Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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162 FXUS63 KLOT 241647 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1147 AM CDT Fri May 24 2024 .KEY MESSAGES... - Expect multiple rounds of showers and storms today with some severe weather threat. A severe thunderstorm watch goes through early afternoon, and additional severe weather is also possible later today. - The main threats for today will be damaging winds, hail, and even a few tornadoes - There are increasing chances for another round of severe thunderstorms on Sunday && .MESOSCALE... Issued at 1148 AM CDT Fri May 24 2024 The line of storms that moved out of Iowa earlier today is passing through the central portion of the local CWA as of late morning. Generally this activity is staying below severe levels but we are watching it closely for some local intensification as well as possible embedded rotation. The recent cell warned in Kendall/Will counties is an example of the continuing hail/wind potential as well. Deep layer shear in the 30-40 kt range and low level helicity thresholds 100-200 m2/s2 are somewhat marginal for widespread organized convection and rotation. The air ahead of this line is uncontaminated by existing convection and characterized by a ridge of SBCAPE values exceeding 2000 J/kg which would support sustained or new convection as the activity continues east. IR satellite imagery shows warming cloud tops for most of the existing convection, but new growth along the southern and western edges, likely related to ongoing support from a lingering low level jet and an approaching cold front, respectively. The question for later this afternoon into the evening is how much redevelopment to expect in the wake of this initial round. CAMs have generally not been handling this first line very well at all, which raises concerns about putting too much trust in their depiction of storms later today. But thinking of the synoptic and mesoscale influences, we are looking at height falls aloft in conjunction with a cold front approaching from the west which is already supporting renewed activity across central and eastern Iowa. This is despite not having the advantage of peak heating, and also being closer to the back edge of the morning activity. Additional support would be provided from warm and moist boundary layer advection essentially in the form of a warm front as the outflow from this initial round stalls out and eventually gets pushed back north. Better deep layer shear along the approaching cold front and backing of low level winds along the stalled outflow boundary and/or advancing warm front would support better organization and rotation potential for storms that do develop later in the afternoon or evening. Hail/wind and possibly tornadoes would all be in play, but given the potential lingering influence of this initial round through the early part of the afternoon period, there is less confidence of the second round being a widespread severe event. Lenning && .DISCUSSION... Issued at 310 AM CDT Fri May 24 2024 Through Saturday... After a fairly nice Thursday with warm temperatures and dry conditions, a quick look at the satellite loops this morning portends a bit of a change to kick off the Memorial Day weekend. A sharp upper trough is working its way through the northern Plains early this morning. Out ahead of the trough is a complex of strong to severe thunderstorms, a bit more potent along and south of the effective warm front, and there are some additional storms ahead of the line as well. The surface pattern is a bit more complicated, with a series of lows analyzed across northern Kansas from which emanates a warm frontal boundary that extends east into central Illinois and toward the eastern seaboard. A second low is across the northern Plains, with a cold front to its south and a warm front across the northern Great Lakes. With time these lows will consolidate into NW Minnesota on Friday. The upper trough will be moving to the northeast, thus the low level mass response will drive the warm front will north this morning. With the warm front lifting north, warm moist sector and its associated instability will likely allow the thunderstorm complex to intensify as well, adding some confidence that this line will be ongoing toward daybreak. There will be some shortwave ridging in the morning. Also, the warm sector will not quite be established across northern Illinois in the morning. Therefore we have at least medium confidence in some weakening to the MCS, and even some lifting to the north this morning. Still, these will pose some threat for some hail given forecast soundings show some low level inhibition may be present, though there could still some lower end damaging wind threat. Things get a bit more complicated depending on where the outflow from this complex will setup, and this will have implications for the severe threat in the afternoon, and what types of hazards will be in play. Conceptually, if there is no major convection in the morning, the warm sector will become established and at least a modest increase in the low to mid level wind fields, providing a favorable instability-shear space. This is where messaging the best thunderstorm, and severe thunderstorm potential, becomes more challenging. In the afternoon it does appear that forcing is a bit more nebulous, more associated with warm advection and shortwave activity and suggestive of scattered festering convection. However, the instability will be the highest in the afternoon, and would be the most likely timing for severe weather. Plus, any convectively induced vort max could put a wrinkle in things, and would certainly increase the convective coverage potential. Our highest concern area is in initially toward the Rockford area and I-39 corridor closer to the better forcing, with this spreading closer to the Chicago area late in the afternoon. Deep layer shear is slightly higher to our northwest, thus instances of damaging winds and hail would be concerns, but anywhere close to the warm front or composite warm front/remnant outflow boundary from morning convection would also pose a tornado threat. With some uncertainty to this location, this threat would exist farther south into IL, which is what prompted the SPC expansion of the tornado hazard in the Day 1 outlook. Also, discrete cells or clusters appear the most likely mode, given the shear vector more normal to the advancing cold front. Finally in the evening, better height falls will overspread the area from west to east along with the cold front. This appears to be another period for thunder getting into the Chicago area and especially south of I-80, depending on the footprint of the afternoon convection. The severe threat is still highest to the west where instability may not get pinched off as early. Behind the front tonight, the upper low will exit northeast of the area, bringing a period of shortwave ridging/surface high pressure. Temperatures will drop back a bit, especially lakeside with onshore flow, but modest temperature recoveries will occur inland to support mid 70s given plentiful sunshine and very dry conditions. KMD Saturday Night through Thursday: Another period of active weather is likely (60%+ chance) for the area on Sunday as another potent short-wave trough drives a deepening sub-995 mb surface low northeastward into southern WI during the day. Ensemble solutions continue to show some spread in the actual track of the surface low across northern IL into WI Sunday afternoon. However, while this is the case, the overall synoptic setup of a deepening sub 995 mb surface low tracking northward into southern WI through the afternoon is certainly one that is concerning for the potential for severe thunderstorms in our area. For this reason, this is certainly a period we will have to monitor closely. The main weather message at this point for Sunday is to pay close attention to the later forecast updates regarding thunderstorms, and in particular severe thunderstorms. The SPC currently has much of the area highlighted in a level 2 out of 5 for severe weather, with southern sections of the area in a level 3 out of 5. The entire day will not be washout, but the potential continues for a couple of rounds of thunderstorms, one potentially early in day, and a potentially more significant round of severe thunderstorms in the afternoon and evening. Following Sunday`s storm system, a building upper-level ridge across the Rockies is expected to setup a cooler, but still active, northwesterly upper-level flow pattern across the Great Lakes region early next week. Ensemble guidance suggests that this pattern will act to steer a few smaller scale impulses southeastward into our area for Monday and Tuesday. Accordingly, there are chances (30-50%) for some scattered afternoon showers (and possibly a few non-severe storms) both days. A period of dry (and rather pleasant) weather is expected mid to late week (especially Wednesday and Thursday of next week) as surface high pressure sets up shop over the Great Lakes region. Daytime temperatures are forecast to be in the low to middle 70s both days under mainly sunny skies. KJB && .AVIATION /12Z TAFS THROUGH 18Z SATURDAY/... Issued at 630 AM CDT Fri May 24 2024 Key messages/concerns: - Line of gusty thunderstorms mid to late morning (14-17z). - Potential for another round of strong thunderstorms late this afternoon into the evening. A line of severe wind producing storms have maintained themselves through the night across western and central IA. Unfortunately, forecast guidance has not been handling the evolution of these storms through the night very well at all, which has lead to overall lower forecast confidence then is typical. However, given the observational trends with the bowing line of storms through the night, we currently see no reason why it will not continue eastward across northern IL through the mid to late morning hours. Its current movement would bring it across KRFD in the 14-16z timeframe, and in the 15 to 17z timeframe for the main Chicago terminals. Strong gusty winds, potentially up to around 50 KT can be expected with this line of storms. Winds will then likely become somewhat chaotic in direction for a period following these storms before directions gradually settle back into an east-southeasterly direction through the afternoon. Given the current expectations with this morning line of storms, confidence has decreased with the prospects for the second advertised round of severe storms later this afternoon and evening. However, in spite of this, we have opted to make no changes to the going forecast at this time. Quieter weather is expected following the cold frontal passage later this evening through Saturday. KJB && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago