Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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547 FXUS63 KLOT 151756 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1256 PM CDT Mon Jul 15 2024 .KEY MESSAGES... - Heat Advisory in effect from noon to 7 pm CDT for much of our IL forecast area. Hot and humid with peak afternoon heat indices of 100- 110F. - Another round of strong to severe thunderstorms expected late this afternoon into tonight. Locally heavy rainfall possible, which could lead to additional flooding. Additional thunderstorm chances continue Tuesday, mainly south of I-80/east of I-55. - Pattern breaks by Wednesday, with dry weather and seasonable temperature and humidity through the weekend. && .MESOSCALE... Issued at 1142 AM CDT Mon Jul 15 2024 Satellite imagery late this morning shows an apparent MCV over western WI moving eastward. This feature seems likely to remain north of our CWA this afternoon and probably a non-factor. Upstream, there is another subtle shortwave apparent over South Dakota which is progged to move east-southeast into northern IA later this afternoon. At the surface, a very warm and humid air mass blankets the Midwest. There is some evidence of a lingering outflow/moisture discontinuity boundary roughly along the I-80 corridor in our CWA. To the south of this boundary, dewpoints are in the mid-upper 70s with lower 70 dewpoints more common to the north of the boundary. This boundary should continue drifting north and likely dissipate with mid to upper 70 degree dewpoints expected to be fairly common across the area this afternoon. Morning soundings and SPC objective analysis both show an expansive EML from the Plains east into the mid-Mississippi Valley. These steep lapse rates juxtaposed overtop the very theta-e rich boundary layer will result in extreme instability with 4-5k+ J/kg of MLCAPE this afternoon in an axis from Iowa east into northern Illinois. As ascent from the South Dakota shortwave trough begins to overtake the extreme instability and weakening inhibition this afternoon, expect explosive severe thunderstorm development over Iowa later this afternoon. Belt of moderate westerly mid-level flow (around 40 kt at 6km) should result in 30-35kt of 0-6km bulk shear. Given the extreme instability, this should allow for supercells with the initial development, but given DCAPE over 1000 J/kg would anticipate an evolution into one or two bowing QLCSs by early evening as the storms approach and move east of the Mississippi River. Conditions continue to look favorable for a QLCS with damaging winds across much of the CWA this evening. Given combination of extreme instability and moderate shear, cannot rule out some swaths of significant wind damage with gusts over 75mph in spots. In addition, strengthening low level jet this evening should result in favorable environment for QLCS type tornadoes as well. Finally, PWATs over 2" and very high freezing levels will result in storms being extreme efficient heavy rainfall producers capable of hourly rainfall rates of 2-3" per hour. Should any "warm air advection" wing type cells develop ahead of the approaching QLCS or upwind propagating storms develop on the tail end of the QLCS, then they would pose a potentially significant flash flood threat. Think it is likely that a (flash) flood watch will eventually be needed for portions of the CWA once the mesoscale evolution of things becomes clearer this afternoon. Of particular concern would be areas near and north of I-88, from the Rockford into the Chicago metro where there have been multiple rounds of heavy rain over the weekend, making them particularly susceptible to flash flooding with any heavy rainfall. - Izzi && .DISCUSSION... Issued at 405 AM CDT Mon Jul 15 2024 Through Tuesday night... Linear MCS which brought severe weather and heavy rainfall to the area earlier in the night continues to move off to the east and southeast of the forecast area early this morning. Trailing stratiform rain and embedded thunder linger southeast of roughly a Gibson City to Chebanse to Rensselaer line as of 330 am CDT, but should exit the area by 5 am CDT. Clearing of extensive anvil cloud cover from the west may allow for patchy fog development across northern IL through sunrise given moist ground and high humidity, but should burn off fairly quickly after the sun comes up. Mostly sunny skies are then expected this morning, allowing temperatures to warm into the low-mid 90s area-wide through this afternoon. The potential exists for increasing high clouds to increase by this afternoon from decaying upstream convection, leading to somewhat lower confidence in widespread mid-90s high temps. However, combined with surface dew points in the mid- upper 70s especially across our IL rural farmland, peak afternoon heat indices between 105-110F are likely at least for a time. Have elected to issue a Heat Advisory for our IL counties roughly south of a line from Forreston to Joliet to Beecher line from noon through 7 pm CDT, where values of 105F and above are most likely. If cloud cover is thinner or of less coverage and allows temps to overperform, the advisory could be expanded farther north and east if necessary. Attention then turns to the potential for another round of strong to severe thunderstorms expected to affect the area from late this afternoon into tonight. The expected evolution of convection across the region remains complex, and depends upon how thunderstorm clusters currently ongoing across the eastern Dakotas evolve through the remainder of the morning hours. Synoptically, an area of surface low pressure is expected to deepen across the upper Midwest and northern Great Lakes region through this evening, in response to a mid-level short wave transiting the south/southeast periphery of an upper low over eastern Manitoba/western Ontario. A weak cold front trailing this low is progged to slowly move southeast across the upper Mississippi Valley, eventually reaching northern IL later tonight. Re-intensification or redevelopment of upstream convection is expected by mid-late afternoon ahead of the cold front, and within a very warm/humid high Theta-E airmass characterized by dew points in the low-mid 70s across IA/WI/northern IL. Latest 06Z CAM guidance suggests clusters of storms to our west/northwest will congeal into another organized forward-propagating linear MCS which would move into our area either very late this afternoon or early this evening. (CAM trends have been to slow this development and arrival from earlier runs, though some differences remain in just where/when convection initiates this afternoon). Higher-end severe winds appear to be the greatest threat, though forecast soundings do indicate sufficient low-level shear which would conditionally support a tornado threat with earlier discreet storms or embedded QLCS circulations within the evolving linear mode. Severe threat looks to be mainly focused within the evening hours, perhaps a bit longer in our far east/southeast (similar to Sunday night). A locally heavy rainfall/flash flood threat will exist as well, especially along the trailing portion of the linear MCS where it may tend to become more west-east oriented after midnight. Our forecast area remains in SPC`s Day 1 Enhanced (level 3 of 5) and WPC`s slight (level 2 of 5) risks of severe and excessive rainfall. Scattered thunderstorm chances appear lower across northern parts of the area by Tuesday, mainly focused along/south of the slowly advancing cold front along and south of I-80. A few storms could linger across our far southern cwa into Tuesday evening. Between cloud cover south, and cool advection behind the front to the north, cooler temperatures in the 80s are expected Tuesday. Wednesday through Sunday... The main upper trough axis is expected to shift east across the forecast area Wednesday. Despite steep low-level lapse rates depicted in forecast soundings, warm mid-level temps (700-600 mb) look to provide a capping inversion with respect to surface based convective development. Therefore, forecast for Wednesday is dry at this point. Broad surface high pressure then builds across the area through the end of the week, setting the stage for a well-deserved period of dry and seasonably mild but less- humid weather conditions, downstream of a high-amplitude upper ridge which develops over western North America. Ratzer && .AVIATION /12Z TAFS THROUGH 18Z TUESDAY/... Issued by National Weather Service Gaylord, MI 1256 PM CDT Mon Jul 15 2024 The primary aviation weather concerns for the 18Z TAF period include: * Severe squall line in the 00z-05z time frame * Period of MVFR ceilings (at least?) ahead of and behind a cold front expected to pass through in the 12z-18z time frame.. Main concern heading into the evening hours is the development of a strong line of thunderstorms over eastern Iowa/northwest Illinois probably after 21z...expected to organize into a squall line that is expected to race across northern Illinois in the 00z-05z time frame. Wind gusts in excess of 40kts will be possible as this line moves through. Behind this and ahead of a cold front moving in from the northwest...some signals for at least MVFR ceilings (and perhaps some drizzle) after 04z...which would persist into Tuesday morning before ceilings thin out after 15z. Cold front wind shift should move into far northwest Illinois around 12z...and into northwest Indiana by 18z. JPB && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Heat Advisory until 7 PM CDT this evening for ILZ008-ILZ010- ILZ011-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039- ILZ106-ILZ107-ILZ108. IN...None. LM...None. && $$ Visit us at weather.gov/chicago