Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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360 FXUS63 KLOT 020626 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 126 AM CDT Thu May 2 2024 .KEY MESSAGES... - Widely scattered showers with a few embedded non-severe storms are expected late Wednesday night into Thursday morning - Showers and storms are expected late Thursday into Friday. A few storms late Thursday afternoon into Thursday night could be strong to severe - Wet antecedent conditions and strong storms may also occur Thursday into Friday, most likely near and west of the I-39 corridor - Periodic chances for showers and thunderstorms next week, with the best chance possibly Tuesday afternoon/evening when there may be a severe potential. && .UPDATE... Issued at 956 PM CDT Wed May 1 2024 Going forecast remains largely on track for tonight into Thursday morning. A surface low continues to take shape over the western Plains and will begin to move eastward overnight as its parent upper trough, currently over the northern Rockies, ejects into the northern Plains and upper Midwest. In doing so, this will lift the warm front draped across the Missouri Valley northward into northern IL and northwest IN Thursday morning. The warm-moist advection associated with the front should aid in saturating the parched lower atmosphere and give way to isolated and widely scattered showers late tonight through Thursday morning. While guidance continues to be rather unimpressed with any showers developing, forecast soundings do show some modest saturation occurring in the mid to lower atmosphere in conjunction with steepening lapse rates which I suspect will be sufficient and thus have maintained the going 20 to 30 percent POPs. A better coverage of showers is still expected for Thursday afternoon as the surface low tracks into the upper Mississippi Valley and the warm front stalls in our area. In fact, forecast soundings are showing a decent amount of destabilization occurring, especially in the warm sector, which will promote scattered thunderstorms as well. However, there continues to be uncertainty as to how the warm front will interact with the cooler air advecting off Lake Michigan which is making it difficult to pin point where the front will stall. Depending on where the front stalls will dictate where the greatest coverage of showers and storms sets up. The latest forecast trend has for the warm front to stall near the IL-WI line (as opposed to I-80 as before) which would keep the better storm coverage more into southern Wisconsin and northern IL, but if the front gets hung up on the lake then it could cause the greater coverage to occur over the Chicago metro into northwest IN. Additionally, there is also a threat for a few storms to become strong to possibly severe given the presence of modest 25 to 35 kts of shear. If any severe storms are to materialize the greatest threat would be gusty winds and some hail, but any surface based storms that occur along the warm front may also contain a brief tornado threat. Thankfully, instability is forecast to diminish rather quickly after sunset so the severe threat window should be confined to the 3 PM to 7 PM CDT timeframe. Though, additional non-severe showers and storms are expected to persist through Thursday night. Otherwise, expect another seasonably warm day with highs in the 70s to around 80, especially for those south of the warm front, with cooler conditions likely along the lakeshore. Yack && .DISCUSSION... Issued at 235 PM CDT Wed May 1 2024 Through Friday: Despite the clearer skies over Illinois, stratus clouds over central Iowa are slowly moving east. While there are some weak returns showing up on radar, drier air in place will keep conditions dry. As additional moisture moves north with isentropic upglide providing enough lift that may result in some showers or even an elevated thunderstorm overnight, though the highest probability (around 30 percent) will be in northwest Illinois. Winds are expected to turn east-northeasterly overnight in response to the next area of low pressure shifting towards the Mid-Missouri Valley. The continued influx of a drier low-level airmass on these easterly winds is leading to models b backing off on rain potential tonight. With a surface low out in the Central Plains today, it will move north and east through the end of the week, drawing a warm front northward Thursday. As the warm front moves, temperatures and dew points to its south will steadily increase through the afternoon, with highs in the upper 70s and low 80s forecast. However, exact temperature values across far northern IL will be strongly dependent on how far north the warm front actual reaches. However, as winds will be out of the east to northeast on Thursday north of the warm front, cooler conditions (in the 60s) can be expected along the immediate Illinois lakeshore. Model soundings show a strong capping inversion for much of the day with all instability aloft. While scattered showers and even isolated elevated thunderstorms are possible through much of the day, there is increasing chances for storms to develop in the late afternoon if the cap does erode. While deep layer shear does not look particularly strong tomorrow, there would be enough instability with steeper lapse rates loft to allow for some strong to even severe convection to develop. While it appears to be a conditional threat, it prompted the expansion for the convective threat to a level 1 out of 5 for tomorrow. By Thursday night, the cold front associated with a surface low will arrive and finally move east through northern Illinois through the overnight. This would allow for another chance for showers and some storms to pass over the area. Additionally, with the anomalously high moisture content for this time of the year (above 1.3") and the antecedent wet soils, there is a potential for localized flooding to occur in the area where heaviest rain rates develop. The front is expected to move east of the area Friday morning as winds switch back to the west to northwest. Lingering showers may provide pockets of light rain through the early afternoon, though probability for precipitation diminishes through the day. Cooler air behind the front will keep high temperatures in the upper 60s to 70s on Friday. DK Friday Night through Wednesday: The extended period looks active with various systems expected to transverse the region, the first on Saturday/Saturday evening and a second Sunday night into Monday. Still uncertainty for timing and neither of these systems look all that strong and instability/ shear also remain limited. Nevertheless, low chance pops for both time periods are warranted. The pattern begins to shift on Monday, with warmer and more humid air expected to spread north and across the region. Precip chances certainly warranted during this time period but trends are unclear for timing/coverage Monday and Monday night. Its possible that convection is focused further west of the area on Monday afternoon/evening and if it moves into the local area, it would be in a decaying phase later Monday night. However, by Tuesday, there has been some consensus that thunderstorms will develop across/near the area in both previous runs and the ensembles. With highs possibly tagging 80 and dewpoints in the 60s, there should be plenty of instability. While not yet shown in SPC outlooks, this time period will need to be monitored for a severe potential. Precipitable water values also look to increase into the 1.5 inch range, which if these materialize would lead to the potential for heavy rain. cms && .AVIATION /06Z TAFS THROUGH 12Z FRIDAY/... Issued at 126 AM CDT Thu May 2 2024 Key Messages: - Periods of showers likely today and tonight. - Potential for thunderstorms this afternoon into this evening. - Low confidence in wind direction/wind speed this afternoon into this evening. Periods of showers are likely to be observed over a good chunk of the TAF period as a low pressure system and its associated fronts approach from the southwest. An initial wave of showers is possible after daybreak this morning out ahead of a warm front lifting northward into the area. However, confidence in these showers developing is only medium, and their coverage may be isolated enough to not affect the terminals much anyways. There will probably be a break in the shower activity late this morning into the early afternoon before an additional wave of showers and scattered thunderstorms approaches from the southwest ahead of a cold front. While confidence is medium to high in thunderstorms occurring in northern Illinois today, particularly along the leading edge of this second wave of precipitation, confidence in the timing of these storms and whether the storms will affect any particular TAF site remains somewhat low due to the somewhat disorganized character of this convective activity. Have elected to largely retain the inherited PROB30 groups for TSRA for now and will continue to reassess. If storms do end up passing over the terminals, they may produce some stronger winds and will likely reduce visibility below VFR levels. Going into tonight, shower coverage is expected to become more widespread, but thunderstorm coverage should dwindle due to the loss of instability with time. Ceilings will likely lower as the showers continue over the course of the night, with MVFR ceilings possible towards the end of the current TAF period. Lastly, the wind forecast for this afternoon into this evening is not an easy one. Initially easterly winds should veer more southeasterly later this morning into the early afternoon as the aforementioned warm front approaches. Winds may then become southerly south of the warm front as it passes by, but some uncertainty remains as to how far north the front will get as the influence of onshore flow off of Lake Michigan will probably cause it to get hung up somewhere in the Chicago metro, which would tend to favor winds remaining east of south at the Chicago metro terminals. If thunderstorms end up passing near or over the terminals this afternoon and evening, then that will likely cause winds to become quite messy for a time before a general trend towards a southwesterly wind direction is expected tonight. Finally, winds should turn westerly towards the end of the current TAF period as the cold front passes through. Ogorek && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago