Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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814 FXUS63 KLOT 041948 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 248 PM CDT Thu Jul 4 2024 .KEY MESSAGES... - Lingering showers, possibly an isolated storm, this afternoon with diminishing coverage through this evening. Additional storm chances arrive after midnight tonight. - 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 202 PM CDT Thu Jul 4 2024 Through Friday night: The earlier cohesive/widespread light-moderate rain has diminished. With several vorticity maxima and embedded jet streaks still in the vicinity, however, additional isolated to scattered showers will likely continue this afternoon. Locally greater coverage appears plausible 1) near/north of the WI state line in the vicinity of the lake breeze, 2) near the I-39 corridor and points west where a bit more instability resides, and 3) well south of I-80 where broad warm advection remains north of a modest 25 knot southwesterly 850 mb LLJ in the vicinity of a subtle surface boundary. While we can`t rule out a few lightning strikes, forecast soundings depict gradually- warming and drying mid-levels through the rest of the afternoon, likely in response to developing subsidence on the backside of the departing parent shortwave. Localized downpours will be possible owing to very slow cell motions. Finally, while not an overly-conducive setup, we`ve received several funnel cloud reports from central Illinois. Based on radar motions, there`s clearly a subtle boundary that showers are tied to, providing a little low-level vorticity/spin boost, so this potential will continue until showers diminish. Through the 5-7 PM timeframe, it looks like most if not all of our forecast area will trend precipitation-free as subsidence shifts across the region yielding favorable conditions for firework displays. Overnight (likely after midnight), shower and storm chances will increase as the shortwave currently pinwheeling into western Minnesota scoots east across Iowa. Have delayed the arrival time of precip chances based on latest guidance which generally matches current upstream conditions well. Suspect the greatest storm coverage will focus north of I-88 in closer proximity to large scale forcing, but sufficient MUCAPE exists for isolated to widely scattered storms to "zipper" farther south ahead of the incoming front/trough. While mid and upper winds are strong, shallow effective inflow depths and equilibrium levels should curtail storm organization. Can`t rule out a stronger cell with some small hail/gusty winds, however. Activity will push east of the region through Friday morning. By midday and into the afternoon, scattered showers will develop in the deeply mixed cold advection regime with coverage increasing the farther north of I-80 you go. Continue to advertised isolated thunder chances (20%), but guidance does depict a region of mid- level warming which may tend to inhibit more widespread electrification. Activity will tend to gradually diminish through the evening hours. Carlaw Saturday through Thursday: Following a likely quiet Saturday, a unsettled period is in store Sunday through Tuesday. On Saturday, a weak trough axis will push southward and weak high pressure over southern Lake Michigan in the afternoon will enable a lake breeze to develop and push inland. Despite relatively cool mid-level temps, height rises and dry mid-level air should preclude any showers developing out of deeper Cumulus. Highs will be a bit below normal, in the upper 70s to lower 80s inland, and mid 70s lakeside. Dew points in the lower 60s will make it feel seasonable. On Saturday night, while it will quiet and seasonably mild locally, attention will turn to the possible development of an at least loosely organized convective complex west of the MS River from northwest Missouri to southern Minnesota. This activity will be associated with a short-wave pivoting toward Lake Superior. It appears probable that the showers and thunderstorms will be on a weakening trend as they outrun the instability axis while tracking off to the east-northeast in the morning. Highest PoPs in the 30-50% range for the northwest 1/4 to 1/3 of the CWA in the morning fit the above thinking reasonably well. There may be a window toward mid day into the early afternoon, possibly aided by remnant outflow from the decaying morning convection, for widely scattered showers and thunderstorms to develop northwest of I-55 as MLCAPE in that area builds to up to 1000 J/kg with minimal capping. A sharp CAPE gradient to the southeast should greatly limit any development for the southeast 1/2 to 2/3 of the CWA. More pronounced height rises in the mid- late afternoon amidst modest mid-level flow and lapse rates only around 6C/km should cause showers and storms to diminish. Forecast highs on Sunday are generally well into the 80s, except along the immediate Illinois shore, with a bit more sticky feeling from dew points 2-5F higher than on Saturday. The Sunday night into Monday timeframe is appearing a bit messy as a slow moving cold front works its way eastward. Weak perturbations streaming over the area in southwest flow aloft should be able to generate another round of scattered showers and embedded thunderstorms late in the evening and overnight, with the highest PoPs/coverage (30-60%), near and northwest of I-55. Following a possible lull in coverage Monday morning, a slightly more robust impulse will encounter a minimally (if that) capped and moist air mass with temps in the 80s and dew points 65-70F. Scattered showers and non-severe thunderstorms appear probable (40-60% PoPs) in the afternoon into the evening. The trough axis to our west will finally push east Monday night, which may keep convection going a bit longer into the night than is typical diurnally. A cold front associated with the short-wave and surface low moving into the Great Lakes region Monday night will exit the CWA Tuesday morning. Despite the frontal passage, relatively cool mid-level temps associated with the short-wave trough and dew points well into the 60s should yield modest destabilization and the potential for isolated to widely scattered showers and thunderstorms (20-30% PoPs) from the late morning through mid afternoon. Mid-level height rises and parched air aloft will likely suppress thunderstorm chances on Wednesday and Thursday, and possibly precip. chances altogether. We are officially showing some low (20-30%) PoPs, however. Near seasonable temperatures in the lower to mid 80s away from the lake will continue. Castro && .AVIATION /18Z TAFS THROUGH 00Z SATURDAY/... Issued at 1247 PM CDT Thu Jul 4 2024 The primary aviation weather concerns for the 18Z TAF period include: * On and off pockets of non-impactful rain throughout the period * The likelihood of MVFR cigs beginning early Friday morning Intermittent periods of light rain or sprinkles will continue through this afternoon and into this evening with no related impacts expected. Additional pockets of light rain will be possible overnight, although perhaps the most appreciable chance comes ahead of an impending cold front in the roughly 08-12Z timeframe. At RFD, this potential will arrive closer to 06Z and a light thunderstorm cannot be ruled out up that way tonight either. Confidence is rather high in a several hour period of MVFR cigs beginning predawn on Friday and lasting through the bulk of the morning. Most models suggest the MVFR should clear away by noon but will likely return during the latter part of the afternoon as more pockets of rain look to move in. There is also a signal that the cigs refuse to budge during the afternoon and we remain MVFR throughout the day. Meanwhile, light southeasterlies will persist into this evening. Confidence on wind direction takes a tumble for the overnight period, but best guess is SSW while magnitude should remain below 10 kt. It`s very possible winds even go variable for several hours overnight. Winds will build through the morning on Friday as they establish a WSW direction and get gusting to around 20 kt by late morning. Doom && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago