Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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859 FXUS63 KLOT 150814 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 314 AM CDT Sat Jun 15 2024 .KEY MESSAGES... - Multi-day stretch of hot and muggy conditions beginning on Sunday. The highest heat indices (100-105 degrees) are currently expected on Monday. - Daily chances for thunderstorms will return Sunday and continue into next week, though coverage is expected to be isolated to scattered at best. && .DISCUSSION... Issued at 313 AM CDT Sat Jun 15 2024 Through Sunday: Today should be quite similar to yesterday weather-wise as surface high pressure continues to dominate over the Great Lakes and the axis of an amplifying upper-level ridge scoots east of the Mississippi River. High temperatures should once again top out in the mid 80s at most locations within our forecast area, though southeasterly return flow will keep temperatures cooler along the Illinois lakeshore, where highs are unlikely to reach 80F. There should be more high-level cloud cover today as a convectively- enhanced shortwave begins to crest the aforementioned ridge, but no precipitation is expected. Tomorrow (Sunday) will mark the beginning of our upcoming stretch of hot and muggy weather as a tightening pressure gradient between the departing Great Lakes surface high and a strengthening surface low over the northern Great Plains induces breezy southerly flow that will begin to more rapidly transport warmth and moisture of Gulf origin into the Midwest. Dew points should be at quite comfortable levels in the low-mid 50s to start off the day, but by the late afternoon, the combination of moisture advection and evapotranspiration from crops will make it feel a lot more humid outside with dew points approaching and potentially even reaching the 70 degree mark. Just how high they will climb will depend on how much mixing occurs during the daytime, which NCEP and global guidance are still somewhat at odds about. However, even the HRRR model, which is notorious for often being too aggressive with mixing dew points out, advertises 70+ degree dew points being achieved Sunday afternoon outside of the Chicago metro in its now in-range 00Z run, so felt comfortable nudging forecast dew points for Sunday upward a few degrees in our gridded forecast database. Assuming that these higher dew points are realized, air temperatures are likely to top out near 90F/in the low 90s, which would translate to heat indices peaking in the mid to upper 90 degree range Sunday afternoon. While most of the area should remain dry on Sunday, there is a lower-end chance for showers and maybe even an isolated storm or two to be observed in our northern counties during the daytime. Whether that comes to fruition or not appears to be largely dependent on how convection across Iowa plays out tonight. Some model solutions have been depicting the development of an MCV from the Iowa convection that ends up lagging behind the main shortwave trough and tracks into southern Wisconsin after daybreak tomorrow. This kind of outcome would be the most auspicious for showers and/or storms being observed in northern Illinois tomorrow, though even if a trailing MCV did not develop, the main shortwave may pass by just close enough for some of its associated shower activity to graze part of our CWA. At any rate, PoPs in the slight chance range best characterize Sunday`s precipitation potential at this time, so have maintained those along and north of I-88 with this latest forecast package. Ogorek Sunday Night through Friday: Hot and muggy conditions are expected on Monday, although questions remain regarding temperature and dewpoint trends through the day. While there`s high confidence peak heat indices will near or locally/briefly exceed 100 degrees, how we get there still remains a bit unclear (is it highs in the upper 90s and dewpoints in the mid 60s, or highs in the mid 80s and dewpoints in the mid 70s?). The GFS continues to be a moist outlier with a notably shallower PBL depth compared to the ECMWF, Canadian, UKMET. Even the NAM, which has a propensity to severely undermix is advertising higher air temperatures and lower dewpoints compared to the GFS. For these reasons, and based on how strongly we`ve been mixing recently, continue to generally toss the GFS solutions on Monday. This results in a warmer high temperatures in the mid and upper 90s and mid-upper 60s dewpoints driving peak heat indices near 100, locally a bit higher. While conditions will be hot and uncomfortable, these conditions aren`t uncommon in our area in June and the latest guidance continues to point towards things settling just under Heat Advisory criteria on Monday. Regarding thunderstorm chances: suspect the GFS continues to depict a bit too much in the way of convection owing to its less deeply mixed PBL which is driving higher dewpoints and less afternoon MLCIN, particularly on Monday. Obviously, if we do indeed fail to mix as deeply as the rest of the global guidance indicates, a bit more in the way of "airmass" and pulse-type afternoon activity would be possible. Additionally, there really don`t look to be any significant large scale forcing mechanisms nearby, so capping things to isolated/widely scattered seems like the way to go. Surface-700 mb theta-e deficits nearing 30 C might support a gusty downburst wind threat, but again storm coverage is not expected to be significant. Tuesday may feature some slightly better chances for scattered showers and storms as a plume of deeper moisture emanating from the central Gulf of Mexico surges northward as an anomalously deep high builds a bit farther east across southern New England. In addition, there`s a potential for increased mid-level cloud cover with this moist plume, and it seems like most guidance is suggesting high temperatures may "cool" a few degrees from Monday`s as a result. The GFS may once again be advertising too much in the way of convection with resulting highs only in the upper 70s/to near 80. The blended offering of highs near 90 seemed like a good middle ground. On Wednesday, a cold front is expected to press southward into Iowa and Wisconsin and will serve as a focus for greater coverage of showers and storms, some of which may meander into our forecast area. This boundary looks like it`ll stall or become quasistationary into Friday which will keep some degree of storm chances running, primarily in our north and northwest. Upper-level winds are not expected to be all that strong, and while a southward-oozing convective complex can`t be ruled out during this period, conditions don`t currently look supportive of organized severe MCS development. Carlaw && .AVIATION /06Z TAFS THROUGH 12Z SUNDAY/... Issued at 1207 AM CDT Sat Jun 15 2024 VFR conditions are expected with no significant aviation weather concerns. Variable/light NW winds overnight will become SE just after daybreak. Small potential winds veer 180-190 infrequently at ORD/MDW in the afternoon, but winds are expected to remain largely east-of-south. Carlaw && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago