Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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011 FXUS63 KLOT 190713 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 213 AM CDT Thu Sep 19 2024 .KEY MESSAGES... - Today will feature highs in the 80s, low humidity levels, and a lake breeze. - A few showers or thunderstorms may sneak into northern Illinois toward daybreak Friday. Additional showers and storms may develop during the afternoon along and east of I-55. - A pattern change is expected this weekend into early next week with the arrival of several waves of showers and storms (favoring Sunday and Monday). && .DISCUSSION... Issued at 213 AM CDT Thu Sep 19 2024 Through Wednesday: National water vapor imagery shows a wound-up low pressure system lifting northward into southern Saskatchewan, a second low pressure system drifting southeastward along the California Coast, and a broad remnant cyclonic circulation along the Atlantic Seaboard. A recent surface analysis depicts a deep plume of low-level moisture extending from the Gulf of Mexico through the central Plains and into southern Manitoba and southwestern Ontario, east of a surface cold front arcing southeastward from the aforementioned surface low in southern Saskatchewan. Meanwhile, the Great Lakes remain within a relative stagnant zone in the surface-to-upper- level pattern, which will delivery yet another day of uninteresting weather. Today will be similar to the past week with highs in the upper 80s, a lake breeze, and low humidity levels. A few clouds should dot the sky this afternoon and evening, closing out what will likely be the last meteorologically quiet day for a while. Tonight, the passage of a subtle upper-level shortwave will encourage the explosive development of thunderstorms along the southeastward moving cold front in southeastern Minnesota, western Wisconsin, and northeastern Iowa. With time, the thunderstorms are expected to grow upscale into a cluster or two while moving southeastward toward northern Illinois. With the convection seemingly likely to outrun the cold front and moisture axis, it should begin to wane while moving into Illinois toward daybreak Friday. With that said, areas near the Wisconsin state line or near I-39 may see periods of showers and hear perhaps a clap of thunder Friday morning. After daybreak on Friday, the moisture axis will slide through northern Illinois and northwestern Indiana leading to noticeably more humidity conditions. Compared to 24 hours ago, model guidance has slowed the arrival and passage of the cold front, now favoring the afternoon hours. Based on how fast morning clouds (from the decaying showers) erode and when the front actually moves through our area, there now appears to be a window of time for the redevelopment of showers and thunderstorms along the front, favoring areas along and east of I-55. While low-level shear looks pretty NIL along the front, the nose of a 100kt 250mb upper-level jet will poke into our area behind the front providing some 40 to 50kt of straight-line deep-layer shear. When combined with around 1000 J/kg of MLCAPE and DCAPE, any sustained storm along the front Friday afternoon will have the opportunity to develop mid-level rotation supporting a threat for both damaging wind- and hail-producing splitting supercells. In all, felt there was enough of a signal for a localized severe weather threat to collaborate an introduction of a "Day 2" Level 1/5 threat level area generally along and east of I-55, valid for Friday afternoon. (Keep in mind that a Level 1/5 threat level equates to a 95% chance that there -won`t- be severe weather within 25 miles of any given point). This weekend, the upper-level trough responsible for the system along the California Coast will eject into the Plains and Midwest. While spread in the ensemble space has decreased appreciably in the past 24 hours, there remains uncertainty in exactly how the trough will evolve and when it will arrive owing to opportunities for phasing with other upper-level shortwaves near the US/Canadian border. With that said, a signal is emerging that a surface low may develop and track close to our area sometime in the Sunday to Monday timeframe, supporting waves of much-needed rainfall. Even if not drought-busting, ensemble guidance continues to tick upward in the range of total rainfall amounts with EPS 75th percentile QPF values (which are probably more representative than the mean or 50th percentile values given thunderstorms will be in play) now exceeding an inch across most of our area. And, while certainly not the only scenario out there, we will have to keep an eye on the timing and track of the surface low particularly in the Monday timeframe as several individual ensemble members depict an evolution that can support severe weather in our area. Regardless, the message at this time is to expect waves of showers and thunderstorms sometime from this weekend into early next week. After the system early next week, ensemble model guidance is locked in on seasonable temperatures (highs in the low to mid 70s, lows in the upper 50s to lower 60s) and continued opportunities for rainfall to close out the month. Borchardt && .AVIATION /06Z TAFS THROUGH 12Z FRIDAY/... Issued at 1228 AM CDT Thu Sep 19 2024 Forecast concerns include... Lake breeze/southeast wind shift mid/late afternoon. Chance of showers early Friday morning with thunder possible. Light/calm winds overnight will become south/southwest after daybreak with speeds into the 6-10kt range. A lake breeze will develop by early afternoon and there is uncertainty for how far inland the lake breeze will move, likely reaching both ORD/MDW though only medium confidence for timing, which could be late afternoon at ORD. Even if the lake breeze doesn`t make it to ORD or MDW, winds will shift back to southeast by early this evening across the entire area. Winds are then expected to slowly turn back to the south/southwest early Friday morning. There will be a chance of showers early Friday morning, likely just after 06z at RFD and in the predawn hours for ORD/MDW. Opted to add prob mention for the 30 hour ORD/MDW tafs though whatever precipitation does develop may be dissipating as it moves across northeast IL. There is also a small chance for an isolated thunderstorm, with the best chance, though still low, across northwest IL and at RFD. Fog potential early this morning looks rather low with perhaps the best chance across far northeast IL and into southeast WI. Not planning any fog mention for the terminals at this time. cms && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago