Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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552 FXUS63 KLOT 020802 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 302 AM CDT Tue Jul 2 2024 .KEY MESSAGES... - Showers and isolated thunderstorms this morning, mainly north of I-88 - Additional showers/storms expected to reach the I-39 corridor this evening, then decreasing in coverage and intensity as they attempt to move east toward/east of the I-55 corridor - Typical July warmth and humidity through the week with periodic chances for showers and storms late Wednesday night through Friday && .DISCUSSION... Issued at 302 AM CDT Tue Jul 2 2024 Through Wednesday: Shortwave trough extending from southern MN to southwest Iowa will continue to move east-northeastward today across the western Great Lakes. There appears to be at least a couple of small MCVs embedded within this larger trough with fairly widespread convection from WI southwestward across IA. This activity is being fed by an axis of higher moisture and moderate elevated instability which is gradually spreading east toward northern IL. Observational trends and short range model guidance suggest that this convection will outpace the better moisture and instability as it spreads eastward. That, plus weakening moisture transport and convergence due to the diurnal weakening and veering of the low level jet should result in convection weakening as IA convection spreads east into IL early this morning. Given current radar trends and latest CAM guidance, have bumped up pops this morning north of I-88 where some showers and perhaps a couple thunderstorms are looking increasingly likely, particularly western CWA. Shortwave trough and embedded MCVs should push east of the area this afternoon likely bring an end to the rain by late morning or early afternoon. Clearing skies and strengthening warm air advection with brisk southerly winds should help boost high temps well into the 80s this afternoon most of the area. Pair of shortwave troughs, one over southern Colorado and a second more robust one over Wyoming, will both translate east and northeastward into the Midwest later today. Combination of these waves and heating/destabilizing air mass ahead of an associated cold front should allow for strong-severe convection to break out over northern MO into IA later this afternoon. While this activity is expected to push eastward toward northern IL this evening, guidance is in strong agreement that it will encounter an increasingly hostile environment likely leading to substantial weakening possibly its demise overnight. Have sharpened up the pop gradient from east to west across the CWA tonight, maintaining categorical pops by evening toward the I-39 corridor, tapering off to just chance/slight chance pops late tonight into northwest IN and adjacent portions of eastern IL where rain is looking pretty unlikely tonight. There has been a sizable trend in guidance toward pushing the effective surface boundary safely south of our CWA Wednesday afternoon, taking the threat of re-newed storm development along the front during the afternoon out of our CWA. Did maintain some chance pops southeastern CWA in case front slows and guidance reverse course. Also, kept some slight chance pops for showers in the morning, in case any lingering showers can survive into the morning hours. Increasingly, it is looking like all or most of our CWA will be rain free Wednesday. While front will move across the area Wednesday, in true summer cold front fashion, the cooler and drier air will lag well behind the front. So with skies clearing out Wednesday, it should be another warm to very warm day with highs well into mid-upper 80s and moderate humidity with afternoon dewpoints in the 60s. - Izzi Wednesday Night through Monday: The front is expected to stall out south of our area Wednesday night which should yield mostly dry conditions. Despite the area being north of the front, dew points are expected to remain in the mid to upper 60s Wednesday night which won`t allow for much cooling with temperatures projected to bottom out in the upper 60s to around 70s by Thursday morning. While our area remains dry, guidance continues to show a storm complex developing across the central Plains and tracking eastward along the stalled front Wednesday night into the day on Thursday. Given that guidance continues to favor the front stalling more into southern IL, the bulk of any associated shower and/or storm activity from the aforementioned complex should remain south of our area. However, the front will be trying to lift back northward during the day on Thursday which may allow a few storms to materialize in our southern CWA Thursday afternoon. Therefore, have maintained some 30 to 40 percent chances for thunderstorms for areas south of I-80 on Thursday, but suspect we may be able to reduce these values further if new forecast trends continue to favor the southern solutions. Regardless, many dry hours our expected for outdoor July 4th festivities, especially for areas north of I-80, with high temperatures in the low to mid-80s. Heading into Thursday night, an upper-level trough is progged to pivot into the Upper Midwest and eventually the Great Lakes by Friday morning. As the trough pivots overhead, a cold front is expected to develop near the Mississippi River and get pushed southward through northern IL and northwest IN Thursday night into the day on Friday. Given that the environment ahead of the cold front is forecast to remain warm and humid, showers and thunderstorms are expected to develop along the boundary and track across the area. While there is still some timing uncertainties as to exactly when the front and associated precipitation will arrive Thursday night, the general consensus is that the front should hold off until after 10 PM Thursday evening. Therefore, suspect that conditions for viewing fireworks should be decent aside from increasing cloud cover ahead of the front but still recommend keeping an eye on forecast trends in case something changes. Showers and thunderstorms are expected to persist through much of the day on Friday as the aforementioned front continues to progress its way across the area. Though, precipitation is forecast to gradually taper from northwest to southeast Friday evening as the front exits. A surface high is then expected to move overhead in the wake of the front Friday night and persist through a good portion of the upcoming weekend. However, guidance continues to show the upper trough stalling over the Great Lakes through the weekend which may be able to kick off the occasional shower and/or thunderstorm if sufficient instability can materialize. Given that this period is still 5+ days out, have maintained the 20 to 30 percent POPs offered by the NBM but suspect that these POPs may be overdone since the region will be under northwest flow and beneath rather dry mid- levels. Regardless of how the precipitation chances play out, temperatures and humidity are expected to remain near typical early July readings with daily highs in the low to mid-80s and dew points in the 60s to around 70. Yack && .AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/... Issued at 103 AM CDT Tue Jul 2 2024 Forecast concerns for the terminals... - Periods of showers tonight through Tuesday morning, especially near RFD - Breezy south-southeast winds Tuesday afternoon - Another round of showers and thunderstorms Tuesday evening and overnight with a cold front Showers continue to develop ahead of an upper-level disturbance that is pivoting into western Iowa. However, the copious amounts of dry air in the sub-cloud layer are preventing many of the showers over northern Illinois from reaching the surface. The expectation is for this dry air to gradually erode across northern Illinois as the disturbance gets closer around daybreak which should allow a better coverage of showers to be observed through Tuesday morning, especially for areas in and around RFD. While some guidance continues to suggest that the shower coverage should make it into ORD, MDW, and DPA; confidence on this occurring remains low due to aforementioned dry low-levels and lack of more defined forcing with southward extent. Though, there was at least enough of a signal in guidance to justify the inclusion of a VCSH mention at ORD, MDW, and DPA for a few hours Tuesday morning. Any showers that do materialize Tuesday morning are expected to diminish by 18z at the latest leaving dry and VFR conditions for the rest of the afternoon. However, winds will become more south-southeasterly and increase in speed with gusts in the 20-25 kt range expected through the afternoon. Gusts will subside Tuesday evening as directions become due south and eventually southwesterly ahead of a cold front. Finally, another round of showers and thunderstorms is expected to develop along the cold front Tuesday evening and persist through the overnight hours. While guidance continues to vary on the magnitude and southward extent of the better instability, confidence is sufficient to introduce PROB30s for thunder at RFD, ORD, and MDW. Note DPA and GYY do not have any mention for this second round due to it being beyond their respective TAF periods, but will likely need to add with next issuance. Regardless of the extent of thunder, light to moderate showers are expected at the terminals into Wednesday morning as the front moves through. Additionally, there is also the potential for some MVFR ceilings (and visibilities) with the showers/storms Tuesday night. Yack && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Beach Hazards Statement until noon CDT today for ILZ006. IN...None. LM...Small Craft Advisory until noon CDT today for Winthrop Harbor to Wilmette Harbor IL. && $$ Visit us at weather.gov/chicago