Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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367 FXUS63 KLOT 260806 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 306 AM CDT Sun May 26 2024 .KEY MESSAGES... - Periods of showers and thunderstorms today, with pockets of gusty winds possible in the morning and a highly conditional risk for strong to severe storms late afternoon into the evening. - Widely scattered afternoon and early evening showers (and possibly a few non-severe storms) on Memorial Day and again Tuesday. - Gradual warming and drying conditions for the end of the week. && .DISCUSSION... Issued at 306 AM CDT Sun May 26 2024 Through Monday: Early this morning, we find a center of low pressure spinning up out over the central Plains with the storm`s warm front extending across central Missouri and into southern Illinois. On either side of the front and as far northward as central and eastern IA, efficient low- mid level warm advection is driving a large swath of showers and thunderstorms. A handful of stronger storms can be found south of the front beneath an impressive EML plume which will eventually be working over our area this afternoon. As the low treks northeastward, these showers and storms will continue to be pushed across the Midwest and fall on the area this morning. Precip should begin to approach the I-39 corridor sometime around dawn. The storms currently out in eastern IA are expected to weaken some after crossing the Mississippi as they outrun the better instability, especially north of I-80. Some late CAM guidance suggests that we could see a mature linear MCS develop ahead of the low prior to moving across northern Illinois later in the morning. If organized convection can get going this morning, locally strong to marginally severe wind gusts may manifest, particularly in the far southern reaches of the CWA. The HRRR is one such model and even takes it a step further resolving a mesolow with a tight pressure rise/fall couplet on the leading edge. Such an artifact would bump the severe wind potential up a bit and could also bring the potential a little farther north during the morning, possibly across portions of the Chicago metro. In either case, the severe wind threat looks notably better just south of the CWA where we`ll find much more instability and shear. Embedded thunderstorms should be on the overall lighter side north of I-80 where far less instability will reside. The brunt of the morning activity will be out of the area by early afternoon. A lot of big question marks continue to revolve around how our environment will recover following the morning. The aforementioned EML will overspread the region during the afternoon building up our mid level instability. Our afternoon and evening storm potential, including the severe threat, will be heavily reliant on how much clearing we can see behind the morning push. Even then it`s tough to say how the low levels will respond with some models having similar looks regarding sky cover but very different ideas for low level thermo profiles. Undoubtedly, the best potential for seeing surface-based convection resides south of the effective warm front which should remain well south of the area. North of the front, guidance is messy but most can agree that we will struggle to storms based in the lowest 100 mb or so owing to very poor low level lapse rates if not a low level capping inversion. The HRRR is an outlier being the only one showing an uncapped environment with steep low level lapse rates during the late afternoon. Should we manage to get anything surface-based going, an impressive 0-1 km hodograph could mean a few tornadoes attempt to spin up. If storms remain even a bit elevated, which looks to be the best bet, then severe wind and hail would be the biggest concern later today. The greatest severe potential later today will again be across the southern CWA in a few hour window between late afternoon and early evening, though odds are again better down across central and southern IL. The storm threat should pretty much wrap up behind the system`s cold front which will sweep across during the evening. Monday will be a bit cooler for the holiday with highs forecast to top out in the lower 70s. The better part of the day should see a fair mix of clouds and sunshine with good westerly breeze. Several models are now resolving scattered showers or even a couple of thunderstorms during the afternoon. Diurnal heating should do away with a morning cap and steep low level lapse rates will extend up to a reservoir of modest mid level instability. With upwards of around 50 kt of deep layer shear expected, it`s possible that a few thunderstorms could feature some locally strong wind gusts. It`s possible that precip struggles to combat a rather dry profile but could get a boost from an encroaching 500mb jet max. Storm chances should dissolve by mid-evening as instability wanes. Doom Monday Night through Saturday: An upper level ridge axis building across the Rockies early in the week will drive an active belt of enhanced northwesterly flow aloft across the Great Lakes region through Tuesday. Within this pattern, two distinct impulses will track southeastward across our area. The first will track into our area later on Monday, and that is talked about in the discussion above. Following quickly on its heals, a stronger impulse (currently seen spinning in the water vapor imagery across central Manitoba) will eject southeastward into the western Great Lakes on Tuesday along the eastern periphery of the building upper ridge axis. A chilly late May airmass accompanying this impulse is likely to make Tuesday one of the coolest days of the week. In fact, breezy northwesterly winds through the day, combined with increased afternoon cloud cover could keep temperatures in some parts of northern IL from climbing out of the 60s! There will also be another good shot for scattered afternoon showers and even a few storms as lapse rates steepen under the cold mid-level trough (temps at 500 mb around -20C). Following the eastward passage of this second impulse on Tuesday, the ridge axis over the Rockies will shift eastward, likely becoming centered over the western Great Lakes later in the week. As this occurs, we are looking to see at least a few day period of dry (and rather pleasant) weather from Wednesday through the later part of the week as surface high pressure sets up shop over the Great Lakes region. Mostly sunny skies are expected, and while temperatures will remain in the low to mid 70s for Wednesday and Thursday, readings are expected to rebound back to near 80 for the end of the week into next weekend. KJB && .AVIATION /06Z TAFS THROUGH 12Z MONDAY/... Issued at 108 AM CDT Sun May 26 2024 Key messages/forecast concerns: - Couple hour period of showers and thunderstorms expected mid to late this morning through early afternoon. - Chance for a few additional widely scattered thunderstorms late this afternoon afternoon into early evening. - Wind trends this afternoon and evening following midday storms. - Low MVFR CIGs Potential tonight. Thunderstorms continue to flourish across the Plains eastward into parts of the lower Missouri Valley early this morning. This activity is expected to continue eastward overnight, likely reaching areas near the Mississippi around daybreak. Thereafter, we expected a line of storms to shift eastward across KRFD during the mid to late morning hours (~14 to 17z) and across the Chicago area terminals from late morning through early afternoon (~16 to 19z). The main threats from these storms will be locally gusty winds and some heavy rainfall. The heavy rainfall may create some temporarily IFR VSBYs. After this band of storms shift east of the area after 19z, we will have to monitor the threat from some renewed widely scattered storm development later in the afternoon (after 21z). Confidence is currently low on the extent of this redevelopment, across northern IL, but with the main weather impulse right across the region, it is certainly plausible that some activity will be around later today through early evening. We thus continue to carry a PROB30 mention for more potential late day storms. Winds will start the day from the east-southeast this morning, then are expected to gradually shift to a west-southwesterly direction sometime either later this afternoon, or early this evening. The exact timing of this wind shift remains a bit unclear at time, and will ultimately be dependent on where the surface low tracks across northern IL later today. Given the uncertainty, I did not stray from the going forecast of a west- southwesterly wind shift early evening around 01z. However, we will have to monitor the potential for it to occur earlier then currently forecast. Ample moisture wrapping around the backside of the surface low may also foster some lower MVFR CIGs across the terminals tonight into early Monday morning. Currently looks like the best timing for these would come later this evening and overnight. KJB && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago