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Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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518 FXUS63 KLOT 060818 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 318 AM CDT Sat Jul 6 2024 .KEY MESSAGES... - Mostly dry conditions are expected, but cannot completely rule out the chance for a light pop-up sprinkle/shower during the afternoon - 25 to 45 percent chance for showers and storms Sunday morning/afternoon, mainly northwest of a La Salle to Lake County line - Unsettled Sunday night-Tuesday afternoon with periodic showers and thunderstorms, highest chance/coverage on Monday. - Seasonable temperatures will continue through next week. && .DISCUSSION... Issued at 318 AM CDT Sat Jul 6 2024 Through Sunday: An upper-level trough continues to gradually move to the east away from Chicago. Upper-level height rises in its wake suggests that much of the area should stay dry through the day. However, soundings have kept a swath of mid-level moisture lingering around the area. This in combination with daytime heating along a weak lake breeze could be enough to support the development of a few isolated to widely scattered sprinkles or a rogue shower this afternoon. The next upper-level shortwave trough currently dropping southward into the western Dakotas will grow into a much longer wave through Saturday. As it moves into Minnesota Sunday morning, a weak shortwave is expected to extend southeastward over Iowa and move into northern Illinois. Winds on Sunday will turn to the south-southwest and advect in higher moisture content. Lapse rates aloft and wind shear through the afternoon are not overly impressive, so the threat of severe weather will be low. However, there should be enough instability as the better area of vorticity moves in ahead of the wave to allow for showers and thunderstorms to move into the area. The favored track of the impulse toward southeastern Wisconsin will favor the higher chances (higher PoPs) generally northwest of a La Salle to Lake County line. The long wave trough continues to move eastward for renewed chances for showers and storms Sunday night, covered in the discussion below. DK Sunday Night through Friday: The threat and coverage of showers and thunderstorms is expected to increase from northwest to southeast across the area Sunday night through at Monday night owing to the approach of a slow eastward moving cold front. During this period, several waves of scattered northeastward moving showers and thunderstorms are favored along and ahead of the front, with each wave being driven by a series of mid-level impulses tracking northeastward along the surface frontal boundary. The highest chances for these showers and storms are expected to remain primarily northwest of I-55 Sunday night into Monday morning, before the chances begin to ramp up farther east and southeast later Monday into Monday night. The threat of severe weather looks to remain rather low with this activity. However, we will have to keep an eye on the possibility for some locally heavy rainfall given the potential for some northeastward training along the slow moving cold front within an environment featuring high PWATs (up to 150% of normal). The surface cold front should begin to shift east of the area early on Tuesday as the main mid-level trough axis makes steady eastward progress into the Western Great Lakes. However, in spite of the early frontal passage, expect the chances for scattered showers and storms to continue into Tuesday as relatively cool mid- level temps under the upper trough shift over a still warm and fairly humid post frontal airmass. Accordingly, we continue to carry 30-40% PoPs. The threat of these showers and storms should come to a quick end later in the afternoon, however, as the trough shifts to our east. For the Wednesday-Friday period, the threat of organized showers and thunderstorms looks to become rather low owing to prevailing mid-upper level height rises, overall parched air aloft, and surface high pressure centered over the northern Great Lakes. However, we will be keeping a close eye on the remnants of Beryl later in the week after it moves inland from the TX Gulf Coast. While it appears the remnants may track primarily south of our area, we cannot at this point rule out the possibility of some showers and/or storms making it into parts of the area for a period later in the week. Nevertheless, at this time we will continue with a more optimistic and mainly dry forecast for Thursday into Friday. Temperatures through the period are expected to remain seasonably mild, with daily highs generally in the 80s. However, onshore flow Tuesday PM through Friday will result in locations near the Lake Michigan shore remaining several degrees cooler. KJB && .AVIATION /06Z TAFS THROUGH 12Z SUNDAY/... Issued at 1227 AM CDT Sat Jul 6 2024 Key messages: - A brief (1 to 3 hour) period of MVFR CIGs and north- northeasterly winds early this morning. - Easterly wind shift with lake breeze expected at main Chicago terminals this afternoon. Currently a weak back door front is shifting southward across northeastern IL. While this is not expected to cause a major change in the going forecast, it does appear that a wind shift to a north-northeasterly direction will occur at the main terminals for a brief hour or two period just after 06z early this morning as the boundary passes. This wind shift is also likely to be accompanied by a short period of MVFR CIGs (2,000-3,000 ft AGL). Opted to message this wind shift within the going tempo for MVFR CIGs from 06-09z. Otherwise, prevailing winds will settle back into a light (8kt or less) northwesterly direction by daybreak, and will remain that way through at least early this afternoon. Thereafter, a lake breeze is expected to shift inland across northeastern IL. This will result in an easterly wind shift at ORD and MDW sometime in the 20-21z timeframe. Winds are then expected to become very light (under 5 kt) from the south-southeast tonight. KJB && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago