Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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102 FXUS63 KLOT 210824 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 324 AM CDT Sat Sep 21 2024 .KEY MESSAGES... - Near record heat expected today, fittingly for the last day of astronomical summer - Scattered showers/t-storms tonight likely to build to a steadier beneficial rain with some embedded thunderstorms Sunday - Another bout or two of rain possible Sunday night through Tuesday, especially south of I-80 && .DISCUSSION... Issued at 324 AM CDT Sat Sep 21 2024 Dense fog blankets northwest Indiana and extreme eastern IL near the IN border early this morning. Dense fog should persist until after sunrise with some potential for a bit more westward expansion into IL. Will monitor trends for possible westward expansion of the dense fog advisory. Once the fog burns off this morning, expecting mostly sunny skies with just some passing mid-high level cloudiness today. Guidance is in good agreement that 925mb temps will peak in the mid 20s Celsius this afternoon, which is about degree or so warmer than yesterday. Guidance pretty unanimously exhibited a low bias with high temps Friday and have trended highs today close to the warmest guidance, with forecast highs potentially threatening the record highs for both Rockford (94F in 1920) and Chicago (94F in 2017). Our CWA will lie east of the axis of deeper moisture which should be creeping into western IL this afternoon. The shallow low level moisture over moist of our CWA should mix out with dewpoints falling at least into the 50s, if not locally into the 40s this afternoon. Scattered thunderstorms could develop this afternoon over western Illinois as the richer low level theta-e air mass advects eastward, with a chance that some of these storms could creep into our western CWA very late this afternoon. Expectation is that this activity would weaken as it outruns the better moisture and likely meets its demise as it chokes on the drier air mass over our CWA early this evening. As a cold front begins to move gradually southeast toward northern IL tonight, the axis of deeper moisture pooled ahead of the boundary is progged to overspread the area later this evening into the overnight hours. While details in timing and strength exists among the various models, there is a pretty consistent signal in guidance that there could be a lower amplitude shortwave potentially enhancing coverage of scattered showers and thunderstorms at times tonight across the CWA. Somewhat complex pattern Sunday with strong northern stream trough over Manitoba with weaker trough axis extending south into the mid-Mississippi Valley potentially interacting favorably with a lead impulse emanating from an upper low emerging out into the central Plains. Confidence is increasing that there could be a period or two of some beneficial rains Sunday with potentially a couple of embedded thunderstorms as well. Rain should have a tendency to move east and potentially end from the west during the afternoon. Given the slow moving front and unseasonably rich moisture pooling near the front, there is growing confidence that many areas could see 24 hour rainfall totals of a half inch to around an inch of rain tonight into Sunday. Embedded thunderstorms could result in some totals well over an inch to approaching 2 inches. Front and better rain chances should push south and probably out of the CWA Sunday night into Monday. The complexity of the pattern with a northern stream trough interacting with the southern stream upper low over the Plains lends itself to higher than normal uncertainty in precip chances. Increasingly, it looks like late Sunday night through Monday will be mostly rain free, particularly north of I-80. NBM pops have trended downward, but are probably still too high, but opted to let them ride given the uncertainty in this pattern. The central Plains upper low is progged to weaken to an open wave as it moves across the Mississippi Valley Monday night into Tuesday. This could result in another period of rain and possibly even some thunder depending on how far north this wave tracks. Still uncertainty in how that will play out and NBM pops reflect this well with mostly chance pops, highest southern CWA. Cooler, more seasonable temperatures are expected Sunday into early next week behind tonight`s front. Medium range guidance is advertising a very blocky pattern to develop over North America later this week. Models often struggle with the evolution of blocky patterns like what is progged later this week, so not a lot of confidence in the details. For now, it looks like Rex Block type pattern with northern stream ridging over the upper Great Lakes and southern stream upper low somewhere in the central Plains or lower Missouri Valley region. If this Rex Block develops as ECMWF suggests with pretty good ensemble agreement, then our weather will hinge on the exact placement of these features. If upper low remains far enough to our south and west, more dry and warmer than average weather is possible later in the week into next weekend, but stay tuned, pattern evolutions like this are prone to significant model swings from run to run. - Izzi && .AVIATION /06Z TAFS THROUGH 12Z SUNDAY/... Issued at 1233 AM CDT Sat Sep 21 2024 - Dense fog over northwest IN, including GYY, until just after sunrise - Lake breeze expected to result in wind shift to easterly (ORD/MDW) and northeasterly (GYY) this afternoon - SCTD SHRA chances will increase this evening for RFD and late evening into the overnight at immediate Chicago area terminals Dense fog bank over northwest Indiana is expected to persist and likely expand westward into far eastern Illinois until quickly burning off just after sunrise. Suspect urban heat island warmth will keep MDW warm/dry enough to limit threat of fog, but will need to monitor for a brief period of fog around sunrise. Otherwise, VFR conditions today, before some SCTD SHRA begin affecting northern Illinois tonight. Initially, highest chances are west in the RFD area this evening, but spreading east into the immediate Chicago area terminals as well by late this evening into the overnight. Light/variable winds should become light southerly this morning, but lake breeze is likely to develop and move inland resulting in a wind shift to easterly at MDW and ORD later this afternoon. - Izzi && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...Dense Fog Advisory until 9 AM CDT /10 AM EDT/ this morning for INZ001-INZ002-INZ010-INZ011-INZ019. LM...None. && $$ Visit us at weather.gov/chicago