to northwest Iowa with a dryline extending southward from this front. ...Front Range into the central Plains and Upper Midwest... A significant void in low-level moisture is present across western Kansas and eastern Colorado early this morning in the wake of the MCS. By late morning, moisture should start to increase across Kansas in response to low-level moisture advection. This moisture advection will intensify during the afternoon as the primary lee cyclone develops across southeast Colorado and low-level flow strengthens. By late afternoon, low-level upslope flow will develop across northeast Colorado with dewpoints into the 50s. Within this region of upslope flow, supercells are expected to develop by late afternoon to early evening. Long hodographs will support the threat for large to very large hail initially. Any storms which are surface based, along or south of the frontal boundary, will pose some tornado threat, particularly after 00Z as the low-level hodograph enlarges and low-level moisture increases. Multiple rounds of supercells are possible within the Enhanced risk, as the initial supercells progress east, before additional supercells likely develop in a similar region as the primary trough ejects into the Plains. Eventually, supercells are expected to congeal into one or more bowing MCSs with an increasing severe-wind threat into south-central Nebraska. Some CAMs suggest the greater coverage severe-wind threat may persist into eastern Nebraska during the overnight period. Strong low-level moisture advection will maintain moderate to strong instability south of the frontal zone during the overnight period, and a strengthening low-level jet will also aid in MCS maintenance and intensity. However, the stable boundary layer during the overnight period casts doubt on how widespread the severe-wind threat will be after midnight. Therefore, the Enhanced risk is focused during the initial stages of supercell-to-bow transition where low-level thermodynamics will be more favorable during the evening hours. ...Northern Illinois vicinity into Wisconsin and Michigan... A compact mid-level shortwave trough over northeastern Kansas this morning will traverse the Midwest with ascent overspreading northern Illinois by mid-afternoon. Once the cap erodes, expect scattered thunderstorms to develop across central Illinois and move into northern Illinois, southeast Wisconsin and northwest Indiana. NAM/RAP forecast soundings show around 2000 to 2500 J/kg MLCAPE and around 30 knots of effective shear (in a confined region near the upper shortwave trough). This combination of instability, shear, and ascent with the passing wave should be sufficient for severe thunderstorms capable of primarily large hail and damaging wind gusts. These storms are expected to weaken as they move over the colder water of Lake Michigan and the boundary layer cools near sunset. ...South Florida... Scattered thunderstorms are expected across south Florida during the afternoon and evening hours amid 1500 to 2000 J/kg MLCAPE. Shear will be relatively weak (20 to 25 knots) and lapse rates will be quite meager. Therefore, multicells with isolated large hail and damaging wind gusts will be the primary hazards. ..Bentley/Grams.. 05/20/2024 Read more