Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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135 FXUS63 KLOT 261641 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1141 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. && .MESOSCALE... Issued at 1141 AM CDT Sun May 26 2024 An initial batch of rain and embedded thunderstorms associated with a lead shortwave interacting with a plume of steep mid- level lapse rates will be pushing out of our region through noon to 1 pm or so. This may yield a bit of a lull in activity across our region. Noting at least modest pressure falls in and behind the trailing stratiform precipitation region, on the order of 2 to 4 mb/1-2 hour, and there`s also clearly a circulation pinwheeling across La Salle County (in regional radar mosaics) which is indicative of subsidence warming occurring in the mid- levels. This is suggestive of at least a modest wake low (or a pair of mesolows), and have seen commensurate increases in peak gusts across our I-39 observations (to about 35 mph). Typically we need a more robust/severe MCS before we get concerned about more significant trailing non-thunderstorm wind gusts. That said, recent HRRR runs are depicting two swaths of strong southeasterly gusts in excess of 40-45 mph (across our NW and far SE), and this model tends to sniff out these types of non- thunderstorm gusts pretty well. We`ll keep an eye on observation trends. The main focus remains on what happens later this afternoon. Model guidance now hints at a fairly swift airmass recovery given the slightly earlier arrival and departure of this first batch of precip. Skies look like they may try to clear out across our west through early this afternoon, and seeing breezy southwesterly winds developing across SE Iowa where clouds have eroded with temperatures jumping into the 70s. HRRR/RAP guidance continues to advect mid and even upper 60 degree dewpoints into the region this afternoon which seems perhaps a smidge high based on upstream observations. Even accounting for this, instability is forecast to build, with MLCAPE climbing back towards 1500 J/kg late in the afternoon, focusing in a relative corridor south of I-88 and east of about I-55. The main surface low (convectively-augmented) will be lifting northeast into southern Wisconsin, with only a glancing blow in our area of the more concentrated region of DCVA overlapping the best instability/kinematic parameter space south of I-80. This suggests storm coverage will probably become increasingly isolated the farther south you go through our forecast area, with the greatest coverage tied to the surface low across our north and towards the Wisconsin state line. All of this said, there is some growing concern that a late-day severe threat may indeed materialize in the 3 to 10 pm time frame as a result. Low-level shear doesn`t look particularly impressive, but deep layer shear values nearing 40 knots suggest a potential for mid-level updraft rotation/organization. It looks like the main threats would be from large hail and strong wind gusts, but can`t rule out a brief tornado potential in the vicinity of lingering backed southeasterly winds. Carlaw && .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 /12Z TAFS THROUGH 18Z MONDAY/... Issued at 637 AM CDT Sun May 26 2024 Key messages/forecast concerns: - Periods of showers and thunderstorms this morning through early afternoon. - Chance for a few additional widely scattered thunderstorms into early evening with a cold frontal passage. - East-southeast winds through the day, shifting westerly this evening. - Low MVFR CIGs Potential tonight. Thunderstorms have moved into northern IL a bit faster then previously forecast. For this reason, we have moved up the timing of precipitation for the Chicago area terminals to 13z. This initial "hit" of storms may only last a hour or so, before the main batch of showers and storms moves into the area from the southwest during the mid to late morning hours. So, beyond moving up the start time, not much has changed with the forecast thinking today. The main threats from these storms will be locally gusty winds and some heavy rainfall. The heavy rainfall may create some temporarily IFR VSBYs. Following the periods of morning thunderstorms, we will have to monitor the threat from some renewed widely scattered storm development later in the afternoon and evening (especially after 22z) as a cold front approaches. Confidence remains low on the extent of this redevelopment, but with the main weather impulse and surface cold front approaching, it remains plausible that some additional thunderstorms could develop into early this evening. Winds will generally be east-southeasterly through much of the day, though the thunderstorms could lead to some periods of more erratic winds for a period later this morning. Winds will then shift to a west-southwesterly direction into this evening following a cold frontal passage. West to northwesterly winds, gusting up to 20 kt at times, are then expected late tonight and on Monday. 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, with a gradual improvement likely later Monday morning. KJB && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago