Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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734 FXUS63 KLOT 251658 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1158 AM CDT Wed Sep 25 2024 .KEY MESSAGES... - Patchy dense ground fog over rural areas of interior northern IL will dissipate shortly after sunrise - Chances for showers return Friday night into early next week, but confidence in specifics remains low. && .DISCUSSION... Issued at 339 AM CDT Wed Sep 25 2024 Through Thursday: Probably jumped the gun a bit with dense fog advisory issuance for our northwestern CWA earlier when multiple METAR were down to 1/4SM and spotter reports of near zero visibility came in. Given recent rainfall, calm/nearly calm winds, and longer nighttime was concerned dense fog was going to become widespread. Since that time, VSBYs have come up considerably in METARs and it is pretty hard to find much fog in area webcams. A couple of fortuitously timed ACARS soundings out of RFD just after 06z showed a negative hydrolapse right off the surface. Guidance indicates continued dry air advection just off the above the sfc early this morning, so that may help keep fog in check. Even though there are serious doubts about how much dense fog there will be, course of least regret is to leave the dense fog advisory in place through sunrise, as there is still a chance for a "sunrise surprise" and an expansion of dense fog development. In fact, just in the past 10 minutes, dense fog has rolled in here at the WFO, so there is some dense fog threat still early this morning. Given the shallow nature of the fog and dry air just off the deck, expect any fog to rapidly dissipate within an hour or so after sunrise. Otherwise, lake effect showers should remain mostly east of our CWA early this morning, possibly just nicking northeast Porter County at times. Lake effect should shut down later this morning. Otherwise, expecting a couple days of sunshine and seasonably mild temperatures today and tomorrow. Could be some patchy shallow ground fog again late tonight in the typical rural low lying areas, but boundary layer will only dry further today, so fog should be less prominent tonight than early this morning. - Izzi Thursday Night through Tuesday: The synoptic weather pattern from the end of this week through early next week continues to look complex with a high-amplitude ridge over the Great Lakes, a cut-off upper-level low settling in to our south, and then Tropical Storm (and soon-to-be Hurricane) Helene jumping into the fray. However, some noticeable trends have continued to emerge in the latest model runs and ensemble means, allowing us to refine our forecast a little bit. After making landfall, Helene will encounter the upper low and likely engage in a Fujiwhara-like interaction that will see the tropical cyclone rapidly get torn apart with its remnants then getting slung anticyclonically around the low and eventually absorbed into the low itself. It looks increasingly likely that this interaction will play out too late in the game for it to rain pretty much anywhere in our forecast area during the daytime on Friday, especially with the Great Lakes ridge still trying to suppress everything southward, but did still keep some sub-30% PoPs going south of the Kankakee River in the odd event that the increasingly diminutive ensemble support for a faster/farther north progression to Helene ends up verifying. Regardless of exactly how quickly Helene arrives and interacts with the upper low, high temperatures on Friday should top out in the mid-upper 70s, while tightening pressure fields in the vicinity of these two interacting systems will yield a period of breezy northeasterly winds Friday into Saturday that will induce high wave action on Lake Michigan, leading to dangerous conditions along the lakeshore. The latest trends in forecast guidance also suggest that the time period when we`d be most likely to see rain would be Friday night into Saturday as Helene`s remnants get whipped around the northern periphery of the conjoining low from east to west. There is still a fair amount of spread amongst the various ensemble suites in how far northwest the QPF footprint will get, with some solutions still keeping it entirely south and southeast of our forecast area, but there appears to be enough support to carry high-end chance PoPs across the southeast third or so of our CWA with lower chance and slight chance PoPs farther to the northwest, where the northern periphery of the precipitation shield will be less likely to reach. Whether we`ll see rain Saturday night into early next week will depend on where the center of the merged low ends up wobbling to after the merger of the two cyclones has concluded. If it remains in close enough proximity to our CWA, then we could see periodic showers continue all the way into Monday. On the other hand, if it meanders too far to the east or the aforementioned ridge keeps it bottled up farther to the south, then our forecast area would be favored to remain dry into early next week. The general trend in forecast guidance has been towards the latter scenario, and the latest NBM PoPs have decreased accordingly, with slight chance and low-end chance percentages now appearing to appropriately characterize the likelihood of precipitation on Sunday and on Monday based on this trend and the existing spread of forecast solutions in the latest ensemble suites. A fairly strong cold front currently pegged to arrive sometime Monday night into Tuesday should then shunt whatever is left of the upper low eastward and bring in more autumn-like temperatures for the middle of next week. Ogorek && .AVIATION /18Z TAFS THROUGH 00Z FRIDAY/... Issued at 1159 AM CDT Wed Sep 25 2024 The only minor concerns are: - Timing a light east/northeast wind shift with a lake breeze late this afternoon/evening at the Chicago-area sites. - Potential for shallow BR/FG tonight/Thursday morning, mainly near RFD. Fair weather cumulus with bases near 040-050 will continue to develop today. VFR conditions will prevail at the terminals. Northwesterly winds will become east/northeast this afternoon at ORD, MDW, and GYY with the passage of a weak lake breeze. Timing at ORD/MDW remains unchanged from previous forecasts. Tonight, winds will become calm/light and variable. Some shallow BR/FG will again be possible, with the strongest signal at this time near RFD. Some potential this could extend towards DPA and GYY, but confidence is too low to include a mention with the outgoing 18z TAFs at this time. Thereafter, winds will become northeasterly through Thursday morning. Carlaw && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago