Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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259 FXUS63 KLOT 020603 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 103 AM CDT Tue Jul 2 2024 .KEY MESSAGES... - Dangerous swimming conditions Tuesday morning for the Lake County (IL) shoreline. - Typical July warmth and humidity through the week with periodic chances for showers and storms Tuesday through Friday - Showers and storms are expected to move across the area Tuesday night through Wednesday morning. Some storms could be strong to severe. && .DISCUSSION... Issued at 304 PM CDT Mon Jul 1 2024 Through Tuesday: Convective debris clouds continue to filter over the region as a surface high drifts eastward across the Great Lakes region this afternoon. Expect partly to mostly cloudy skies tonight combined with steady S/SSE flow to support temps in the upper 50s to low 60s. With a slight onshore flow increasing over southern Lake Michigan tonight, dangerous swimming conditions will develop from building waves and long-shore currents along Lake (IL) County beaches from around sunrise through late Tuesday morning. While a NW to SE oriented arc of low-level isentropic ascent will lift across northern Illinois late tonight through sunrise Tuesday, it appears moisture will be insufficient for saturation and realization of elevated instability from marginal mid-level lapse rates. Have put slight chance of thunderstorms in the forecast roughly north of a Mendota to Chicago line from around 3-7am. Effective shear around 30 knots with low MUCAPE suggests the primary threats will be limited to perhaps very small hail and some sporadic gusts under 40mph. An area of convection is expected to develop across northwest KS and southwest NE late this afternoon, with the remnant convective wave progged to lift ENE across southern Wisconsin late Tuesday morning into the mid-afternoon. Marginal lapse rates around 6-6.5 C/km, poor deep layer moisture profiles, and stagnant if not slightly rising mid-level heights all point toward a notable weakening trend with any residual convection as it shifts toward eastern Iowa Tuesday morning. However, continued low-level isentropic ascent extending southward into far northern Illinois supports maintaining a chance of showers and some embedded thunderstorms north of a Dixon to Evanston line through the afternoon hours on Tuesday. Kluber Tuesday Night through Monday: Convection will be ongoing across Iowa and into western Illinois Tuesday evening as a cold front pushes east through the Mississippi River Valley. This convection will advance towards the CWA through the evening and into the overnight hours. Dry air in the low levels and poor mid-level lapse rates (5-6 C/km) will limit instability from building in our area. This will result in the storms to be weakening as they reach the western portion of the CWA, however some storms producing severe weather is not out of the question given deep layer shear values of 40-45 kts. SPC has placed the far western portion of our CWA in a level 1 out of 5 risk of severe weather for this threat. The cold front should pass through the area sometime early Wednesday morning with shower and storm chances dropping off through the morning for most of the area. Wednesday afternoon is expected to be dry for most areas though a few lingering showers are possible. Despite the frontal passage temperatures will still climb into the mid-to-upper 80s with dew points in the low 70s making conditions hot and humid. Model guidance depicts the front slowing down and eventually stalling out just south of the area through central Illinois. A couple of the models show the possibility of the front actually stalling out within the southern third of our CWA. If this scenario were to play out then showers and storms would likely continue through the day on Wednesday, though they would primarily be confined to areas south of I-80. On Thursday cyclogenesis in the lee of the Rockies will latch onto the stalled front and allow it to slowly meander back to the north as a warm frontal boundary. Modest instability (1000-2000 J/kg of MLCAPE) and deep layer shear (25-35 kts) will develop behind the front, in the open warm sector of the developing low pressure system. Thus, depending how far north the warm frontal boundary is able to traverse, this could put the area under the gun for another round of showers and storms on Thursday. For now have gone with likely (50-70%) PoPs across the southern CWA with chance (30-50%) PoPs north of I-80, however, as previously mentioned, this is highly dependent on the ability of the boundary to move northward. Chance (30-40%) PoPs will once again be over the entire area on Friday as the cold front of the low pressure system moves across the area. A plume of modest (6-7 C/km) mid-level lapse rates and instability (1000-1500 J/kg of MLCAPE) are expected to precede this frontal passage, bringing the potential for thunderstorms. However, uncertainty still remains in the timing and progression of this front and system. Moving into the weekend, dry conditions are expected behind the cold front with temperatures in the low-to-mid 80s. Chances for showers and storms return late Sunday into Monday as there is a signal for a shortwave to move across the area, although uncertainty is quite high in how this will play out. Carothers && .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 from 5 AM early this morning to noon CDT today for ILZ006. IN...None. LM...Small Craft Advisory from 5 AM early this morning to noon CDT today for Winthrop Harbor to Wilmette Harbor IL. && $$ Visit us at weather.gov/chicago