Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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175 FXUS63 KLOT 040809 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 309 AM CDT Thu Jul 4 2024 .KEY MESSAGES... - Showers and isolated thunderstorms mainly along and well south of I-80 mid morning into early afternoon with brief moderate rainfall - Isolated thunderstorms late this afternoon into early evening, low confidence on coverage. - Mostly dry conditions for the weekend, but there will be increasing rain chances late Sunday through Monday. - Seasonable temperatures will continue through next week. && .DISCUSSION... Issued at 309 AM CDT Thu Jul 4 2024 Through Friday: The first weather feature of interest for Independence Day is a convectively induced vort/MCV across northern Kansas/western Missouri. Satellite/radar depiction also nicely portrays some warm advection induced showers and thunderstorms ahead of a broad upper trough axis back across the northern Rockies. At the surface, a cold/stationary front extends from southern Missouri northeastward to the southern/eastern Great Lakes, with a high PWAT airmass along and south of the front feeding into the southwest flow. The lead upper jet core will drive through northern IL this morning and with the continued southwest flow this morning, the frontal boundary will lift a bit farther north as well and try to bring some precipitation with it. Upper air analysis depicts a relatively dry airmass below 500 mb, and there is some ridging closer to the surface, analyzed in the 850-925 mb and surface charts. A bit deeper saturation is forecasted to be focused south of I-80 in response. There may be a decent amount of echoes on the radar later this morning initially with the warm advection, however with the dry air coupled with mid level lapse rates being quite poor and not very strong low level winds, whatever reaches the ground will be more showery/sprinkely in nature. The main convectively induced wave may try to drive some of this farther north in the late morning to early afternoon. Whatever does make it through could have some brief moderate rainfall given PWATs around 1.75", especially along and south of Highway 24. With the passing of the main wave, nebulous forcing would then be limited to the front and the lake breeze boundary. Thus we would expect precipitation coverage would still be somewhat limited, and thunder coverage would not be high either - isolated at best. For tonight, the mid/upper level trough axis will encroach on the mid Mississippi Valley. Associated height falls and upper jet segment look to lead to an expansion of showers and potentially a few thunderstorms along the frontal boundary. There is some uncertainty as to how quickly this will expand into northern Illinois, plus the better warm advection Chances look to be around 15-30% by mid evening (9 PM) along the I-39 corridor, with higher chances later. Coverage would expand more so late in the evening and overnight toward the Chicago area. The main low will pass to our north and the stronger warm advection will be across central Illinois in Indiana, thus we get somewhat dry slotted, but will need to maintain some widely scattered shower chances into Friday morning mainly along and east of I-55. Scattered showers would then spread in from the northwest to areas north of I-80/88 in the afternoon as the trough axis will be passing over area. The core of colder temperatures aloft do slide by across Wisconsin, thus the thunderstorm probability is low (20% or less). It will also be breezy behind the system as the surface low continues to deepen across lower Michigan. KMD Friday Night through Wednesday: The upper-level trough that brought us the rain chances on Friday will begin to pivot into Lower Michigan and the eastern Great Lakes Friday night. As this occurs, modest upper-level height rises are expected to develop overhead by Saturday morning which will diminish any lingering showers Friday evening and promote rain-free conditions through the day on Saturday. Given that the area will also be behind a cold front during this time, dew points should fall into the lower 60s generating modest humidity levels. Though temperatures are still expected to be in the seasonable category with readings forecast to be in the upper 70s and lower 80s Saturday afternoon and low to mid-60s both Friday and Saturday nights. The modest ridging is forecast to drift eastward Saturday night into Sunday as an upper-level shortwave trough ejects into the upper Midwest. While initially it looked as if this trough would struggle to have much rain with it, guidance has started to show a complex of showers and thunderstorms developing over the northern Plains Saturday night and moving eastward towards IL by Sunday morning. However, the dry low-levels and resultant sparse instability forecast to be over northern IL Sunday morning should weaken (and possibly erode) any showers/storms as they cross the Mississippi River. Thus, suspect that any rain chances Sunday morning will remain low (<15%) for areas east of I-39, but a stray shower in our far western CWA (west of I-39) cannot be ruled out. Heading into Sunday afternoon, the aforementioned shortwave is forecast to pivot into the northern Great Lakes as a secondary trough pivots into southern WI. This second wave is what previous forecasts have been noting could bring us some scattered showers and/or thunderstorms Sunday afternoon. While guidance does continue to produce some slight chance to chance POPs (20-30%) across northern IL and northwest IN, the limited low-level moisture and dry mid-levels may prove sufficient to hold any rain off until better moisture arrives Sunday evening. Therefore, expectation for many dry hours on Sunday remains valid with temperatures forecast to be slightly warmer than Saturday as highs warm into the low to mid-80s. Rain chances are expected to increase Sunday night as the aforementioned trough becomes more situated overhead and better moisture filters in at the surface. This trough looks to be notably slower moving with guidance continuing to fester showers and storms overhead through much of the day on Monday and possibly into Tuesday. Therefore, have maintained the 40-50% POPs offered by the NBM for this period which looks reasonable. While rain chances should diminish on Tuesday as the core of the trough pushes east, guidance does show at least broad troughing hanging around the Great Lakes through much of next week. So additional periods of showers and/or thunderstorms may linger throughout the week depending on available moisture and instability. Otherwise, expect temperatures to remain near typical early July readings in the low to mid-80s. Yack && .AVIATION /06Z TAFS THROUGH 12Z FRIDAY/... Issued at 102 AM CDT Thu Jul 4 2024 Forecast concerns for the terminals... - Lake breeze turning winds easterly at Chicago area terminals Thursday afternoon - Spotty showers and/or storms possible Thursday afternoon, but better chances after midnight Thursday night Generally quiet weather conditions are expected for tonight into Thursday morning with light west-northwest winds and BKN to OVC VFR cirrus. A thunderstorm complex, currently over eastern Kansas, is expected to move into central IL around daybreak Thursday morning which may develop some isolated showers along a stalled frontal boundary roughly from a KPNT to KRZL line. While this should keep any notable rain south of the terminals, some guidance is trying to bring sprinkles into at least the Chicago sites but confidence on this occurring is low. Furthermore, guidance also continues to hint that some spotty showers and/or thunderstorms may attempt to develop across northern IL near a lake breeze Thursday afternoon. However, given the rather meager instability and lack of robust forcing suspect the guidance is overdone and put the chance of any showers/storms Thursday afternoon <20%. Therefore, have maintained dry forecasts at the terminals during this timeframe. Given that dry conditions should prevail through Thursday afternoon, the main forecast concern is the aforementioned lake breeze that is expected to develop and move through the Chicago terminals during the late morning and early afternoon. As per typical, guidance does vary a bit on exact timing but the advertised easterly wind shifts at ORD, MDW, and GYY between 14z and 16z seem reasonable. Elsewhere, winds will settle into a southeasterly direction Thursday afternoon with speeds remaining generally under 10 kts areawide. Heading into Thursday evening, a more robust upper disturbance is expected to pivot into the Upper Midwest and move a cold front through northern IL and northwest IN late Thursday evening and overnight. The warm and humid airmass ahead of the front should allow showers and thunderstorms to develop along the front and persist overnight as it tracks through the area. However, timing on when exactly the front and showers/storms will arrive remains uncertain especially near RFD. Therefore, have decided to maintain a dry forecast at the 24-hour TAF sites for now but did introduce a PROB30 at ORD and MDW where confidence is slightly higher. Yack && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago