Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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052 FXUS63 KLOT 220552 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1252 AM CDT Sat Jun 22 2024 .KEY MESSAGES... - Potential for strong to severe storms with damaging winds, torrential downpours and localized flooding late Saturday afternoon through Saturday evening. - A brief reprieve in heat and humidity is expected Sunday and Monday, before it returns (with chances for thunderstorms) on Tuesday. && .DISCUSSION... Issued at 305 PM CDT Fri Jun 21 2024 Through Saturday night: The remainder of the afternoon will remain very warm and humid, especially away from the cooling influences of Lake Michigan. Expect low to mid 90s inland, with 70s to low 80s closer to the lake. Capping is expected to curtail thunderstorm develop across a majority of the area through the remainder of the. The only exception being areas across far northern IL near the WI state line, where capping is weaker thanks to a corridor of higher surface dew points around 70. Accordingly, a few isolated showers and storms cannot be ruled out in this area late this afternoon into early evening. However, expect the better focus for these storms to largely remain north of the WI state line into this evening. Warm air advection will increase tonight along an strengthening southwesterly low-level jet across much of the Corn Belt, as low pressure shifts out across the Mid-Missouri Valley. This is expected to foster an significant increase in the areal coverage of elevated thunderstorms tonight, particularly across IA eastward across southern WI. While the primarily focus for these storms will be north of the area tonight, it does appear that a period of showers and some scattered storms may develop into northern IL after midnight tonight. Severe weather is not anticipated with these storms, though a high PWAT environment (values around 2") will support the potential for some torrential downpours with any storms. Another very warm (hot) and humid day is on tap for the area on Saturday, as temperatures areawide top out in the low to mid 90s. However, unlike today, offshore gusty southwest winds up to 30 mph on Saturday will curtail any chances for lake cooling near the shore, so expect these very warm conditions to extend all the way to the lake. The threat of thunderstorms looks rather low during most of the day on Saturday as we await the arrival of an upper trough and surface cold front expected late Saturday into the evening. While this is the case, severe storms are expected to develop to our west-northwest Saturday afternoon as the moist airmass in advance of the approaching cold front destabilizes through the day. A favorable kinematic environment with effective shear values up to 40 kt will support organized storm structures and clusters capable of producing strong damaging wind gusts. Damaging wind gusts are likely to remain the primarily threat with these storms as they shift eastward across southern WI and northern IL Saturday evening. Rather poor mid-level lapse rates in combination with a very high PWAT airmass (up to 2.25") suggests the threat of hail will be rather low outside of any well developed supercell structures. Torrential downpours will also accompany these storms given the near record PWATs. Accordingly, even short instances of training storms could result in localized corridors of 2-3"+ amounts and possible flooding. This will especially be problematic if it occurs over the urban areas in and around Chicago and Rockford. The arrival of these storm clusters could be as early as ~5pm Saturday afternoon across northwestern parts of the area (including the Rockford area), and as early as 6 or 7 pm in and around the Chicago metro area. This threat will then come to an end from northwest to southeast later in the evening. KJB Sunday through Friday: Behind the frontal passage early Sunday morning, temperatures and humidity finally turn more seasonal, with highs back in the 80s Sunday and mid 80s to 90 Monday...with upper 70s to low 80s along the lakeshore. Northwest winds arrive on the backside of the frontal system, lingering into the start of the new work week, before sfc high pressure drifts over the Great Lakes. While this sfc high drifts into the region, a rapid moving upper level low will track across the northern Great Lakes into the ne CONUS. With sfc high pressure around during this timeframe, limited moisture at the sfc should prevent any precip from developing. The sfc high will then be ushered quickly to the mid-Atlantic coast Tuesday as the next frontal system deepens across the upper Midwest. Southwest winds return Tuesday ahead of the main frontal boundary, bringing a brief return of hot and humid conditions to the region. Heat indices are currently forecast in the upper 90s to low 100s, with high temps in the mid 90s. The low pressure system driving this frontal boundary will track eastward across southern Canada, while the trailing boundary will drape through from the Great Lakes into the eastern Plains. Dependent on moisture axis flowing into the region during Tuesday, could produce early pre-frontal showers and storms across portions of the Midwest, with focus on southern Great Lakes into the mid- Mississippi River Valley. The main wave of showers and storms is forecast to track through the area late Tuesday evening into early Wednesday morning. Some model depictions are showing a weak (almost non-existent) upper level wave developing as this system moves out of the southern Great Lakes into the Ohio River Valley region, with a slow southward progression Wednesday could occur. This would linger precipitation chances through Wednesday morning for areas south of the I-80 corridor. After the front pushes out of the region, the remainder of the week looks dry with seasonal conditions prevailing across the region. Baker && .AVIATION /06Z TAFS THROUGH 12Z SUNDAY/... Issued at 1252 AM CDT Sat Jun 22 2024 Aviation Key Messages Include: - Scattered showers and a few thunderstorms will move near RFD/DPA/ORD this morning (high confidence). - Breezy southwest winds with gusts of 25-30kt will prevail from mid-morning through the evening (high confidence). - A line of thunderstorms will sweep across the terminals this evening (high confidence) accompanied by northwesterly winds gusting 25-35kt (medium confidence), and trailed by MVFR cigs (medium confidence). Discussion: Through 12Z: Ongoing scattered showers and thunderstorms over northwestern Illinois will continue moving slowly northeastward throughout the night. Opted to introduce targeted TEMPO groups between 07-09Z at DPA/ORD for TSRA. The showers and thunderstorms may graze the 10-mile vicinity ring of MDW, but for now, opted to keep all precipitation out of their TAF this morning. Meanwhile, at RFD, will go out the gate with VCTS through 09Z as the current cells appear to be on a trajectory to just miss the airfield. Outside of showers and thunderstorms this morning, a slowly- moving outflow boundary with a northeasterly wind shift is in the process of stalling along a line from Elgin to MDW. Confidence is low on whether it will actually reach MDW, though if it does, speeds should be less than 5 kt (given a forward motion of the boundary of a mere 3 kt). Finally, will have to keep a close eye on an area of marine mist and LIFR/IFR cigs lagging behind the outflow boundary near PWK. The forward progress of the BR/IFR cigs is slowing, so confidence is cautiously growing that it will not reach ORD, let alone MDW. Regardless, trends will be monitored throughout the night (visby may lower to 2-3SM and cigs may lower to 400-500ft if the bank does reach ORD). After 12Z: Southwest winds will become increasingly breezy after sunrise as the low-level pressure gradient gradually packs overhead. Confidence is high in gusts over 25kt, and medium in a few gusts nearing 30kt, particularly this evening. The expectation is that all terminals should be dry from daybreak through at least mid-afternoon. However, will have to keep an eye on ongoing thunderstorms in northwestern Iowa for signs that they build southward into the feeding LLJ and roll along the Wisconsin state line from mid-morning through mid-afternoon. In fact, there may be a somewhat continuous west-to-east line of convection near the Wisconsin state line at some point this afternoon. All things considered, felt introducing a 6-hour PROB30 for TSRA at RFD from 16-22Z was appropriate given a low confidence but high impact event. This evening, a line of thunderstorms is then expected to sweep southeastward across the terminals (generally between 23Z to 05Z across the airspace). Confidence is high enough in thunderstorms to convert the inherited PROB30 groups into targeted TEMPO groups, albeit maintaining longer than needed 4-hour long windows to account for continued wiggle room in the time of arrival of the line. Later TAF packages should offer refinements in the timing (to be 2 hours or so) at all terminals. A northwesterly wind shift with gusts of 25 to 35kt appears likely with the line of storms. Gusts may even locally exceed 40kt (particularly at RFD). In addition, visibility may drop to 2 miles or lower in the most vigorous convective cells given very high environmental moisture content. Confidence is high that there will be thunderstorms this evening, and medium in the magnitude of wind gusts and drops in visibility as they arrive. Finally, cigs will attempt to build downward into MVFR behind the line of storms this evening and prevail through the remainder of the TAF period. In fact, pockets of IFR may develop, as well. Confidence is medium on cig trends after 00Z. Borchardt && .CLIMATE... Issued at 337 AM CDT Fri Jun 21 2024 Here are the daily high and warmest low temperature records for Chicago and Rockford for today and tomorrow: Chicago ----------------------------- Day: 6/21 6/22 ----------------------------- Record High: 101 97 Record Warm Low: 74 76 Rockford ----------------------------- Day: 6/21 6/22 ----------------------------- Record High: 100 97 Record Warm Low: 71 73 NWS Chicago && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Dense Fog Advisory until 4 AM CDT early this morning for the IL nearshore waters. && $$ Visit us at weather.gov/chicago