Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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537 FXUS63 KLOT 041530 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1030 AM CDT Thu Jul 4 2024 .KEY MESSAGES... - Showers/rain this morning, highest coverage along/south of I-80. Diminishing threat for late-PM/early-evening storms. - Mostly dry conditions for the weekend, but there will be increasing rain chances late Sunday through Monday. - Seasonable temperatures will continue through next week. && .UPDATE... Issued at 1029 AM CDT Thu Jul 4 2024 A respectable mid-level wave/remnant MCV continues to shift across central Illinois late this morning--a northward jog from earlier expectations. The result of this has been to toss a shield of stratiform rain into our area, and earlier updated PoPs to reflect greater coverage/chances, particularly south of about I-80 this morning. Farther to the north, a more appreciable dry layer under 10,000 feet as seen on Davenport`s 12z sounding has helped curtail the northward progression of the stratiform precip shield, although have recently seen evidence of sprinkles/light rain breaching this dry layer up towards about I-88. The latest forecast will reflect 20 to 50 percent chances north of I-80 into the early afternoon, with likelies and categoricals farther south towards central Illinois. The main area we`ll be monitoring for an embedded thunder threat will be across our far south (Ford - Benton counties) early this afternoon in the vicinity of a weak meso low which may drag a higher theta-e airmass northward. Latest observations and satellite imagery, however, suggest the main corridor of concern may remain south of the forecast area entirely. Based on the current wave`s progression, it looks like the bulk of precipitation should come to an end through the mid afternoon, which may end up leaving us with a quiet late afternoon and early/mid evening across the entire area. Given the expansive cloud cover, temperatures may struggle to get into the mid 80s, which will result in much lower instability and increased MLCIN. This, in combination with likely mesoscale subsidence on the backside of the departing wave suggests very limited to nil storm coverage into the mid evening. Carlaw && .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 /12Z TAFS THROUGH 18Z FRIDAY/... Issued at 648 AM CDT Thu Jul 4 2024 Forecast concerns for the terminals... - Lake breeze to turn winds easterly at Chicago terminals later this morning - Low end chance for showers and storms along the lake breeze this afternoon with better chance for showers late tonight into early Friday morning - Lowering ceilings with showers overnight through Friday morning - Breezy westerly winds Friday afternoon Generally quiet weather conditions are expected at the terminals this morning as light north-northwest and VFR cirrus prevail. Though, winds will gradually become easterly at ORD, MDW, and GYY late this morning as a lake breeze develops and surges inland. While the more pronounced winds with the lake breeze may hold off until 18z-19z this afternoon, the wind shift looks to occur between 14z and 15z. Elsewhere winds will become more southeasterly this morning before turning southerly this afternoon and evening. Dry conditions should prevail through early evening at the terminals, but the potential for isolated showers and thunderstorms to develop along the lake breeze remains. Given that instability this afternoon looks modest and the lack of upper-level forcing, confidence in any showers/storms developing this afternoon remains too low for a formal mention at this time. However, better shower and thunderstorm chances are expected late this evening and overnight as a cold front moves through the area. While confidence is higher on rain occurring overnight, the timing and coverage of thunder tonight at the terminals remains uncertain. Therefore, have decided to maintain the PROB30s for -SHRA at the Chicago area terminals but did introduce a PROB30 for TSRA at RFD where potential looks to be a bit higher. Regardless, showers/storms should come to an end around daybreak Friday morning leaving mostly dry conditions for the rest of the TAF period. Additionally, ceilings are also expected to lower with the showers overnight and persist into Friday morning. Guidance remains in decent agreement that MVFR ceilings will be the most common, but pockets of IFR are possible especially near RFD. While ceilings should lift back to VFR by mid-morning on Friday, there is a signal that the lingering upper trough overhead may keep lower ceilings around through the rest of the period. Since winds are also expected to increase Friday morning I have decided to lean more towards the quicker improvement. Finally, winds will become westerly behind the front Friday morning with speeds increasing into the afternoon. Gusts are expected to top out in the 20 to 25 kt range Friday afternoon but there is the potential for gusts to near 30 kts at times. Yack && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago