Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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924 FXUS63 KLOT 131735 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1235 PM CDT Thu Jun 13 2024 .KEY MESSAGES... - There is a threat for severe weather this afternoon and evening. Hazards include destructive hail (2"+), wind (75mph+), and flash flooding. - Hot and humid conditions are expected next week. - Periodic low chances for thunderstorms will return next week. && .UPDATE... Issued at 1031 AM CDT Thu Jun 13 2024 The forecast for this afternoon and evening appears to be on track. A recent hand surface analysis depicts a surface pressure trough and associated cold front extending from central Wisconsin to northeastern Kansas with several embedded areas of relative low pressure minima. Ahead of the cold front, a somewhat tight low-level pressure gradient and associated breezy southwest winds are facilitating northeastward moisture advection into northern Illinois with surface dew points rising into the mid to upper 60s. When combined with seasonably warm surface temperatures already nosing into the mid 80s, a pool of instability is quickly building across the Upper Mississippi River Valley ahead of the cold front. (With that said, low-level thermal capping remains over our area as confirmed by a dearth of growing cumulus fields and recent AMDAR soundings). Early this afternoon, an upper-level shortwave and related upper-level gravity waves (all seen via visible and water vapor satellite imagery along the Iowa/Minnesota borders) will move across southern Wisconsin and provide a "glancing blow" to the destabilizing airmass over northern Illinois. Our expectation is that the shortwave should be able to initiate thunderstorms this afternoon across northern Illinois, though the exact coverage, when, and where thunderstorms initially develop remain tricky items to tack down. Initial thunderstorms may develop out of the growing ACCAS field over southern Wisconsin as early as 12-1pm, or later on this afternoon over the I-80 or I-88 corridors between 3-5 pm during "peak heating." There is also a scenario where initial thunderstorm development across northern Illinois is relatively sparse due to the (too early?) arrival of the shortwave and otherwise nebulous forcing as the cold front stalls overhead. It may not be until any convectively-generated gravity waves or an MCV in Iowa this afternoon moves into northern Illinois toward or after sunset that thunderstorms really take off in our local area, by that point along and south of I-80. Generally speaking, the later it takes for storms to develop, the further south the threat will be. Regardless of when and were thunderstorms develop, an extremely unstable airmass characterized by forecast MLCAPE >3000 J/kg and convective- layer shear >45kt will support supercell structures with a threat for damaging to significantly damaging hail (locally >2" in diameter). In fact, with the majority of the shear profile centered in the "mid- levels" and >1000 J/kg of CAPE in the hail-growth layer, a few giant hailstones >3" cannot be ruled out today somewhere in the general region (Iowa or Illinois). And, DCAPE >1200 J/kg will support damaging to destructive downbursts (locally >75 mph), especially where any localized clustering or upscale growth can occur. This is all to say that the SPC level 2/5 to 3/5 severe weather threat (including relatively rare 10% "hatching" for both destructive hail and winds) remains valid across most of the area. Finally, we will have to keep a close eye on any mesoscale corridors in which training convection can occur as the high PWAT airmass will be favorable for rainfall rates >2"/hr, particularly after sunset as the cold front stalls south of I-80. Indeed, a few individual CAM runs depict swaths of 3 to locally 6" of rain this evening in the general region. It is not out of question that a targeted flash flood watch may be needed as confidence increases in where, when, and at what coverage thunderstorms will develop. In all, the message remains the same to remain weather aware this afternoon and evening. Updated products are being sent. Borchardt && .DISCUSSION... Issued at 320 AM CDT Thu Jun 13 2024 Through Tonight: Strong warm and moist advection continues early this morning across the area which has held temperatures in the 70s and dewpoints in the lower 60s. After daybreak expect dewpoints to further increase into the upper 60s and perhaps even near 70. This will make for a warm and muggy day with highs in the upper 80s to around 90 and heat indices in the low-mid 90s. We continue to monitor a couple areas of upstream convection at this hour, one small cluster moving east southeast across east central Iowa and another broader axis of convection extending from southeast Minnesota eastward across central Wisconsin. The former may bring a few showers (maybe a rogue lightning strike) into areas generally southwest of a Dixon to Paxton, IL line around daybreak. The latter may just clip areas near the WI/IL stateline near to shortly after daybreak as well. Most areas will remain dry but a general increase in cloud cover is likely for the first part of the morning. Given an overall lower shower/storm coverage, while acknowledging a brief period of increased cloud cover may delay destabilization initially this morning, earlier concerns of morning showers shunting the effective front well south of the Chicago metro are becoming a less likely outcome with each passing hour. Accordingly, confidence is increasing in scattered to potentially numerous thunderstorm development this afternoon along a cold front, potentially as early as 1-2 PM CDT across southeast Wisconsin and along the WI/IL stateline. These storms would then likely increase in coverage with southward extent as they move across the area into the evening. As expected, there remain subtle timing, coverage, and placement differences among the various suite of available model guidance but there is enough consistency to boost shower/storm chances into the "likely to definite" range (60-80%), highest south of I-80. Instability upwards of 2000-3000+ J/kg combined with deep layer shear of 35-45+ kts will support supercell structures with initial storm development this afternoon. Main hazards include significant hail (2"+) and wind gusts (75mph+). With the arrival of a subtle mid-level wave, further weakening of any remaining convective inhibition should occur toward late afternoon/early evening, suggesting storm mode transitions to congealing multi- cell clusters and bowing segments as storm coverage increases, particularly south of I-80. The SPC Day 1 Outlook for severe weather remains mostly unchanged with a level 2 of 5 risk for much of northern and northeast Illinois and northwest Indiana and a level 3 of 5 risk confined to areas southwest of the Chicago metro where the greatest instability is expected. A northeastward expansion of this area into the metro certainly cannot be ruled out with later updates. Any storms that develop across the entire area will be capable of destructive hail (2"+) and wind gusts (75+ mph). A tornado cannot be fully ruled out today given an initial supercellular mode in the afternoon, and then later into the evening after storms grow upscale, but weak low-level shear and high LCLs support no more than a 2% tornado risk. Lastly, high PWATs and the potential for a period of training storms this evening, supports a continued mention of a localized flash flooding risk. Petr Friday through Wednesday: Primary forecast concerns for the extended remain heat/humidity and thunderstorm chances. High pressure will be moving across the upper Great Lakes region Friday and this will send a weak cold front down Lake Michigan, arriving in the mid/late afternoon. While winds will already be northeast near the lake, along with cooler temps, this front will usher in cooler air across the entire cwa through Friday evening. After highs in the lower/mid 80s for most locations Friday, temps will be falling into the 60s by sunset along with lower humidity levels. Low temps by Saturday morning will be in the 50s for most areas. High temps will be back in the 80s for most areas on Saturday with dewpoints still only in the 50s. East/southeast winds will keep the IL shore of Lake Michigan cooler. As has been advertised for several days, the upper ridge will begin to build north across the region on Sunday. To some degree, the models and their ensembles have had a chance of thunderstorms from early Sunday morning through early Monday morning, as the ridge has not yet fully spread north of the cwa. And while confidence remains low, this time period, Sunday-ish, seems to be a time period to monitor for the potential for some storms, with uncertainty for timing/location. Highs on Sunday could easily be in the mid 90s for most of the area absent of any significant cloud cover/precipitation. After Sunday, confidence is very low for thunderstorm chances. The ECMWF and its ensembles are hot and dry for several days next week with the upper ridge firmly in place while the GFS continues to show frequent precip chances, which would also impact high temps, pushing them lower then currently advertised. Blended pops have some form of low chance pops from Saturday night onward and while any one of these periods may have some precip, much of the extended is likely to be dry with little skill trying to time any impulse. Uncertainty increases further by Wednesday/Thursday when a cold front may approach from the northwest bringing what will likely be the next best chance of precipitation (after Sunday) and if the front does move through the area, a break from the hot and humid conditions. As for heat index values, current highs in the lower/mid 90s and dewpoints generally in the upper 60s to around 70 yield heat index values in the upper 90s to around 100. Again, with no precip influences/effects. Plan to starting mentioning this heat potential in the HWO this morning. cms && .AVIATION /18Z TAFS THROUGH 00Z SATURDAY/... Issued at 1235 PM CDT Thu Jun 13 2024 Forecast concerns for the terminals... - Scattered to widespread thunderstorms this afternoon and evening, some storms could be severe - Winds become northerly behind a cold front this evening - Lake breeze to move through the Chicago terminals Friday afternoon Breezy west-southwest winds have developed at the terminals, especially the Chicago ones, this morning as the atmosphere has mixed into a low-level jet overhead. While this jet is gradually diminishing, gusts in the lower 20 kt range will likely persist through early afternoon. At the same time, an upper disturbance and associated cold front will continue to move towards northern IL from WI which will aid in developing scattered to widespread thunderstorms this afternoon and evening. While confidence is high that storms will develop, the confidence on exact timing and coverage over the terminals, especially at RFD, is lower. Therefore, I have maintained the TEMPOs and their respective timings which seem reasonable based on latest guidance trends. However, I did extend the VCTS mentions at MDW and GYY an hour later to account for the potential for the front and associated storms to stall near I-80 this evening. Additionally, there is also the threat that any storms this afternoon and evening could become severe with the threats of large hail (possibly in excess of 2 inches), damaging winds (possibly in excess of 60 kts), and locally heavy rain which will result in reduced visibilities. Storms will gradually come to an end from north to south late this evening as the front moves through. As a result winds will become northerly (possibly varying between 350 and 010) with speeds in the 6 to 8 kt range overnight and persist into Friday morning. Though a lake breeze is expected to develop early Friday afternoon and push through the Chicago terminals between 18z and 20z. Otherwise, expect mainly VFR conditions outside of storms with gradually scattering clouds overnight into Friday. Yack && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago