Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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071 FXUS63 KLOT 040602 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 102 AM CDT Thu Jul 4 2024 .KEY MESSAGES... - Widely isolated thunderstorms possible south of I-80 Thursday morning and areawide by late afternoon. Conditions do not favor widespread thunderstorm potential. - For prime July 4th fireworks display time Thursday evening (through ~10 PM CDT), thunderstorm chances appear low but not zero, with higher t-storm coverage expected overnight/early Fri. - Next favorable window for scattered or higher thunderstorm coverage appears to be Sunday PM and on Monday. - Temperatures averaging near seasonable through mid next week. && .UPDATE... Issued at 814 PM CDT Wed Jul 3 2024 No significant changes to going forecast for tonight. Main focus continues to be on the potential for isolated (10-20% coverage) showers and thunderstorms south of about a Pontiac IL to Wheatfield IN line this evening, and along the WFO LOT/ILX border region overnight. Latest surface analysis depicts a weak, nearly-stationary cold front along the above mentioned line. Surface temps in the low-80s and dew points in the low-70s characterize the low-level air mass ahead of the front, though RAP forecast soundings indicate fairly poor lapse rates above a shallow boundary layer maintaining some weak capping. Low-level winds are very light, less than 10 kts on either side of the front, providing only weak low-level convergence as well. Mid- level winds (and resulting deep layer shear) have increased a bit since this afternoon however, in association with a weak mid-level short wave tracking east-northeast out of Missouri, which may yet aid in developing a few isolated showers or thunderstorms across our far southeast counties through mid-late evening and a couple of shallow, isolated showers have developed over far southeast Iroquois and Benton counties recently. The weak front is expected to slowly sag toward a Bloomington IL to Rensselaer line overnight, which along with the aforementioned mid-level wave, may result in a few additional isolated showers or storms across our far south cwa overnight. Going forecast has this well-covered, with slight (20%) pops along the boundary through the night. Forecast into Thursday continues to look reasonable as well, with a gradual increase of convective chances south of the Illinois and Kankakee river valleys by mid-late morning as the front begins to lift slowly back north in association with a potential MCV developing from activity currently ongoing across western KS. Ratzer && .DISCUSSION... Issued at 300 PM CDT Wed Jul 3 2024 Through Thursday: A cold front drifting SSE has continued to force isolated warm- core showers within an axis of pooling low to mid 70s dew points roughly south of a Pontiac to Valparaiso line. Modest mid-level warming around 500-600 hPa should greatly mitigate thunderstorm chances, but a few sporadic lightning strikes cannot be ruled out with the strongest cores. This activity will continue to shift SSE with the front into early evening. The cold front will stall/wash out across the far southern CWA and over central Illinois tonight. Isolated showers and perhaps a few lightning strikes cannot be ruled out at anytime tonight south of a Bloomington to Rensselaer line as the right entrance of a 80 knot upper speed max remains generally stationary above a weakly capped and higher theta-e airmass. A mid-level wave crossing southern Colorado this afternoon will force a cluster of convection along a boundary lifting northward across southern Kansas/Missouri overnight. Guidance depicts a decent amount of spread in the downstream trajectory of the (now convectively enhanced) wave is it tracks into central/southern Illinois by late Thursday morning. This is likely the result of the MCV favoring a WNW to ESE MUCAPE gradient toward far southern Illinois in opposition to the stronger WSW steering flow aloft directed into central Illinois. Overall expectations are for the better coverage of convection to remain south of the forecast area, but for the footprint of the wave to potentially force isolated to widely scattered storms south of the IL/Kankakee River Valleys late morning into the early afternoon. Beyond early afternoon, diurnal heating and modest low-level moistening will support a weakly capped airmass across much of the CWA. Overall forcing by this time looks to be quite nebulous with little mid to upper-level forcing in place. However, any subtle low-level convergent boundaries like residual outflows, an expected lake breeze, or axes of differential heating from scattered mid/upper-level clouds could ultimately force isolated showers or possibly a few storms mid-afternoon into early evening. This does not appear to be a wash-out by any means, but enough for those with holiday plans to keep mindful of forecast updates through the day. Kluber Thursday Night through Wednesday: Early to mid Thursday evening, surface low pressure will be centered over southern Minnesota, with its trailing cold front extending southward across central Iowa and northern Missouri. The parent short-wave trough will also be centered well back to the north and west, resulting in only minor mid-level height falls into the western Great Lakes. A majority of forecast soundings depict a largely capped environment, at least to any deeper convection capable of producing lightning. With relatively dry mid-level air being drawn northeastward and sunset, it`s questionable if just enough moisture may materialize at the right level to tap into 250-1000 J/kg of MUCAPE. Given the above with frontal convergence/forcing well west but modest large scale forcing and near climo PWATs, there may be spotty isolated to perhaps widely scattered showers (highest chance/30-40%) near and west of I-39 in the 7-10pm timeframe. Current signs point toward the peak fireworks display time probably being dry in much of the area, though as noted, can`t rule out a few showers in the area. Confidence is lower in any thunderstorms being able to develop, at roughly 15-30%, highest near/west of I-39. This period will needless to say be monitored closely. Shower chances may uptick some/ooze farther east in the late evening, though thunder potential is still questionable. As the cold front sweeps across the region after midnight, stronger large forcing (increased height falls and frontal convergence) and a bit better column moisture should more readily erode MUCIN to offset the diurnally unfavorable timing for numerous showers and scattered thunderstorms. A seasonably muggy night can be expected out ahead of the cold front, with temperatures in the 70s most of the night and dew points well into the 60s. On Friday, following a quieter balance of the morning, the seasonably stout mid- upper trough (500 mb heights in the mid 560s DaM) will translate east from Wisconsin to northern Lake Michigan. Steepened low-mid level lapse rates from cold air advection and maximized mid-level height falls should result in isolated to widely scattered showers near and north of I-80. Can`t rule out an isolated rumble of thunder or two, though diminishing lower level moisture (decreasing Td) looks to be a limiting factor. A fairly tight pressure gradient over the region and deeper mixing will result in westerly winds gusting up to 30-35 mph. Looking at highs in the mid 70s to lower 80s, coolest for the northwest 1/3 or so of the CWA. Friday night through Saturday evening will be quiet from weak surface high pressure sliding east across the area, along with seasonable temperatures, albeit with lower humidity levels. Broad west-southwest to southwest flow aloft and seasonable to above normal column moisture (PWATs 100-150% of normal) will likely open the door for periodic scattered shower and thunderstorm chances Sunday through Monday evening. While there may be sufficient deep layer shear for some storm organization during peak diurnal favorability, it doesn`t appear to be a favorable synoptic pattern for noteworthy severe weather. In the wake of the cold front passage Monday night, humidity levels may come down some, with primarily dry conditions. Castro && .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